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.