Human Biology (Zoology): digestive, respiratory, circulatory, excretory, nervous, sense organs, endocrine, reproductive

(c) Myopia due to elongated eyeball

The student sees near objects (books) clearly but not distant ones (the board), which is myopia (short-sightedness). The eyeball is elongated (or the lens too convex), so the image of a distant object forms in front of the retina. Hypermetropia and presbyopia blur near vision instead.

(b) Right atrium

The coronary sinus collects deoxygenated blood from the cardiac veins of the myocardium and opens into the right atrium, which sends it to the right ventricle and then to the lungs.

(a) Labelling the parts

• A — Liver
• B — Pancreas

(b) Why part B (pancreas) is called a mixed gland

The pancreas is called a mixed gland because it is both exocrine and endocrine in function:

• Exocrine part: The acinar cells secrete pancreatic juice (containing digestive enzymes like trypsin, amylase and lipase) which is carried by the pancreatic duct into the duodenum.
• Endocrine part: The islets of Langerhans (α and β cells) secrete hormones — glucagon and insulin — directly into the blood.

Since it performs both exocrine (duct-bearing) and endocrine (ductless) secretion, it is called a mixed gland.

(c) Functions of A and C

Part A (Liver):

1. Secretes bile (stored in the gall bladder), which emulsifies fats and provides an alkaline medium in the duodenum.
2. Site of glycogenesis, glycogenolysis and gluconeogenesis — thus maintains blood sugar level.
3. Detoxifies harmful substances (drugs, alcohol, ammonia) by converting ammonia into urea.
4. Stores glycogen, vitamins (A, D, E, K) and iron; produces plasma proteins, fibrinogen and heparin.

Part C (Gall bladder):

1. Stores and concentrates bile secreted by the liver.
2. Releases bile through the cystic duct into the duodenum when fatty food enters the intestine.

(d) Sphincter at the hepato-pancreatic ampulla

The sphincter of Oddi (also called the sphincter of the ampulla of Vater) guards the opening of the hepato-pancreatic ampulla into the duodenum.

Differentiation between sympathetic and parasympathetic nervous systems (3 marks)

Well labelled diagram of human brain (3 marks)

                    CEREBRUM
        _______________________________
       /                               \
      /   (longitudinal fissure)         \
     /___________________________________\
    |  CORPUS CALLOSUM (below fissure)     |
    |                                      |
    |   THALAMUS        HYPOTHALAMUS       |
    |        \             /               |
    |         \  PINEAL   /                |
    |          \  GLAND   /                |
    |           \________/                 |
    |            MIDBRAIN                  |
    |           /        \                 |
    |     PONS  |        | CEREBELLUM      |
    |           \        /                 |
    |            \______/                  |
    |          MEDULLA OBLONGATA           |
    |______________________________________|
                    SPINAL CORD

Labels required: Cerebrum, longitudinal fissure, corpus callosum, thalamus, hypothalamus, pineal gland, midbrain, pons, cerebellum, medulla oblongata, spinal cord.

Brain ventricles (2 marks)

The brain ventricles are fluid-filled cavities within the brain that are continuous with the central canal of the spinal cord. They are lined by ependymal cells and contain cerebrospinal fluid (CSF).

• Lateral ventricles (first and second): A pair of C-shaped cavities, one in each cerebral hemisphere.
• Third ventricle: A narrow cavity in the diencephalon, between the two thalami. It communicates with the lateral ventricles through the interventricular foramen (foramen of Monro).
• Fourth ventricle: A cavity between the cerebellum and the medulla/pons. It connects with the third ventricle through the cerebral aqueduct (aqueduct of Sylvius) and continues downwards into the central canal of the spinal cord.

Functions: The CSF secreted by the choroid plexus of these ventricles cushions the brain against shock, maintains a constant pressure, supplies nutrients and removes metabolic wastes. Blockage of the aqueduct causes accumulation of CSF (hydrocephalus).

Hormones of the adrenal cortex and their functions

The adrenal cortex is divided into three zones, each secreting a different group of steroid (corticosteroid) hormones:

1. Zona glomerulosa — Mineralocorticoids (mainly aldosterone)

   • Regulate water and electrolyte balance by promoting reabsorption of Na⁺ and excretion of K⁺ in the renal tubules.
   • Maintain blood volume, blood pressure and osmotic balance of body fluids.

2. Zona fasciculata — Glucocorticoids (mainly cortisol, also corticosterone)

   • Promote gluconeogenesis from proteins and fats, thus raising blood glucose.
   • Suppress inflammatory and allergic reactions (used therapeutically as anti-inflammatory drugs).
   • Help the body resist stress and maintain normal blood pressure.

3. Zona reticularis — Sex corticoids (gonadocorticoids) (androgens and small amounts of estrogens)

   • Androgens stimulate the development of male secondary sexual characters and sexual drive in both sexes.
   • In females they contribute to libido, body-hair growth and the onset of puberty.

Disorders of adrenal cortical hormones

1. Addison's disease (hyposecretion of cortical hormones): Caused by atrophy or destruction of the adrenal cortex (often by tuberculosis or autoimmune damage). Symptoms: bronzed (bronze-coloured) skin due to excess MSH/ACTH, muscular weakness, low blood pressure, low blood sugar, loss of appetite, weight loss, nausea and increased K⁺ with decreased Na⁺. It is treated by giving cortisol and aldosterone (corticosteroid therapy).

2. Cushing's syndrome (hypersecretion of glucocorticoids): Caused by a tumour of the adrenal cortex or excessive ACTH from the pituitary, or prolonged use of corticosteroid drugs. Symptoms: obesity of the trunk and face (moon face, buffalo hump), thin arms and legs, high blood sugar (steroid diabetes), high blood pressure, purple abdominal striae, weak bones and easy bruising. Treated by removal of the tumour or reducing steroid dose.

3. Conn's syndrome (hypersecretion of aldosterone): Due to an aldosterone-secreting tumour of the zona glomerulosa. Symptoms: excessive Na⁺ and water retention, severe hypertension, oedema, hypokalaemia (low K⁺), muscular weakness and alkalosis. Treated by surgical removal of the tumour or aldosterone antagonists.

4. Adrenogenital syndrome (hypersecretion of sex corticoids): Caused by over-secretion of androgens by the zona reticularis (congenital adrenal hyperplasia). In children it causes precocious puberty; in adult females it causes masculinisation — deepening of voice, beard growth, acne and enlargement of the clitoris. Treated by cortisol replacement, which suppresses ACTH and hence androgen output.

(d) Chyle

The acidic semi-fluid chyme from the stomach is neutralised and acted upon by bile and pancreatic juice in the duodenum; the emulsified fats give it a milky appearance, and this milky fluid is called chyle.

(a) Labelling the parts

• A — Cochlea (the spiral, snail-shell-shaped structure)
• B — Semicircular canals (the three loop-like tubes above)
• C — Vestibule (the central sac-like region containing the utricle and saccule)
• D — Auditory nerve (the nerve bundle leaving the internal ear toward the brain)

(b) Functions

Function of A (Cochlea):

• It is the main organ of hearing. It contains the organ of Corti with hair cells resting on the basilar membrane.
• When sound vibrations reach the cochlea through the oval window and perilymph, the hair cells are stimulated. This generates nerve impulses carried by the auditory (cochlear) nerve to the brain, where sound is perceived.

Function of D (Auditory nerve):

• It carries nerve impulses from the internal ear to the brain.
• Its cochlear part carries hearing impulses from the cochlea, and its vestibular part carries balance impulses from the vestibule and semicircular canals.

I'll solve this question myself first, then compare with the model answer.

