Essentials of Anatomy and Physiology

peritubular capillaries, HCO 3  ions follow. Notice
what has happened: Two Hions have been excreted
in urine, and two Naions and two HCO 3 ions have
been returned to the blood. As reactions like these
take place, the body fluids are prevented from becom-
ing too acidic.
Another mechanism used by the cells of the kidney
tubules to regulate pH is the phosphate buffer system,
which is described in Chapter 19.

OTHER FUNCTIONS

OF THE KIDNEYS

In addition to the functions described thus far, the kid-
neys have other functions, some of which are not

directly related to the formation of urine. These func-

tions are secretion of renin (which does influence

urine formation), production of erythropoietin, and

activation of vitamin D.

1.Secretion of renin—When blood pressure de-

creases, the juxtaglomerular(juxtameans “next

to”) cells in the walls of the afferent arterioles

secrete the enzyme renin. Renin then initiates the

renin-angiotensin mechanism to raise blood pres-

sure. This was first described in Chapter 13, and

the sequence of events is presented in Table 18–2.

The end product of this mechanism is angiotensin

II, which causes vasoconstriction and increases the

secretion of aldosterone, both of which help raise

blood pressure.

The Urinary System 429

ADH Increases reabsorption of H 2 O

ANP

PTH

Aldosterone

Blood

Urine

Increases reabsorption of Ca+^2

Increases reabsorption of Na+

and excretion of K+

Decreases reabsorption of Na+

Figure 18–5. Effects of hormones on the kidneys.

QUESTION:Do any of these hormones affect both reabsorption and secretion? If so, how?