Homeostasis of osmolarity in blood plasma and interstitial fluid is referred to as osmoregulation. A feedback control loop connecting posterior pituitary lobe and kidneys acts as main component of osmoregulation.
In positive dependence from plasma osmolality the posterior pituitary lobe secrets the hormone ADH (also referred to as adiuretin or vasopressin). By stimulating water resorption in distal renal tubuli ADH decreases the osmolarity of body fluids again thus closing the loop.
Important disorders of osmoregulation are central diabetes insipidus (point a in the diagram), where posterior pituitary is not able to produce ADH and renal diabetes insipidus (point b in the diagram) which is caused by ADH-resistency (e. g. by mutation of V2-receptors or certain diseases of the kidney).
The feedback control's function may be tested by the following procedures:
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