Diffusion is the fundamental transport process supporting life in the human body, governing the movement of molecules across cell membranes and fluid spaces. This passive mechanism involves the net movement of substances from an area of higher concentration to an area of lower concentration, driven by the random kinetic energy of the molecules. Moving molecules “downhill” along a concentration gradient, diffusion requires no cellular energy input, such as adenosine triphosphate (ATP). This principle maintains biological equilibrium and is harnessed across systems, from gas exchange in the lungs to signaling in the brain.
Gas Exchange in the Respiratory System
The respiratory system employs diffusion for the continuous supply of oxygen and the removal of carbon dioxide. This exchange occurs across the respiratory membrane, a barrier formed by the walls of the alveoli and the surrounding pulmonary capillaries. The process is highly efficient due to the membrane’s minimal thickness, typically only \(0.5\) to \(1\) micrometer wide.
The driving force for gas exchange is the steep difference in partial pressures between the alveolar air and the deoxygenated blood. Oxygen moves from an alveolar partial pressure of approximately \(104 \text{ mm Hg}\) into the blood (\(40 \text{ mm Hg}\)). Carbon dioxide, a byproduct of cellular metabolism, moves in the opposite direction, from blood (\(45 \text{ mm Hg}\)) into the alveoli (\(40 \text{ mm Hg}\)). Although the pressure gradient for carbon dioxide is much smaller, it diffuses efficiently because it is roughly \(20\) times more soluble in blood than oxygen.
Movement of Substances at the Tissue Level
Once oxygenated blood reaches the systemic capillary beds, diffusion facilitates the exchange of resources and waste with active tissues. Nutrients like glucose and amino acids diffuse out of the blood, through the interstitial fluid, and into the cells where they are needed. The concentration gradient that draws glucose into the cell is maintained by the cell’s metabolic machinery.
As glucose enters the cell, enzymes immediately convert it into glucose-6-phosphate through phosphorylation. Because the glucose is chemically altered, the concentration of free glucose inside the cell remains close to zero, ensuring a continuous, favorable gradient for facilitated diffusion from the blood. Metabolic waste products, such as carbon dioxide and lactic acid, follow the reverse gradient, diffusing from their high concentration within the tissue and interstitial fluid back into the capillary blood for excretion.
Regulation of Fluid and Waste in the Kidneys
The kidney utilizes diffusion to regulate fluid balance and manage waste within the microscopic filtering units called nephrons. As blood plasma is filtered, essential substances must be recovered from the tubular fluid before excretion as urine. For instance, urea, a waste product of protein metabolism, is freely filtered but partially reabsorbed to maintain the kidney’s concentrating ability.
Water reabsorption by osmosis in the proximal tubules concentrates the remaining urea in the tubular fluid. This establishes a gradient that drives approximately \(50\%\) of the urea to passively diffuse back into the blood, often facilitated by specialized urea transporters. The active transport of electrolytes like sodium establishes electrical and chemical gradients that drive the passive diffusion of other ions, such as chloride, as well as water movement (osmosis) across the tubular cells.
Signaling Across the Nervous System
Diffusion transmits chemical signals between neurons at the synapse. The synaptic cleft, the gap separating the presynaptic and postsynaptic neurons, is exceptionally narrow, measuring only about \(20\) nanometers (\(\text{nm}\)) across. This ultra-short distance is the key to rapid communication.
When an electrical signal reaches the end of a neuron, it triggers the release of neurotransmitter molecules into this cleft. These molecules rely on simple diffusion to cross the space and bind to receptors on the receiving cell. Because the distance is so small, the diffusion process is extremely fast, allowing the cycle of neurotransmitter release, diffusion, and receptor binding to occur in \(0.5\) to \(4.0 \text{ milliseconds}\) (\(\text{ms}\)).