Renal physiology: regulation of water balance
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.
TBW balance
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.
: regulation of water balance){kind=link}