The Structure of Artery and Vein Walls
Because veins transport blood far from the heart, they maintain much lower blood pressures than arteries. Their thin walls reflect this difference. Veins have much less elastic tissue and muscle tissue than arteries and collapse if empty. The thin walls enhance veins' ability to expand; they can carry higher volumes of blood than arteries.
Blood pressure drops by the time blood reaches the capillaries. This is because there is such a greater volume of capillaries compared to arteries. The large amount of surface area the capillaries cover allows the blood a lot of room to dissipate, thereby reducing the pressure. Capillary composition—an endothelium and underlying membrane—is far simpler than the three-layer arrangement of the other vessels. Capillaries have very thin walls, as thin as 0.2–0.4 μm. They are also abundant and arranged very close together. Unlike arteries and veins, capillaries are specialized for permeability. Combined with the simple wall construction, permeability allows exchange of materials between blood and interstitial fluid, which surrounds cells found in tissues within the body. Continuous capillaries, the type found in most tissues, contain narrow intercellular clefts, or openings, between endothelial cells. These allow passage of small solutes such as glucose. Fenestrated capillaries are important in organs carrying out filtration and absorption. The endothelial cells of fenestrated capillaries include many holes, or fenestrations, that allow small molecules to pass while containing larger ones.
Vasodilation and Vasoconstriction
The smooth muscle in the tunica media (middle layer) of blood vessel walls enables vessels to contract or relax, decreasing or increasing their diameter. Vasoconstriction, the decrease in vessel diameter produced by smooth muscle contraction, decreases blood flow. Vasoconstriction increases blood pressure upstream of the contraction and decreases the blood pressure downstream of it. Vasodilation refers to the increase in vessel diameter resulting from smooth muscle relaxation. This process increases blood flow and decreases blood pressure by widening the blood vessels and allowing more blood to flow through.Blood flow across capillary beds is adjusted by precapillary sphincters. A precapillary sphincter is a smooth muscular tube found at the opening of each capillary that controls whether the capillary is open or closed to arriving blood. Blood from arteries flows through a capillary bed in a thoroughfare channel, an arteriole connected directly to a postcapillary venule. The thoroughfare channel supplies the 10–100 capillaries that typically form a capillary bed. Blood in the thoroughfare channel perfuses (flows across) capillaries when the precapillary sphincters are open and bypasses the bed when the precapillary sphincters are closed. Blood pressure is already low by the time blood reaches capillaries, and it drops much further when blood is blocked by the vasoconstriction of precapillary sphincters.
Vasodilation and Vasoconstriction in Capillaries
Redirection of blood at capillaries allows the body to move blood where it is needed, from one organ to another. Arterial vasoconstriction decreases blood flow to capillaries and increases blood pressure along that route. Blood instead takes a path with less resistance, shifting to where vessels are more dilated and blood pressure is lower. For example, in a person relaxing after a meal, vasoconstriction halts most blood flow to leg capillaries. Blood pressure above the legs then rises, and higher resistance causes blood to flow in an arterial path with less resistance. Blood redirects to the small intestine, where it enables digestion. In contrast, arteries dilate in a person running on a treadmill. Blood is directed away from kidneys and digestive organs to the heart, lungs, and skeletal muscles.
At capillary beds, nutrients and gases are exchanged directly between interstitial fluid and blood by four processes: diffusion, transcytosis, filtration, and reabsorption. Diffusion is the most important of these, enabling materials to move across the plasma membrane from areas of higher concentration to areas of lower concentration. Oxygen and glucose diffuse from blood through interstitial fluid, found between cells in the body, to the cells of tissues. Carbon dioxide and other wastes diffuse from tissues to blood. Through transcytosis, where molecules are transported across the interior of a cell, endothelial cells move fluids across the plasma membrane. The plasma membrane uses pinocytosis, pinching off vesicles containing fluid droplets. Once transported across the plasma membrane, fluid is released by exocytosis, the vesicles expelling their contents. Though not common, transcytosis moves albumin, fatty acids, and hormones such as insulin.