Blood Composition: Plasma Proteins PDF

1 Blood Composition: plasma proteins ILOs By the end of this lecture, students will be able to 1. Correlate the normal values of different blood components to their functions in health and disease. 2. Explain the role of plasma proteins in establishing blood functions Composition & function of blood...

1 Blood Composition: plasma proteins ILOs By the end of this lecture, students will be able to 1. Correlate the normal values of different blood components to their functions in health and disease. 2. Explain the role of plasma proteins in establishing blood functions Composition & function of blood: Blood contains the blood cells and the liquid portion known as plasma. Blood cells are of 3 types: red blood cells (RBCs) or erythrocytes, white blood cells (WBCs) or leukocytes and platelets or thrombocytes. The fluid portion of the blood, the plasma, is a remarkable solution containing an immense number of ions, inorganic molecules, and organic molecules that are in transit to various parts of the body or aid in the transport of other substances. The volume of blood in a normal adult is about 5 litres. The plasma forms 55% of the whole blood volume. Blood carries many important functions: The respiratory function which is responsible for transport of O2 and CO2 between lungs and tissues. This function is carried mainly by RBCs. Defensive function, where the blood plays an important role in the defense of the body against microorganisms. This function is carried by WBCs and immunoglobulins Haemostatic function which is responsible for stoppage of bleeding through platelets. In addition to many other functions as nutrient function, excretory function, storage function, transport function, and regulation of body temperature. Constituents of plasma 1.Water (90%) 2. Inorganic constituents (1%) e.g. sodium, chloride, calcium, potassium 3. Organic constituents (9%) which include; Plasma proteins (7%), Nutrients & waste products (2%) Plasma proteins Their concentration ranges from 7-9 g/dL. The major types of protein present in the plasma are albumin (~4.8 g/dL), globulin (α, β, γ) (~2.7 g/dL), and fibrinogen (~0.2 g/dL). Origin of plasma proteins Circulating antibodies are manufactured by lymphocytes. Most of the other plasma proteins are synthesized in the liver. Data on the turnover of albumin show that its synthesis plays an important role in the maintenance of normal levels. In normal adult humans, the plasma albumin level is 3.5 to 5.0 g/dL, and the total exchangeable albumin pool is 4.0 to 5.0 g/ kg body weight; 38–45% of this albumin is intravascular, and much of the rest of it is in the skin. Between 6% and 10% of the exchangeable pool is degraded per day, and the degraded albumin is replaced by hepatic synthesis of 200 to 400 mg/kg/ d. The albumin is probably transported to the extravascular areas by vesicular transport across the walls of the capillaries. Albumin synthesis is carefully regulated. It is decreased during fasting and increased in conditions such as nephrosis in which there is excessive albumin loss. Functions of plasma proteins 1- Protein metabolism Plasma proteins are constantly used by the tissues for their protein metabolism (thus they are dynamic). 2- Regulation of blood volume A major function of albumin is to provide colloid osmotic pressure in the plasma, which prevents plasma loss from the capillaries. The capillary walls are relatively impermeable to plasma proteins, so this osmotic force tends to pull water into the blood, thus maintaining blood volume. 3- Immunity The globulins perform a number of enzymatic functions in the plasma, but equally important, they are principally responsible for the body’s both natural and acquired immunity against invading organisms. 4- Blood coagulation Some of the plasma proteins (the clotting factors) are needed for blood coagulation. The most important of these are prothrombin and fibrinogen. Fibrinogen polymerizes into long fibrin threads during blood coagulation, thereby forming blood clots that help repair leaks in the circulatory system. Plasma clots on standing, remaining fluid only if an anticoagulant is added. If whole blood is allowed to clot and the clot is removed, the remaining fluid is called serum. Serum has essentially the same composition as plasma, except that its fibrinogen and clotting factors II, V, and VIII have been removed and it has a higher serotonin content because of the breakdown of platelets during clotting. 5- Buffering action The plasma proteins are also responsible for 15% of the buffering capacity of the blood because of the weak ionization of their substituent COOH and NH2 groups. At the normal plasma pH of 7.40, the proteins are mostly in the anionic form. 6- Regulation of arterial blood pressure Whole blood is five times and plasma is two times as viscous as water. Blood viscosity is due to red blood cells and plasma proteins. Blood viscosity is responsible partially for the peripheral resistance, which is the resistance that the blood meets during its passage through the peripheral narrow blood vessel. It prevents the rapid outflow of blood through the vessels, thus maintaining normal arterial blood pressure. 7- Transport of important substances Some plasma proteins function as carriers for various hormones, other solutes, and drugs. Plasma proteins and their principal functions are listed in Table 1. Table 1; Major Plasma Proteins