98. Regional Circulations – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Regional circulation refers to the specialized blood supply of individual organs. Blood flow is distributed according to the metabolic and functional needs of each tissue. Local autoregulation ensures adequate perfusion. The kidneys receive the highest blood flow among major organs, whereas the carotid body has the highest flow per unit weight.
• Cerebral circulation is essential because the brain has minimal energy reserves and depends on a continuous supply of oxygen and glucose. Cerebral blood flow is about 750 mL/min (15% of cardiac output). Loss of blood flow for 15–30 seconds causes unconsciousness, while ischemia lasting more than 4–6 minutes produces irreversible neuronal injury. The blood-brain barrier (BBB) is formed by tight endothelial junctions, basement membrane, and astrocyte end-feet, protecting the brain from toxins and fluctuations in plasma composition.
• Cerebral blood flow is supplied by the internal carotid and vertebrobasilar systems, which form the Circle of Willis. Cerebral vessels behave functionally as end arteries. Regulation depends mainly on metabolic factors (carbon dioxide is the most important), autoregulation, and intracranial pressure. Cerebral blood flow remains nearly constant between 60–150 mm Hg mean arterial pressure. Hypercapnia increases cerebral blood flow, whereas hyperventilation reduces it. The Monro–Kellie doctrine explains the relationship among brain tissue, cerebrospinal fluid, and intracranial blood volume. Stroke may be ischemic or hemorrhagic.
• Coronary circulation supplies the myocardium through the right and left coronary arteries, which function as functional end arteries. Coronary blood flow is about 250 mL/min (4–5% of cardiac output) and occurs mainly during diastole. The myocardium extracts 70–80% of arterial oxygen at rest; therefore, increased oxygen demand is met by increasing coronary blood flow rather than oxygen extraction. Metabolic regulation is the most important mechanism, with adenosine acting as the major vasodilator. Coronary autoregulation occurs between 70–110 mm Hg. The subendocardium is most susceptible to ischemia because systolic compression reduces its blood flow.
• Coronary artery disease is usually caused by atherosclerosis. Angina pectoris results from transient myocardial ischemia without necrosis, whereas myocardial infarction is irreversible myocardial necrosis due to prolonged coronary occlusion. Diagnosis is based on electrocardiogram, cardiac troponins, and coronary angiography. Early reperfusion with tissue plasminogen activator or primary angioplasty limits myocardial damage.
• Cutaneous circulation is important for thermoregulation. Skin vessels receive only sympathetic vasoconstrictor fibers. Increased body temperature causes vasodilation, whereas cold causes vasoconstriction. Apical areas contain numerous arteriovenous anastomoses, which help regulate heat loss. The triple response consists of red reaction, wheal, and flare; the flare is produced by the axon reflex, while reactive hyperemia occurs after temporary arterial occlusion.
• Splanchnic circulation includes intestinal, hepatic, and splenic blood flow. The intestine receives about 20% of cardiac output at rest, increasing markedly after meals (functional hyperemia). Intestinal blood flow is regulated by sympathetic nerves, local metabolites, and gastrointestinal activity. The villous countercurrent exchange mechanism makes villus tips highly susceptible to ischemia during shock.
• Hepatic circulation is unique because the liver has a dual blood supply: the portal vein provides about 70–75% of blood flow, while the hepatic artery supplies 25–30% and most of the oxygen required by the liver.