[037] Colloid osmotic pressure (GB#102A02) | 基礎医学教育研究会(KIKKEN)Lab

● What is osmotic pressure and something different?

We study the difficult “osmotic pressure” at the beginning of the textbook, but as soon as we study the blood, the word “colloid osmotic pressure” usually comes out without any particular explanation. At that time, the people heard are not sure whether they are the same or different. To tell the truth, they look similar and do not look alike. Although the fundamental principle is the same, “osmotic pressure” is a force related to all cells, “colloid osmotic pressure” is a force which is practically only a problem on the wall of a capillary.

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What is colloid? It is a term that refers to a state in which irregularly sized fine particles are dispersed and floating in liquids and the like.In the case of body fluids, the identity of this microparticle is basically protein particles such as floating albumin and globulin. The colloid osmotic pressure is the osmotic pressure generated by the protein dissolved in water.

● Capillaries remove water and regain it

When blood reaches the capillary, gas exchange of oxygen and carbon dioxide occurs, and at the same time, a part of the blood solution (blood plasma) is pushed out of the blood vessel by blood pressure. This aqueous solution becomes a tissue fluid directly wrapping around the cell, or a body fluid called interstitial fluid. On the other hand, blood actually reabsorbs most of the tissue fluid extruded through the wall of the capillary. The colloid osmotic pressure plays an important role as a force to recover this tissue fluid in the blood.

● Capillary walls are full of holes

The wall of the capillary is made by sticking flat endothelial cells like tiles.The wall itself made of this cell is the inner wall which is continuous inside all the blood vessels, including in the heart. Although the outside is wrapped in the net of the connective tissue called the basement membrane, the wall made of the capillary cells is made of this thin inner wall only. The seams of cells on this wall have portions where there are slight gaps (there are also capillaries without gaps). In addition, some cells have windows that communicate the inside and the outside of blood vessels in some places in the cells themselves. Therefore, while the blood vessel cells do not pass ions such as Na⁺ ion and Cl^– ion freely, the gap of the blood vessel wall, together with water, the ions pass through easily. In capillaries without wall gaps, transporters of cell membranes actively carry water and ions. It is practically free to move. But the wall of the capillary is almost impermeable to plasma proteins.
※ Basement membrane omitted in animation

● If plasma proteins are removed, interstitial fluid

The blood is bright red, the tissue fluid that directly wraps around the cell, or the body fluid called interstitial fluid is colorless and transparent, so it looks like a different thing at all. But the red one is red because of the red blood cell, the plasma which is the liquid without the cellular components basically is almost colorless and transparent like the tissue liquid. The tissue fluid source is plasma, so it should be similar and natural. However, although it is similar, there is an important difference that the amount of dissolved protein is large in plasma and less in tissue fluid. This difference is because water and ions flow when plasma is extruded by blood pressure, but plasma protein hardly passes and remains in plasma. For this reason, the phenomenon that a liquid traverses a capillary is called “filtration”.

● Salinity concentration is not related to colloid osmotic pressure

As a result of the filtration, the plasma with the protein left and the tissue liquid with a small amount of protein will face each other across the wall of the capillary. Then the power to try to resolve the imbalance will occur naturally this time. This is colloid osmotic pressure.　Just “osmotic pressure” refers to the force that acts on cell membranes in the physiological world. Since the cell membrane can not pass water-soluble substances other than water molecules and gas molecules freely, water molecules move according to the total concentration and become a force to balance. On the other hand, osmotic pressure due to ions such as cell membranes is not created in the walls of capillary vessels because molecules with small amounts of water and ions pass freely. There is no effect on the colloid osmotic pressure unless it affects the protein concentration, irrespective of salt concentration. The osmotic pressure generated in the capillary wall is due only to the difference in protein concentration, so it is distinguished as “colloid osmotic pressure”.

● Plasma is always trying to absorb tissue fluid

Since the protein remains in the plasma by filtration in the normal range, the colloid osmotic pressure will always be high in the capillary. That is, the colloid osmotic pressure serves as a force for the plasma to draw water. However, since it can also pass ions, it is not only water but also liquid that is actually filtered and filtered, including ions and others, that is, mostly interstitial fluid itself.

