Blood is a fluid circulating through the tissues of the body, performing the functions of transporting oxygen, nutrients, electrolytes, hormones, vitamins, antibodies, heat and oxygen and carrying waste products to the organs of excretion by flowing through the arteries and veins. It is considered as a type of connective tissue because in it the cellular or formed elements are suspended in a large amount of matrix or the intercellular substance and some of its cells show close affinities with the cells of other types of connective tissues. The blood flowing through the arteries is bright red coloured whereas, that following through the veins is dark red coloured. A normal adult man has about 5 liters of blood flowing in his body. Blood constitutes about 7% to 8% of his total body weight. Its pH is about 7.35 to 7.45 and is has somewhat metallic odour.

Blood differs from the connective tissue proper in the following ways: having the matrix which is a fluid, not having fibers in the plasma, Cells are free floating in the matrix and the matrix is not secreted by the cells.

When the blood is kept in a test tube and is allowed to stand for sometime, two layers are distinctly marked, if prevented from clotting. The upper layer i.e., the fluid protein is known as plasma which is straw coloured. The lower layer is somewhat dense and is dark red coloured, consisting of large number of cells of different types. In other words, a large number of various types of cells (blood corpuscles) are floating or suspended in watery fluid like plasma.

Plasma

Plasma is a straw-coloured fluid and it constitutes about 52% to 62% of blood. It contains about 90% water, about 7% soluble proteins, about 1% mineral salts and about 2% of other substances. The plasma resembles the interstitial fluid in certain details, but the most important point of differences existing between the two is that the plasma contains about 7% proteins whereas, the interstitial fluid has about 2% proteins.

The inorganic salts include the chlorides, bicarbonates and phosphates of sodium, potassium, calcium, magnesium and iron. Out of these salts sodium chloride and sodium bicarbonates form the major constituents. The alkaline nature of plasma is due to the presence of these salts.

Functions of Blood plasma: Plasma serves as the medium for transport of materials to various part of the body and at the same time, it also transports waste products to various sites such as lungs, liver, kidneys and spleen for clearance. Albumin is a major plasma protein, helps in retaining water in the blood stream by its osmotic effect. Albumin also binds to certain other substances that are transported in the plasma and thus, it serves as a nonspecific carrier protein.

It can also act as a source for rapid replacement of tissue proteins. Some globulins, present in the blood plasma, perform important functions by acting as antibodies and the others perform their functions in the transport of lipids, iron or copper in the blood. Fibrinogen is concerned with the coagulation of blood. In addition to these, the plasma proteins also help in maintaining pH of blood by neutralizing the acids and bases.

Blood cells or corpuscles

The plasma contains two main kinds of corpuscles namely, red blood corpuscles or erythrocytes and white blood corpuscles or leucocytes and a third kind, called blood platelets. These blood corpuscles form about 38% to 48% of blood. Out of the total number of cells in the body of a normal adult man, the blood corpuscles constitute about 1/3^rd of it.

(I) Erythrocytes

Shape and size: The erythrocytes (or the Red Blood Corpuscles RBCs) are biconcave and disc like in shape. The biconcave shape of the RBCs provides a large surface-volume-ratio. This is an advantage for carrying more oxygen. The erythrocytes are non-nucleated in all mammals including man, but in camel they are nucleated and oval in outline. In vertebrates, other than mammals, the erythrocytes are oval and biconvex and are nucleated.

Number: The number of RBCs per cubic millimeter of blood is about 5,500,000 in man and about 4,500,000 in woman. Persons living at high altitudes may have about 8,000,000 per cubic million of blood. Erythremia is a disease in which the number of erythrocytes in the blood is greatly increased. The total surface area occupied by all the RBCs of an adult man is about 3820 square meter, i.e. about 2000 times more than the surface area of the body.

Haematocrit: It is volume percentage of erythrocytes in whole blood, i.e., the volume of RBCs per 100 ml of blood. The value ranges from 40 to 54% in man and the average value is 47%. In woman the average value is 42% and the range is 37% to 47%

ESR (Erythrocyte Sedimentation Rate): it is the rate at which the RBCs settle in a column of unclotted blood in one hour. It is a non-specific indicator of inflammatory disease and is increased in anaemia. Its value is 0-15 in normal men and 0-20 in normal women.

Formation of RBCs: Formation of RBCs takes place in the red bone marrow, mainly the vertebrae, ribs, sternum, parts of the cranium and proximal ends of humerus and femur. In the immature condition the RBCs are nucleated, but as they continue to become mature, the nucleus and all extra nuclear organelles, except ribosomes, are extruded out of them. Then the RBCs are passed out into the blood capillaries by the process know as diapedesis. Therefore, when a fresh blood is examined, the erythrocytes appear as yellow-green discs with the center containing no internal structure.

Haemoglobin: An RBC has a typical cell membrane and an internal stoma or framework formed of lipids and proteins to which more than 200 million molecules of haemoglobin remain attached. The haemoglobin constitutes about 95% of the dry weight of each RBC. It is a red coloured respiratory pigment of blood, consisting of haem and a simple protein named globin. Because of the presence of this pigment the cells are named as RBCs and when they are seen in bulk, they impart a red colour. A normal adult man has about 14 to 18 gms of haemoglobin in the RBCs per 100 ml. Polycythemia refers to an increase in the haemoglobin concentration of the blood. It may be caused either due to an increase in the total volume of RBCs in the blood or due to a decrease in the total volume of plasma. A reduction in the quantity of haemoglobin in the blood causes anaemia.

