DIFFUSION THROUGH THE CAPILLARY MEMBRANE

 By far the most important means by which substances are transferred between the plasma and the interstitial fluid is diffusion. Figure 1 illustrates this process, showing that as the blood flows along the lumen of the capillary, tremendous numbers of water molecules and dissolved particles diffuse back and forth through the capillary wall, providing continual mixing between the interstitial fluid and the plasma. Diffusion results from thermal motion of the water molecules and dissolved substances in the fluid, with the different molecules and ions moving first in one direction and then another, bouncing randomly in every direction.

Fig1. . Diffusion of fluid molecules and dissolved substances  between the capillary and interstitial fluid spaces.

Lipid-Soluble Substances Diffuse Directly Through the Cell Membranes of the Capillary Endothelium. If a substance is lipid soluble, it can diffuse directly through the cell membranes of the capillary without having to go through the pores. Such substances include oxygen and carbon dioxide. Because these substances can permeate all areas of the capillary membrane, their rates of transport through the capillary membrane are many times faster than the rates for lipid-insoluble substances, such as sodium ions and glucose that can go only through the pores.

Water-Soluble, Non–Lipid-Soluble Substances Diffuse Through Intercellular “Pores” in the Capillary Membrane. Many substances needed by the tissues are soluble in water but cannot pass through the lipid membranes of the endothelial cells; such substances include water molecules, sodium ions, chloride ions, and glucose. Although only 1/1000 of the surface area of the capillaries is represented by the intercellular clefts between the endothelial cells, the velocity of thermal molecular motion in the clefts is so great that even this small area is sufficient to allow tremendous diffusion of water and water-soluble substances through these cleft-pores. To give one an idea of the rapidity with which these sub stances diffuse, the rate at which water molecules diffuse through the capillary membrane is about 80 times as great as the rate at which plasma itself flows linearly along the capillary. That is, the water of the plasma is exchanged with the water of the interstitial fluid 80 times before the plasma can flow the entire distance through the capillary.

Effect of Molecular Size on Passage Through the Pores. The width of the capillary intercellular cleft-pores, 6 to 7 nanometers, is about 20 times the diameter of the water molecule, which is the smallest molecule that normally passes through the capillary pores. The diameters of plasma protein molecules, however, are slightly greater than the width of the pores. Other substances, such as sodium ions, chloride ions, glucose, and urea, have inter mediate diameters. Therefore, the permeability of the capillary pores for different substances varies according to their molecular diameters.

Table 1 lists the relative permeabilities of the capillary pores in skeletal muscle for substances commonly encountered, demonstrating, for instance, that the permeability for glucose molecules is 0.6 times that for water molecules, whereas the permeability for albumin molecules is very slight—only 1/1000 that for water molecules.

Table1. Relative Permeability of Skeletal  Muscle Capillary Pores to Different-Sized  Molecules

A word of caution must be issued at this point. The capillaries in various tissues have extreme differences in their permeabilities. For instance, the membranes of the liver capillary sinusoids are so permeable that even plasma proteins pass through these walls, almost as easily as water and other substances. Also, the permeability of the renal glomerular membrane for water and electrolytes is about 500 times the permeability of the muscle capillaries, but this is not true for the plasma proteins; for these proteins, the capillary permeabilities are very slight, as in other tissues and organs. When we study these different organs later in this text, it should become clear why some tissues require greater degrees of capillary permeability than do other tissues. For example, greater degrees of capillary permeability are required for the liver to transfer tremendous amounts of nutrients between the blood and liver parenchymal cells and for the kidneys to allow filtration of large quantities of fluid for the formation of urine.

Effect of Concentration Difference on Net Rate of Diffusion Through the Capillary Membrane. The “net” rate of diffusion of a substance through any mem brane is proportional to the concentration difference of the substance between the two sides of the membrane. That is, the greater the difference between the concentrations of any given substance on the two sides of the capillary membrane, the greater the net movement of the sub stance in one direction through the membrane. For instance, the concentration of oxygen in capillary blood is normally greater than in the interstitial fluid. Therefore, large quantities of oxygen normally move from the blood toward the tissues. Conversely, the concentration of carbon dioxide is greater in the tissues than in the blood, which causes excess carbon dioxide to move into the blood and to be carried away from the tissues.

The rates of diffusion through the capillary membranes of most nutritionally important substances are so great that only slight concentration differences suffice to cause more than adequate transport between the plasma and interstitial fluid. For instance, the concentration of oxygen in the interstitial fluid immediately outside the capillary is no more than a few percent less than its concentration in the plasma of the blood, yet this slight difference causes enough oxygen to move from the blood into the interstitial spaces to provide all the oxygen required for tissue metabolism—often as much as several liters of oxygen per minute during very active states of the body.

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