Special Circulation

Different organs have specialised circulatory arrangements reflecting their unique functions.

Coronary circulation: 95% of coronary venous blood returns via the coronary sinus; the remainder via anterior coronary and thebesian veins (the latter draining into the left ventricle, slightly deoxygenating arterial blood). Blood flow to the left ventricle occurs almost exclusively during diastole because high systolic intramural pressure compresses intramural vessels, completely interrupting flow during early systole. The right ventricle, with lower intramural pressure, receives flow in both systole and diastole. The heart extracts ~75% of delivered O2 at rest — far more than other organs (skeletal muscle extracts ~25%) — leaving little reserve. During exercise, increased O2 demand must be met by increased coronary flow via vasodilation. The main stimulus for coronary vasodilation is O2 deficiency; even a 5% fall in O2 content triggers dilation. Adenosine (from ATP breakdown) is a major vasodilator, acting via NO release from endothelium. Beta-blockers increase diastolic duration and improve subendocardial perfusion. The coronary arteries are functional end-arteries with limited anastomoses, so stenosis >70% dramatically increases resistance (∝ 1/r4) and causes ischaemia.

Cutaneous circulation: at rest the skin receives ~8% of cardiac output for thermoregulation, not metabolic needs. Below ~24°C ambient temperature, cutaneous vessels vasoconstrict to retain heat; above ~24°C they vasodilate. Arteriovenous anastomoses in extremities (normally closed by sympathetic tone) bypass capillaries and allow rapid heat removal during exercise. Paradoxical cold vasodilation occurs after prolonged cold exposure (to prevent ischaemia) — a cyclic vasoconstriction/dilation cycle. Raynaud''s phenomenon is an exaggerated vasoconstrictor response to cold.

Cerebral circulation: the brain receives ~13% of cardiac output; cerebral blood flow (CBF) is held constant at ~50 mL/100 g/min via tight myogenic autoregulation over a MAP range of 50-150 mmHg. The brain has no capacity for anaerobic metabolism. The Circle of Willis provides collateral protection if a carotid or vertebral artery is occluded. Local CBF is regulated by metabolic factors: ↑PaCO2 → vasodilation (CO2 lowers pH, main local stimulus); ↓PaO2 below 50 mmHg → vasodilation.

Renal circulation: kidneys receive 20-25% of cardiac output at rest (~400 mL/100 g/min) for filtration, not metabolism. The unique series arrangement (glomerular → efferent arteriole → peritubular capillaries) allows high glomerular hydrostatic pressure for filtration. Autoregulation is via the myogenic response and tubuloglomerular feedback (macula densa senses NaCl delivery → signals afferent arteriole to constrict if flow is excessive).