(a) Explain in detail the absorption and hormonal regulation of blood calcium in body. Discuss in brief the clinical…

(a) Calcium absorption, hormonal regulation and hypocalcaemia

Total plasma calcium is 2.2–2.6 mmol/L (ionised fraction about 1.1–1.3 mmol/L), held within narrow limits by gut, bone and kidney.

Absorption: of roughly 1 g dietary calcium, about 30% is absorbed. Active transcellular uptake in the duodenum and proximal jejunum is vitamin D dependent: calcium enters through apical TRPV6 channels, is carried by calbindin-D9k and extruded basolaterally by PMCA1b and the Na+/Ca2+ exchanger. Passive paracellular absorption occurs along the whole small intestine and rises with intake. Phytates and oxalates reduce absorption; lactose and acid favour it.

Hormonal regulation:

• Vitamin D: skin 7-dehydrocholesterol (UVB) gives cholecalciferol, which is 25-hydroxylated in the liver and 1α-hydroxylated in the kidney to calcitriol. PTH, low calcium and low phosphate stimulate this step. Calcitriol raises intestinal calcium and phosphate absorption and supports bone mineralisation.
• PTH, from the chief cells, is released when calcium-sensing receptors detect a fall in ionised calcium. It stimulates osteoclastic resorption indirectly (osteoblast RANKL), increases distal tubular calcium reabsorption, reduces proximal phosphate reabsorption and increases 1α-hydroxylase.
• Calcitonin, from thyroid C cells, is released when calcium rises and inhibits osteoclasts; its role in humans is minor.

Hypocalcaemia: causes include hypoparathyroidism, vitamin D deficiency and CKD. Features reflect neuromuscular hyperexcitability: perioral and fingertip paraesthesia, muscle cramps, carpopedal spasm, tetany, laryngospasm and seizures. Chvostek's sign (facial twitch on tapping the facial nerve) and Trousseau's sign (carpal spasm with a cuff inflated above systolic pressure for 3 minutes) are positive. ECG shows QT prolongation, and chronic cases may show cataract, basal ganglia calcification and dry skin.

(b) Regulation of cardiac output and changes in moderate isotonic exercise

Cardiac output (CO) = heart rate (HR) × stroke volume (SV), normally about 5 L/min at rest.

Mechanisms:

• Preload: by the Frank–Starling law, greater end-diastolic volume gives a stronger contraction. Venous return is aided by the skeletal-muscle pump, respiratory pump, venous tone and blood volume.
• Contractility: sympathetic β1 stimulation, circulating adrenaline and raised intracellular calcium increase contractility at a given preload; hypoxia, acidosis and negative inotropes reduce it.
• Heart rate: sympathetic activity raises it and vagal tone lowers it, with baroreceptor reflex modulation.
• Afterload: a higher arterial pressure or peripheral resistance reduces SV.

Moderate isotonic (dynamic) exercise:

• Central command and muscle receptors raise sympathetic outflow and withdraw vagal tone, so HR rises roughly in proportion to workload.
• SV rises early (muscle and respiratory pumps, greater contractility) and then plateaus at about 40–50% of maximal effort. CO rises to about 10–15 L/min.
• Vasodilation in active muscle from local metabolites lowers total peripheral resistance, while splanchnic and renal beds constrict, so muscle blood flow rises sharply.
• Systolic pressure rises modestly, diastolic pressure stays unchanged or falls slightly, mean pressure rises little, and the arteriovenous oxygen difference widens.

(c)(i) Hepatic segments

Couinaud's system divides the liver into eight functionally independent segments, each with its own portal pedicle (portal vein branch, hepatic artery and bile duct) and hepatic vein drainage. Three hepatic veins separate the segments vertically; a transverse plane through the right and left portal vein branches divides them into upper and lower groups. Segment I is the caudate lobe, which drains directly into the IVC. Segments II (left lateral superior), III (left lateral inferior) and IV (left medial, IVa and IVb) make up the left liver. Segments V (right anterior inferior), VI (right posterior inferior), VII (right posterior superior) and VIII (right anterior superior) make up the right liver. Segmental independence allows surgical resection of individual segments.

(c)(ii) Cranial nerve nuclei and functional components

Eight functional columns are recognised (motor columns medial, sensory columns lateral):

• GSE (somatic motor): oculomotor (III), trochlear (IV), abducens (VI) and hypoglossal (XII) nuclei.
• GVE (parasympathetic): Edinger–Westphal (III), superior salivatory and lacrimatory (VII), inferior salivatory (IX) and dorsal motor nucleus of the vagus (X).
• SVE (branchial motor): motor nucleus of V, facial nucleus (VII), nucleus ambiguus (IX, X and cranial root of XI). The spinal root of XI arises from the spinal accessory nucleus in C1–C5.
• GSA (general sensation of face): principal sensory, spinal and mesencephalic (proprioceptive) nuclei of V.
• GVA (visceral sensation): caudal nucleus tractus solitarius (IX, X).
• SVA (taste): rostral nucleus tractus solitarius or gustatory nucleus (VII, IX, X).
• SSA (special senses): cochlear (VIII) and vestibular (VIII) nuclei.

CN I and II have no brainstem nuclei; olfactory fibres end in the olfactory bulb and cortex, and optic fibres in the lateral geniculate body.