Water, Electrolyte, and Acid-Base Balance
4/8/14
Balance
• Water and electrolytes (molecules that release ions in
water) must maintain a balance of quantities coming in and leaving.
• Mechanisms in the body are responsible for this balance
• Water and electrolyte balance interdependent– Electrolytes are dissolved in water– Anything that alters electrolyte concentration will
alter the concentration of water.
Fluid Compartments
• Intracellular fluid compartments– All the water and electrolytes that cell membranes
enclose (fluid inside of cells)– 63% of total body water by volume
• Extracellular fluid compartments – 37% by volume– all of the fluids outside cells.
• Tissues, blood vessels, lymphatic vessels• Transcellular fluid = cerebrospinal fluid, fluids in the eye,
joints, glands, and body cavities.
Water Balance
• Exists when water intake equals water output.• Depends on our thirst centers in the brain to
vary water intake and on the kidney’s ability to vary water output.
Water Intake
• Average adult takes in about 2,500 milliliters of water daily.– 60% by drinking water or beverages– 30% comes from moist foods– 10% is a by-product of the oxidative metabolism of
nutrients = water of metabolism
Regulation of Water Intake
• Primary regulator of water intake is thirst.– Thirst center is in the hypothalamus of the brain.– A thirsty person usually has a dry mouth caused by
loss of extracellular water and the resulting decreased flow of saliva.
– Thirst mechanism is normally triggered whenever the total body water decreases by as little as 1%.
– Act of drinking water distends the stomach triggering nerve impulses that inhibit the thirst mechanism.
Water Output
• Water normally enters the body through the mouth, but can be lost through a variety of routes.– Urine, feces, sweat, evaporation from the skin,
lungs during breathing– 60% urine, 6% feces, 6% sweat. 28% lost through
skin and lungs. • These percentages will change with level of physical
activity, environment, etc.
Regulation of Water Output
• Sweat, feces, and evaporation are necessary functions (cooling, waste elimination).
• Thus, primary regulation of water output is urine production.– If a person takes in too much water, urine
production increases to maintain the balance, vice versa.• Caffeine inhibits the reabsorption of sodium ions and
other solutes, resulting in increased urine volume.