| Popis: |
Considerations of body fluid homeostasis inevitably focus on water and sodium homeostasis. It has long been recognized that animals do not monitor body water or sodium content per se but become aware of dehydration or sodium loss when appropriate sensors detect specific consequences of each deficiency. There are two dimensions in which such deficiencies become manifest and can be detected, related to the osmolality and volume of blood. The receptors responsive to changes in plasma osmolality appear to be located in the anterior hypothalamic area, and stimulation by osmotic dehydration elicits thirst and secretion of the antidiuretic hormone, arginine vasopressin (AVP). The regulation of blood volume is considerably more complex than osmoregulation. The same considerations of thirst and AVP secretion associated with osmoregulation are also relevant to volume regulation, but additionally there are parallel issues involved in the maintenance of sodium balance, namely, sodium chloride intake motivated by sodium appetite and renal sodium conservation mediated in large part by the adrenocortical hormone, aldosterone. Reduction in plasma volume (hypovolaemia) elicits each of these compensatory responses, which are stimulated by neural signals from vascular baroreceptors and/or by the renin-angiotensin system (see Chapter 6). |
