Porth's Essentials of Pathophysiology, 4e

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Circulatory Function

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volume and causes blood to accumulate in the systemic veins and right heart. The veins and venules are thin-walled, distensible, and collapsible vessels. The veins are capable of enlarg- ing and storing large quantities of blood, which can be made available to the circulation as needed. Even though the veins are thin walled, they are muscular. This allows them to contract or expand to accommodate varying amounts of blood. Veins are innervated by the sympa- thetic nervous system. When blood is lost from the cir- culation, the veins constrict as a means of maintaining the circulating blood volume. Because the venous system is a low-pressure system, blood flow must oppose the effects of gravity. In a per- son in the standing position, the weight of the blood in the vascular column causes an increase of 1 mm Hg in pressure for every 13.6 mm of distance below the level of the heart. Were it not for the valves in the veins and the action of the skeletal muscles, the venous pressure in the feet would be about +90 mm Hg in the standing adult. Gravity has no effect on the venous pressure in a person in the recumbent position because the blood in the veins is then at the level of the heart. Valves in the veins of extremities counteract the effects of gravity (Fig. 17-19), and with the help of skeletal muscles that surround and intermittently compress the leg veins in a milking manner, move blood forward to the heart. This pumping action is known as the venous or muscle pump and is efficient enough that under nor- mal circumstances, the pressure in the feet of a walking

Basal lamina (cut)

Nucleus of endothelial cell

Lumen

Capillary pores

Red blood cell

Endothelial cell

Intercellular junctions

FIGURE 17-20. Endothelial cells and intercellular junctions in a section of capillary.

adult remains less than 20 mm Hg. There are no valves in the abdominal or thoracic veins, and blood flow in these veins is heavily influenced by the pressure in the abdominal and thoracic cavities, respectively. Capillaries Capillaries are microscopic, single-cell–thick vessels that connect the arterial and venous segments of the circula- tion. The capillary wall is composed of a single layer of endothelial cells surrounded by a basement membrane or basal lamina (Fig. 17-20). Intercellular junctions join the capillary endothelial cells; these are called the capillary pores. Lipid-soluble materials diffuse directly through the capillary cell membrane. Water and water-soluble mate- rials leave and enter the capillary through the capillary pores. The size of the capillary pores varies with capillary function. In the brain, the endothelial cells are joined by tight junctions that form the blood–brain barrier. This prevents substances that would alter neural excitability from leaving the capillary. In organs that process blood contents, such as the liver, capillaries have large pores so that substances can pass easily through the capillary wall. In the kidneys, the glomerular capillaries have small openings called fenestrations that pass directly through the middle of the endothelial cells, a feature that is con- sistent with the filtration function of the glomerulus. Lymphatic System The lymphatic system, commonly called the lymphatics, represents an accessory route through which fluid can flow through the tissue spaces into the blood and then

Toward heart

Open valve

Closed valve

FIGURE 17-19. Portion of a femoral vein opened, to show the valves.The direction of flow is upward. Backward flow closes the valve.