Blood
Blood performs several vital functions of the blood; hence it has been frequently referred to as the seat of the soul. It distributes nutrients, oxygen, detoxicating compounds, and maintains a uniforms body temperature. It is a carrier of hormones which control and integrate the activities of the various organs. It conveys antibodies and cells that protect the body from invading pathogens and also transport drugs administered to the body so that they may produce their therapeutic effects on specific sites of ailment.
The blood also collects the waste material generated by the body's metabolic activities. The blood can carry out its functions efficiently only if it is kept circulating in the body. This is affected by the heart which acts as a pump, and an intricate system of distributing pipes the blood vessels, viz, arteries, veins and capillaries. The heart and the blood vessels together form the cardio-vascular system.
Blood as a fluid Tissue: -
In human beings there is about 90 ml of blood per kilogram of body weight. However, this volume depends on the age and health of the Individual. Blood has a specific gravity of 1.05 to 1.06, and a pH of 7.4. It is made up of a fluid matrix called Plasma, and several types of cells termed formed elements. These two components separate when blood is centrifuged. The plasma forms 54% of the blood volume and the cells 45%. The latter is reffered to as the hematocrit or packed cell volume(PCV). The plasma contains many inorganic and organic substances. The formed elements consist of red blood cells(RBC) or erythrocytes, white blood cells(WBC) or leucocytes, and platelets or thrombocytes.
PLASMA
Blood plasma is the straw colored fluid obtained when all formed element are removed from blood. The ratio of plasma to formed elements is held relatively constant by the homeostatic controls of the body. The plasma contains inorganic salts with a dominance of chloride (Cl), sodium (Na+), and calcium (Ca++) ions, and lesser quantities of potassium(K+) and magnesium (Mg++) ions. Smaller concentrations of bicarbonate, sulphate, and phosphate, and traces of iron, copper, and iodine are also present.
The Plasma also contain glucose which is used as a principal energy source in cellular metabolism. A variety of other substances, such as amino acid, vitamins, lipids (neutral fats and cholesterol), hormones, urea, uric acid, and other metabolite waste produces are also present in the plasma. In addition, it contains dissolved oxygen, carbon dioxide, and nitrogen. Plasma also reflects certain abnormal conditions occurring in cells and tissue of the body. For example, within few hours following a heart attack, there is a noticeable increase in the level of the enzymes transaminase and lactic dehydrogenase in the plasma arising from the affected heart muscles. This has proved to be of significant value to a clinician is the diagnosis of heart attacks.
The plasma of a man contains about 70g proteins/L. Some of these proteins possess enzymic activity (e.g., phosphatases), While other function shows no activity at all. The function of the plasma proteins can be summarized as follow: -
- Lipoproteins carry lipids and also transport lipid soluble vitamins and steroids hormones.
- They are amphoteric compound because they act both as acids and bases. At the pH of human blood (pH- 7.4) proteins behave as acids and combines with cations such as Na+ and form a buffer, they are however, weak buffers.
- Being macromolecules plasma proteins create a colloidal osmotic pressure in the blood which is higher than the colloidal osmotic pressure in the blood which is higher than the colloidal osmotic pressure in the interstitial fluid. This favors the passage of interstitial fluid into the plasma Finally (4) plasma, proteins, prothrombin and fibrinogen participate in blood clotting.
FORMED ELEMENT
The formed elements consist of the erythrocytes, leucocytes, and the thrombocytes.
ERYTHROCYTES: -
The red blood cells or erythrocytes are small (diameter: -7.5 in man and 9.0 in elephant) biconcave dices. Mammalian erythrocyte, except for the camel does mot possess a nucleus. In contrast, other vertebrates possess a nucleus. The erythrocytes are immobile and move with the flow of blood stream. The average concentration of erythrocytes is approximately 5000000/mm3 of blood in the adult human female. The erythrocytes are reddish in colour due to the presence of a respiratory pigment hemoglobin. Each erythrocyte contains an estimated 200 to 300 million molecules of hemoglobin. The latter serves to transport oxygen and, do some extent carbon dioxide, to and form the lungs. Hemoglobin is easily removed when the cell is broken (or lysed) by exposure to a hypotonic medium, or water. the resulting structure of the erythrocyte is called a "ghost" which consists of a network of protein and lipids.
