Properties of blood The colour of blood is red due to haemoglobin. Arterial blood is scarlet red (due to oxygen) and venous blood is purple red (due to carbon dioxide) The pH under normal condition is 7.4 (venous blood 7.36) Blood is opaque due to its cellular elements, particularly the red blood cells. After haemolysis blood becomes transparent.

Blood has specific gravity of 1060 (that of cells is about 1090 while that of plasma is 1025-1030) Blood is five times more viscous than water due to its cellular elements and some plasma proteins. The viscosity of plasma is twice as water. The osmotic pressure of the plasma is about 5500 mmHg. It is mostly due to crystalloids.

Functions of blood It is the major transport medium in the body. This is essential for: Nutrition: from the intestine to all the tissues Waste products: From the cells to the excretory organs, e.g. urea to the kidneys, CO2 to the lungs, and bilirubin to the liver. Hormones : from the endocrine glands to their target cells.

Regulation of: Water content of the tissues: by fluid exchange between the plasma and the interstitial fluid. pH of tissue fluids: by removing any acidic metabolites from the tissue fluids and buffering them by the buffering systems of the blood, e.g. the carbonic/ bicarbonate buffering system (H2CO3/NaHCO3).

Hormonal secretion: by feedback mechanism; e. g Hormonal secretion: by feedback mechanism; e.g. parathormone raises the plasma calcium level, a rise in the plasma calcium level inhibits parathormone secretion, and a drop of plasma calcium level stimulates parathormone secretion.

Body temperature: by heat transfer from deep organs to the skin. This is facilitated by the high heat conductivity and the high specific heat of water. Blood also informs the hypothalamic thermostat about the body core temperature. The hypothalamic thermostat then reacts accordingly to adjust body temperature at its normal level.

Defence against: Microorganisms: blood phagocytes, with the help of antibodies, attack and kill any invading microorganisms. Toxic substances: antitoxins, i.e. antibodies against toxins, neutralize toxins and poisons. Blood loss: blood clotting factors seal wounds and prevent blood loss.

Plasma Blood plasma is a mixture of proteins, enzymes, nutrients, wastes, hormones and gases. The specific composition of its components are as follows:  

Composition of Plasma

Plasma Proteins The total plasma protein concentration is 7 gm/dL (range 6-8). Of this, 4 gm is albumin, 2.7 gm is a mixture of globulins and 0.3 gm is fibrinogen. Globulins are classified according to their physicochemical properties into 4 different types; α1, α2, β and γ- globulins.

Separation and measurement of plasma proteins Several methods are used to separate and measure the different component of plasma proteins. One method is chemical precipitation by ammonium sulphate. A 25% saturation with ammonium sulphate precipitates fibrinogen, 50% saturation precipitates globulins, and full saturation precipitates albumin.

Electrophoresis is another method Electrophoresis is another method. If a mixture of plasma proteins is put in an electric field between two electrodes, different types of proteins migrate towards the anode at different speeds. Thus proteins can be spread and separated on a paper strip and their concentration determined. Other methods include; isoelectric focusing, ultracentrifugation and immunoelectrophoresis.

The A/G ratio The A/G is the ratio of albumin/globulin in the plasma. The normal value depends on the method of determination. It is 1.5 by electrophoresis and 1.2 by the chemical precipitation method. The A/G is a non specific test that could help clinically in diagnosis of some diseases, e.g. it is decreased in liver diseases (due to decrease in albumin synthesis) and in infections as tuberculosis and typhoid (due to increase in γ- globulin synthesis). It is increased in hypogamma-globulinemia and in AIDS.  

Formation of the Plasma Proteins. Essentially all the albumin and fibrinogen of the plasma proteins, as well as 50 to 80 per cent of the globulins, are formed in the liver. The remainder of the globulins are formed almost entirely in the lymphoid tissues. They are mainly the gamma globulins that constitute the antibodies used in the immune system.

The rate of plasma protein formation by the liver can be extremely high, as much as 30 g/day. Certain disease conditions cause rapid loss of plasma proteins; severe burns that denude large surface areas of the skin can cause the loss of several liters of plasma through the denuded areas each day.

The rapid production of plasma proteins by the liver is valuable in preventing death in such states. Occasionally, a person with severe renal disease loses as much as 20 grams of plasma protein in the urine each day for months, and it is continually replaced mainly by liver production of the required proteins.

In cirrhosis of the liver, large amounts of fibrous tissue develop among the liver parenchymal cells, causing a reduction in their ability to synthesize plasma proteins. This leads to decreased plasma colloid osmotic pressure, which causes generalized oedema.

Functions of plasma proteins Haemostasis: fibrinogen, prothrombin and most of other blood clotting factors are plasma proteins. Tissue fluid formation: Fluid interchange between plasma and interstitial fluids occurs by filtration and absorption at the capillaries. The filtration force is mainly the hydrostatic capillary pressure. The absorption force is the oncotic pressure of the plasma proteins.

In hypoproteinemia, the oncotic pressure decreases leading to more filtration of interstitial fluid than reapsorption which causes accumulation of excess amounts of interstitial fluids, i.e. edema

Peripheral vascular resistance: The peripheral vascular resistance is determined by two main factors; i.e. the diameter of the arterioles and the viscosity of the blood. The viscosity of the blood is the function of two factors; i.e. the RBCs and plasma proteins. The globulins and fibrinogen are particularly important for the viscosity of the plasma. The peripheral vascular resistance is important for maintaining the arterial blood pressure.

Buffering action: At the normal pH of arterial blood (7.4), plasma proteins act as weak acids (proteinic acids) that combine with bases, mainly sodium. The mixture of a weak acid and its salt with a strong base (Na+) makes a blood buffering system. 15% of the buffering capacity of the blood is due to plasma proteins.

Defence and protection: Gamma globulins are the antibodies which attack any invading bacteria or foreign bodies and neutralize toxins. Regulating the activity of the biologically active substances: plasma proteins combine loosely with many chemical substances including hormones (e.g. thyroxin and cortisol), minerals (e.g. iron and copper) and numerous drugs.

The bonds are firm enough to limit free diffusion of the small molecular substances, and thus reduce their biological activity. The bond form serves as a reservoir from which the free hormone, metal, or drug is slowly released.

Effect on capillary permeability: Proteins clot the pores in the capillary wall. Hypoproteinemia leads to increase in capillary permeability which favours the development of edema. Carbon dioxide transport: 10% of the tidal CO2 is carried from the tissues to the lungs combined with plasma proteins in the form of carbaminocompounds. At the lung, CO2 is released and proteins restore their amino structure.

Nutritional function during starvation: Plasma proteins are used by the tissues during prolonged starvation after the body protein reserves have been depleted.