Human Physiology: Body Fluid Balance

Study Notes

Balance: A state of equilibrium where substances are maintained in the right amounts and locations within the body.

Osmosis: The primary method of water movement into and out of body fluid compartments.
Osmosis: The movement of water molecules across a selectively permeable membrane from a region of high water concentration to a region of lower water concentration.
Solute Concentration: The concentration of solutes determines the direction of water movement.
Electrolytes: Most solutes in the body are electrolytes—inorganic compounds that dissociate into ions in solution.
Sodium and Water: "Where sodium goes, water follows."

Total Body Water: Approximately 40 liters (10.56 gallons).
Water Content by Age:
- Babies: 75% water
- Men: 63% water
- Women: 52% water

Fluid Compartments: Separated by selectively permeable membranes.
Intracellular Fluid: 2/3 (63%) of total body water.
Extracellular Fluid: 1/3 (37%) of total body water.
- Interstitial Fluid: 80% of extracellular water.
- Blood Plasma: 20% of extracellular water.

Extracellular Fluids: High in Na+, Cl-, Ca++, HCO3-.
- Blood Plasma: Contains more protein than interstitial fluid and lymph.
Intracellular Fluids: High in K+, phosphate, Mg++, and more protein than plasma.

Thirst: The main regulator of water intake.
Dehydration: A decrease in body water as little as 1% can cause:
- Decreased saliva production.
- Increased blood osmotic pressure, stimulating osmoreceptors in the hypothalamus.
- Decreased blood volume, leading to renin production.

Urine Formation: Water output is regulated through urine formation.
Antidiuretic Hormone (ADH): Production is stimulated by increased blood tonicity or decreased blood volume.
- ADH Action: Acts on distal convoluted tubules and collecting ducts of the kidney, increasing water reabsorption.
Aldosterone: Production is stimulated by angiotensin II, which is produced via the renin-angiotensin system.
- Aldosterone Action: Causes sodium (and water) reabsorption.
Atrial Natriuretic Peptide (ANP): Causes sodium (and water) loss when pressure in the right atrium is too high.