The average life span of an erythrocyte in man is about 120days the old erythrocytes are continuously breaking down and new once are formed. The erythrocytes are destroyed by the phagocytic macrophages found in the reticulo-endothelial system (in bone marrow, spleen, liver, etc.). The porphyrin section of the hemoglobin is catabolized and formed bile pigment (bilirubin and biliverdin). However, most of the iron is retained in the bone marrow to be again incorporated into new erythrocytes.
LEUCOCYTES: -
The leucocytes are nucleated cells and are somewhat larger than the erythrocytes the white blood cells are also less abundant than the red blood cells. There are 5000 to 9000 leucocytes/mm3 of blood. An increase in the number of leucocytes above 10000/mm3 of blood is known as leukocytosis is caused by violent exercise, infection, hemorrhage, and a high protein diet. Leucopenia results from malnutrition, deficiency of vitamin B12, folic acid, and pyridoxine.
The leucocytes are classified into granulocytes and agranulocytes depending on the presence or absence of granules in the cytoplasm.
Granulocytes are divided into three basic types, viz., neutrophils, eosinophils, and basophils. This classification of the granulocytes is based on the nuclear structure and affinity of the cytoplasmic granules to dyes (acid or base). In the neutrophils the cytoplasm is filled with fine uniformly distributed granules which do not show any affinity to either acid or basic dyes, and hence the name neutrophil. The nucleus is polymorphic divided unequally in 3-5 lobes and separated by deep indentations or by narrow strands of chromatin, therefore neutrophils are also referred to as polymorphocytes. These form 65-75% of the total leucocytes. The size of each neutrophil range from 10-12u. The eosinophils are loaded with granules which are relatively bigger in size than in the neutrophils. The granules show affinity to the acid dyes such as eosin the latter imparts them an intense red colour, and hence the name eosinophils. These are also referred to as acidophils. The nucleus is bilobed and the two lobes are connected by a thin stand of chromatin. The eosinophils form 2-4% of the total number of leucocytes. In the basophil, the cytoplasm takes an intense blue colored with basic dyes. The nucleus in the basophil is irregularly polymorphic. The basophils have the least representation in the total leucocyte count the normal percentage is only 0.15.
Agranulocytes are of two types, viz, lymphocytes and monocytes. The structure of the nucleus of agranulocytes is not polymorphic and is therefore, different form that of the granulocytes. Lymphocytes fall into two categories- small and large lymphocytes. The nucleus of the small lymphocyte is larger and it almost occupied the entire cell leaving only a peripheral cytoplasm, whereas the largest lymphocytes have relatively more cytoplasm. The lymphocytes form the second largest group of leucocytes; the small lymphocytes constitute about 25% and largest lymphocytes about 3% of the total leucocytes. The size of the small lymphocyte is about 7u, Whereas it is 10-12u in the large lymphocytes.
In monocytes the cytoplasm is more abundant in relation to the nucleus; the nucleus is ovoid, kidney shaped or with many indentations. Moreover, characteristically, the nucleus is always placed eccentrically in the cells. The monocytes form 5-7% of total leucocytes. There size range from 10-15u.
The average life span of leucocytes is two days. All leucocytes defend the body against invasion by microorganism which are engulfed and digested by the leucocytes by a process known as phagocytosis. Additionally, the lymphocytes form antibodies against proteinaceous matter gaining entry into the body. The relative ratio of the leucocytes changes during many diseases. This phenomenon is of great use to pathologists in diagnosis of diseases. For example, the concentration of eosinophils increases significantly during infection by worms, and in allergic conditions such as asthma and hay fever.
Leukemia is very serious disorder of the body. This is a form of blood cancer and is characterized by an uncontrolled increase in the number of leucocytes (up to 1000000/mm3 of blood), with the appearance of immature forms in the blood stream. Leukemia is accompanied by anemia and is eventually fatal because the bone marrow cells that normally produce erythrocytes are displaced by leukemia cells.
Thrombocytes or platelets: - In addition to the erythrocytes and the leucocytes, thrombocytes or platelets are another group of a formed the leucocytes, thrombocytes or platelets are another group of formed elements observed in the blood. These are small fragments (2-5u) of cells which lake nuclei, and function in blood clotting. They are oval in shape and have the capability of amoeboid movements. There are 250000 platelets/mm3 of blood. The life span of the thrombocytes is also very short perhaps only a few days in duration.
