General composition and structure
The liquid part of blood consists of 90% water, with the remaining 10% being the dry residue. Organic substances absolutely dominate the dry residue (9%), while inorganic components account for only 1%.
Looking more closely at the structure of the solid residue, approximately two-thirds of its mass consists of proteins, and one-third consists of low-molecular-weight compounds and electrolytes.
In its chemical profile, plasma closely resembles interstitial (intercellular) fluid. Sodium ($Na^+$) is the main cation, while chlorides ($Cl^-$) and bicarbonates ($HCO_3^-$) predominate among anions. The key difference lies in the protein concentration: in the bloodstream, it reaches approximately 70 g/L, which is significantly higher than in the intercellular space.
In addition, the liquid medium acts as a carrier for hormones, nutrient components, and metabolic products (nitrogenous substances, organic acids).
Protein fractions
More than a hundred different proteins circulate in the vascular bed. They are produced primarily in the liver, but are also synthesized by the bone marrow, spleen, or released during the breakdown of blood cells. During electrophoresis, they are traditionally divided into five fractions: albumins and four groups of globulins.
1. Albumins Their concentration is 40–45 g/L. These are molecules with a relatively low molecular weight. The main task of albumins is to generate oncotic (colloid-osmotic) pressure. This force counteracts hydrostatic pressure and prevents fluid from leaving the vessels into the tissues. With massive loss of albumin through the kidneys, "renal" edema develops, and with prolonged nutritional deficiency, "hunger" edema.
2. Globulins Contained in an amount of 25–30 g/L, characterized by large molecular sizes.
- $\alpha_1$-globulins: prothrombin, $\alpha_1$-antitrypsin, high-density lipoproteins (HDL).
- $\alpha_2$-globulins: plasminogen, $\alpha_2$-macroglobulin, $\alpha_2$-antithrombin III, $\alpha_2$-haptoglobin.
- $\beta$-globulins: fibrinogen, transferrin, low-density lipoproteins (LDL).
- $\gamma$-globulins: represented by immunoglobulins (IgA, IgD, IgE, IgG, IgM).
Functions of proteins
Proteins perform critically important physiological tasks:
- Pressure regulation: formation of the oncotic gradient (normally 25 mm Hg).
- Hemostasis: participation in all links of the regulation of the aggregate state of blood (prothrombin and fibrinogen provide coagulation, plasminogen provides fibrinolysis, antithrombin III works in the anticoagulation system).
- Immune defense: work of immunoglobulins, complement system proteins, and macroglobulins.
- Transport function: transport of lipids, hormones, vitamins, heavy metal salts. For example, transferrin binds iron, transcortin binds cortisol, transcobalamins transport vitamin B12, lipoproteins transport cholesterol, and albumins transport fatty acids.
- Rheological: maintaining optimal blood flow viscosity.
- Trophic (reserve): serve as a depot of amino acids for building tissue proteins.
- Buffer: direct participation in the regulation of acid-base balance.
Lipoproteins
These complexes are responsible for the transport of cholesterol, triacylglycerol, phosphoglycerides, and other substances. They are divided according to density into chylomicrons, VLDL (very low density), LDL (low density), and HDL (high density).
Density is directly and inversely dependent on the proportion of fats in the composition: while VLDL contains about 90% of them (hence low density), HDL contains about 50%.
In clinical practice, special attention is paid to the level of LDL: its concentration has a direct relationship with the likelihood of developing atherosclerosis, which determines the high atherogenicity of this fraction.
Briefly: answers to topical questions
How does blood plasma differ from blood serum?
Blood serum is plasma devoid of fibrinogen.
| Characteristic | Blood plasma | Blood serum |
|---|---|---|
| Method of obtaining | Centrifugation of blood with anticoagulants | Separation of fluid during clot formation |
| Presence of fibrinogen | Present | Absent |
What is the normal osmotic pressure of blood plasma and what is it created by?
The normal osmotic pressure of blood is 7.6–8.1 atm (768–818 kPa, 295–310 mosm/L). It is created primarily due to the concentration of mineral salts, the main ones being sodium and chlorine. An integral part of osmotic pressure is also oncotic pressure (25–30 mm Hg), which is created by plasma proteins, primarily albumins.
Which buffer systems ensure the maintenance of blood plasma pH?
To maintain the acid-base balance of plasma, chemical buffer systems are involved, acting as the first mobile compensation system.
- Bicarbonate — consists of carbonic acid and bicarbonates (1:20 ratio), is the main buffer system of plasma.
- Protein — based on the amphoteric nature of proteins: they bind H+ or OH-.
Blood buffer systems in general also include:
- Phosphate — salts of mono- and disubstituted phosphates.
- Hemoglobin — provides 75% of the blood buffer capacity, includes reduced and oxidized hemoglobin.
How to easily remember some transport proteins: Transferrin transports ferrum (iron), transcortin transports cortisol, and transcobalamins transport vitamin B12 (since it contains cobalt).
Class and exam questions
How does plasma differ from intercellular (interstitial) fluid?
In terms of electrolyte chemical composition, they are very similar (sodium, chlorides, and bicarbonates predominate in both media). The main difference is that there is significantly more protein in the vascular bed (about 70 g/L).
Why do edema occur during prolonged fasting?
With nutritional deficiency, albumin levels drop. Because of this, oncotic pressure, which normally keeps water in the vessels, falls, and fluid leaks into the tissues, causing "hunger" edema.
Which plasma proteins are responsible for blood clotting?
Prothrombin ($\alpha_1$-globulin) and fibrinogen ($\beta$-globulin) participate in the coagulation process. At the same time, plasminogen is responsible for dissolving the thrombus, and antithrombin III for preventing excessive clotting.
What else to study on this topic
- Mechanisms of oncotic pressure formation and transcapillary exchange
- The role of $\gamma$-globulins (immunoglobulins) in humoral immunity
- The work of the protein buffer system and maintenance of acid-base balance
- Interaction of the coagulation, anticoagulation, and fibrinolytic systems of blood
- Lipoprotein metabolism and mechanisms of atherosclerosis development
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