Fick's law can also be expressed as Equation 17.18 for diffusion across a capillary wall. In this equation, P represents the capillary permeability of the substance, Co represents the concentration of the substance outside the capillary, and Ci represents the concentration of the substance inside the capillary. The PS product is a convenient way to express the available capillary surface area, as the intrinsic permeability of the capillary is typically not significantly altered under normal conditions. However, in pathological conditions such as a bee sting, the capillary permeability may be affected.
In capillaries, the diffusion of lipid-insoluble molecules is restricted to water-filled channels or pores. The movement of solute across the capillary endothelium is a complex process that involves considerations of attractions between solute and solvent molecules, interactions between solute molecules, pore configuration, and charge on the molecules in relation to charge on the endothelial cells. This solute motion is not simply a result of random thermal movement of molecules along a concentration gradient. For small molecules like water, NaCl, urea, and glucose, the capillary pores present little restriction to diffusion, as they have a low reflection coefficient. The diffusion of these substances is so rapid that the concentration gradient across the capillary endothelium is extremely small. The diffusion of larger lipid-insoluble molecules becomes more restricted in capillaries, and it eventually becomes minimal when the molecular weight exceeds approximately 60,000. In the case of small molecules, the only limitation to their movement across the capillary wall is the rate at which blood flow transports them to the capillary. This type of transport is known as flow-limited. 
For flow-limited small molecules, their concentration in the blood reaches equilibrium with their concentration in interstitial fluid near the origin of the capillary from its parent arteriole. As the small molecule travels towards the arterial end of the capillary, its concentration decreases to negligible levels. However, if the flow is substantial, the small molecule can still be present at a distant location downstream in the capillary. A slightly larger molecule can travel further along the capillary before reaching an insignificant concentration in the blood. The number of larger molecules that enter the arterial end of the capillary but cannot pass through the capillary pores is equal to the number that leaves the venous end of the capillary.
With large molecules, diffusion across the capillaries becomes the limiting factor. This means that the permeability of a capillary to a large solute molecule determines its transport across the capillary wall. The diffusion of small lipid-insoluble molecules is so rapid that it only limits blood-tissue exchange when the distances between capillaries and parenchymal cells are large, such as in tissue edema or very low capillary density. The movement of lipid-soluble molecules across the capillary wall is not restricted to capillary pores, which only account for approximately 0.02% of the capillary surface. Instead, lipid-soluble molecules can directly pass through the lipid membranes of the entire capillary endothelium, facilitating rapid transfer between blood and tissue. The degree of lipid solubility, as indicated by the oil-to-water partition coefficient, serves as a good indicator of the ease of transfer of lipid molecules through the capillary endothelium.
Both oxygen (O2) and carbon dioxide (CO2) are lipid soluble and can easily pass through endothelial cells. Calculations based on the diffusion coefficient for O2, capillary density and diffusion distances, blood flow, and tissue O2 consumption indicate that the supply of oxygen to normal tissue at rest and during activity is not limited by diffusion or the number of open capillaries.
For diffusion across a capillary wall, Fick’s law can also be expressed as
Equation 17.18
P = capillary permeability by the substance
Co = concentration of the substance outside the
Ci = concentration of the substance inside the capillary
The PS product provides a convenient expression of available capillary surface area because the intrinsic permeability of the capillary is rarely altered much under physiological conditions. However, in pathological conditions, as with a bee sting, capillary permeability may be altered.
In capillaries, diffusion of lipid-insoluble molecules is restricted to water-filled channels or pores. Movement of solute across the capillary endothelium is complex and involves corrections for attractions between solute and solvent molecules, interactions between solute molecules, pore configuration, and charge on the molecules in relation to charge on the endothelial cells. Such solute motion is not simply a matter of random thermal movement of molecules that seemingly run down a concentration gradient. For small molecules, such as water, NaCl, urea, and glucose, the capillary pores offer little restriction to diffusion (i.e., they have a low reflection coefficient; see the section “ ”). Diffusion of these substances is so rapid that the mean concentration gradient across the capillary endothelium is extremely small. The larger the lipid-insoluble molecules are, the more restricted is their diffusion through capillaries. Diffusion eventually becomes minimal when the molecular weight of the molecules exceeds approximately 60,000. With small molecules, the only limitation to net movement across the capillary wall is the rate at which blood flow transports the molecules to the capillary. Transport of these molecules is said to be flow limited.
With flow-limited small molecules, the concentration of the molecule in blood reaches equilibrium with its concentration in interstitial fluid at a location near the origin of the capillary from its parent arteriole. Its concentration falls to negligible levels near the arterial end of the capillary (
Fig.
17.25A ). If the flow is large, the small molecule can still be present at a distant locus downstream in the capillary. A somewhat larger molecule moves farther along the capillary before it reaches an insignificant concentration in blood. Furthermore, the number of still larger molecules that enter the arterial end of the capillary but cannot pass through the capillary pores equals the number that leaves the venous end of the capillary (see
Fig. 17.25A
With large molecules, diffusion across the capillaries becomes the limiting factor (diffusion limited); that is, the permeability of a capillary to a large solute molecule limits its transport across the capillary wall. Diffusion of small lipid-insoluble molecules is so rapid that diffusion limits blood-tissue exchange only when distances between capillaries and parenchymal cells are great (e.g., as in tissue edema or very low capillary density; see
Fig. 17.25B
Movement of lipid-soluble molecules across the capillary wall is not limited to capillary pores (only ≈0.02% of the capillary surface); it also occurs directly through the lipid membranes of the entire capillary endothelium. Consequently, lipid-soluble molecules move rapidly between blood and tissue. The degree of lipid solubility (oil-to-water partition coefficient) provides a good index of the ease of transfer of lipid molecules through the capillary endothelium.
Both O2 and CO2 are lipid soluble, and they readily pass through endothelial cells. Calculations based on (1) the diffusion coefficient for O2, (2) capillary density and diffusion distances, (3) blood flow, and (4) tissue O2 consumption indicate that the O2 supply of normal tissue at rest and during activity is not limited by diffusion or by the number of open capillaries.