Essentials of Anatomy and Physiology

A normal blood pressure is essential to normal
body functioning. Perhaps the most serious change
is a sudden, drastic decrease in blood pressure, such
as would follow a severe hemorrhage. In response to
such a decrease, the kidneys will decrease filtration
and urinary output and will initiate the formation of
angiotensin II. In these ways the kidneys help
ensure that the heart has enough blood to pump to
maintain cardiac output and blood pressure.
2.Secretion of erythropoietin—This hormone is
secreted whenever the blood oxygen level decreases
(a state of hypoxia). Erythropoietin stimulates
the red bone marrow to increase the rate of RBC
production. With more RBCs in circulation, the
oxygen-carrying capacity of the blood is greater,
and the hypoxic state may be corrected (see also
Box 18–3: Erythropoietin).
3.Activation of vitamin D—This vitamin exists in
several structural forms that are converted to cal-

citriol(D 2 ) by the kidneys. Calcitriol is the most

active form of vitamin D, which increases the

absorption of calcium and phosphate in the small

intestine.

430 The Urinary System

Peritubular

capillary Filtrate

Renal tubule

Na+ Na+ Na+

HCO 3 

Na+

HCO 3 

HCO 3 

HCO 3 

CICI



Na+

Na+

H CI

+

H+

Na+

Na+

CO 2 H 2 O H 2 CO 3

CO 2 H 2 O H 2 CO 3

H+

H+







+

NH 2 NH 3

NH 4

NH 3

NH 4 Cl

Blood Urine

Figure 18–6. Renal regulation of acid–base

balance. The cells of the renal tubule secrete H

ions and ammonia into the filtrate and return

Naions and HCO 3 ions to the blood in the

peritubular capillaries. See text for further des-

cription.

QUESTION:The cells of the renal tubule make

good use of CO 2. What do the cells use CO 2 for?

Table 18–2 THE RENIN-ANGIOTENSIN

MECHANISM

Sequence

1. Decreased blood pressure stimulates the kidneys to
   secrete renin.


2. Renin splits the plasma protein angiotensinogen
   (synthesized by the liver) to angiotensin I.


3. Angiotensin I is converted to angiotensin II by an
   enzyme found in lung tissue and vascular endothe-
   lium.


4. Angiotensin II causes vasoconstriction and stimulates
   the adrenal cortex to secrete aldosterone.