Page 183 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

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Salt and Water Regulation Na content lead to changes in ECF volume. It is
regulated mainly by the following factors:
Osmoregulation. The osmolality of most body ! Renin–angiotensin system (RAS) (! p. 184).
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fluids is about 290 mOsm/kg H 2O. Any increase Its activation promotes the retention of Na via
in the osmolality of extracellular fluid (ECF) angiotensin II (AT II; lowers GFR), aldosterone
due, for example, to NaCl absorption or water (! A4) and ADH.
loss, results in an outflow of water from the in- ! Atriopeptin (atrial natriuretic peptide; ANP)
tracellular space, because the intracellular is a peptide hormone secreted by specific cells
fluid (ICF) and ECF are in osmotic balance of the cardiac atrium in response to rises in ECF +
Kidneys, Salt, and Water Balance controlled by osmosensors (or osmoreceptors) reabsorption from the collecting duct.
(! p. 173; B2, B6). The osmolality of the ECF
volume and hence atrial pressure. ANP pro-
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must be tightly regulated to protect cells from
motes the renal excretion of Na by raising the
filtration fraction (! p. 152) and inhibits Na
large volume fluctuations. Osmoregulation is
found mainly in the hypothalamus, hormones
! ADH. ADH secretion is stimulated by (a) in-
creased plasma and CSF osmolality; (b) the
(e.g., antidiuretic hormone = ADH = adiuretin =
Gauer-Henry reflex, which occurs when stretch
vasopressin) and the kidney, the target organ
of ADH (! p. 166).
receptors in the atrium warn the hy-
Water deficit (! A1). Net water losses (hy-
pothalamus of a decrease (" 10%) in ECF
povolemia) due, for example, to sweating, uri-
volume (~ atrial pressure); (c) angiotensin II
tonic. Osmolality rises of 1% or more
! Pressure diuresis (! p. 172), caused by an
(! 3 mOsm/kg H 2O) are sufficient to stimulate
elevated arterial blood pressure, e.g. due to an
7 nation or respiration, make the ECF hyper- (! p. 184).
elevated ECF volume, results in increased ex-
the secretion of ADH from the posterior lobe of
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the pituitary (! p. 280). ADH decreases uri- cretion of Na and water, thereby lowering ECF
nary H 2O excretion (! p. 166). The likewise hy- volume and hence blood pressure. This control
pertonic cerebrospinal fluid (CSF) stimulates circuit is thought to be the major mechanism
central osmosensors in the hypothalamus, for long term blood pressure regulation.
which trigger hyperosmotic thirst. The percep- Salt deficit (! A3). When hyponatremia occurs
tion of thirst results in an urge to replenish the in the presence of a primarily normal H 2O con-
body’s water reserves. Peripheral osmosensors tent of the body, blood osmolality and there-
in the portal vein region and vagal afferent fore ADH secretion decrease, thereby increas-
neurons warn the hypothalamus of water ing transiently the excretion of H 2O. The ECF
shifts in the gastrointestinal tract. volume, plasma volume, and blood pressure
Water excess (! A2). The absorption of hy- consequently decrease (! A4). This, in turn,
potonic fluid reduces the osmolality of ECF. activates the RAS, which triggers hypovolemic
This signal inhibits the secretion of ADH, re- thirst by secreting AT II and induces Na reten-
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sulting in water diuresis (! p. 166) and nor- tion by secreting aldosterone. The retention of
malization of plasma osmolality within less Na increases plasma osmolality leading to
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than 1 hour. secretion of ADH and, ultimately, to the reten-
tion of water. The additional intake of fluids in
Water intoxication occurs when excessive volumes
of water are absorbed too quickly, leading to symp- response to thirst also helps to normalize the
toms of nausea, vomiting and shock. The condition is ECF volume.
caused by an undue drop in the plasma osmolality Salt excess (! A4). An abnormally high NaCl
before adequate inhibition of ADH secretion has oc- content of the body in the presence of a normal
curred. H 2O volume leads to increased plasma
osmolality (thirst) and ADH secretion. Thus,
Volume regulation. Around 8–15 g of NaCl are
absorbed each day. The kidneys have to excrete the ECF volume rises and RAS activity is
the same amount over time to maintain Na + curbed. The additional secretion of ANP, per-
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and ECF homeostasis (! p. 168). Since Na is haps together with a natriuretic hormone with
170 the major extracellular ion (Cl – balance is a longer half-life than ANP (ouabain?), leads to
increased excretion of NaCl and H 2O and, con-
maintained secondarily), changes in total body
sequently, to normalization of the ECF volume.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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