section 39.3
Osmolality Controls
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
Amino acid sequence of atrial natriuretic peptide. It has a one-disulfide
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
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