section 39.3

Water

and

Osmolality Controls

931

FIGURE 39-3

Principal transport processes in the renal nephron. ADH, Antidiuretic hormone. [Reproduced with permission from

M. B. Burg, The nephron in transport of sodium, amino acids, and glucose.

H osp. P ract.

13(10), 99 (1978).

A. Iselin, illustrator.]

membrane receptor. The ANP receptor itself is a guanylyl

cyclase with its ligand binding domain located in the ex-

tracellular space and its catalytic domain in the cytsolic

FIGURE 39-4

Amino acid sequence of atrial natriuretic peptide. It has a one-disulfide

linkage.

domain. The receptor has only one membrane spanning

domain. This ANP receptor-activated cGMP complex is

unique and does not involve any G-proteins. A soluble cy-

tosolic guanylyl cyclase that is activated by nitric oxide

binding to the heme group of the enzyme causes vascu-

lar relaxation (Chapter 17). The intracellular formation of

cGMP causes activation cGMP-dependent protein kinases

which mediate the actions of ANP. An ANP of 32 amino

acid residues (known as B type-ANP) found in the ventri-

cles of the heart (and in the brain) is secreted in response

to ventricular expansion and pressure. B type-ANP has

similar physiologic function compared to atrial ANP.

The pH of extracellular fluid is kept within very nar-

row limits (7.35-7.45) by buffering mechanisms (see also

Chapter 1), the lungs, and the kidneys. These three systems

do not act independently. For example, in acute blood loss

release of ADH and aldosterone restores the blood volume

and renal regulation of the pH leads to shifts in K+ and

Na+ levels.

39.3 Water and Osmolality Controls

Despite considerable variation in fluid intake, an individ-

ual maintains water balance and a constant composition

of body fluids. The homeostatic regulation of water is