Notes
Water Balance
Sections
The kidneys play a key role in osmoregulation, which maintains the osmolarity of the body's fluids.
Homeostatic osmolarity of blood is approximately 290 milliosomoles/liter.
Shifts in blood osmolarity trigger renal mechanisms that alter water reabsorption in urine to return to blood osmolarity to homeostasis.
Thus, urine osmolarity reflects these changes.
Key definitions: Relate urine osmolarity to blood osmolarity
Isosmotic urine has the same osmolarity as blood.
Hyperosmotic urine has higher osmolarity than blood.
Hypoosmotic urine has lower osmolarity than blood.
Water Deprivation
Water deprivation triggers the production of hyperosmotic urine.
High solute concentration raises blood osmolarity.
Osmoreceptors in hypothalamus are activated. In response, osmoreceptors trigger increased thirst.
Osmoreceptors also trigger the pituitary gland to release ADH.
In the kidney, ADH increases the number of nephron principal cell aquaporins.
More water is reabsorbed and Blood osmolarity returns to homeostasis.
Urine volume is reduced; osmolarity is increased.
Clinical Correlations:
One of the treatments for bedwetting is to give exogenous ADH (vasopressin) in order to reduce urine volume at night.
In the syndrome of inappropriate ADH (SIADH), circulating levels of ADH are abnormally elevated, which increases the amount of water reabsorption and produces hyperosmotic urine; ADH inhibitors can correct this.
Water Excess
Water excess triggers production of hyposmotic urine.
High body water content reduces blood osmolarity.
Hypothalamic osmoreceptors are not activated, so pituitary gland is not stimulated to release ADH.
In absence of ADH, fewer aquaporins in the nephron reabsorb water and excess water is released in the urine.
Blood osmolarity returns to homeostasis.
Urine volume is high, its osmolarity is low.
Clinical Correlations:
In diabetes insipidus, there is either pathologic failure of ADH release or failure of kidney detection of ADH; as a result, large volumes of dilute urine are produced. DI can be treated with exogenous ADH or other medications.