Blood Vessels: Arteries, Veins, and Capillaries – Structure and Function

The circulatory system's highways: three vessel types with distinct structure and function.

Arteries

• Function: Carry oxygenated blood away from heart (except pulmonary artery) under high pressure.
• Structure: Thick tunica media (smooth muscle, elastin) → withstands pressure, regulates flow via vasoconstriction/dilation.
• Types: Elastic (aorta, carotids) – Windkessel effect; Muscular (distributing) – resistance regulation.

Veins

• Function: Return deoxygenated blood to heart (except pulmonary veins) under low pressure.
• Structure: Thinner walls, larger lumen, valves (prevent backflow), capacitance vessels (hold ~65% blood volume).
• Return Mechanisms: Skeletal muscle pump, respiratory pump, venous tone.

Capillaries

• Function: Exchange of gases, nutrients, waste, hormones between blood and tissues.
• Structure: Single endothelial cell layer + basement membrane; diameter ~5–10 µm (RBCs deform to pass).
• Types: Continuous (muscle, lung, BBB), Fenestrated (kidney, endocrine, gut), Sinusoidal (liver, spleen, marrow).

Capillary Exchange (Starling Forces)

Net filtration = K_f [(P_c – P_i) – σ(π_c – π_i)]. Balanced by lymphatics.

Together, these vessels form a closed loop of ~60,000 miles in adults.