RENAL REGULATION OF BODY FLUID Dr. Eman El Eter

What is the impact of the following on your body fluid volume and osmolarity?  What happens when you are exposed to hot weather?  What if you have a stomach bug and excessive diarrhea?  What if you drink 2 liters of fluids?

ECF  Constant concentration of electrolytes & solutes  They create the osmolarity of ECF  Na+, Cl- most abundant osmostic molecules.  Osmolality is determined by the amount of extracellular NaCl and water which depends upon balance between intake and excretion of these substances.  Normal plasma Na+ = mEq/L  Osmolarity = 300 mOsm/L

Control of ECF osmolarity & Na+ concentration  1-Osmoreceptor –ADH mechanism.  2- Thirst mechanism

Osmoreceptor mechanism Changes in plasma osmolarity can lead to a cascade of events to return it back to normal.

What happens if ECF osmolarity increased?  ECF osmolarity  (+) osmoreceptors in Ant. Hypothalamus & Send signals to Supraoptic nucleus, Then to posterior Pituitary (+) ADH To increase H2O absorption

Where in the brain ADH is formed? Main site for ADH synthesis ADH is stored in posterior pituitary

Osmotic vs non-osmotic stimuli; effect on ADH Rapid response AVP=arginine vasopressin=ADH

Non-osmotic stimuli releasing ADH  Arterial baroreceptor reflex  Chemoreceptor reflex  Whenever BP & blood volume reduced, ADH is released water retention by the kidney to restore BP to normal.  Day-to-day regulation of ADH secretion is effected mainly by changes in plasma osmolarity.

Role of thirst in controlling ECF osmolarity & Na+ concentration  To stay in balance water intake = water loss  Fluid intake is regulated by thirst mechanism  Stimuli of thirst mechanism:  A. Increase thirst:  Increased osmolarity ECF.  Decreased ECF volume.  Decreased blood pressure.  Angiotensin II.  Dryness of the mouth.  B. Decrease thirst? -Gastric distention decreases thirst.

Role of thirst in controlling ECF osmolarity & Na+ concentration, cont….  The desire to thirst is completely satisfied when: Plasma osmolarity, Blood volume, Or both return to normal

Thirst mechanism

Role of Ang II & aldosterone  They do not normally play a major role in controlling ECF osmolarity and Na+ concentration.  Their major role is to absorb sodium through distal convoluted tubules, leading to greater extracellular fluid volume and sodium quantity.

Take home message  ADH-thirst is the most powerful feedback system in the body for controlling extracellular fluid osmolarity and sodium concentration