The Circulatory System

Blood vessels are classified by structure and function. Elastic arteries (e.g. aorta, pulmonary trunk) have a tunica media rich in elastic tissue with interspersed smooth muscle. During systole, they expand to absorb the ejected stroke volume; during diastole, elastic recoil maintains forward blood flow and sustained arterial pressure. This "Windkessel" function smooths the pulsatile output of the heart into a more continuous flow. Muscular arteries have a tunica media dominated by smooth muscle, allowing regulation of blood distribution to peripheral territories.

All large vessels share three-layer wall construction. The tunica intima is the innermost layer — a single layer of non-thrombogenic squamous endothelial cells. The tunica media is the thickest layer in arteries, containing elastic tissue and/or smooth muscle. The tunica adventitia is the outer connective tissue sheath; it is thickest in veins, limiting venous compliance.

Veins have much thinner walls and lower internal pressure than arteries. They possess bicuspid venous valves that prevent retrograde blood flow. Because veins are highly distensible, they act as capacitance vessels (blood reservoirs). On cross-section, histological specimens of veins appear crinkled post-mortem because surrounding tissues compress the compliant wall. Lymphatic vessels also carry valves and transport excess interstitial fluid back into the circulation; lymph fluid lacks red blood cells.

Capillaries are the exchange vessels, with walls consisting of a single endothelial layer. There are three types. Continuous capillaries (muscle, brain, skin) have tight intercellular junctions and moderate permeability. Fenestrated capillaries (kidneys, gut mucosa, glands) have pores allowing rapid water filtration. Sinusoidal (discontinuous) capillaries (liver, spleen, bone marrow) have gaps allowing large proteins and cells to pass freely.

Substances cross capillary walls by several mechanisms: direct diffusion through lipid-soluble endothelium (O2, CO2), diffusion through intercellular clefts (ions, polar molecules), active vesicular transport (pinocytosis), and structural pores in fenestrated/sinusoidal capillaries. Proteins are normally excluded by the cleft, except in sinusoidal capillaries.

Clinically, venous access is used to reach the right heart; arterial access is used to reach the left heart. The great saphenous vein — the longest vein in the body — runs anterior to the medial malleolus and joins the femoral vein at the saphenofemoral junction near the groin.