My solution:

The question asks for three things: (1) description of the female reproductive system, (2) a well-labelled diagram, and (3) explanation of ovulation. This is an 8-mark question.

Female reproductive system — I'd describe the primary sex organs (ovaries) and accessory organs (oviducts, uterus, cervix, vagina, external genitalia, mammary glands), covering structure and function of each.

Diagram — A proper labelled diagram is required. The model answer provides an ASCII schematic, not a proper biological diagram. For a board exam, a hand-drawn labelled diagram is expected. The ASCII art is crude and not a genuine "well labelled diagram" — though as a text-based response it's a reasonable substitute. The labels list is comprehensive.

Ovulation — The model answer covers: FSH-driven follicular development, oocyte maturation to secondary oocyte (arrested at metaphase-II), estrogen feedback, LH surge, follicle rupture, release of oocyte with zona pellucida and corona radiata, fimbriae pickup, ciliary/peristaltic transport, viability (~24 hours), corpus luteum formation, progesterone secretion, and significance. This is thorough and accurate.

Checking accuracy:

• Ovary size 2–4 cm ✓
• Oviduct 10–12 cm ✓
• Uterus ~7.5 × 5 × 2.5 cm ✓
• Three layers: perimetrium, myometrium, endometrium ✓
• Ampulla = usual fertilization site ✓
• Ovulation on day 14 of 28-day cycle ✓
• Secondary oocyte arrested at metaphase-II ✓
• LH surge triggers ovulation ✓
• Ovum viable ~24 hours ✓
• Corpus luteum secretes progesterone ✓

The content is accurate and complete for an 8-mark answer. The only weakness is the diagram being ASCII rather than a proper biological drawing, but within the constraints of a text-based response, this is acceptable and the labelling is thorough. The description, functions, and ovulation explanation are all correct and well-organized.

The answer is correct and complete.

OK

Structure of a nephron

The nephron is the structural and functional unit of the kidney; each human kidney has about one million. A nephron has a renal corpuscle and a renal tubule.

1. Renal corpuscle (Malpighian body): It lies in the cortex.

• Glomerulus: a tuft of capillaries formed from the afferent arteriole, which is wider than the efferent arteriole that drains it; this keeps the pressure high for filtration.
• Bowman's capsule: a double-walled cup around the glomerulus. The inner wall has podocytes with slit pores, the outer wall is squamous epithelium, and the space between is the capsular space collecting the filtrate.

2. Renal tubule:

• Proximal convoluted tubule (PCT): coiled, cuboidal cells with a brush border; reabsorbs most water, glucose, amino acids and salts.
• Loop of Henle: U-shaped, dipping into the medulla; the descending limb is permeable to water and the ascending limb to salts but not water. It maintains the medullary gradient for concentrating urine.
• Distal convoluted tubule (DCT): cuboidal cells without brush border; reabsorbs Na⁺ and water and secretes K⁺ and H⁺.
• The DCT opens into a collecting duct, which passes through the medulla to the renal pelvis.

Labelled diagram

 Afferent arteriole ─► GLOMERULUS ─► Efferent arteriole ─► peritubular capillaries
                         │ (inside Bowman's capsule)
                         ▼
                 Bowman's capsule
                         │
             Proximal convoluted tubule (PCT)   ← CORTEX
   - - - - - - - - - - - │ - - - - - - - - - - - - - -
        Descending limb ─┴─ Ascending limb      ← MEDULLA
              (Loop of Henle)       │
                                    ▼
             Distal convoluted tubule (DCT)      ← CORTEX
                         │
                  Collecting duct ─► renal pelvis

If ADH is secreted insufficiently

ADH (vasopressin), from the posterior pituitary, makes the DCT and collecting duct permeable to water. With too little ADH:

• Water is not reabsorbed, so a large volume of dilute urine is passed (polyuria).
• The body becomes dehydrated, with intense thirst (polydipsia) and rise in blood osmotic pressure.
• This condition is diabetes insipidus; it is treated with ADH (vasopressin) replacement.

(d) Amylase

Pancreatic amylase is poured into the duodenum through the pancreatic duct, where it digests carbohydrates (starch) into maltose. Lipase acts on fats, while pepsin and trypsin act on proteins.

(c) Accumulation of carbon-dioxide in room

In a poorly ventilated room, respiration by the trapped people steadily uses up oxygen and releases carbon-dioxide. The rising CO₂ (and falling O₂) in the air makes breathing hard and difficult.

(b) Seminal vesicle

The seminal vesicle secretes a fructose-rich alkaline fluid that nourishes and provides energy to the sperm; the prostate and Cowper's glands contribute other secretions, while the testis produces sperm and testosterone.

The figure shows a section of the human stomach (gastric gland region).

(a) Labelling [2]

(b) Function of A [1]

The gastric glands (A) secrete gastric juice into the stomach lumen. Gastric juice contains HCl, which kills ingested microbes, provides the acidic pH needed for pepsin to act and converts pepsinogen into pepsin; pepsinogen, which digests proteins into peptones and polypeptides; mucus, which protects the stomach lining from HCl and from self-digestion; and rennin (in infants), which curdles milk.

(c) Enzyme released from B or C [1]

Pepsinogen is released from B (chief cells). It is the inactive enzyme precursor which, on contact with HCl secreted by C (parietal cells), is converted into the active enzyme pepsin, which digests proteins. (In infants, chief cells also release rennin, which curdles milk.)

Four renal disorders:

1. Nephritis – inflammation of the glomeruli (glomerulonephritis), often following streptococcal infection.
2. Kidney stones (renal calculi) – crystallisation of calcium oxalate, uric acid or phosphate salts in the renal pelvis or ureter.
3. Uremia – accumulation of urea and other nitrogenous wastes in blood due to renal failure.
4. Renal failure / Bright's disease – loss of the kidney's filtering ability, leading to anuria and oedema. (Other accepted answers: polycystic kidney disease, nephrotic syndrome, urinary tract infection, kidney tumour.)

Lifestyle modifications suggested to a family member suffering from such a disorder:

1. Drink plenty of water (2.5–3 litres/day) to keep urine dilute and flush out salts, preventing stone formation and infection.
2. Restrict salt and protein intake – a low-sodium, moderate-protein diet reduces fluid retention (oedema), blood pressure and the load of nitrogenous waste on damaged nephrons.
3. Avoid alcohol, smoking and self-medication – especially painkillers like NSAIDs and aminoglycoside antibiotics, which are nephrotoxic.
4. Control blood pressure and blood sugar – regular check-ups, weight control and prescribed medicines, since hypertension and diabetes are the leading causes of kidney damage.
5. Take regular moderate exercise, avoid a sedentary life and treat any urinary infection or throat infection promptly to prevent further renal damage.

Structure of the Cerebrum

The cerebrum is the largest part of the human brain, forming about four-fifths of its total weight. It develops from the telencephalon of the forebrain and is divided into two cerebral hemispheres by a deep median cleft called the longitudinal fissure. The floor of this fissure is formed by a thick band of white fibres called the corpus callosum, which connects the two hemispheres and allows communication between them.

External features:

• The surface shows convolutions — raised ridges called gyri and grooves called sulci (deeper grooves are called fissures). These folds greatly increase the surface area without increasing the volume.
• Each hemisphere is divided into four lobes by sulci:
  • Frontal lobe — anterior, in front of the central sulcus
  • Parietal lobe — behind the central sulcus
  • Temporal lobe — below the lateral sulcus
  • Occipital lobe — posterior part
• The surface is made of grey matter (cerebral cortex), about 2–4 mm thick, containing cell bodies of neurons. The interior is white matter, made of myelinated axons that carry impulses to and from the cortex. Deep within the white matter lie the basal ganglia (masses of grey matter).

