Water Balance
Key points:
- 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 the kidney, ADH increases the number of nephron principal cell aquaporins
- 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.
- In absence of ADH, fewer aquaporins in the nephron reabsorb water.
- 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.