BLOOD PLASMA: FORMATION, COMPONENTS AND FUNCTIONS - ANATOMY AND PHYSIOLOGY - 2024

The blood plasma is in large proportion the aqueous fraction of the blood. It is a connective tissue in a liquid phase, which moves through capillaries, veins and arteries both in humans and in other groups of vertebrates in the circulation process. The function of plasma is the transport of respiratory gases and various nutrients that cells need for their function.

Within the human body, plasma is an extracellular fluid. Together with the interstitial or tissue fluid (as it is also called) they are outside or surrounding cells. However, the interstitial fluid is formed from the plasma, thanks to the pumping by circulation from the small vessels and microcapillaries near the cell.

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Plasma contains many dissolved organic and inorganic compounds that are used by cells in their metabolism, as well as containing many waste substances as a consequence of cellular activity.

Components

Blood plasma, like other body fluids, is made up mostly of water. This aqueous solution is composed of 10% of solutes, of which 0.9% corresponds to inorganic salts, 2% to non-protein organic compounds and approximately 7% corresponds to proteins. The remaining 90% is made up of water.

Among the inorganic salts and ions that make up blood plasma we find bicarbonates, chlorides, phosphates and / or sulfates as anionic compounds. And also some cationic molecules such as Ca ⁺, Mg ²⁺, K ⁺, Na ⁺, Fe ⁺ and Cu ⁺.

There are also many organic compounds such as urea, creatine, creatinine, bilirubin, uric acid, glucose, citric acid, lactic acid, cholesterol, cholesterol, fatty acids, amino acids, antibodies and hormones.

Among the proteins found in plasma are albumin, globulin, and fibrinogen. In addition to solid components, there are dissolved gaseous compounds such as O ₂, CO ₂ and N.

Plasma proteins

Plasma proteins are a diverse group of small and large molecules with numerous functions. Currently about 100 plasma component proteins have been characterized.

The most abundant protein group in plasma is albumin, which constitutes between 54 and 58% of the total proteins found in said solution, and acts in the regulation of the osmotic pressure between plasma and body cells.

Enzymes are also found in plasma. These come from the process of cellular apoptosis, although they do not carry out any metabolic activity within the plasma, with the exception of those that participate in the coagulation process.

Globulins

Globulins make up about 35% of the proteins in plasma. This diverse group of proteins is subdivided into several types, according to electrophoretic characteristics, being able to find between 6 and 7% of α ₁ -globulins, 8 and 9% of α ₂ -globulins, 13 and 14% of β-globulins, and between 11 and 12% γ-globulins.

Fibrinogen (a β-globulin) represents approximately 5% of proteins and together with prothrombin also found in plasma, it is responsible for blood clotting.

Ceruloplasmines transport Cu ²⁺ and it is also an oxidase enzyme. Low levels of this protein in plasma are associated with Wilson's disease, which causes neurological and liver damage due to the accumulation of Cu ²⁺ in these tissues.

Some lipoproteins (of the α-globulin type) are found to transport important lipids (cholesterol) and fat-soluble vitamins. Immunoglobulins (γ-globulin) or antibodies are involved in defense against antigens.

In total, this group of globulins represents around 35% of the total proteins, and they are characterized, like some metal-binding proteins that are also present, in being a group of high molecular weight.

How much plasma is there?

The fluids present in the body, whether intracellular or not, are mainly made up of water. The human body, as well as that of other vertebrate organisms, is made up of 70% water or more by body weight.

This amount of liquid is divided into 50% of water present in the cytoplasm of the cells, 15% of water present in the interstices and 5% corresponding to plasma. The plasma in the human body would represent approximately 5 liters of water (more or less 5 kilograms of our body weight).

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Plasma represents approximately 55% of blood by volume. As we mentioned, of this percentage, basically 90% is water and the remaining 10% are dissolved solids. It is also the transport medium for the body's immune cells.

When we separate a volume of blood by centrifugation, we can easily see three layers in which one can distinguish an amber-colored one that is plasma, a lower layer made up of erythrocytes (red blood cells) and in the middle a whitish layer where the cells are included. platelets and white blood cells.

Most plasma is formed through intestinal absorption of fluid, solutes, and organic substances. In addition to this, plasma fluid is incorporated as well as several of its components through renal absorption. In this way, blood pressure is regulated by the amount of plasma present in the blood.

