Structure and Function of Blood Vessels

Core Concepts

The three tunics

Most vessels have three concentric layers:

• Tunica intima — the innermost layer: a single layer of endothelium on a basement membrane. It is the only layer present in all vessels and more than a passive lining: it secretes signals that influence vessel diameter and clotting.
• Tunica media — the middle layer of smooth muscle and elastic fibers. It vasoconstricts (lumen narrows) and vasodilates (lumen widens); innervated by sympathetic nerves, it is the main control point for blood pressure and flow distribution.
• Tunica externa (adventitia) — the outer connective-tissue layer anchoring the vessel; in large vessels it carries small vessels (vasa vasorum) that nourish the thick wall.

Arteries: the pressure vessels

• Elastic arteries (aorta, pulmonary trunk, largest branches) have abundant elastin; they stretch during ejection and recoil during diastole, smoothing the heart's pulsatile output into more continuous flow (the "windkessel" effect).
• Muscular arteries (brachial, femoral, coronary) have more smooth muscle; they distribute blood to specific organs and can adjust their diameter.
• Arterioles are the smallest arterial branches, just before capillaries; their muscular walls make them the primary site of resistance — tiny diameter changes produce large changes in downstream flow.

Capillaries: the exchange vessels

Capillaries are microscopic tubes whose wall is essentially endothelium plus a basement membrane — no tunica media or externa. That thinness is the point: oxygen, nutrients, and wastes cross by diffusion and bulk flow. Capillaries form capillary beds fed by a metarteriole and drained by venules; a precapillary sphincter (a smooth-muscle ring at the entrance) controls how many capillaries are open at once. Capillary walls differ by location:

• Continuous capillaries — uninterrupted lining with tight junctions; found in muscle, skin, and the brain (where junctions are especially tight — the blood–brain barrier).
• Fenestrated capillaries — endothelial cells with small pores that speed passage of water and small solutes; found in kidneys, intestines, and endocrine glands.
• Sinusoids — wide channels with large gaps and incomplete basement membranes, allowing even large molecules and cells to pass; found in liver, spleen, and bone marrow.

Veins: the capacitance vessels

Venules collect blood from capillary beds; veins carry it back toward the heart. Their tunica media is thin relative to the lumen, making veins distensible — able to hold large volumes at low pressure. Veins therefore hold most of the body's blood (around 60% of total volume in a resting adult, a commonly taught figure) and act as the blood reservoir. Two features compensate:

• Venous valves — one-way flaps of tunica intima that prevent backflow, especially against gravity in the limbs.
• Skeletal muscle and respiratory pumps — contracting leg muscles squeeze veins upward; breathing changes thoracic pressure and draws blood into the chest.

Vascular anastomoses and vessel tone

Most tissues receive blood from more than one route. An anastomosis connects vessels: arterial anastomoses (around joints, in heart and brain) provide collateral circulation if one artery is blocked; venous anastomoses are even more common. Areas with end arteries (retina, kidneys) have few or no anastomoses, so blockage there causes infarction more readily.

Finally, the tunica media's smooth muscle is under continuous control: vasoconstriction narrows the lumen (raising resistance), vasodilation widens it (lowering resistance, increasing flow). These changes — driven by neural signals, hormones, and local chemicals — are the central mechanism of the homeostatic regulation covered in Topic 4.

Common Confusions