Mechanism of Water Balance PDF

Mechanism of water balance Prof. Tarig Hakim Merghani Objectives  Explainthe mechanism of water balance and relate to common conditions like diarrhea, excessive sweating, burns and diabetes insipidus Water Balance  Water intake= Water loss Water intake/ day  Drinking 1.3 L/da...

Mechanism of water balance Prof. Tarig Hakim Merghani Objectives  Explainthe mechanism of water balance and relate to common conditions like diarrhea, excessive sweating, burns and diabetes insipidus Water Balance  Water intake= Water loss Water intake/ day  Drinking 1.3 L/day  Solid food 0.9 L/day  Metabolism 0.3 L/day  Net= 2.5 L/day Water loss/ day  Urine 1.4 L/day  Stool 0.2 L/day  Sweating & insensible loss 0.9 L/day Net= 2.5 L/day Water Balance  Water intake (esp. drinking) = Water loss (esp. urine)  Values vary greatly in different physiological and pathological conditions Physiological Abnormalities of water balance  Exercise There is an increased water loss Sweating & Hyperventilation Balanced by: Decreased urine volume (& increased intake) Physiological Abnormalities of water balance  Fasting There is a decreased water intake Balanced by: decreased urine volume (& increased intake)  High temperature There is an increased water loss Balanced by: decreased urine volume (& increased intake)  Pathological Abnormalities of water balance  Vomiting (loss of isotonic fluid)  Diarrhea (Loss of isotonic fluid)  Diabetes insipidus (loss of hypotonic fluid)  Burn (loss of hypotonic fluid)  Pathological Abnormalities of water balance  Heat exhaustion or excessive sweating (loss of hypotonic fluid)  Hyperventilation (loss of hypotonic fluid)  Excessive I.V. fluid (excess isotonic fluid)  Excessive drinking (Psycogenic) (Excess hypotonic fluid) Control of water intake = Control of thirst  The thirst center is located in the hypothalamus Control of water intake  Stimuli of thirst: Hyperosmolarity Hypovolemia Hypotension Angiotensin II  Inhibitors of Thirst (Opposite factors + ANP) Hyperosmolarity stimulates osmoreceptors in hypothalamus Volume receptors are found at junction of veins with the heart (= inhibitory to thirst)  Hypervolemia stretches volume receptors, which inhibit the thirst center through the vagus nerve. Conversely, hypovolemia causes less stretch; and therefore, no inhibition of thirst Baroreceptors are found in aortic arch and carotid bifurcation (= inhibitory to thirst)  Hypertension stretches baroreceptors, which inhibit the thirst center through the vagus and glossopharyngeal nerves. Conversely, hypotension causes less stretch; and therefore, no inhibition of thirst Formation of Angiotensin II hormone Atrial natriuretic peptide (ANP)  ANP is released by atria (the heart) in response to hypervolemia  Inhibits effect of angiotensin II on thirst  Inhibits renin secretion  Inhibits aldosterone production,  Inhibits renal reabsorption of sodium. Overall effect= Reduction of ECF volume Control of water loss = Control of urine volume  Involves: The kidneys Certain hormones: E.g., Anti-diuretic hormone (ADH) The kidneys  Reabsorb > 99% of water filtered in the glomeruli  About 7-13% of the filtered water is reabsorbed by the action ADH Anti-diuretic hormone (ADH)  Deficiency of ADH= Urine volume increases and may reach up to 23 L/day (causing excessive thirst)  = Diabetes insipidus (DI).  Anti-diuretic hormone (ADH)  Excessive ADH secretion = causes a reduction in urine volume= hypertension, and edema due to water retention.  = Syndrome of inappropriate ADH secretion (SIADH). Aldosterone acts in the kidney to reabsorb sodium (followed by water) Dehydration = Water +/- salt depletion  Types: Isotonic dehydration (isonatremic) Hypotonic (hyponatremic) Hypertonic (hypernatremic) Dehydration  Causes of fluid loss: 1. The intestinal tract (diarrhea, vomiting, or bleeding) 2. The skin (sweating, burns) 3. The urine (diuretic therapy, diabetes insipidus or mellitus). 4. Others (hyperventilation, decreased intake, …) Isotonic dehydration (By diarrhea or vomiting) Isotonic dehydration (By diarrhea or vomiting) ECF osmolarity: No change ICF volume: No change ICF osmolarity: No change Hypotonic dehydration (By adrenal insufficiency= low aldosterone) Hypotonic dehydration (By adrenal insufficiency= low aldosterone) ECF osmolarity: Decreases ICF volume: Increases ICF osmolarity: Decreases Hypertonic dehydration (By Diabetes, sweating or diuretics) Hypertonic dehydration (By Diabetes, sweating or diuretics) ECF osmolarity: Increases ICF volume: Decreases ICF osmolarity: Increases General symptoms & Signs of dehydration Treatment of dehydration  Depending on the type: Treatment of the cause Intravenous (I.V. fluids) Oral Rehydration Salts (ORS) Effects of I.V. solutions on body fluid compartments