● Filtration and absorption is a competition between blood pressure and colloid osmotic pressure

The concentration of protein in plasma is approximately 1.5 mM. On the other hand, in the interstitial fluid, it is only about one tenth of that. As a result, the colloid osmotic pressure of plasma is said to be approximately 25 mmHg. This value can be regarded as the water absorption capacity of the plasma as it is. On the other hand, since the force to filter depends on the blood pressure, it is about 35 mmHg on the arterial side of the capillary, that is, at the beginning, but since it has resistance of the blood vessel, it continues to descend down to 15 mmHg as going to the vein side. On the arterial side of the capillary, the blood pressure is higher than the colloid osmotic pressure, so that the water is filtered out and discharged to the outside of the blood vessel, but on the vein side the colloid osmotic pressure is higher than the blood pressure, so the tissue fluid is absorbed by the capillary. The extent to be filtered for urine production in the kidney is very special and should be excluded. Other than that, about 20 liters of water per day throughout the body is filtered from the walls of the capillaries, most of which 16 to 18 liters of water are also collected in the capillary. The remaining 2 to 3 liters of water per day is absorbed by the lymphatic vessels and finally collected in the vena cava near the heart so that the circulation of the water is adjusted so that the water circulation is consistent .

● The recovery power of interstitial fluid depends on the amount of plasma protein

Collection of interstitial fluid in capillaries is due to osmotic pressure of plasma. What produces colloid osmotic pressure is the difference between the amount of protein in plasma and tissue fluid. In short, the number of particles that can not pass through the capillary walls is related to the osmotic pressure. Most of this protein is produced in the liver and supplied to the plasma. Plasma protein amount is usually said to be 7.0 g/dl (deciliter). It is 7 g protein per 1/10 liter of plasma. About two-thirds of it is albumin, most of the rest is globulin.In terms of weight ratio, it is 66 : 33. However, since the molecular weight of albumin is about two thirds of the average molecular weight of globulin, the number of molecules is 66/0.66 : 33 = 1.0 : 0.33 = 75 : 25. That is, the amount of albumin is effective for over 70% of the colloid osmotic pressure. Liver damage causes poor production of these proteins, and in kidney diseases plasma proteins are mixed in the urine in large quantities and leak out of the body, which also leads to the deficiency of plasma proteins. Then, the colloid osmotic pressure decreases and the recovery force of water from the interstitial fluid decreases. On the other hand, because the recovery power of water does not increase, as a result water continues to accumulate in the tissue, causing so-called edema (edema). Besides that, the plasma protein amount decreases pathologically for various reasons.

After all, when the plasma protein is sufficient, water is absorbed in the blood, the tissue fluid is kept at an appropriate amount, and when the plasma protein is deficient, the water of the interstitial fluid will trickle and cause edema.

Plasma protein leaks slightly

Basically, the wall of the capillary does not pass plasma proteins, but it does not pass through at all, and the degree is also different depending on the location of the blood vessel. If it is still normal, overall the balance of body fluid circulation can be taken. However, if the wall of the blood vessel can not successfully restrict the protein, if it flows as it is, the plasma colloid osmotic pressure decreases and the colloid osmotic pressure of the interstitial fluid rises. As a result, plasma recovery power of plasma decreases, resulting in edema. The nature of capillaries that do not pass proteins is not outstanding, but they are doing very important work.

〇 Referenced sites

・Albumin, Wikipedia, 30th June 2017.
・Serum albumin, molecules of this month, 2003-1.
・Globin, Wikipedia, July 1, 2017.

○ Related articles

◆[006] Osmotic pressure　
◆[004] Cation and anion, attraction and repulsion　
◆[016] 血液循環　blood circulation
◆[023] 赤血球とヘモグロビン　erythrocyte and hemoglobin
◆[043] 糸球体のろ過 glomerular filtration
◆[025] 血管の運動　vasomotor
◆[013] 細胞膜の脂質二重層 lipid bilayer of the cell membrane

○ Referenced books

・Lehninger Principles of Biochemistry 6th, International Edition, Macmillan Higher Education, England.
・カラー版 ボロン ブールペープ 「生理学」， 西村書店
・カラー図解 人体の正常構造と機能 全10巻縮刷版，坂井 建雄，日本医事新報社
・人体機能生理学，杉 晴夫，南江堂
・トートラ人体解剖生理学　原書8版，丸善
・イラスト解剖学，松村 讓兒，中外医学社
・柔道整復学校協会編「生理学」，南江堂
・東洋療法学校協会編「生理学」，医歯薬出版株式会社

rev.20140525，rev.20150122，rev.20150717，rev.20160713, rev.20180513.