159. Regulation of Volume, Composition and Osmolality of Body Fluid Compartments – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Total body water is about 60% of body weight in a healthy adult. Intracellular fluid forms about 40% and extracellular fluid about 20%. Of extracellular fluid, about 75% is interstitial fluid and 25% is plasma.
• Sodium and its accompanying anions are the major determinants of extracellular fluid volume and osmolality. Total body sodium mainly determines extracellular fluid volume, whereas plasma sodium concentration is the major determinant of plasma osmolality.
• Water moves freely across cell membranes according to osmotic gradients. At steady state, intracellular and extracellular fluids have nearly equal osmolality. Exchange between plasma and interstitial fluid is governed mainly by Starling forces across capillary walls.
• Water balance is maintained when water intake equals water loss. Major sources of water are drinking, food and metabolic water. Major losses occur through urine, feces, sweat and insensible evaporation. The kidneys are the principal organs controlling water excretion.
• Thirst and antidiuretic hormone are the major regulators of water balance. Increased plasma osmolality stimulates hypothalamic osmoreceptors, thirst and antidiuretic hormone secretion. Antidiuretic hormone increases water reabsorption in the collecting ducts and reduces urine volume.
• Effective circulating volume refers to the portion of blood that effectively perfuses tissues. It is maintained by adequate cardiac output, arterial pressure and vascular tone. Sodium retention expands extracellular fluid volume, whereas sodium loss reduces it.
• Low-pressure volume receptors are present mainly in the atria, great veins and pulmonary circulation. High-pressure receptors are present in the carotid sinus and aortic arch. Reduced vascular filling increases sympathetic activity and promotes antidiuretic hormone release and renal sodium and water retention.
• The renin–angiotensin–aldosterone system is activated during reduced renal perfusion or hypovolemia. Angiotensin II increases sodium reabsorption and stimulates aldosterone, while aldosterone increases sodium reabsorption and potassium secretion. Atrial natriuretic peptide promotes sodium and water excretion.
• Normal plasma osmolality is approximately 280–295 milliosmoles/kg water. Sodium and its accompanying anions contribute about 90% of plasma osmolality. Hypothalamic osmoreceptors regulate osmolality through changes in thirst and antidiuretic hormone secretion.
• Tonicity refers to the effect of a solution on cell volume and depends on effective, non-penetrating solutes. Isotonic fluid does not alter cell volume, hypotonic fluid causes cellular swelling, and hypertonic fluid causes cellular shrinkage. Normal saline is isotonic, whereas 5% glucose becomes physiologically hypotonic after glucose metabolism.
• Iso-osmotic dehydration causes loss of extracellular fluid without significant change in intracellular volume. Hypo-osmotic dehydration results mainly from sodium loss, causing water to enter cells. Hyperosmotic dehydration results mainly from water loss, causing water to leave cells and cellular shrinkage.
• Iso-osmotic volume expansion increases extracellular and plasma volume without changing intracellular volume. Hypo-osmotic expansion causes water to enter cells and may produce cerebral edema. Hyperosmotic expansion draws water out of cells and is usually caused by excessive administration of hypertonic sodium solutions.
• In volume depletion, compensatory responses include increased sympathetic activity, antidiuretic hormone, renin–angiotensin–aldosterone system and thirst, with reduced atrial natriuretic peptide. In volume expansion, antidiuretic hormone is suppressed and atrial natriuretic peptide promotes natriuresis and diuresis.