Another way by which materials are added for the formation of plasma is by endocytosis, or to be precise by pinocytosis. Many cells in the endothelium of blood vessels form large numbers of transport vesicles that release large amounts of solutes and lipoproteins into the bloodstream.

Differences with interstitial fluid

Plasma and interstitial fluid have fairly similar compositions, however, blood plasma has a large amount of proteins, which in most cases are too large to pass from capillaries to interstitial fluid during blood circulation.

Plasma-like body fluids

Primitive urine and blood serum have aspects of coloration and concentration of solutes very similar to those present in plasma.

However, the difference lies in the absence of proteins or substances of high molecular weight in the first case and in the second, it would constitute the liquid part of the blood when the coagulation factors (fibrinogen) are consumed after this occurs.

Features

The different proteins that make up plasma perform different activities, but they all carry out general functions together. The maintenance of osmotic pressure and electrolyte balance are part of the most important functions of blood plasma.

They also play a major role in the mobilization of biological molecules, the turnover of proteins in the tissues and the maintenance of the balance of the buffer system or blood buffer.

Blood clotting

When a blood vessel is damaged, there is a loss of blood whose duration depends on the response of the system to activate and carry out mechanisms that prevent said loss, which if prolonged can affect the system. Blood coagulation is the dominant hemostatic defense against these situations.

The blood clots that cover the blood leak form as a network of fibers from fibrinogen.

This network called fibrin, is formed by the enzymatic action of thrombin on fibrinogen, which breaks peptide bonds releasing fibrinopeptides that transform said protein into fibrin monomers, which associate with each other to form the network.

Thrombin is found in an inactive form in plasma as prothrombin. When a blood vessel ruptures, platelets, calcium ions, and clotting factors such as thromboplastin are rapidly released into the plasma. This triggers a series of reactions that carry out the transformation of prothrombin to thrombin.

Immune response

Immunoglobulins or antibodies present in plasma play a fundamental role in the body's immune responses. They are synthesized by plasma cells in response to the detection of a foreign substance or an antigen.

These proteins are recognized by the cells of the immune system, being able to respond to them and generate an immune response. Immunoglobulins are transported in plasma, being available for use in any region where a threat of infection is detected.

There are several types of immunoglobulins, each with specific actions. Immunoglobulin M (IgM) is the first class of antibody to appear in plasma after infection. IgG is the main antibody in plasma and is capable of crossing the placental membrane and being transferred to the fetal circulation.

IgA is an antibody of external secretions (mucus, tears and saliva) being the first line of defense against bacterial and viral antigens. IgE intervenes in anaphylactic hypersensitivity reactions, being responsible for allergies and is the main defense against parasites.

Regulation

Components of blood plasma play an important role as regulators in the system. Among the most important regulations are osmotic regulation, ionic regulation and volume regulation.

Osmotic regulation tries to keep the plasma osmotic pressure stable, regardless of the amount of fluids that the body consumes. For example, in humans a pressure stability of about 300 mOsm (micro osmoles) is maintained.

Ion regulation refers to the stability of inorganic ion concentrations in plasma.

The third regulation consists of maintaining a constant volume of water in the blood plasma. These three types of regulation within plasma are closely related and are due in part to the presence of albumin.

Albumin is responsible for fixing water in its molecule, preventing it from escaping from the blood vessels and thus regulating the osmotic pressure and the volume of water. On the other hand, it establishes ionic bonds transporting inorganic ions, keeping their concentrations stable within plasma and in blood cells and other tissues.

Other important functions of plasma

The excretory function of the kidneys is related to the composition of plasma. In the formation of urine, the transfer of organic and inorganic molecules occurs that have been excreted by cells and tissues in the blood plasma.

Thus, many other metabolic functions carried out in different body tissues and cells are only possible thanks to the transport of the molecules and substrates necessary for these processes through plasma.

Importance of blood plasma in evolution

Blood plasma is essentially the watery portion of the blood that carries metabolites and wastes from cells. What began as a simple and easily satisfied requirement for molecule transport resulted in the evolution of several complex and essential respiratory and circulatory adaptations.

For example, the solubility of oxygen in blood plasma is so low that plasma alone cannot carry enough oxygen to support metabolic demands.

With the evolution of special oxygen-carrying blood proteins, such as hemoglobin, which appears to have evolved along with the circulatory system, the oxygen-carrying capacity of the blood increased considerably.

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