Porth's Essentials of Pathophysiology, 4e

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Control of Cardiovascular Function

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back to the heart. Importantly, the lymphatics can carry proteins and large particulate matter away from the tis- sue spaces, neither of which can be removed by absorp- tion into the venous system. The lymphatic system is also the main route for absorption of fats and fat soluble vitamins from the gastrointestinal tract. The lymphatic system is made up of vessels similar to those of the blood vessels in the circulatory system. These vessels commonly travel along with an arteriole or venule or with a companion artery and vein. The ter- minal lymphatic vessels are made up of a single layer of connective tissue with an endothelial lining and resemble blood capillaries. The lymphatic vessels lack tight junc- tions and are loosely anchored to the surrounding tis- sues by fine filaments (Fig. 17-21). The loose junctions permit the entry of large particles, and the filaments hold the vessels open under conditions of edema, when the pressure of the surrounding tissues would otherwise cause them to collapse. The lymph capillaries drain into larger lymph vessels that ultimately empty into the right and left thoracic ducts (Fig. 17-22). The thoracic ducts empty into the circulation at the junctions of the subcla- vian and internal jugular veins. Although the divisions are not as distinct as in the cir- culatory system, the larger lymph vessels show evidence of having intimal, medial, and adventitial layers simi- lar to those of blood vessels. Contraction of the smooth muscle in the medial layer of the larger collecting lymph channels assists in propelling lymph fluid toward the thorax. External compression of the lymph channels by active and passive movements of body parts also aids in forward propulsion of lymph fluid. The rate of

flow through the lymphatic system by way of all of the various lymph channels, approximately 120 mL/hour, is determined by the interstitial fluid pressure and the activity of lymph pumps. The Microcirculation The most important function of the circulatory sys- tem occurs in the microcirculation, which consists of the arterioles, capillaries, and venules. It is here that the transport of nutrients to the tissues and removal of metabolites takes place. Blood enters the microcircula- tion through an arteriole, passes through the capillar- ies, and leaves by way of a small venule (Fig. 17-23). Small cuffs of smooth muscle, the precapillary sphinc- ters, are positioned at the arterial end of the capillary. The smooth muscle tone of the arterioles, precapillary sphincters, and venules controls blood flow through the capillary bed. Depending on venous pressure, blood flows through the capillary channels when the precapil- lary sphincters are open. An important aspect of the circulatory system, which occurs at the level of the microcirculation, is the abil- ity of organs and tissues to regulate their blood flow based on metabolic needs. Local control is particularly important in tissues such as skeletal muscle and in the heart, organs in which the metabolic activity and need for blood flow vary extensively; and in the brain where metabolic activity and need for blood flow remain rela- tively constant. Autoregulation of Blood Flow Autoregulation is a local control mechanism that auto- matically adjusts tissue blood flow independent of sys- temic factors. For example, blood flow to organs such as the heart, brain, and kidneys remains relatively con- stant, although blood pressure may vary over a range of 60 to 180 mm Hg. In contrast to the mean arterial pressure, which is controlled by systemic mechanisms that adjust the cardiac output to maintain that pressure, changes in blood flow to the individual body tissues are controlled intrinsically by modifying the diameter of local arterioles feeding the capillaries. There are two mechanisms that control autoregula- tion: metabolic and myogenic. In most tissues, declining levels of nutrients, particularly oxygen, are the stron- gest stimuli for autoregulation. Substances released by metabolically active tissues (such as potassium and hydrogen ions, lactic acid, and adenosine, which is a breakdown product of ATP) serve as autoregulation stimuli. Whatever the precise stimuli, the net result is an immediate vasodilation of the arterioles serving the cap- illaries of the metabolically deprived tissues. Inadequate blood perfusion to an organ is quickly followed by a decline in its metabolic rate and, if prolonged, death of its cells. Likewise, excessively high arterial pressure and tissue perfusion can be dangerous because it may damage the more fragile blood vessels. The myogenic (myo = muscle; gen = organ) control mechanisms rely on stretch of the vascular smooth muscle in the vessel wall.

B

Interstitial fluid Opening

Tissue cell

Anchoring filament

Venule

Blood capillary

Lymph

Tissue cell

Endothelium of lymphatic capillary

Interstitial fluid

Arteriole

Lymphatic capillary

A

FIGURE 17-21. (A) Location of the lymphatic capillary. Blood from the arterial side of the capillary moves into the interstitial spaces and is reabsorbed in the venous side of the capillary bed. (B) Details of the lymphatic capillary with its anchoring filaments and overlapping edges that serve as valves and can be pushed open, allowing the inflow of interstitial fluids and suspended particles.