Fluid loss
The regulation of fluid loss from the body occurs
in the kidneys, with the volume of body fluid
determined by the loss of sodium chloride in urine
(Tortora and Derrickson 2009b). Daily intake of
sodium chloride can vary significantly, and the
kidneys alter levels of sodium chloride excreted
in urine through hormonal control. The three
main hormones that alter the reabsorption rates
of sodium and chloride are antidiuretic hormone
(ADH) angiotensin II, aldosterone and atrial
natriuretic peptide (ANP) (Tortora and Derrickson
2009b). A negative feedback mechanism involving
secretion of ADH maintains the blood osmotic
pressure, and sodium and water concentrations,
within normal limits.
Changes in sodium levels in plasma will alter
blood volume, for example a significant increase in
sodium will lead to an increase in circulating blood
volume and therefore blood pressure, causing the
blood vessel walls to stretch. This increased stretch
is detected by baroreceptors in the carotid bodies,
arch of the aorta and walls of the atria (Porth
2011). Once stimulated by the increase in blood
volume, these receptors initiate the sympathetic
nervous system response and lead to a reduction in
the release of ADH from the pituitary gland. The
sympathetic nervous system response increases the
glomerular filtration rate along with a decrease in
renin production, and less reabsorption of sodium
and water. Sensors in the kidney are also stimulated
by an increase in renal perfusion, which decreases
the secretion of renin (Speakman and Weldy 2002)