Body Fluids and Their Functions

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Study Notes

About 60% of an adult human body is fluid, primarily a water solution of ions and other substances.
Most of this fluid is inside cells (intracellular fluid).
One-third is outside cells (extracellular fluid).
Extracellular fluid is constantly moving throughout the body.
This fluid is transported via blood and then mixes with tissue fluids through capillary walls by diffusion.
Adult females have 50% of total body weight as fluid.
Adult males have 60% of total body weight as fluid.

Every body part needs water to function.
A healthy body balances water intake and output.
Fluid imbalance occurs when water or fluid loss exceeds intake or intake exceeds output.

Heart failure can cause fluid to collect in lungs, liver, blood vessels, and body tissues, as the heart has a poor job of pumping to the kidneys.
Kidney disease can cause the body to not function correctly and lose fluids due to difficulty processing fluids.
Diarrhea, vomiting, or high fever can cause excessive loss of fluids.
Lack of antidiuretic hormone (ADH) can cause the kidneys to excessively lose fluid and lead to dehydration as a result.
Medications can affect fluid balance.

Lipids form a barrier in cell membranes to impede water and water-soluble substance movement between cell compartments.
Proteins in membranes frequently penetrate completely and form pathways or channels for the passage of substances.
Many membrane proteins are enzymes that catalyze various chemical reactions.

The lipid layer of the cell membrane is semipermeable.
Fat-soluble substances are the only substances that can pass through the lipid layer.

Passive movement of substances across the membrane due to random molecular movement.

Cellular uptake of large particles (e.g., bacteria, whole cells via phagocytosis).
Pinocytosis is the ingestion of minute particles that form vesicles inside the cell cytoplasm.
Phagocytosis is the ingestion of larger particles.