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Blood is made of  two  components

 

1. A fluid component- plasma
2. Cellular components-  consisting of red blood cells (RBC), White blood cells (WBC), Platelets or thrombocytes

 

Blood is the primary medium for transport of oxygen, nutrients  and hormones to the tissues . It also plays an important role in carrying carbon dioxide and other metabolic wastes away from the tissues.  8% of our total body weight is the circulating blood.  In a man weighing 70 Kg it is nearly 5.6 L. Of this volume 55 % is water and the remaining 45% is the cellular component.

 

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It is a translucent and pale yellow colored fluid. The pale yellow color is due to the presence of small quantities of bilirubin. 90% of plasma is made up of water and the remaining is made up of proteins, electrolytes and small organic molecules. 100 ml plasma contains 7 grams of protein, 900 mg of electrolyte like sodium, chloride, bicarbonate, potassium, calcium, magnesium and phosphate. It also contains organic molecules like 100 mg of glucose, 25 mg of non-nitrogenous waste products including urea,  lipid in the form or triglycerides, phospholipids and cholesterol.

 

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The cellular components

Red blood cells:  RBC 's are the most common cells in blood. The normal RBC count is approximately 4.7 to 6.1 million cells per microliter in males, where as in females it is  4.2–5.4 million cells per microliter.  These are cells with out nucleus and are biconcave in shape. RBC 's  contain the oxygen carrying pigment called hemoglobin.  Red cells have a life span of approximately 120 days; and approximately 1% of red cells are replaced each day. Red cells are involved in transport of  Oxygen from the lungs to the tissues and carbondioxide from the tissues back to the lungs. 

 

The parameters by which RBC 's are usually measured are

 

• Blood hemoglobin (Hg) in grams per deciliter (g/dL),
• The hematocrit (Hct) or packed cell volume(PCV) (volume of RBCs as a percent of total blood volume),
• The RBC count (millions of cells per microliter).

 

The size of red cells is measured as the mean corpuscular volume (MCV), reported in femto liters. The normal MCV is ~80 to 100 fL. Red blood cells that are smaller than 80 fL are called microcytic; those that are larger than 100 fL are called macrocytic.

 

The red cells are mainly produced from the bone marrow of all bone till 5 years of age and after that only flat bones  of the vertebrae, sternum, ribs and hip bones are involved in the production. The production involves various stages and the immediate precursors of matured are called as reticulocytes.

 

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White blood cells

 

Several types of leukocytes, or white blood cells (WBC' s), are found in the blood. WBC are involved in fighting infections affecting the body. The normal WBC count is ~ 4,000 to 10,000/micro liters.

WBC' s are usually divided into granulocytes, which have specific granules, and agranulocytes, which lack specific granules. 

 

Granulocytes are further classified as

• Neutrophils
• Basophils
• Eosinophils

Agranulocytes are classified as

• Lymphocytes
• Monocytes 

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Neutrophils

 

They are the most commonest white blood cells in adults. Two types are commonly observed

1. Segmented neutrophils, also called also called polymorphonuclear neutrophil(PMN or polys). Polys normally comprise ~50 to 70% of total WBC' s
2. Banded neutrophils , these  are an earlier stage than segmented neutrophils but are fully functional. They normally represent ~2 to 6% of all WBC' s; the number of band cells increases with acute stress or infection.

Eosinophils

 

These cells are mainly  involved in defense against parasitic infection. The normal Eosinophil count is ~2 to 4% of total WBC. The number of eosinophils increases in allergic reactions and parasitic infections.

 

Basophils

 

Basophils are the least common type of leukocytes, normally ≤1% of total WBC' s. Basophils appear to be involved in immediate hypersensitivity reactions related to immunoglobulin class E. 

 

Lymphocytes

 

Lymphocytes are the second most common type of WBC' s in adults (~20–40% of WBC). The lymphocyte number is usually  higher in children and they are found to  increases with viral infections.

 

Functionally, there are two main types of lymphocytes:

 

 

1.  B cells
2. T cells.

 

B cells are the primary effectors of the antibody-mediated immune system.   They develop in the bone marrow and are found in lymph nodes, the spleen and other organs, as well as the blood.  After antigen stimulation, B cells may develop into plasma cells, whichare the primary antibody-producing cells.

 

T cells are the main effectors of cell-mediated immunity. T cell precursors originate in the bone marrow but develop and mature in the thymus.  T cells are the command and control cells of the entire immune system as they stimulate or inhibit the function of other cells of the immune system, including B cells, monocytes and macrophages, and other T cells. Normally, the majority of circulating lymphocytes are T cells.

 

 

Monocytes

 

Monocytes normally comprise ~3 to 8% of WBC' s. They have two main function,  Phagocytosis of microorganisms (particularly fungi and mycobacteria) and  antigen processing and presentation. In this role, monocytes are critical in initiation of immune reactions. 

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Platelets (Thrombocytes).

 

The normal platelet count is ~150,000 to 350,000 cells/mL. These cells are involved in preventing bleeding and formation of clots. Platelets have a life span of approximately 10 days. Aged platelets are removed from the blood by the spleen.