Key Terms
Adults
50-60% of body mass is water. Infants: ~75%.
Intracellular Fluid (ICF)
Fluid inside cells. About 60% of total body water (~25 liters in an average adult male).
Extracellular Fluid (ECF)
Fluid outside cells. About 1/3 of total body water.
Other ECF examples
Cerebrospinal fluid, lymph, synovial fluid, pleural fluid, pericardial fluid, peritoneal fluid, aqueous humor of the eye
ICF is high in
Potassium (K+), phosphate, magnesium, protein. ECF (plasma and IF) is high in: sodium (Na+), chloride (Cl-), bicarbonate
Active transport
Requires ATP; moves substances against their concentration gradient. Example: sodium-potassium pump.
Passive transport
No energy required; moves substances down a concentration gradient. Subtypes:
Pulmonary edema
Excess fluid in lung air sacs; compromises gas exchange; can be life-threatening. Test for pitting edema: press a finger
Intake
~2500 mL/day total
Output
~2500 mL/day total
Insensible water loss
Water lost through skin evaporation and exhaled air that a person is typically unaware of.
Minimum urine output required for normal function
~0.47 liters/day. Below this threshold, metabolic wastes cannot be adequately removed.
Plasma osmolality
Ratio of solutes to water in blood plasma; reflects hydration state.
When blood osmolality rises (dehydration)
1. Osmoreceptors in the hypothalamus detect the increase.
ADH has two major effects
1. Constricts peripheral arterioles, redirecting blood to the body's core.