Total body water (TBW) distribution
TBW is 42L. It is contained in 2 major compartments:
1. The intracellular fluid (ICF): the water inside cells, which accounts for 28L
2. The extracellular fluid (ECF): the water outside cells, representing 14L. which is further divided into:
a. interstitial fluid (ISF, 11L),
b. transcellular fluid (1L),
c. plasma (3L).
Hydrostatic and osmotic pressures influence movement between the compartments.
Water intake: from fluids, food, and oxidation of food.
Water loss: from urine, faeces, and insensible losses (e.g, sweating).
Intake and losses are balanced and TBW remains relatively constant.
Urine concentration and dilution
The ability to concentrate or dilute urine depends on
1- Medullary hypertonicity: (by the active transport of NaCl) which provides the osmotic driving force for the reabsorption of water.
2- ADH: causes water reabsorbtion from the lumen of the collecting duct in presence of medullary hypertonicity.
Antidiuretic hormone (ADH or vasopressin)
ADH is secreted from the posterior pituitary in response to:
· changes in plasma osmolarity (detected by osmoreceptors in the hypothalamus)
· changes in blood pressure or volume (detected by baroreceptors in the left atrium, aortic arch, and carotid sinus).
The action of ADH:
· stimulate thirst center in brain.
· Increases CD permeability to water and urea.
· Increases LLH and CD reabsorption of NaCl.
Children have a circadian rhythm in ADH secretion high at night and low during the day. Adults essentially have a constant ADH secretion over a 24-h period.
Response of kidney to water imbalance to restore plasma osmolarity
1- Response to water excess
Body fluids become hypotonic causing decrease ADH secretion resulting in:
· Suppression of thirst
· decrease CD permeability with water reabsorbtion into the lumen, and excretion of a large volume of hypotonic urine.
2- Response to water deficit
Body fluids become hypertonic causing increase ADH secretion resulting in:
· stimulation of thirst sensation.
· increase CD permeability with water reabsorbtion into the lumen, and excretion of a small volume of hypertonic urine.