Functional areas of the cortex:

• Sensory areas — receive impulses from receptors (touch, temperature, pain, vision, hearing, smell, taste).
• Motor areas — send impulses to muscles, controlling voluntary movements.
• Association areas — integrate sensory information with past experience; seat of intelligence, memory, reasoning, and emotions.

Cerebrum vs. Cerebellum

Conclusion

The cerebrum is the seat of intelligence and voluntary control, while the cerebellum coordinates movement and balance — together they make the brain the master organ of the body.

Ionic theory (of nerve conduction) states that the resting potential and the action potential of a nerve fibre arise from unequal distribution of ions (mainly Na⁺, K⁺, Cl⁻) across the semipermeable axon membrane and from changes in its permeability to these ions. In the resting state the membrane is more permeable to K⁺ than to Na⁺, so K⁺ leaks out and the inside becomes negative (about −70 mV, the resting potential).

Mechanism of impulse conduction in a nerve fibre

1. Resting (polarised) state: The axon membrane is polarised — outside positive, inside negative (−70 mV). The Na⁺–K⁺ pump actively pumps 3 Na⁺ out for every 2 K⁺ in, maintaining the ionic gradient.
2. Stimulus and depolarisation: A stimulus makes the membrane permeable to Na⁺ at that point. Na⁺ rushes in, the inner side becomes positive (+40 mV) — depolarisation — and this region is now an action potential.
3. Local circuit / current flow: The depolarised region is opposite in charge to the neighbouring polarised region. A local current flows from the depolarised area to the adjacent resting area, acting as a stimulus there.
4. Propagation: The same sequence (Na⁺ influx → depolarisation → local current) repeats in the next segment, so the impulse travels along the whole length of the fibre. It is self-propagating and travels in one direction only, because the region just behind is in the refractory period.
5. Repolarisation: After the impulse passes, K⁺ moves out (and the Na⁺–K⁺ pump works), restoring the original polarity, so the fibre is ready for the next impulse.

Transmission through a synapse

A synapse is the junction between two neurons (or a neuron and an effector). The impulse crosses it chemically:

1. The action potential arrives at the presynaptic knob and depolarises it.
2. This opens voltage-gated Ca²⁺ channels; Ca²⁺ enters the knob.
3. Ca²⁺ causes the synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitters (e.g. acetylcholine) into the synaptic cleft.
4. The transmitter diffuses across the cleft and binds to receptors on the postsynaptic membrane.
5. Binding opens Na⁺ channels, causing depolarisation of the postsynaptic membrane and generation of a new action potential, which then travels along the next neuron.
6. The transmitter is quickly removed by acetylcholinesterase (or reuptake), so the synapse is ready for the next impulse.

Because transmission is chemical, the impulse can pass in one direction only (presynaptic → postsynaptic) and shows a short synaptic delay.

(i) Labelling [1]

• A: Hypothalamus
• B: Pituitary gland (hypophysis)

(ii) Two hormones released from B [1] Any two: growth hormone (GH), TSH, ACTH, FSH, LH, prolactin (anterior lobe); oxytocin, ADH/vasopressin (posterior lobe).

(iii) Birth hormone [2] Oxytocin is called the birth hormone. At the end of pregnancy it strongly stimulates the smooth muscle of the uterus to contract (labour contractions), which expels the foetus during childbirth. It also causes milk ejection from the mammary glands during suckling.

Organs of the male reproductive system

The male reproductive system consists of a pair of testes, accessory ducts, accessory glands and the external genitalia (penis and scrotum).

1. Testes (primary sex organs): A pair of oval organs (about 4–5 cm long) lying in the scrotal sac. Each has about 250 lobules containing coiled seminiferous tubules, where sperms are produced from spermatogonia, supported by Sertoli cells. Leydig (interstitial) cells between the tubules secrete testosterone.

2. Accessory ducts:

• Rete testis and vasa efferentia carry sperms out of the testis.
• Epididymis: a coiled tube on the testis in which sperms mature and are stored.
• Vas deferens: carries sperms up into the abdomen to join the duct of the seminal vesicle.
• Ejaculatory duct: formed by the vas deferens and seminal vesicle duct; it opens into the urethra.
• Urethra: passes through the penis and carries both urine and semen.

3. Accessory glands:

• Seminal vesicles: secrete an alkaline, fructose-rich fluid that nourishes the sperms (about 60% of semen).
• Prostate gland: secretes a milky alkaline fluid that activates the sperms and neutralises the acidity of the urethra and vagina.
• Cowper's (bulbourethral) glands: secrete a mucus-like lubricating fluid before ejaculation.

4. External genitalia: The penis is the copulatory organ made of erectile tissue, with the glans covered by the foreskin. The scrotum is the skin pouch holding the testes.

Labelled diagram

   Urinary bladder
        │
   Vas deferens ── Seminal vesicle
        │               │
        └─ Ejaculatory duct ─ Prostate gland
                             │
   Urethra ── Cowper's gland
        │
     Penis (glans, foreskin)
   Epididymis ── Testis (in Scrotum)

Why the testes lie outside the abdominal cavity Spermatogenesis needs a temperature about 2–3 °C lower than body temperature. The scrotum keeps the testes outside the abdomen at this lower temperature; if the testes stay in the abdomen (cryptorchidism), sperm production fails and sterility results.

V.S. of the human eye

          Cornea
   Aqueous  │   Ciliary body ─ Suspensory ligament
   humour ─►│ ┌──────────────────────────┐ Sclera
   Pupil ─► ( │ Lens     Vitreous humour │ Choroid
   Iris ─►    │                          │ Retina
              └────────┬───────── Fovea  ┘
                       │          centralis
              Blind spot (optic disc)
                       │
                  Optic nerve

Labelled parts: cornea, sclera, choroid, retina, iris, pupil, lens, ciliary body, suspensory ligament, aqueous humour, vitreous humour, fovea centralis, blind spot, optic nerve.

Structure of the eye

The eye is a nearly spherical organ (about 2.5 cm across) in the bony orbit, with three coats and refractive media.

1. Fibrous coat (outer)

• Sclera: tough, white, opaque posterior 5/6; protects the eye and gives attachment to the extrinsic muscles.
• Cornea: transparent anterior 1/6; avascular; the chief refracting surface.

2. Vascular coat (middle, uvea)

• Choroid: dark, vascular layer; nourishes the retina and absorbs stray light.
• Ciliary body: ring of ciliary muscles holding the suspensory ligaments of the lens; changes lens curvature (accommodation) and secretes aqueous humour.
• Iris: pigmented muscular diaphragm giving eye colour; its muscles change the size of the pupil and so regulate the light entering.

3. Nervous coat (retina, inner) Contains rods (rhodopsin; dim light, black and white vision) and cones (iodopsin; bright light and colour vision). The fovea centralis (yellow spot) has only cones and gives the sharpest vision.

4. Refractive media

• Aqueous humour: in front of the lens; nourishes cornea and lens and maintains pressure.
• Lens: transparent, elastic, biconvex; focuses light on the retina.
• Vitreous humour: jelly filling the cavity behind the lens; keeps the eyeball in shape and the retina in place.

Binocular vision: the overlapping fields of the two eyes are fused by the brain into one three-dimensional image, giving depth perception.

Blind spot is devoid of vision At the blind spot (optic disc) the optic nerve leaves the eye and blood vessels enter. This area has no rods or cones, so light falling on it cannot be converted into nerve impulses and no vision is possible there.

(b) Piamater

The pia mater is the innermost and most delicate of the three meninges; it is highly vascular and closely adheres to the surface of the brain, dipping into all its sulci and fissures.

