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.