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Importance of Proteins
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Importance of Proteins

Transport function. There are special spots in the molecule of protein, which are able to bind inorganic substances (for example, ions, water) and organic substances (for example, hormones, biologically active substances) and to transport them. Binding of this substances to proteins provides:

Keeping small molecules in bloodstream when blood is passing through kidneys.

Predicts its destructing by blood enzymes.

There are specific and non-specific transport proteins. Not-specific – are able to bind different substances and transport those (most of albumins transport hormones, calcium). Specific – transport only one kind of substances. For example, cerulloplasmin – ions of cuprum (Cu), transferrin – ions of iron (Fe), haptoglobulin – bilirubin.

Trophic function. Proteins are the source of amino acids,

which with peripheral tissues, are used for the formation of proper, specified for the organ proteins. Proteins are the source of energy. When breaking down 1 g of protein in the organism 4,1 kkal.

Trophic function of proteins is used clinically when disorder of natural way of nourishment, in parenteral nourishment, when protein suspensions are injected in bloodstream.

Enzymatic function. There are a lot of protein-enzymes into the plasma. There are secretory and indicatory (cellular) enzymes. Secretory enzymes are synthesized in liver and are excreted in the blood plasma, where they perform their function. Typical representatives of this group are protein-enzymes of blood clotting. Indicatory enzymes are entering the blood from other organs. Their activity is not high. In conditions of pathological states, enzymes are taken from cells into the blood and its activity increases, which indicate the level of lesion. That’s why quantitative definition of blood enzymes is one of the available laboratory methods of diagnostics. For example, activity of AlAt (alaninaminotransferase) increases during liver sickness. Activity of AsAt (aspartateaminotransferase) increases up to 20 times during myocardial infarction. Activity of lactatedehydrogenase increases during myocardial infarction, hepatitis, myopathy, tumors, and leucosis.

Participating hemostasis. Proteins comprise biochemical systems of blood plasma, which provides hemostasis, namely:

Blood clotting system;

Anticoagulatory system;

Fibrinolytic system;

Kallikrein-kinine system.

Participating in maintaining pH of the blood. Proteins form protein buffer. In acidic medium, they work as bases, binding acids; in base, they react like acids, binding bases. This property of the proteins is called amphoteric. Mostly buffer properties belong to carboxyl groups and amino groups. Plasma proteins are responsible for 15% of the buffering capacity of the blood and carriage of CO2.

Protein – NH2 +CO2 ® NHCOOC (Carbamino protein);

Proteinic acid: Na proteinate buffer:

a) Na proteinate +H2CO 3    ®  NaHCO3 + Proteinic acid.

b) Na proteinate + lactic acid ® Na lactate + Proteinic acid.

Lactic acid (strong acid) is converted to proteinic acid (weak acid).

Maintenance of the reological properties of the blood, namely its stickness. When increase in protein quantity, stickness increases, when decrease in protein quantity, stickness decreases.

Proteins are the source of biologically active substances.

For example, kinins and angiotensin.

Protective function. Proteins participate in non-specific and specific protection of the organism. Non-specific protection is represented by complement system proteins, interferons, orosomucoid and viruses’ inhibitors. Specific – by antibodies: congenital (agglutinins) and acquired.

Making creative connections. Proteins participate transferring of the information, which affects genetic apparatus of cells, provides growth, development and differentiation of tissues. For example, proteins are growth factor of nervous tissue, erythropoietin etc.

Capillary Permeability. Plasma proteins close the pores in the cement substance (between the endothelial cells) of the capillary wall. Hypoproteinaemia increases the capillary permeability.

Creation of oncotic. (colloid-osmotic) pressure

Ponc= 25 – 30 mmHg. 80% of oncotic pressure is made by albumins (molecule of albumin has small size and in volume of plasma, its quantity is the highest).

Role of oncotic pressure in redistribution of water in the organism (fig. 1.3):

 

Figure 1.3 – Redistribution of water in the capillaries

Wall of capillaries is permeable to small molecules and water. That is why osmotic pressure in blood plasma and in intercellular liquid is almost equal. Big molecules, first of all protein molecules, can not pass through the capillary wall. That’s why there is gradient of protein concentration (oncotic pressure gradient - Ponc) between plasma and intercellular liquid. Ponc inside capillary is higher than in intercellular liquid. Hydrostatic pressure – pressure of the liquid on the capillary wall (from one side blood is making pressure on capillary wall, from other side – intercellular liquid), it also plays important role in redistribution of the water. Hydrostatic pressure of the blood is higher, than hydrostatic pressure of intercellular liquid.

Water exchange occurs in two ways:

filtration (transition of the water from the capillary to the tissue);

reabsorbtion (transition of the water from the tissue to the capillary).

The direction of the water movement is defined by the filtrative pressure (Pf).

Pf = (Phbp + Pop) – (Php + Pobp)

Phbp – hydrostatic pressure of the blood;

Pop – oncotic pressure of the intercellular liquid;

Php – hydrostatic pressure of the intercellular liquid;

Pobp – oncotic pressure of the blood.

If Pf >0 – filtration occurs.

If Pf <0 – reabsorbtion occurs.

Increasing of the Phbp and Pop leads to filtration, increasing of the Php and Pobp leads to reabsorbtion.

In the arterial end of the capillary:

Pf = (32,5 + 4,5) – (25 + 3) = 9 mmHg – filtration occurs, water is transited to the tissue.

As the blood passes in the capillary, in the result of transition of the water to the tissue, hydrostatic pressure decreases. In the middle of the capillary Pf = 0 and water transition stops.

In the venous end of the capillary:

Pf = (17,5 + 4,5) – (25 + 3) = - 6 mmHg – reabsorbtion occurs, water passes into the capillary.

At the beginning of the capillary approximately 0,5% of blood plasma passes to the tissues. Pf in the arterial part of the capillary (Pf = 9 mmHg) is higher than in the venous part (Pf = - 6 mmHg), that is why the bloodstream returns not the whole 100% of the liquid, but nearly 90%. 10% are excreted through the lymphatic vessels.

This pressure values may vary in different organs and it depends on organ activity. Described mechanism of filtration – reabsorbtion is called Starling’s mechanism.

Changes in any of the parameters may cause disorder in filtration and reabsorbtion correlation.

For example, the decrease in protein concentration in blood plasma will lead to the decrease of the reabsorbtion, delay of the water in intercellular medium and development of the intercellular oedema. This can happen during starvation (cachexy oedemas); when pathological processes in kidneys, in the consequence of which proteinuria can occur and loss of proteins (nephrotic oedemas); when disorder in albumin synthesis by liver (hepatic oedemas); when allergic and inflammatory processes, when there is the increase of vessel wall permeability and other plasma proteins are leaving to the intercellular space (membranogenic oedemas) and other.



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