(c) Cornea, Pupil, Lens, Retina

Light first enters through the cornea, then passes through the pupil (the opening in the iris), is focused by the lens, and finally strikes the retina where the image is formed.

(a) Hyperglycemia

Insulin lowers blood glucose by converting glucose into glycogen; when insulin is deficient, glucose is not taken up and stored, so blood glucose rises above normal — a condition called hyperglycemia (as seen in diabetes mellitus).

(i) Identification [1] The figure shows the human respiratory system, with the air passage leading to the lungs, in which breathing (gaseous exchange) takes place.

(ii) Labelling [2] In the standard figure the parts are, in order along the air path:

• A: Nasal cavity
• B: Pharynx / larynx
• C: Trachea (windpipe)
• D: Lung (with the diaphragm below)

(iii) Role of the process [1] Breathing brings oxygen into the lungs, where it diffuses into the blood for release of energy (ATP) in cells by cellular respiration, and removes the carbon dioxide produced by the tissues from the body.

Internal Structure of the Human Heart

The human heart is a four-chambered, muscular organ lying in the mediastinum of the thoracic cavity, slightly tilted to the left. It is enclosed in a double membrane, the pericardium, and is divided by septa into two thin-walled upper atria and two thick-walled lower ventricles.

Chambers and septa

• Right atrium: receives deoxygenated blood from the body through the superior vena cava (head and upper body), inferior vena cava (lower body) and coronary sinus (heart wall).
• Right ventricle: receives blood from the right atrium through the tricuspid valve and pumps it to the lungs through the pulmonary artery. Its wall is thinner because it supplies only the pulmonary circuit.
• Left atrium: receives oxygenated blood from the lungs through four pulmonary veins.
• Left ventricle: receives blood from the left atrium through the bicuspid (mitral) valve and pumps it into the aorta. Its wall is the thickest because it supplies the whole body (systemic circulation).
• The interatrial septum separates the atria and bears the fossa ovalis (remnant of the foetal foramen ovale). The thick interventricular septum separates the ventricles.

Valves

• Atrioventricular valves (tricuspid and bicuspid) prevent backflow from ventricles to atria. Their cusps are held by chordae tendineae attached to papillary muscles.
• Semilunar valves (pulmonary valve and aortic valve) prevent backflow from the arteries into the ventricles.

Conducting system: The SA node (pacemaker) in the right atrium starts each beat; the impulse reaches the AV node, then the bundle of His and Purkinje fibres in the ventricular walls. The first branches of the aorta, the coronary arteries, supply the heart muscle itself.

Labelled diagram (frontal section, as seen from the front)

Sudden Interruption of Blood Supply to the Heart

If a coronary artery is suddenly blocked (by a thrombus or atherosclerotic plaque), the part of the myocardium it supplies gets no oxygen. This is myocardial infarction (heart attack).

• The affected cardiac muscle cells die (necrosis) because they cannot respire aerobically.
• The pumping power falls, so cardiac output and the blood supply to other organs decrease.
• Symptoms are severe chest pain spreading to the left arm and jaw, sweating, breathlessness and anxiety.
• A large block, or damage to the conducting system, may cause arrhythmia, cardiac arrest and death unless treated at once.

---

OR

Parts of the Human Respiratory System

Air passes from the atmosphere to the lungs through the following cavities and tubes.

1. Nostrils (external nares): openings of the nose lined with hair and mucus that trap dust and microbes.
2. Nasal cavity: lined with ciliated epithelium and mucous glands; it warms, moistens and filters the air and opens into the pharynx through the internal nares.
3. Pharynx: muscular passage common to food and air (nasopharynx, oropharynx, laryngopharynx).
4. Larynx (voice box): cartilaginous box containing the vocal cords and the opening glottis, which is guarded by the epiglottis so that food does not enter the trachea.
5. Trachea (windpipe): tube about 10-12 cm long, kept open by C-shaped cartilage rings, lined with ciliated mucous epithelium that sweeps dust upward. It divides into two bronchi.
6. Bronchi and bronchioles: the primary bronchi enter the lungs and divide repeatedly into smaller bronchi and finally bronchioles, which lack cartilage.
7. Lungs: paired, spongy organs covered by the double-layered pleura; the right lung has three lobes and the left has two. Each contains millions of thin-walled, richly vascular alveoli, where O$_2$ and CO$_2$ are exchanged.
8. Diaphragm and intercostal muscles: bring about breathing movements.

Labelled diagram: draw the nostril and nasal cavity at the top, then pharynx, larynx (with epiglottis and glottis), trachea with cartilage rings, the two bronchi branching into bronchioles ending in alveoli inside the right lung (three lobes) and left lung (two lobes), the pleura around the lungs, and the diaphragm below.

Respiratory Tubes in an Asthma Attack

During an asthma attack the smooth muscle of the bronchi and bronchioles contracts (bronchospasm), their lining becomes inflamed and secretes excess mucus. The tubes therefore narrow and partly block, air (especially during exhalation) cannot pass freely, and the person has wheezing, coughing, chest tightness and breathlessness. Unless relieved by a bronchodilator, oxygen supply falls and the attack can become life-threatening.

(a) Malleus, Incus, Stapes

The three ear-ossicles of the middle ear are arranged in sequence from the tympanic membrane inward as malleus (hammer), incus (anvil) and stapes (stirrup). The stapes is the smallest bone and its footplate fits into the oval window.

(b) Islet of Langerhans

The endocrine part of the pancreas is the islet of Langerhans (islets of Langerhans), which consists of alpha, beta and delta cells that secrete glucagon, insulin and somatostatin respectively. The other options are parts of the adrenal cortex (zona glomerulosa, zona reticularis) and the pituitary gland (adenohypophysis).

(a) Labelling (standard V.S. of the human eye)

(b) Functions

• A, Cornea: the transparent front layer; it refracts most of the incoming light towards the lens and protects the front of the eye.
• B, Lens: changes its curvature (accommodation) to focus light sharply on the retina.
• C, Retina: contains photoreceptors (rods and cones) that convert light into nerve impulses.
• D, Optic nerve: carries the visual impulses from the retina to the brain.

Two common renal disorders in human beings are:

1. Kidney stones (renal calculi)

• Insoluble crystalline masses of salts (mainly calcium oxalate, also uric acid or calcium phosphate) form in the renal pelvis or ureter.
• Cause: concentrated urine (less water intake), high intake of oxalate-rich food, hypercalcaemia, urinary tract infection, or hyperparathyroidism.
• Effect: severe pain in the lumbar region (renal colic), blood in urine (haematuria), obstructed urine flow, and possible infection or damage to the kidney.
• Treatment: plenty of water, lithotripsy (breaking stones by shock waves), or surgical removal.

2. Uraemia / renal failure

• When the nephrons are destroyed (by infection, diabetes, hypertension, or toxic substances), the kidneys fail to filter blood properly.
• Effect: urea and other nitrogenous wastes accumulate in the blood — a condition called uraemia. It leads to oedema, high blood pressure, anaemia, and finally death if untreated.
• Treatment: dialysis (artificial kidney) to remove the wastes, or kidney transplantation from a compatible donor.

(Other acceptable answers: glomerulonephritis, nephritis, polyuria, haematuria, glycosuria, albuminuria, and renal hypertension.)

Diagram: Human alimentary canal (draw and label as follows)

Draw a long tube from mouth to anus, with these parts labelled:

• Mouth (buccal cavity) with teeth and tongue
• Pharynx
• Oesophagus (food pipe)
• Stomach (cardiac region, fundus, body, pyloric region)
• Small intestine: duodenum, jejunum, ileum
• Large intestine: caecum, colon (ascending, transverse, descending), rectum
• Anus
• Associated glands labelled alongside: salivary glands, liver (with gall bladder), pancreas

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Digestion of proteins in the gut

Proteins are digested by proteolytic enzymes (proteases) acting at different sites, each with its own pH requirement.

