Editing Anatomy and Physiology (section)

=== Transport Across Membranes ===

* '''Diffusion''': solutes move from higher to lower concentration. Gas exchange in the alveoli is diffusion.
* '''Osmosis''': water moves across a semipermeable membrane toward the higher solute concentration. This is the mechanism behind every IV fluid decision.
* '''Facilitated diffusion''': carrier-assisted movement down a gradient, such as glucose entry into most cells with insulin's help.
* '''Active transport''': movement against a gradient using ATP. The sodium-potassium pump moves three sodium ions out for every two potassium ions in, maintaining the resting membrane potential.
* '''Filtration''': movement driven by hydrostatic pressure, as in the glomerulus and at the arterial end of capillaries.

'''Nursing application:''' IV fluid tonicity follows directly from osmosis.
* '''Isotonic''' (0.9% sodium chloride, lactated Ringer's): stays in the extracellular space, expands vascular volume, used for dehydration and hypovolemia. Risk is fluid overload.
* '''Hypotonic''' (0.45% sodium chloride): water shifts into cells; used for cellular dehydration. Risk is cerebral edema and worsening hypotension, so it is avoided in head injury and in hypovolemic shock.
* '''Hypertonic''' (3% sodium chloride, dextrose 5% in 0.9% sodium chloride): pulls water out of cells into the vascular space. Used cautiously for severe hyponatremia and cerebral edema. Risk is fluid overload and phlebitis, so infuse slowly and monitor closely.