Structure & Function of Blood and Lymph

The components of blood

Blood is the main medium of transport within the body and consists of a liquid phase called plasma in which are floating cells and formed elements.

About 55% of the blood volume consists of a straw coloured liquid called plasma, the other 45% being made up of cells. The cells are either erythrocytes (red cells) or leucocytes (white cells). In addition there are

also platelets (thrombocytes) present which are fragments of cytoplasm.

Blood has several major functions:

1. Transport

2. Defence

3. Formation of lymph and tissue fluid

4. Homeostasis

 

Blood plasma

The composition of plasma e.g. its pH, salt concentration and osmotic pressure are precisely regulated by the kidney. This helps to provide an optimum environment for the cells which are bathed in tissue fluid, itself derived from blood. Plasma contains: Plasma proteins e.g. albumins, globulins and clotting factors, such as fibrinogen. Albumins and fibrinogen are made by the liver and secreted into the blood, while the gamma globulins are made by plasma cells (activated lymphocytes) already present in the blood and lymphatic system. The gamma globulins are antibodies which protect against disease organisms.

Albumins are the most abundant of the plasma proteins and are responsible for much of the osmotic pressure of blood, therefore holding water in the blood, thus maintaining blood pressure and volume. Albumins and some globulins also act as carrier proteins for substances which are insoluble in plasma. For example, the hormone thyroxine is transported from the thyroid gland by attaching onto a specific globulin and cholesterol is carried on lipoproteins (protein plus lipid units). Absorbed digested food products such as glucose, excretory products such as urea, salts such as sodium chloride and many hormones are dissolved and carried in the plasma.

Erythrocytes

Erythrocytes are little more than packets of the red coloured protein, haemoglobin, surrounded by a cell membrane. The nucleus is lost during differentiation in the red bone marrow so that a mature red cell is shaped like a biconcave disc. This disc shape gives it a large surface area in relation to volume, as required for efficient gas exchange. The shape can also be distorted to enable cells to squeeze through capillaries and sinusoids as narrow as 6 micrometres in diameter. Thus the passage of red cells is slowed up allowing more efficiency in gas exchange in the capillaries.  The respiratory pigment, haemoglobin, allows the blood to carry enough oxygen to meet the body’s needs, buffers the blood pH to between 7.2 and 7.6, and carries some waste carbon dioxide. The red cells also contain the enzyme carbonic anhydrase, which is involved with blood carbon dioxide transport in the form of hydrogen carbonate ions.

In order to synthesise haemoglobin when red blood cells are made in the red bone marrow, there must be a supply of iron, vitamin B12 and folic acid. A lack of any of these results in some form of anaemia. Before birth, red cells are also formed in the fetal liver and in the placenta. As red cells age they become fragile and eventually rupture when squeezing through the narrow blood sinusoids of the liver and spleen. Eventually, they are broken down in the liver.

Leucocytes

There 5 structures of leucocytes.. The granulocytes have granules in their cytoplasm and multilobed nuclei (i.e. they are polymorphonuclear), whereas the agranulocytes have no cytoplasmic granules and have either an oval or horseshoe shaped nucleus. The granulocytes and monocytes can all move through the tissues by moeboid action. Lymphocytes cannot, since they do not have enough cytoplasm to form pseudopodia. They move passively with blood and tissue fluid flow.

 

Intercellular fluid or lymph is derived from blood plasma but contains virtually no protein, since the plasma proteins are too large to escape through the capillary walls. Thus they are retained within the blood where they exert an osmotic pressure. Lymph is formed at the arterial end of the capillary bed where it is forced out of the capillaries by a relatively high blood pressure of around 4.8 kilopascals. This carries glucose, salts, other nutrients, oxygen and hormones to the cells. The proteins, retained in the blood plasma, exert an osmotic pressure of around 4.3 kPa which tends to draw some of the water of the lymph back into the blood, thus concentrating the lymph. The effective filtration pressure forming lymph is thus 4.8 - 4.3 = 0.5 kPa.

