diffusion
Summary
Diffusion is the passive movement of molecules from areas of high concentration to low concentration across a concentration gradient, requiring no energy input. It's fundamental to gas exchange in lungs, drug absorption, and cellular transport processes.
Detail
Diffusion is a passive transport mechanism driven by kinetic energy and concentration gradients, following Fick's law of diffusion. The rate depends on concentration gradient, surface area, membrane thickness, and molecular size/lipophilicity. In medicine, diffusion is crucial for: (1) Pulmonary gas exchange - O2 and CO2 diffusion across alveolar-capillary membrane, (2) Drug absorption - lipophilic drugs diffuse across cell membranes, (3) Renal function - small molecules diffuse across glomerular filtration barrier, (4) Cellular metabolism - nutrients and waste products diffuse across cell membranes. Simple diffusion occurs directly through lipid bilayers (O2, CO2, steroid hormones), while facilitated diffusion uses transport proteins (glucose via GLUT transporters). Pathologically, diffusion can be impaired in conditions like pulmonary fibrosis (thickened alveolar membrane) or drug malabsorption syndromes.
Sources
- Guyton and Hall Textbook of Medical Physiology
- Costanzo Physiology
- First Aid for the USMLE Step 1
- Robbins Basic Pathology
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