The Renin Angiotensin Mechanism

When blood pressure drops, the sympathetic nerves also stimulate the juxtaglomerular cells to secrete the enzyme renin (REE-nin). Renin acts on a plasma protein, angiotensinogen, to remove a fragment called angiotensin I, a chain of 10 amino acids. In the lungs and kidneys, angiotensin-converting enzyme (ACE) removes two more

Saladin: Anatomy & I 23. The Urinary System I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 23 The Urinary System 891

Reduced GFR

High GFR

Reduced GFR

High GFR

Renin Angiotensin Control Saladin
Figure 23.12 Negative Feedback Control of Glomerular Filtration Rate.

amino acids, converting it to angiotensin II, a hormone with multiple effects (fig. 23.13):

It stimulates widespread vasoconstriction, which raises the MAP throughout the body.

It constricts both the afferent and efferent arterioles. The net effect of this is to reduce GFR and water loss.

It stimulates the secretion of antidiuretic hormone, which promotes water reabsorption.

It stimulates the adrenal cortex to secrete aldosterone, which in turn promotes sodium and water retention.

It stimulates the sense of thirst and encourages water intake.

Some of these effects are explained more fully later in this chapter and in chapter 24.

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