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0 Glomerular filtration rate to normal

An increase in MAP leads to an increase in RBF, PGC, and GFR. As a result, the rate of fluid flow through the distal tubule increases, leading to an increase in reabsorption of Na+ and Cl- ions by the cells of the macula densa in the distal tubule. Consequently, these cells release vasoconstrictor substances, primarily adenosine. The subsequent increase in the resistance of the nearby afferent arteriole decreases RBF to normal and, as a result, PGC and therefore GFR decrease to normal. In this way, the distal tubule regulates its own filtrate flow.

Resistance of the afferent arteriole. Many physiological conditions warrant a change in RBF and GFR, even when MAP is within the autoregulatory range. For example, volume overload is resolved with an increase in RBF, GFR, and urine output so that excess water and solutes are eliminated. Conversely, volume depletion, such as that which occurs with hemorrhage or dehydration, is resolved with decreased RBF, GFR, and urine output; in this way, water and solutes are conserved.

The resistance of the afferent arteriole is influenced by several factors, including (see Table 19.1):

• Sympathetic nerves

• Angiotensin II

• Prostaglandins

Sympathetic nerves. The afferent and efferent arterioles are densely innervated with sympathetic nerves. Norepinephrine released directly from the nerves or circulating epinephrine released from the adrenal medulla stimulates a1 adrenergic receptors to cause vasoconstriction. The predominant site of regulation is the afferent arteriole. Under normal resting conditions, there is little sympathetic tone to these vessels so that RBF is comparatively high. As discussed previously, this facilitates glomerular filtration.

However, the degree of sympathetic stimulation to the kidneys is altered under various physiological and pathophysiological conditions. For example, consider a case in which an individual is volume depleted due to hemorrhage or dehydration:

0 Plasma volume

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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