These structures form a network of tubes from the heart, to the tissues of the body, and back again.
Arteries are vessels that carry blood away from the heart. They consist of three layers:
Arteries branch and divide, becoming smaller and smaller. When they become microscopic, they are called arterioles and have a different structure. They consist of a layer of endothelial cells surrounded by a thin layer of smooth muscle cells. The smallest arterioles connect to capillaries. Flow into the capillaries is controlled by precapillary sphincters. Smooth muscle cells encircle the bases of capillaries at the arteriole-capillary junctions and constrict to limit the flow of blood. The human body uses these sphincters to divert blood to the area where it is needed. During physical activity, blood is supplied to the muscles, leaving other parts of the body with lower flow.
Capillaries consist of a single layer of endothelial cells supported by a thin layer of connective tissue. Exchange of nutrients and gases occurs in the capillary bed, a network of microscopic vessels that passes through a region of tissue. The capillary walls are permeable, allowing materials to pass through the cells and through small gaps between cells. Nutrients including fats, carbohydrates, and proteins are used by the cells of the body as fuel. Energy is released when these fuels are combined with oxygen in a controlled chemical reaction to form carbon dioxide, water, and other waste products. Nutrients pass out of the capillaries into the extracellular fluid that surrounds the cells of the body through the process of diffusion. Similarly, waste passes into the cells by diffusion, following the concentration gradient. The concentration gradient is not the only factor influencing material exchange. Pressure is higher in the capillary than in the extracellular fluid creating a pressure gradient out of the vessel. However, protein concentrations within the blood are higher than in the extracellular fluid causing an osmotic pressure that draws fluid into the vessel. Because the capillary has a very small diameter, only slightly larger than a red blood cell, it has high flow resistance. Pressure on the arteriole side of the capillary bed is higher, causing a net flow out of the vessel into the extracellular fluid, overcoming osmotic pressure. At the venule side (beyond the flow restriction) pressure is lower, resulting in a net flow into the vessel due to osmotic pressure exceeding vessel pressure. Approximately 90% of the fluid that passes into the capillary bed is returned to the circulatory system through this process. The remaining 10% is collected by the lymphatic system and eventually delivered back to the circulatory system through a series of lymphatic vessels.
The capillaries combine to form venules. Small venules consist of an inner layer of endothelial cells surrounded by a layer of elastic fibers. Venules combine to form veins, and veins combine to form larger veins eventually leading back to the heart. Veins have a structure similar to arteries, with an inner layer of endothelial cells, a middle layer of smoother muscle, and an outer layer of elastic fibers. However, because the pressure in veins is low, the layer of smooth muscle in thinner. The muscles in the veins serve a different purpose. Approximately 60% of the blood resides in the veins, which act as a reservoir. If blood is lost due to an injury, muscles in the veins can be contracted to reduce the reserve volume and maintain blood pressure to compensate for the blood loss.