1. In the mouth — No protein digestion. Saliva contains only salivary amylase (ptyalin), which acts on starch. Proteins pass unchanged into the stomach.

2. In the stomach — The gastric glands secrete gastric juice containing HCl, pepsinogen and rennin (in infants).

   • HCl makes the medium strongly acidic (pH ~1.8) and converts inactive pepsinogen → active pepsin.
   • Pepsin is an endopeptidase: it breaks proteins into peptones and proteoses.
   • Rennin (in infants) converts milk protein casein into calcium paracaseinate, which pepsin then digests.
   • Mucus protects the stomach wall from HCl and pepsin.

3. In the small intestine — The acidic chyme enters the duodenum and is neutralised by bicarbonate of pancreatic juice and bile. Here:

   • Trypsinogen (pancreatic juice) is activated to trypsin by enterokinase of intestinal juice. Trypsin then activates chymotrypsinogen → chymotrypsin and also activates more trypsinogen.
   • Trypsin, chymotrypsin and carboxypeptidase (pancreatic juice) act on peptones, proteoses and polypeptides, converting them into dipeptides and some amino acids. Carboxypeptidase is an exopeptidase acting at the carboxyl end.
   • Intestinal juice (succus entericus) contains aminopeptidase and dipeptidases, which complete the breakdown of dipeptides into free amino acids.
   • Amino acids are absorbed by the villi of the ileum (by active transport and diffusion) into the blood and carried to the liver via the hepatic portal vein.

So protein digestion is a stepwise process: proteins → peptones/proteoses → polypeptides → dipeptides → amino acids, with pepsin in the stomach and trypsin, chymotrypsin, carboxypeptidase and intestinal peptidases in the small intestine.

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Relationship between amylase and lipase in digestion

Amylase and lipase are complementary, not antagonistic enzymes — they act on different substrates but together they complete the digestion of the two main energy-giving food components.

• Amylase is a carbohydrase: it hydrolyses starch and glycogen into maltose and dextrins. It acts in the mouth (salivary amylase, pH ~6.8) and in the small intestine (pancreatic amylase, pH ~8).
• Lipase is a lipolytic enzyme: it hydrolyses emulsified fats into fatty acids and glycerol. It acts mainly in the small intestine (pancreatic and intestinal lipase), since fats need bile salts for emulsification first.

Their relationship:

1. Same site, same time — Both are present in pancreatic juice and intestinal juice and act in the alkaline medium of the small intestine, so they work together there.
2. Different substrates, same purpose — Amylase handles carbohydrates, lipase handles fats; both break large food molecules into absorbable simple molecules (glucose, fatty acids, glycerol) to supply energy.
3. Mutually supporting — The simple sugars and fatty acids produced are both absorbed by the intestinal villi; a deficiency of either enzyme leaves one class of nutrient undigested, causing poor absorption and fatty or starchy stools.
4. Different optimum pH — Salivary amylase works in a slightly acidic/neutral pH and is destroyed in the stomach, while pancreatic lipase needs an alkaline pH; hence lipase acts only after the chyme is neutralised in the duodenum.

Thus amylase and lipase are synergistic digestive enzymes — each is essential for digesting a different macronutrient, and normal digestion requires the coordinated action of both.

1. Sino-auricular node is a natural pacemaker (3 marks)

The sino-auricular node (SA node) is a small mass of specialised cardiac muscle tissue located in the wall of the right atrium, near the opening of the superior vena cava. It is called the natural pacemaker because of the following reasons:

• It has the property of automaticity (self-excitation): Unlike ordinary cardiac muscle fibres, the SA node cells can spontaneously depolarise on their own without any external nerve stimulus. Their resting membrane potential is unstable and drifts upward until the threshold is reached, producing an action potential.
• It generates the highest rate of impulse: The SA node depolarises at the fastest rate (about 70–75 times per minute in humans) compared to other parts of the conducting system (AV node ≈ 50–60/min, bundle of His ≈ 40/min, Purkinje fibres ≈ 20–35/min).
• It initiates every heartbeat: Because it fires first, the impulse generated by the SA node spreads through the atrial muscle and then to the AV node, bundle of His and Purkinje fibres, setting the rhythm of the whole heart. The slower tissues are thus "driven" by the SA node before they can fire on their own.
• It sets the heart rate: The rate at which the SA node discharges determines the heart rate (sinus rhythm), so it acts as the pacemaker of the heart.

Because it initiates and regulates the normal rhythm of the heart on its own, the SA node is rightly called the natural pacemaker.

2. If the natural pacemaker does not function normally (2 marks)

If the SA node fails to function properly (sick sinus syndrome, heart block, etc.), the heart may beat too slowly, irregularly or may even stop. In such a condition:

• An artificial pacemaker is implanted surgically under the skin of the chest, with its electrodes fixed to the wall of the right ventricle (or atrium).
• It delivers regular electrical impulses to the heart muscle and maintains a normal heart rate and rhythm.
• In some cases, medicines (e.g. atropine, isoproterenol) may be given temporarily, but for permanent defects an artificial pacemaker is the proper remedy.

3. Testing of blood grouping in human beings (3 marks)

Blood grouping is done by the ABO and Rh typing method, using antisera. The procedure is:

1. Preparation: A clean glass slide is taken and divided into two halves with a wax pencil. A drop of blood (from a finger prick or venous sample) is placed on each half.
2. Adding antisera: One drop of anti-A serum is added to the first drop of blood and one drop of anti-B serum to the second drop. (For Rh typing, a third drop is mixed with anti-D serum.)
3. Mixing: Each drop is mixed separately with a separate clean glass rod (or applicator stick) to avoid cross-contamination.
4. Observation: The slide is rocked gently for about a minute and observed for agglutination (clumping):

5. Rh factor: If agglutination occurs with anti-D serum, the person is Rh positive; if not, the person is Rh negative.

Thus, by observing agglutination with anti-A, anti-B and anti-D sera, the ABO and Rh blood group of a person is determined.

(b) Bundles of axons enclosed in connective tissue

A nerve is a bundle of many axons (nerve fibres) wrapped together by connective tissue sheaths — endoneurium around each axon, perineurium around each fascicle and epineurium around the whole nerve.

(b) Cl⁻

In the Hamburger phenomenon (chloride shift), CO₂ entering the RBC is converted by carbonic anhydrase to H₂CO₃, which dissociates into H⁺ and HCO₃⁻. HCO₃⁻ diffuses out of the RBC into the plasma, while Cl⁻ moves from the plasma into the RBC to maintain electrical neutrality. The inward-moving Cl⁻ is label X.

(b) Adrenal medulla

The adrenal medulla secretes adrenaline (epinephrine) and noradrenaline (norepinephrine), which raise heart rate, blood pressure and blood glucose during emergencies — the fight-or-flight response. The adrenal cortex secretes corticosteroids, not these emergency hormones.

(d) No anti-A and anti-B antibodies

Blood group AB has both A and B antigens on the surface of its red blood cells, so the plasma contains neither anti-A nor anti-B antibodies (it is the universal recipient).

(a) Water

The descending limb is permeable to water but nearly impermeable to solutes, so water leaves it by osmosis as the filtrate descends into the increasingly concentrated medulla.

Digestion of carbohydrates in the human alimentary canal

1. In the mouth (buccal cavity). Salivary glands secrete saliva containing the enzyme salivary amylase (ptyalin). It acts on cooked starch at the slightly alkaline pH of saliva and hydrolyses it into maltose (a disaccharide). About 30% of starch is digested here. Since food stays in the mouth only briefly, most starch passes on undigested.

