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.