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+ = 140-145 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.

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 (+) ADHTo increase H2O absorption

Where in the brain ADH is formed?

Main site for ADH synthesis

Main site for ADH synthesis

ADH is stored in posterior pituitary

ADH is stored in posterior pituitary

Osmotic vs non-osmotic stimuli; effect on ADH

Rapid response

Rapid response

AVP=arginine vasopressin=ADH

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