2. In the stomach. No carbohydrate digestion occurs here because the gastric juice is strongly acidic (pH about 1.5–2), which inactivates salivary amylase. The stomach secretes no carbohydrate-splitting enzyme in humans.

3. In the small intestine (duodenum). The acidic chyme is neutralised by alkaline bile and pancreatic juice, restoring a slightly alkaline pH suitable for enzymes. Pancreatic amylase (amylopsin) now converts the remaining starch into maltose. Intestinal juice (succus entericus) supplies the final enzymes:

• Maltase → maltose → glucose + glucose
• Lactase → lactose → glucose + galactose
• Sucrase (invertase) → sucrose → glucose + fructose

Thus all carbohydrates are finally broken down into monosaccharides (mainly glucose) in the ileum.

4. Absorption. The monosaccharides are absorbed by the villi of the ileum — glucose and galactose by active transport, fructose by facilitated diffusion — and carried by the hepatic portal vein to the liver. The large intestine has no role in carbohydrate digestion.

Summary: Starch → (salivary amylase) → maltose → (pancreatic amylase) → maltose → (maltase, lactase, sucrase) → glucose, galactose, fructose → absorbed in ileum.

Structure of the Human Brain

The human brain is the central control organ of the nervous system. It is protected by the skull (cranium) and by three membranes called meninges (dura mater, arachnoid mater, and pia mater). Between the arachnoid and pia mater lies the cerebrospinal fluid (CSF), which cushions the brain and supplies it with nutrients. The brain is divided into three main parts: the forebrain, midbrain, and hindbrain.

1. Forebrain (Prosencephalon) It consists of the cerebrum, thalamus, and hypothalamus.

• Cerebrum: It is the largest part of the brain, forming about two-thirds of its total mass. It is divided into two cerebral hemispheres (left and right) by a deep groove called the median fissure. The surface shows raised folds called gyri and grooves called sulci, which increase the surface area. Each hemisphere has four lobes: frontal, parietal, temporal, and occipital. The outer layer is grey matter (cerebral cortex) made of nerve cell bodies, while the inner part is white matter made of nerve fibres. The two hemispheres are connected by a band of white matter called the corpus callosum.
• Thalamus: A relay centre for sensory impulses going to the cerebrum.
• Hypothalamus: Controls body temperature, hunger, thirst, and hormone release.

2. Midbrain (Mesencephalon) It lies between the forebrain and hindbrain. It contains the optic lobes for vision and reflex centres for pupil reflexes.

3. Hindbrain (Rhombencephalon) It consists of the cerebellum, pons varolii, and medulla oblongata.

• Cerebellum: Located below the cerebrum at the back of the brain. Its surface also has folds (folia). It has two hemispheres and a central vermis.
• Pons varolii: A bridge of nerve fibres connecting the two halves of the cerebellum and the medulla to the cerebrum.
• Medulla oblongata: The lowest part of the brain, continuous with the spinal cord. It contains centres for vital involuntary functions.

Functions of Cerebrum, Cerebellum, and Medulla Oblongata

Cerebrum

• Seat of intelligence, memory, thinking, reasoning, and imagination.
• Controls all voluntary movements of the body.
• Receives and interprets sensory impulses such as touch, pain, temperature, sight, hearing, taste, and smell.
• Controls speech, writing, and emotions (joy, sorrow, fear, anger).
• Different lobes have specific roles: frontal lobe for reasoning and motor control, parietal for touch and taste, temporal for hearing and smell, occipital for vision.

Cerebellum

• Maintains balance and equilibrium of the body.
• Coordinates muscular movements and posture.
• Makes body movements smooth, precise, and coordinated.
• Regulates muscle tone.

Medulla Oblongata

• Controls respiration (breathing movements).
• Regulates heartbeat and blood pressure through the cardiac and vasomotor centres.
• Controls reflex actions such as coughing, sneezing, swallowing, vomiting, and salivation.
• Regulates secretion of digestive juices and other involuntary activities.

In short, the cerebrum governs thought and voluntary action, the cerebellum governs balance and coordination, and the medulla oblongata governs involuntary vital functions essential for life.

Labelled diagram of the female reproductive system

Draw the following (front view): two ovaries at the sides of the pelvis; a fallopian tube (oviduct) curving from each ovary to the upper corners of the uterus, with a funnel-shaped infundibulum bearing finger-like fimbriae near the ovary; the pear-shaped uterus (womb) in the middle with its inner lining, the endometrium, and thick muscular wall, the myometrium; the narrow cervix below the uterus; and the vagina leading to the outside (with the external genitalia, vulva).

Hormonal regulation of the menstrual cycle

The menstrual cycle (about 28 days) is controlled by the hypothalamus-pituitary-ovary axis.

1. Menstrual phase (days 1-5): If the ovum is not fertilised, the corpus luteum degenerates, so progesterone and oestrogen fall. The endometrium breaks down and is shed as menstrual bleeding.

2. Follicular (proliferative) phase (days 6-13):

• The hypothalamus releases GnRH, which stimulates the anterior pituitary to secrete FSH and LH.
• FSH stimulates growth of ovarian follicles; the developing follicle secretes oestrogen.
• Oestrogen rebuilds and thickens the endometrium. Early on it gives negative feedback on FSH; at high levels near mid-cycle it gives positive feedback.

3. Ovulation (about day 14): The peak of oestrogen triggers a sudden LH surge, which makes the mature Graafian follicle rupture and release the ovum.

4. Luteal (secretory) phase (days 15-28):

• LH converts the ruptured follicle into the corpus luteum, which secretes progesterone (and some oestrogen).
• Progesterone maintains the endometrium and makes it glandular and ready for implantation, and inhibits FSH and LH by negative feedback.
• If fertilisation occurs, the corpus luteum persists and later the placenta maintains pregnancy; otherwise the corpus luteum degenerates and menstruation begins again.

(d) Stratified non-keratinized squamous

The vagina is a moist canal that is exposed to friction during intercourse and childbirth. It is lined by several layers of squamous cells that do not contain keratin, which protects the wall and keeps the surface moist. Keratinised epithelium is found in the dry skin (epidermis).

(b) Protein digestion does not take place

Oxyntic (parietal) cells secrete HCl, which provides the acidic pH needed to activate pepsinogen into pepsin. If they stop working, HCl is absent, pepsin is not activated, and protein digestion in the stomach does not occur. (Option (d) is wrong because pepsinogen is secreted by the chief cells, which are still functioning; the stomach pH would rise, not fall.)

(a) Renal pelvis, ureter, urinary bladder, urethra

Urine formed in the kidney collects in the renal pelvis, is carried by the ureter to the urinary bladder where it is stored, and is finally passed out of the body through the urethra.

(c) Myxoedema

Myxoedema is caused by hypothyroidism in adults. Low thyroxine reduces the metabolic rate, which gives a low body temperature, intolerance to cold, a puffy face (moon face) and a swollen appearance. Grave's disease is hyperthyroidism (heat intolerance), Hashimoto's disease is an autoimmune thyroiditis that is a cause of hypothyroidism but is not named for these features, and Conn's disease is an excess of aldosterone.

Diagram of the human digestive system

Draw the alimentary canal as a continuous tube, with labels: mouth (buccal cavity) with teeth, tongue and salivary glands, pharynx, oesophagus, stomach, small intestine (duodenum, jejunum, ileum), liver, gall bladder, bile duct, pancreas, large intestine (caecum, colon, rectum) and anus.

