General Composition and Structure
The liquid portion of blood consists of 90% water, with the remaining 10% representing dry residue. Organic substances dominate the dry residue entirely (9%), leaving only 1% for inorganic components.
Looking closer at the dry residue, approximately two-thirds of its mass consists of proteins, while one-third comprises low-molecular-weight compounds and electrolytes.
Chemically, plasma closely resembles interstitial fluid. Sodium ($Na^+$) is the primary cation, while chlorides ($Cl^-$) and bicarbonates ($HCO_3^-$) predominate among anions. The key difference lies in the protein concentration: within the vascular bed, it reaches approximately 70 g/L, which is significantly higher than in the interstitial space.
Additionally, the liquid medium acts as a carrier for hormones, nutritional components, and metabolic waste products (nitrogenous compounds, organic acids).
Protein Fractions
More than one hundred different proteins circulate within the vascular bed. They are synthesized primarily in the liver, but also by the bone marrow, spleen, or released during blood cell breakdown. During electrophoresis, they are traditionally separated into five fractions: albumins and four globulin groups.
1. Albumins Their concentration is 40–45 g/L. These molecules have a relatively low molecular weight. The main function of albumins is generating oncotic (colloid-osmotic) pressure. This force counteracts hydrostatic pressure and prevents fluid from leaking out of vessels into tissues. A massive loss of albumins via the kidneys leads to "renal" edema, whereas prolonged nutritional deficiency causes "hunger" edema.
2. Globulins Present in amounts of 25–30 g/L, distinguished by larger 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 Plasma Proteins
Proteins perform critically important physiological tasks:
- Pressure Regulation: formation of the oncotic gradient (normally 25 mmHg).
- Hemostasis: participation in all regulatory links of the blood coagulation system (prothrombin and fibrinogen ensure clotting, plasminogen drives fibrinolysis, and antithrombin III acts within the anticoagulation system).
- Immune Defense: functions of immunoglobulins, complement system proteins, and macroglobulins.
- Transport Function: carriage of lipids, hormones, vitamins, and heavy metal salts. For example, transferrin binds iron, transcortin binds cortisol, transcobalamins transport vitamin B12, lipoproteins transport cholesterol, and albumins carry fatty acids.
- Rheological: maintenance of optimal blood flow viscosity.
- Trophic (Reserve): serving as an amino acid pool for tissue protein synthesis.
- Buffering: direct participation in acid-base balance regulation.
Lipoproteins
These complexes are responsible for transporting cholesterol, triacylglycerols, phosphoglycerides, and other substances. They are classified by density into chylomicrons, VLDL (very low-density lipoproteins), LDL (low-density lipoproteins), and HDL (high-density lipoproteins).
Density is inversely proportional to the lipid fraction content: while VLDL contains about 90% lipids (hence low density), HDL contains about 50%.
In clinical practice, special attention is paid to LDL levels: its concentration correlates directly with the likelihood of atherosclerosis development, establishing the high atherogenicity of this fraction.