Function of haemoglobin: Haemoglobin combines with oxygen to form oxyhaemoglobin and in this form it carries oxygen to the tissue cells. It also helps in transporting negligible amount of carbon dioxide from the tissue cells to the respiratory surface by combining with CO2 and forming carbaminohaemoglobin. When haemoglobin combines with carbon monoxide, it forms carboxy haemoglobin, which results in carbon monoxide poisoning.

Haemolysis: When RBCs are placed in distilled water, they continue to swell up until they become spherical and finally get disrupted, releasing the haemoglobin into the surrounding medium. This is known as haemolysis.

Life span: In mammals, including man, the RBCs live for about 120 days. At the end of this period, they are removed by macrophases in the liver, spleen and red bone marrow. The bile pigments are formed from the worn out RBCs in the liver cells and these are excreted in the bile to the intestine and are passed out alongwith faces.

Functions of erythrocytes:

1. They help in transport of oxygen to the various tissues of the body.
2. They help in the removal of carbon dioxide from the tissues during respiration.

(II) Leucocytes

The leucocytes or white blood corpuscles or WBCs are small, semitransparent and devoid of haemoglobin. Each leucocyte consists of nonpigmented, colourless cytoplasm, surrounded by a membrane and contains a nucleus, mitochondria, Golgi complex and other organelles. They are far less in number in comparison to erythrocytes, numbering about 5,300 per cubic millimeter of blood. In other words, there are about 600 erythrocytes to every leucocyte. The blood leucocyte count may increase after exercise, during pain, pregnancy and labour and also during many diseases. Leukemia is a malignant disease of the blood forming tissues characterized by large increase in the number of leucocytes in the circulation. In Leucopenia there is an abnormal decrease of white blood corpuscles in the blood circulation.

Leucocytes are formed in the bone marrow, spleen and lymph nodes. All types of leucocytes may be formed in the bone marrow but those formed in the spleen and lymph nodes may become the lymphocytes or monocytes. More ever, those formed in the thymus, become the specific type of lymphocytes called T-lymphocytes. The process of formation of leucocytes is called leucopoiesis. Most of the leucocytes are phagocytic and their shape is changeable and they show a characteristic amoeboid movement by the flowing action of their cytoplasm like an amoeba. Most leucocytes have a short life span.

They can move independently through the intercellular spaces among the tissues and can pass out of the blood capillaries by crawling in between the endothelial cells of the capillary wall into the lymph and tissue fluid. Leucocytes are found everywhere in the body. Their number increases during infection as they act as scavengers and fighters.

Kinds of leucocytes: The leucocytes are classified on the basis of their nuclear structure and dyes immediately accepted by them. There are mainly two types of leucocytes namely granulocytes and agranulocytes.

Granulocytes: Such leucocytes contain granules in their cytoplasm. The nucleus is irregular and lobed or subdivided. These are also known as polymorpho-nuclear leucocytes. Granulocytes are of three types as follows:

1. Neutrophil- The cytoplasmic granules are stained with neutral strains. The nucleus is many lobed. These are much numerous, constituting about 60 to 70% of total leucocytes. They are phagocytic in nature. Their life span is about 2 to 4 days.

2. Basophil – The granules are stained with basic stains. The nucleus is usually with two lobes. These are non-phagocytic in nature. Their life span is about 12 to 15 days.

3. Eosinophil – The cytoplasmic granules are large and stained with acid stains. The nucleus is two lobed. They form about 3% of total leucocytes and non-phagocytic in nature. Eosinophils help in neutralizing the toxins produced by the bacteria. Their life span is about 8 to 12 days.

Agranulocytes: The cytoplasm of WBC is without granules. The nucleus is round in shape. These are produced in the lymphoid tissues such as spleen, lymphatic glands and in the bone marrow. But in the embryo, these are produced in the liver and spleen. Agranulocytes are of three types:

1. Small lymphocytes – These are about 6 to 9 micron wide and contain scanty cytoplasm and a very large nucleus. These constitute 25% of the total white corpuscles.
2. Large lymphocytes – These are about 9 to 15 micron wide, containing abundant cytoplasm and a very large nucleus. They constitute about 3% of the total leucocytes
3. Monocytes: Monocytes are the largest cells of the blood, measuring about 16 to 20 micron in diameter. They are provided with a kidney shaped nucleus and grayish blue cytoplasm. Their life span is about 3 months.

Functions of leucocytes:

1. The leucocytes help in phagocytosis i.e., killing and eating away the foreign bodies such as bacteria etc.
2. They act as scavengers and mobile body guard. They are also known as body’s microscopic policemen.

(III) Blood Platelets

Blood platelets are also called thrombocytes. They are small, round or oval, disc like in shape and non-nucleated. Their number per cubic millimeter of blood is 130,000 to about 400,000. They are formed when cytoplasmic fragments of megakaryocytes are pinched off form the bone marrow into the general circulation. Blood platelets produce an enzyme thrombokinase, required for the process of clotting of blood. In addition to this, they are also known to store and transport several chemicals.

General Functions of Blood

Blood performs various important functions in the vertebrates such as:

1-     transport of oxygen, carbon dioxide, nutrients, waste products and hormones

2-     Protection from harmful bacteria, protozoa, viruses and other foreign substances

3-     Protection from the excessive loss of blood by coagulation

4-     Regulation of the body temperature in warm blooded animals

5-     Regulates the pH and water content of the tissues.

Lymph: The lymph is a portion of blood in composition. This a clear and colourless fluid containing white blood corpuscles, few blood proteins, excretory matters like carbon dioxide, excess amount of water and digested food substances. When the tissue fluid enters the lymph vessels, it is called as lymph.