Parts of the alimentary canal

1. Mouth and buccal cavity: food is chewed by teeth and mixed with saliva from the salivary glands. Salivary amylase begins starch digestion, and the tongue forms the bolus.
2. Pharynx and oesophagus: the pharynx passes food on by swallowing, and the oesophagus carries the bolus to the stomach by peristalsis.
3. Stomach: a J-shaped muscular bag. Gastric glands secrete HCl, pepsinogen and mucus. Proteins are partly digested to peptones and the food becomes semi-liquid chyme.
4. Small intestine: about 6 m long, with the duodenum (C-shaped, receives bile and pancreatic juice), jejunum and ileum. Digestion is completed here and absorption takes place through villi.
5. Large intestine: caecum, colon and rectum. It absorbs water and salts and forms faeces.
6. Anus: faeces are egested through the anus.

Associated glands (salivary glands, liver and pancreas) pour secretions into the canal.

If the bile duct is blocked

Bile cannot reach the duodenum. Fats are not emulsified, so fat digestion and absorption of fat-soluble vitamins (A, D, E, K) are impaired; faeces become pale and fatty. Bile pigments accumulate in the blood, causing jaundice.

The human ear is the organ of hearing and equilibrium (balance). It has three parts.

Diagram: draw the ear and label pinna, external auditory canal, tympanic membrane, malleus, incus, stapes, oval window, Eustachian tube, cochlea, semicircular canals, vestibule and auditory nerve.

1. External ear

• Pinna collects sound waves.
• External auditory canal leads sound to the tympanic membrane (eardrum), which vibrates. The canal has hair and wax-secreting ceruminous glands that trap dust.

2. Middle ear

• An air-filled cavity containing three ossicles: malleus, incus and stapes. They amplify the vibrations of the eardrum and transmit them to the oval window.
• The Eustachian tube connects the middle ear to the pharynx and equalises air pressure on both sides of the eardrum.

3. Inner ear (membranous labyrinth in the bony labyrinth)

• Cochlea: a coiled tube containing the organ of Corti with hair cells, which convert vibrations into nerve impulses for hearing.
• Vestibule (utricle and saccule) and three semicircular canals: detect the position and movement of the head, which controls balance.
• The auditory nerve carries impulses to the brain.

If the Eustachian tube opening is blocked

Air pressure on the two sides of the eardrum cannot be balanced. The eardrum is pulled inward, which causes pain, a feeling of fullness and reduced hearing. Fluid can also collect, leading to infection (otitis media).

(c) In the form of bicarbonates

About 70% of the $CO_2$ from tissues is carried as bicarbonate ($HCO_3^-$): in red cells carbonic anhydrase converts $CO_2 + H_2O$ into $H_2CO_3$, which dissociates into $H^+$ and $HCO_3^-$. In the lungs the reaction reverses and $CO_2$ is released. Oxyhaemoglobin carries oxygen, not the main $CO_2$ load, and ethyl alcohol and glucose are not $CO_2$ carriers.

(b) Internal ear

The organ of Corti, the receptor of hearing, lies on the basilar membrane inside the cochlea (cochlear duct) of the internal ear. Its hair cells convert sound vibrations into nerve impulses.

(a) Labels

• A: adrenal cortex (outer region)
• B: adrenal medulla (inner region)

(b) Hormones

Adrenal cortex (A)

• Cortisol (glucocorticoid): raises blood glucose by promoting gluconeogenesis and helps the body resist stress.
• Aldosterone (mineralocorticoid): acts on kidney tubules to retain $Na^+$ and water and excrete $K^+$, regulating blood pressure.

Adrenal medulla (B)

• Adrenaline (epinephrine): increases heart rate, blood glucose and blood flow to muscles in emergencies ("fight or flight").
• Noradrenaline (norepinephrine): constricts blood vessels, raising blood pressure.

Ultrafiltration (glomerular filtration) is the first step of urine formation and takes place in the Bowman's capsule of the nephron.

• Blood enters the glomerulus through a wide afferent arteriole and leaves through a narrower efferent arteriole, creating high glomerular hydrostatic pressure (about 55 mm Hg).
• This pressure forces water and small solutes (glucose, amino acids, urea, salts) through the three-layered filtration membrane: fenestrated capillary endothelium, basement membrane and the slit pores between the podocytes of the capsule's inner layer.
• Blood cells and large proteins cannot pass, so the filtrate (glomerular filtrate) is essentially protein-free plasma.
• The net filtration pressure is about 10 mm Hg after subtracting the opposing blood colloid osmotic pressure and capsular pressure.
• About 125 mL of filtrate forms per minute (about 180 L per day), most of which is later reabsorbed.

Myogenic heart (1 mark)

The human heart is myogenic because its beat is initiated by the cardiac muscle itself, in a specialised pacemaker region, and not by nerve impulses. A heart removed from the body keeps beating for some time if supplied with oxygenated blood, showing that the stimulus arises within the heart.

Origin and conduction of the heartbeat (4 marks)

1. The sinoatrial (SA) node, in the wall of the right atrium, is the pacemaker. It spontaneously generates impulses about 70-72 times per minute.
2. The impulse spreads over both atria, which contract (atrial systole), and reaches the atrioventricular (AV) node at the base of the right atrium.
3. The AV node delays the impulse briefly so that the ventricles fill completely.
4. It passes into the bundle of His, which divides into right and left bundle branches in the interventricular septum.
5. The branches end in Purkinje fibres in the ventricular walls, which spread the impulse rapidly so the ventricles contract together (ventricular systole).

Regulation of heartbeat (3 marks)

• Nervous control: the cardiac centres in the medulla oblongata act through the autonomic system. Sympathetic nerves (noradrenaline) increase heart rate and force; parasympathetic (vagus) nerves (acetylcholine) decrease them.
• Hormonal control: adrenaline and noradrenaline from the adrenal medulla increase heart rate and contraction strength; thyroxine also raises it.
• Other factors: baroreceptors and chemoreceptors, body temperature, exercise and emotions adjust the heart rate through the cardiac centre.

Protective layers of the brain (3 marks)

The brain is protected by the skull (cranium) and three membranes, the meninges, from outside to inside:

1. Dura mater: tough, thick, outer fibrous layer, lying against the skull.
2. Arachnoid mater: thin, web-like middle layer; the subarachnoid space below it contains cerebrospinal fluid.
3. Pia mater: thin, delicate inner layer that closely covers the brain and is rich in blood vessels.

Absence of cerebrospinal fluid (2 marks)

CSF cushions the brain against shocks and lets it float, so without it the brain would be damaged by jolts and by its own weight against the skull. Nutrients and oxygen supply and removal of wastes (CO$_2$, metabolites) would be impaired, the intracranial pressure and chemical balance could not be regulated, and the brain would be at higher risk of infection and injury.

Cerebrum vs cerebellum (3 marks)

(b) Decrease the rate of heart beat

Acetylcholine from parasympathetic (vagus) nerve endings slows the heart rate. Increases in blood pressure and myocardial contractility are sympathetic effects (noradrenaline).

(c) Automatically degenerates after sometime

If fertilization and implantation do not occur, the corpus luteum degenerates after about 10-12 days into the corpus albicans. Progesterone falls and menstruation begins. It secretes progesterone and oestrogen, not FSH, LH or oxytocin.

(a) Haemopoiesis

Haemopoiesis (haematopoiesis) is the formation of blood cells from stem cells, mainly in the red bone marrow. Haemolysis is the breakdown of red cells, and lymphopoiesis is only the formation of lymphocytes.

