Nutrition Science and Everyday Application - beta v 0.1

464 ALICE CALLAHAN, PHD, HEATHER LEONARD, MED, RDN, AND TAMBERLY POWELL, MS, RDN
Figure 8.7. Osmosis is the diffusion of water through a semipermeable membrane towards
higher solute concentration. If a membrane is permeable to water but not a solute, water will
equalize its own concentration by diffusing to the side of lower water concentration (and thus the
side of higher solute concentration). In the beaker on the left, the solution on the right side of
the membrane is more concentrated with solutes; therefore, water diffuses to the right side of the
beaker to equalize its concentration.
To maintain water and electrolyte balance, cells control the movement of electrolytes
across their membranes, and water follows the electrolytes by osmosis. The health of the
cell depends on proper fluid and electrolyte balance. If the body’s fluid and electrolyte levels
change too rapidly, cells can struggle to correct the imbalance quickly enough. For example,
consider a person exercising strenuously, losing water and electrolytes in the form of sweat,
and drinking excessive amounts of water. The excess water dilutes the sodium in the blood,
leading to hyponatremia, or low blood sodium concentrations. Sodium levels within the cells
are now more concentrated, leading water to enter the cells by osmosis. As a result, the cells
swell with water and can burst if the imbalance is severe and prolonged.
In contrast, the opposite situation can occur in a person exercising strenuously for a long
duration with inadequate fluid intake. This can lead to dehydration and hypernatremia, or
elevated blood sodium levels. The high concentration of sodium in the extracellular fluid
causes water to leave cells by osmosis, making them shrink (Figure 8.8). This scenario can
also occur anytime a person is dehydrated because of significant fluid loss, such as from
diarrhea and/or vomiting caused by illness.
When a person becomes dehydrated, and solutes like sodium become too concentrated
in the blood, the thirst response is triggered. Sensory receptors in the thirst center in
the hypothalamus monitor the concentration of solutes of the blood. If blood solutes (like
sodium) increase above ideal levels, the hypothalamus transmits signals that result in a
conscious awareness of thirst. The hypothalamus also communicates to the kidneys to
decrease water output through the urine.