T Mean arterial pressure to normal

Loss of plasma volume leads to a decrease in MAP. Baroreceptors located in the aortic and carotid sinuses detect this fall in MAP and elicit reflex responses that include an increase in the overall activity of the sympathetic nervous system. Sympathetic stimulation of the heart and blood vessels leads to an increase in cardiac output (CO) and increased total peripheral resistance (TPR). These adjustments, which increase MAP, are responsible for the short-term regulation of blood pressure. Although increases in CO and TPR are effective in temporary maintenance of MAP and blood flow to the vital organs, these activities cannot persist indefinitely. Ultimately, plasma volume must be returned to normal (see Table 19.1).

An overall increase in sympathetic nerve activity includes an increase in sympathetic input to the kidneys. Consequently, resistance of the afferent arteriole increases, leading to a decrease in RBF. As discussed, this results in a decrease in PGC, GFR, and urine output. As such, the renal excretion of sodium and water is decreased. In other words, sodium and water are conserved by the body, which increases plasma volume and MAP toward normal. These changes are responsible for the long-term regulation of blood pressure (see Table 19.1).

Sympathetic stimulation also increases the resistance of the efferent arte-riole, leading to a decrease in blood pressure in the peritubular capillaries. This fall in pressure facilitates movement of sodium and water from the tubules into these capillaries.

Angiotensin II. Angiotensin II also increases the resistance of the renal arterioles and consequently decreases RBF and GFR. Angiotensin II is synthesized by the following pathway:


Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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