Diffusion is a fundamental biological process in the human body. It is a passive movement, meaning it does not require energy. Substances naturally move from an area of higher concentration to an area of lower concentration, known as moving down a concentration gradient. This movement distributes essential materials throughout the body, facilitating physiological functions.
Gas Exchange in the Lungs
Diffusion primarily occurs in the lungs for gas exchange. The lungs contain millions of tiny air sacs called alveoli, surrounded by an extensive network of capillaries. This interface, the alveolar-capillary membrane, is incredibly thin, typically ranging from 0.2 to 2.5 micrometers. This minimal thickness creates a short diffusion pathway, optimizing gas transfer.
Oxygen from inhaled air, in high concentration within the alveoli, diffuses across this thin membrane into the bloodstream. Carbon dioxide, a waste product, is in higher concentration in the blood within the capillaries. It then diffuses from the blood into the alveoli to be exhaled. The sheer number of alveoli provides a vast surface area for this exchange, estimated to be about 70 to 100 square meters.
Nutrient and Waste Transport in Tissues
Diffusion also occurs throughout the body’s tissues, facilitating the delivery of substances to cells and the removal of metabolic waste. The body’s network of capillaries permeates nearly every tissue, bringing blood close to individual cells. Capillary walls are remarkably thin, often consisting of a single layer of endothelial cells, allowing efficient substance movement.
Oxygen and nutrients like glucose and amino acids are in higher concentrations in capillary blood. They diffuse from the bloodstream into surrounding tissue cells. Metabolic waste products, such as carbon dioxide and urea, accumulate in higher concentrations within tissue cells. These then diffuse from the cells into the bloodstream to be transported away for excretion.
Absorption in the Digestive System
Diffusion also aids in absorbing digested nutrients from the gastrointestinal tract into the bloodstream. The small intestine is the primary site, with its inner lining covered by millions of tiny villi and microvilli. These structures create an enormous surface area for absorption, which can be as large as 300 square meters.
Simple sugars like glucose, amino acids, and some fatty acids are in high concentration within the small intestine’s lumen. They diffuse across the single-cell-thick epithelial lining of the villi into capillaries or lacteals. While some nutrients require specialized transport mechanisms, simple diffusion is a pathway for many small molecules and lipids to enter the body’s circulation.