Blood

Although blood is a fluid it must be seen as a connective tissue which consists of a ground substance, blood plasma, and cell elements, blood corpuscles. The only difference with the other connective tissues is that it does not contain fibre elements .

Blood is a sticky fluid with a slightly salty taste. It has a bright red or scarlet colour when it flows from the arteries but a dark red or purple colour when it flows from the veins. It is slightly alkaline (pH 7.4).

Plasma is a yellowish, straw-coloured liquid which consists mainly of water (>90%). The other 10% of the blood plasma consists of dissolved substances of which the following are the most important: Organic constituents (2%) which include nutrients such as glucose, fats, amino acids and vitamins. Inorganic salts and ions which include ions such as bicarbonates, phosphates, sulphates, chlorides, calcium potassium, sodium and magnesium. Secretions such as enzymes and hormones. Dissolved gases such as oxygen and carbon (IV) oxide, i.e. gases involved with respiration. Antibodies which are protective protein compounds. Plasma proteins (7%), the most important of which are fibrinogen, albumen and globulin.

• Plasma transport the various blood types throughout the body.
• It transports food and nutrients from the digestive system to the various tissues in the body.
• It transports waste products from the tissues to the excretory organs.
• Fibrinogen plays an important role in the clotting of blood.
• Blood plasma plays an important role in regulating the body temperature.
• Hormones are transported by the plasma to their target organs where they bring about a specific function.
• Albumen and globulin regulate the water content of cells and extracellular body fluids .
• Globulin also gives rise to antibodies that provide immunity against various diseases.
• The proper balance of ions allows for the normal functioning of nerves, muscles, etc.

In humans there are about 5 million erythrocytes per cubic millimetre of blood. Erythrocytes are small, round, bi-concave discs which float in the blood plasma. They are actually yellowish in colour but when present in large numbers they are red. Each adult red blood cell represents a cell without a nucleus, which is surrounded by a thin, elastic membrane. They are soft, flexible and elastic and therefore move easily through the narrow blood capillaries. Approximately 90% of the content of each erythrocyte is haemoglobin which supplies the characteristic colour of the red corpuscles. Red blood cells are formed in the red marrow of long and flat bones, especially in the spongy regions in the heads of the long bones. The life span of an erythrocyte is approximately 4 months.

• The erythrocytes transport oxygen in the blood from the lungs to all the cells and tissues of the body.
• Red blood corpuscles also assist with the transport of carbon (IV) oxide from the tissues to the lungs.
• They play an important role in regulating the acid-base balance of the blood, thus preventing large changes in pH.
• Erythrocytes also assist when a blood clot is formed.

Leucocytes are far less numerous than red blood corpuscles. Leucocytes are larger than red blood corpuscles and have a definite nucleus. They are irregular in shape, slightly translucent and nearly colourless.They are able to change their shape because of the fact that they move by means of pseudopodia (false feet). Many are phagocytic, i.e. they are able to engulf micro-organisms and foreign intruders into their cytoplasm by flowing around them. There are five types of leucocytes which can be divided into two groups, namely granular white blood cells where the cytoplasm is granular, and non-granular white cells where the cytoplasm does not contain granules. The two principal types of white blood cells are neutrophils and lymphocytes. Neutrophils are the most abundant and are produced in the red bone marrow. Their nuclei are divided to form 3 to 5 lobes, connected by thin threads of nuclear material. They all have conspicuous granules in their cytoplasm. Eosinophils and basophils also have granules in their cytoplasm and irregular-shaped nuclei. Lymphocytes are produced in the spleen, tonsils and lymph nodes and are the smallest of the white blood cells. There are no granules in the cytoplasm but a large spherical nucleus is present.

• Neutrophils are active in phagocytosis and defend us against harmful viruses, bacteria and other foreign intruders.
• Neutrophils also play a role in the healing of wounds and repairing worn out and damaged tissues.
• Neutrophils prevent infections from spreading to other tissues of the body.
• Lymphocytes are involved in the synthesis and distribution of antibodies in the blood. The B-cells are responsible for humoral or antibody immunity. The T-cells are responsible for cellular immunity.

These are small, colourless, plate-like discs. No nucleus is visible. They are not true cells but are cytoplasmic fragments of large cells found in red bone marrow. When tissue is damaged and the platelets leave the blood vessels, they release a substance which transforms soluble fibrinogen in the plasma to a network of fibrin threads.

• Blood platelets play an important role in initiating the process of blood-clotting and in the plugging up and sealing of damaged blood vessels and form tissues.

Composition of Blood

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