Diffusion across capillary membranes: exchange of water and substances between blood and interstitial fluid

2021-05-29 01:36 PM

Diffusion of water and solute molecules with thermal motion randomly move in one direction and then change in another direction. Lipid-soluble substances diffuse directly across cell membranes in the endothelium of capillaries.

The main function of microcirculation is to transport nutrients to tissues and remove cellular waste products. Small arterioles control blood flow to individual tissues and their local condition, by controlling the diameter of the arterioles. Thus, in most cases, each tissue's regulation of flow is related to its own needs.

The walls of the capillaries are very thin and are composed of a layer of highly permeable endothelial cells. Thus, water, cellular nutrients and cellular secretion products can be exchanged quickly and easily between tissues and circulating blood.

The human body's peripheral circulatory system has about 10 billion capillaries with an estimated total surface area of ​​500 to 700 square meters (about one-eighth of the surface area of ​​a football field). Thus, any functioning cell has a capillary that feeds it no more than 20-30 micrometres away.

Diffusion is by far the most important mode of metabolism between plasma and interstitial fluid.

Figure: Diffusion of soluble and insoluble molecules between capillary and interstitial space

Illustration of this process, showing that as blood flows along the lumen of the capillary, many water molecules and dissolved particles diffuse back and forth through the capillary wall, creating a continuous mixing of plasma and fluid. interstitial. Diffusion phenomenon where water and solute molecules have thermal motion randomly moving in one direction and then changing in another direction.

Lipid-soluble substances diffuse directly across cell membranes in the endothelium of capillaries

If a substance is lipid-soluble, it can diffuse directly across the endothelial cell membrane without having to pass through pores. Among these are oxygen and carbon dioxide. Because these substances can penetrate every site of the endothelial cell membrane, the diffusion rate is many times faster than that of insoluble lipids, such as sodium and glucose ions (which can only pass through the pores). ).

Diffusion of water-soluble, lipid-insoluble substances through "holes" in the endothelial cell membrane

Many substances necessary for tissues are water-soluble but cannot pass through the lipid membranes of endothelial cells; These substances include water molecules, sodium ions, chloride ions, and glucose.

Although only 1/1000 of the surface area of ​​capillaries is the interstitial cleft between endothelial cells, the velocity of thermal motion of the molecules in the cleft is so great that even this small area is sufficient. to allow diffusion of large amounts of water and water-soluble substances through these slits. Such a diffusion rate is 80 times faster than the linear velocity of plasma along the capillary lumen. In other words, in the time it takes for a drop of blood to pass through a capillary, the water in the plasma containing that drop of blood and the water in the interstitial fluid has had enough time to diffuse into each other, mix, and exchange nutrients for 80 years. times already.

Effect of molecular size passing through holes

The hole in the capillary, i.e. the intercellular slit is 6-7 nanometres wide, about 20 times the diameter of the water molecule which is the smallest molecule to pass through the hole. The diameter of the plasma protein molecules is larger than the width of the pores. Other substances, such as sodium ions, chloride ions, glucose, and urea, have intermediate diameters. Therefore, the permeability of the capillary pores to substances varies according to their molecular diameter.

The table lists the relationship of capillary pores in skeletal muscle to common substances.

Board. Relation of capillary pores in skeletal muscle to substances of different sizes

Substance - Molecular Mass - Permeability

Water - 18 - 1.00

NaCl - 58.5 - 0.96

Urea - 60 - 0.8

Glucose - 180 - 0.6

Sucrose - 342 - 0.4

Inulin - 5000 - 0.2

Myoglobin - 17,600 - 0.03

Haemoglobin - 68,000 - 0.01

Albumin - 69,000 - 0.001

Through this, we see that the permeability for glucose molecules is 0.6 times that of water molecules, while the permeability for albumin molecules is very low, only 1/1000 times that of water molecules.

It should be known that capillaries in different tissues have different permeability. For example, the capillary pores of the hepatic capillary sinuses are so highly permeable that even plasma proteins pass almost as easily as water and other substances. In addition, the permeability of the glomerular membrane to water and electrolytes is about 500 times higher than that of the muscle capillaries, but this membrane does not allow plasma proteins; The capillary permeability of proteins is very small, as in other tissues and organs.

As we study the different organs in this book, it becomes easy to understand why some tissues require more capillary permeability than others. For example, a higher degree of capillary permeability is required for the liver to transport large amounts of nutrients between blood cells and the liver and kidney parenchyma to allow for the filtration of large amounts of fluid for the formation of blood cells. into urine.

Effect of concentration difference on the overall rate of diffusion across the endothelial cell membrane

Through the endothelial cell membrane. The actual rate of diffusion of a substance across any cell membrane is directly proportional to the difference in concentration of the substance between the two sides of the cell membrane. That is: the greater the concentration difference of a substance, the greater the direct transport of that substance across the membrane. For example, the concentration of oxygen in capillary blood is usually greater than in the interstitial fluid. As a result, large amounts of oxygen normally move from the blood into the tissues. In contrast, there is a higher concentration of carbon dioxide in the tissues than in the blood, creating excess carbon dioxide to move into the blood and be carried away from the tissues.

Figure. The structure of the interstitial space. Proteoglycan fibres are everywhere in the space between collagen fibrils. Free vesicles and small amounts of fluid from the channels are sometimes present

The rate of diffusion across the capillary membrane of most of the most important nutrients is so great that even a low concentration difference is sufficient to induce more transport in the transport between plasma and interstitial fluid. For example, the concentration of oxygen in the interstitial fluid just outside the capillaries is not more than a few percent of its concentration in the blood plasma, so even a very small concentration difference is enough to supply oxygen. for tissue metabolism—usually as much as a few litres of oxygen per minute in a highly active state of the body.