The lymph then percolates around the cells exchanging oxygen and nutrients for waste products, such as carbon dioxide and urea. The lymph in this situation is known as tissue fluid. Much of the lymph returns to the blood plasma at the venous end of the capillary bed. Here the blood pressure has fallen to around 3.9 kPa, but the osmotic pressure of the plasma proteins remains around 4.3 kPa. Thus the net uptake pressure for drawing lymph back into the capillaries is 4.3 - 3.9 = 0.4 kPa. Since the formation pressure was 0.5 kPa, this means that some of the lymph formed cannot get back into the capillaries at the venous end. This surplus lymph is collected up into open ended vein like vessels called lymphatics. These eventually all join up and empty the lymph into the blood at the left subclavian vein. The lymph vessels contain valves to prevent backflow of lymph which is moved along by the pressure of surrounding muscles and other organs.

Along the lymph vessels, at intervals are small bodies called lymph nodes, through which the lymph drains. Here phagocytes can engulf any bacteria or other debris in the lymph. The lymph nodes also contain and produce lymphocytes, which can be released into the lymph.

Blood group systems

The ABO system concerns the presence or absence of two antigens (actually called agglutinogens since they can agglutinate or clump together the red cells). These agglutinogens are proteins A and B which are situated

on the surface of the red blood cells. The presence or absence of agglutinogens is genetically controlled. Plasma contains specific antigens (called agglutinins) which are also determined genetically. Agglutinin A will cause the clumping together of red cells which contain agglutinogen A. Agglutinin B will clump together red cells which contain agglutinogen B. You do not possess agglutinins against your own red cells but your plasma does contain agglutinins to attack any foreign agglutinogens. Thus, Group A people possess agglutinin B in their plasma and Group B people possess agglutinin A. Thus in blood transfusions it is important to match the bloods correctly so that the recipient’s agglutinins (in their plasma) will not agglutinate the donor’s red cells. If this occurs the clumped red cells block small blood vessels, such as the glomeruli in the kidney, resulting in death. Group AB people have no antibodies and so can, in theory, receive blood of all groups. Thus they are called ‘universal recipients’’. Group O blood can, in theory, be given to all recipients, and thus Group O people are referred to as ‘universal donors’. In transfusion, the agglutinin in the donation is so diluted by entry into the whole blood volume of the recipient, that it does not cause any agglutination problems.

The rhesus system was first discovered in rhesus monkeys but also occurs in humans. People who are rhesus positive possess the rhesus agglutinogen on their red blood cells and make up about 85% of the UK population. 15% of the population have no agglutinogens and are called rhesus negative. Under normal circumstances the plasma does not contain anti rhesus agglutinins (anti D) which only develop if the immune response is invoked by the introduction of rhesus positive cells into a rhesus negative person. This could happen if a rhesus negative person receives a transfusion of rhesus positive blood in error. This is not serious on the first occasion, but if it occurs twice, because the first transfusion sensitised the immune system to the rhesus agglutinogen, a quick serious agglutination reaction occurs.

 

Blood clotting

Excess blood loss is prevented by haemostasis. Damaged blood vessels immediately constrict, decreasing blood flow and loss. Damage to the endothelium of the vessel exposes collagen fibres. Platelets that touch this collagen become large and sticky and rapidly form a plug to cover the exposed area. Clotting then occurs as follows:

1. Damaged tissue cells and platelets release the enzyme thromboplastin.

2. Thromboplastin converts the inactive plasma protein prothrombin into the enzyme thrombin. Ca2+ ions are needed as a cofactor for this reaction.

3. Thrombin converts the soluble plasma protein fibrinogen into insoluble fibrin.

4. Fibrin plugs the damaged vessel.

In the hours and days after clotting the fibrin threads contract and cross bond to pull the walls of the damaged vessel closer together and to retract (shrink) the clot. More fibrin may be added as this ruptures more platelets, and so the clot thickens and hardens to become the scab. As the clot retracts, a yellowish fluid called serum is forced out of it. Serum is blood plasma minus the clotting factors. The scab acts as a scaffold on which the tissue can be repaired. Fibrinolysis is the dissolving of the clot once tissue repair is complete. Scabs on the body surface can fall off, but those deep in the body are broken down by enzymes called plasmins which are formed from plasma proteins known as plasminogens.