Labelled diagram of the human alimentary canal (3 marks)

Draw a long tube from mouth to anus with its associated glands and label:

Mechanism of digestion (4 marks)

• Mouth: teeth chew the food. Saliva (salivary amylase) breaks cooked starch into maltose; mucus lubricates and the food becomes a bolus.
• Stomach: peristalsis churns the food with gastric juice into chyme. HCl kills microbes and activates pepsinogen to pepsin, which breaks proteins into peptones and proteoses. Rennin (in infants) curdles milk; gastric lipase acts weakly on fats.
• Small intestine: here digestion is completed. Bile from the liver emulsifies fats and makes the medium alkaline. Pancreatic juice contains trypsin and chymotrypsin (proteins to peptides), pancreatic amylase (starch to maltose) and lipase (fats to fatty acids and glycerol). Intestinal juice (succus entericus) has peptidases (peptides to amino acids), maltase, sucrase and lactase (to glucose, fructose, galactose) and lipase.
• Absorption: nutrients are absorbed through villi of the small intestine into blood and lymph.
• Large intestine: absorbs water and minerals, and forms and stores faeces for egestion through the anus.

Effect of removing the pancreas (1 mark)

Digestion of carbohydrates, proteins and fats would be incomplete, because pancreatic amylase, trypsin and lipase would be missing. Fat digestion would suffer most, and the person would also get diabetes because insulin would no longer be made.

Labelled diagram of the human respiratory system

Draw the nose and nasal cavity at the top, then the pharynx, larynx (with epiglottis), trachea dividing into two bronchi, bronchioles ending in alveoli in the right (three lobes) and left (two lobes) lungs, the pleural membranes, and the diaphragm below the lungs. Label: nostril, nasal cavity, pharynx, larynx, trachea, bronchus, bronchiole, alveoli, lung, pleura, rib, intercostal muscles, diaphragm.

Why and how gases are exchanged rapidly in the lungs

Oxygen is needed for cellular respiration and $CO_2$ must be removed. Exchange takes place by simple diffusion between alveolar air and blood in the pulmonary capillaries, driven by partial-pressure differences:

• Alveolar $pO_2 \approx 104$ mm Hg, while blood arriving has $pO_2 \approx 40$ mm Hg, so $O_2$ moves into blood.
• Blood arriving has $pCO_2 \approx 45$ mm Hg, while alveoli have about 40 mm Hg, so $CO_2$ moves into the alveoli.

The exchange is rapid because:

1. There are about 300 million alveoli, giving a very large surface area (around $70\ m^2$).
2. The alveolar wall and capillary wall are each one cell thick, so the diffusion distance is under 1 micrometre.
3. The surface is moist, so gases dissolve.
4. A rich capillary network and continuous ventilation maintain the concentration gradient.
5. Haemoglobin quickly binds oxygen, and $CO_2$ is highly soluble and diffuses faster than $O_2$.

At high altitude

Air pressure falls, so the partial pressure of oxygen is low and less $O_2$ diffuses into the blood. This causes hypoxia, called altitude sickness: headache, breathlessness, rapid breathing and heartbeat, nausea, fatigue and in severe cases pulmonary or cerebral oedema.

Views on solving the problem

• Ascend gradually and rest to allow acclimatization, during which the body makes more red blood cells and haemoglobin and breathes faster.
• Drink plenty of fluids, avoid alcohol and heavy exertion, and eat carbohydrate-rich food.
• Carry supplementary oxygen and take medicines such as acetazolamide on medical advice.
• Descend to lower altitude quickly if symptoms become serious.

(a) Rennin

Milk protein (casein) is curdled and digested by rennin, secreted by gastric glands in the stomach of infants. It converts soluble caseinogen into insoluble calcium paracaseinate, which stays longer in the stomach for pepsin to act on. Lactase digests milk sugar and sucrase digests sucrose, so neither acts on protein.

(b) Body movement

The cerebellum coordinates voluntary movement and maintains balance, posture and muscle tone, so injury causes unsteady, uncoordinated movement. Breathing is controlled by the medulla oblongata, memory by the cerebrum (and hippocampus), and smell by the olfactory lobes and cerebrum.

(c) Seminal fluid is without sperms

Sperms are still produced in the testes and stay motile with normal nuclei, but the cut vas deferens blocks their route to the ejaculatory duct. Semen is therefore ejaculated containing only the secretions of the seminal vesicles, prostate and Cowper's glands, with no sperms (the basis of vasectomy).

Conversion of complex foods into simple forms (4 marks)

Complex foods are broken down by enzymes in the alimentary canal:

• Carbohydrates: salivary amylase begins starch digestion in the mouth (starch to maltose). In the small intestine pancreatic amylase continues it, and maltase, sucrase and lactase of intestinal juice change disaccharides into monosaccharides (glucose, fructose, galactose).
• Proteins: pepsin with HCl in the stomach converts proteins into peptones and proteoses. In the small intestine trypsin and chymotrypsin make smaller peptides, and peptidases (aminopeptidase, dipeptidase) make amino acids.
• Fats: bile salts emulsify fat globules into tiny droplets in the duodenum; pancreatic and intestinal lipases then convert them into fatty acids and glycerol (monoglycerides).
• Nucleic acids: nucleases of pancreatic juice and nucleosidases of intestinal juice produce nucleotides, then nucleosides, sugars, bases and phosphates.

These simple molecules are then absorbed by the villi.

Labelled diagram (2 marks)

Draw the alimentary canal from mouth to anus and label: buccal cavity, tongue, salivary glands, pharynx, oesophagus, stomach, liver, gall bladder, bile duct, pancreas, duodenum, jejunum, ileum, caecum, appendix, colon, rectum and anus.

Effect of removing the gall bladder (2 marks)

The gall bladder stores and concentrates bile. After surgical removal, bile still flows from the liver to the duodenum but continuously and in small amounts, not in a large burst when fatty food arrives. Fat emulsification and lipase action become less efficient, so large fatty meals cause poor fat digestion, loose stools and discomfort, and absorption of fat-soluble vitamins (A, D, E, K) may fall. The person therefore needs to eat low-fat meals.

Transport of $CO_2$

$CO_2$ produced in tissue cells diffuses into the blood (higher $pCO_2$ in tissues, about 45 mm Hg, than in blood) and is carried to the lungs in three ways:

1. As bicarbonate ions (about 70%): $CO_2$ diffuses into red blood cells, where it combines with water to form carbonic acid, which dissociates:

$CO_2 + H_2O \underset{\text{carbonic anhydrase}}{\rightleftharpoons} H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$

The $HCO_3^-$ moves out into plasma in exchange for $Cl^-$ (the chloride shift), and is carried in plasma. The $H^+$ is buffered by haemoglobin. 2. As carbaminohaemoglobin (about 20-25%): $CO_2$ binds to the amino groups of haemoglobin globin, forming $HbCO_2$. This binding is favoured when $pCO_2$ is high and $pO_2$ is low. 3. Dissolved in plasma (about 7%): $CO_2$ is about 20-25 times more soluble than $O_2$.

In the lung capillaries, $pCO_2$ is lower, so the reactions reverse: $HCO_3^-$ re-enters the red cells, gives $CO_2$, carbaminohaemoglobin releases $CO_2$, and it diffuses into the alveoli to be breathed out.

Role of carbonic anhydrase

It is an enzyme in red blood cells containing zinc that speeds up the combination of $CO_2$ and water to form carbonic acid (and the reverse reaction in lungs) about 5000 times. It therefore enables the rapid transport of most $CO_2$ as bicarbonate.

Significance of the diaphragm in breathing

The diaphragm is a dome-shaped muscular sheet between the thoracic and abdominal cavities. On inspiration it contracts and flattens, increasing the thoracic volume vertically, which lowers the lung pressure so air enters. On expiration it relaxes and returns to dome shape, reducing the volume and pushing air out. It is the main muscle of quiet breathing.