Diffusion

Definition

Diffusion is the passive movement of molecules from an area of high concentration to one of low concentration down the concentration gradient. It requires no energy (ATP) and is essential for gas exchange, nutrient transfer, and drug absorption.

Clinical Applications

• Pulmonary gas exchange: O2 diffuses from alveoli into capillaries; CO2 diffuses out.
• Cellular metabolism: Nutrients and waste move in/out of cells.
• Drug absorption: Most drugs cross membranes by passive diffusion.
• Dialysis: Waste particles diffuse across the dialyzer membrane.

Factors Affecting Diffusion

Concentration gradient, membrane surface area, membrane thickness, molecular size, lipid solubility, and temperature, such as thickened alveolar-capillary membrane (pulmonary fibrosis, edema) impairs gas exchange.

NCLEX Relevance

Fundamental to understanding pulmonary, renal, and pharmacologic processes. Contrast with osmosis (water) and active transport (requires ATP).