Porth's Essentials of Pathophysiology, 4e

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Disorders of Blood Flow and Blood Pressure

C h a p t e r 1 8

Arterial blood pressure

Cardiac output

Peripheral vascular resistance

Sympathetic activity

Heart rate Stroke volume

Vagal and sympathetic activity

Heart

Baroreceptors

Venous return

Angiotensin II

Adrenal gland

Blood volume

FIGURE 18-13. Mechanisms of blood pressure regulation.The solid lines represent the mechanisms for renal and baroreceptor control of blood pressure through changes in cardiac output and peripheral vascular resistance.The dashed lines represent the stimulus for regulation of blood pressure by the baroreceptors and the kidneys.

Aldosterone

Salt and water retention

Renin-angiotensin mechanism

Kidney

stimulation produces an increase in heart rate and car- diac contractility. Blood vessels are mainly innervated by the sympathetic nervous system, which produces constriction of the small arteries and arterioles with a resultant increase in peripheral vascular resistance. The ANS control of blood pressure is mediated through both intrinsic and extrinsic cardiovascular reflexes, as well as higher neural control centers. 26 The intrinsic reflexes, including the baroreceptor and che- moreceptor reflexes, respond to stimuli originating from within the cardiovascular system and are essential to rapid and short-term regulation of blood pressure. The extrinsic reflexes mediate the cardiovascular response to stimuli originating from outside the cardiovascular sys- tem. They mediate blood pressure responses associated with factors such as pain and temperature changes. The neural pathways for these reactions are more diffuse, and their responses are less consistent than those of the intrinsic reflexes. Many of these responses are channeled through the hypothalamus, which plays an essential role in the control of sympathetic nervous system responses. The baroreceptors are pressure-sensitive receptors located in the walls of blood vessels and the heart. They can be classified as high-pressure and low-pressure baroreceptors based on the type of blood vessel in which they are located. The high-pressure carotid and aortic

baroreceptors are located in strategic position between the heart and the brain (Fig. 18-14). They respond to changes in the stretch of the vessel wall by sending impulses to cardiovascular centers in the brain stem to effect appropriate changes in heart rate and vascu- lar smooth muscle tone. For example, the fall in blood pressure that occurs on moving from the lying to the standing position produces a decrease in the stretch of the baroreceptors with a resultant increase in heart rate and sympathetically induced vasoconstriction that pro- duces an increase in peripheral vascular resistance. The low-pressure baroreceptors, which are located in large systemic veins, pulmonary vessels, and walls of the right atrium and ventricles of the heart, have both circulatory and renal effects. They produce changes in antidiuretic hormone (ADH) secretion, resulting in profound effects on the retention of salt and water. The arterial chemoreceptors are cells that monitor the oxygen, carbon dioxide, and hydrogen ion con- tent of the blood. They are located in the carotid bod- ies, which lie in the bifurcation of the two common carotids, and in the aortic bodies of the aorta 26 (see Fig. 18-14). Because of their location, these chemore- ceptors are always in close contact with the arterial blood. Although the main function of the chemorecep- tors is to regulate ventilation, they also communicate ( text continues on page 423 )