USMLE Step 1: Physiology MEMBRANE TRANSPORT Diffusion Factors that affect the rate of diffusion(D)of a substance between two compartments separat ed by a membrane are given in the following formula D∝ △P×SA×So T×VMW AP= concentration gradient across the membrane. The greater the concentration gradient, the ter tbe rate of diffusio SA= surface area of the membrane. The greater the surface area, the greater the rate of diffu sion (For example, exercise opens additional pulmonary capillaries, increasing the surface area for exchange. Emphysema decreases the surface area for exchange. SOL= solubility in the membrane or permeability. The more soluble the substance, the faster it will diffuse. Generally CO, diffuses faster across membranes than O, because CO, exhibits greater solubility T= thickness of the membrane. The thicker the membrane the slower the rate of diffusion (e. g, lung fibrosis) MW =molecular weight. This factor is not important clinically The molecules of eacb species diffuse independently. There is no durect interaction among molecules during diffusion. If the inspired nitrogen in room air is replaced elium, the rate of oxygen and carbon dioxide diffusion will be unaffected Osmosis Osmosis is the diffusion of water across a semipermeable or selectively permeable membrane. Water will diffuse from a region of higher water concentration to a region of lower water con- centration. The water concentration of a solution is determined by the concentration of solute The greater the solute concentration, the lower the water concentration. The basic principles are demonstrated in Figure 1-1-2. 0;0 o;。 Figure 1-1-2 This figure shows two compartments separated by a membrane that is permeable to water but not to solute. Side b has the greater concentration of solute(circles)and thus a lower water con- centration than side A. As a result, water wi diffuse from A to B, and the height of column B will rise, and that of A will fall 4 medical"... USMLEStep1: Physiology 4 KAPLA~. meulCa I MEMBRANETRANSPORT Diffusion Factors that affectthe rate of diffusion (D) of a substance between two compartments separat￾ed by a membrane are given in the following formula: D ex:LlP X SA X SOL T X VMW LlP=concentration gradient across the membrane. The greater the concentration gradient, the greater the rate of diffusion. SA = surface area of the membrane. The greater the surface area, the greater the rate of diffu￾sion. (For example, exerciseopens additional pulmonary capillaries,increasing the surface area for exchange.Emphysema decreases the surface area for exchange.) SOL = solubility in the membrane or permeability. The more soluble the substance, the faster it will diffuse. Generally CO2 diffuses faster across membranes than °2 because CO2 exhibits greater solubility. T = thickness of the membrane. The thicker the membrane, the slower the rate of diffusion, (e.g., lung fibrosis). MW = molecular weight. This factor is not important clinically. The molecules of each species diffuse independently. There is no direct interaction among molecules during diffusion. If the inspired nitrogen in room air is replaced by helium, the rate of oxygen and carbon dioxide diffusion will be unaffected. Osmosis Osmosis is the diffusion of water across a semipermeable or selectivelypermeable membrane. Water will diffuse from a region of higher water concentration to a region of lower water con￾centration. The water concentration of a solution is determined by the concentration of solute. The greater the solute concentration, the lower the water concentration. The basicprinciples are demonstrated in Figure 1-1-2. A B Figure 1-1-2 This figure shows two compartments separated by a membrane that is permeable to water but not to solute. SideBhas the greater concentration of solute (circles)and thus a lowerwater con￾centration than side A. As a result, water will diffuse from A to B, and the height of column B will rise, and that of A will fall. I 0 0 I 0 0 I I 0 0 0 0 I 0 0 I I 0 0 I 0 0 I 0 0 I