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Blood vessels are classified by structure and function into arteries (thick-walled, elastic, carry blood away from heart), arterioles (primary resistance vessels), capillaries (site of exchange), venules, and veins (thin-walled, capacitance vessels, contain ~65% of blood volume). Blood flow is governed by Ohm's Law analogue: flow = pressure difference / resistance, where resistance is strongly influenced by vessel radius (Poiseuille's Law: R ∝ 1/r⁴). The microcirculation at capillary beds is regulated by precapillary sphincters, and exchange of gases, nutrients, and waste occurs by diffusion and osmosis. Venous return relies on skeletal muscle pump, respiratory pump, and venous valves.
Work through the hemodynamics equations using clinical examples (e.g., how does arteriosclerosis affect blood pressure?). Map the pulmonary and systemic circuits on a diagram, tracing oxygen content changes at each stage.
From your study of the cardiovascular system, you know the heart pumps blood through two circuits. But the heart's pumping is only part of the story — the vessels themselves are precision-engineered to control where blood goes, how fast it flows, and where exchange occurs.
Vessels are organized in a hierarchy of decreasing diameter as blood leaves the heart. Arteries have thick, elastic walls that expand with each heartbeat and recoil to maintain pressure between beats — this elasticity is why you feel a pulse in arteries but not veins. Arterioles, the small arteries just upstream of capillary beds, are the body's primary pressure regulators: by constricting or dilating their smooth muscle walls, they act like adjustable valves controlling flow into each tissue bed. The most important physical principle here is Poiseuille's Law: resistance increases with the fourth power of decreasing radius (R ∝ 1/r⁴). Halving an arteriole's radius increases resistance 16-fold — which is why arteriosclerosis, even modest arterial narrowing, raises blood pressure so dramatically and forces the heart to work harder.
Capillaries are where the circulatory system does its actual job: exchanging gases, nutrients, and waste with tissues. They are optimally built for this role because their walls are only a single cell thick, blood moves slowly through them (maximizing contact time), and their total cross-sectional area is enormous. Precapillary sphincters regulate which capillary beds are open at any time, routing blood toward metabolically active tissues and away from resting ones.
Blood returns to the heart through venules and veins, which hold about 65% of total blood volume — making veins the body's blood reservoir. Unlike arteries, veins are low-pressure vessels that rely on accessory mechanisms for return: the skeletal muscle pump (contracting limb muscles compress veins), the respiratory pump (pressure changes during breathing draw blood toward the chest), and one-way venous valves that prevent backflow. This explains why prolonged immobility causes blood to pool in the legs, and why regular movement is so important to circulatory health.