8.2 Animal Transport

Arteries have thick walls containing elastic fibers and smooth muscle, a relatively narrow lumen and no valves along most of their length. Elastic tissue stretches during systole and recoils during diastole, smoothing intermittent ventricular ejection into continuing flow. Smooth muscle in arterioles changes lumen radius. Because resistance changes very steeply with radius, modest vasoconstriction can greatly reduce flow and raise pressure upstream.

Veins return blood at low pressure. Their wider lumens reduce resistance, thinner walls reflect the smaller pressure, and valves prevent reverse flow. Skeletal-muscle contraction compresses veins, and breathing changes thoracic pressure; with valves, both mechanisms propel blood toward the heart. Capillaries consist of a single thin endothelial layer with a lumen barely wider than a red blood cell. Their immense combined cross-sectional area slows flow and provides a short diffusion path.

Atherosclerosis begins with damage and lipid accumulation in an arterial wall, followed by inflammation and plaque formation. The narrowed lumen increases resistance, and plaque rupture can trigger a clot. A coronary blockage deprives cardiac muscle of oxygen; a cerebral blockage can cause ischemic stroke. The biological danger comes from impaired perfusion and loss of aerobic ATP production, not simply from the physical presence of cholesterol.