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Blood is a specialized body fluid that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells. Approximately 3% of an adult body weight is made up of blood; females have about 4-5 liters of blood while male have about 5-6 liters. Blood has pH of 7.33-7.45 making it slightly basic (less than 7 is considered acidic) while blood is about 4.5 to 5.5 time as vicious of water, indicating that it is more resistance to flow than water. 

 This viscosity is vital to the function of blood because if blood flows too easily or with too much resistance it can strain the heart and cause cardio vascular problems. Blood in the arteries is brighter than blood in the veins because of the higher level of oxygen found in the arteries, but its color is straw yellow.
Blood is composed of blood cells suspended in a liquid called blood plasma; blood plasma which constitutes 55% of body fluid is mostly water (92% by volume) and contains dissipated proteins glucose, mineral ions, hormones, carbon dioxide (plasma being the main medium for excretory product transportation of red cells themselves). Albumin is the main protein in the plasma and it functions to regulate colloidal osmotic pressure of blood. The blood cells are mainly red blood cells (erythrocytes) and white blood cells (leucocytes). (Wikipedia, 2009).

The medical term for blood is hemo - or hemato- (also spelt haemat and haemo form the Greek word (haima) for blood. In terms of anatomy and histology, blood is considered a specialized form of connective tissue, given its origin in the bone and the presence of potential molecular fibers in the form of fibrinogen, (Wikipedia).


          Blood is classified as a connective tissue and consists of two main components, plasma, which is clear extracellular fluid and formed element, which are made up of the blood cells and platelets.

Blood plasma: This is a mixture of proteins, enzymes, nutrients wastes hormones and gases. This specific composition and functions of its component are as follows.

     a-Proteins: They are the most abundant substance in the plasma and plan a part in a variety of roles including clothing, defence, and transportation. 

Collectively they play several functions which are as follows.

-   a-They are important reserve supply of amino acids for cell nutrition; these amino acids are used by other cells to synthesis new products.

     b- Plasma proteins also serve as a carrier for other molecules. Many types of small molecules bind to plasma proteins and are transported from the organs that absorbs these proteins to other tissues for utilization.

       c- Proteins also helps to keep the blood slightly basic at a stable pH, they do these by functioning as weak bases themselves to bind excess H+  ions. By doing so they remove excess H+ from the blood which keeps it slightly basic.

      d- Plasma proteins interact in specific ways to cause the blood to coagulate which is part of the body’s responds to injury to the blood vessels (also known as vascular injury), and help protect against the loss of blood and invasion by foreign microorganisms and viruses.

     e- Plasma proteins govern the distribution of water between the blood and body  fluid by producing what is known as colloidal osmotic pressure.

     There are three main categories of plasma proteins and each individual type of protein has its own specific properties and function in addition to their overall collective role.

  1. Albumins:They are the smallest and most abundant plasma protein in the blood. Reduction in plasma albumin content can result in a loss of fluid in interstitial space (space within the tissue) which may occur in nutritional kidney and liver disease. Albumin also helps much substances dissolve in the plasma by binding to them hence playing an important role in plasma transport of substances such as drugs and fatty acids.  

      2. Globulins: It can be subdivided into three classes from smallest to largest in molecular weight into Alpha, Beta and Gamma globulins. The globulin includes high density lipoproteins (HDL) and alpha -1 globulin and low density lipoproteins (LDL), a beta 1 globulin. HDL function in lipid transport carrying fats cell for use in energy metabolism; membrane construction and hormone function. HDL also appears to prevent cholesterol from invading and settling in the walls of the arteries. LDL carries cholesterol and fat to tissue for uses in manufacturing steroid hormone and building cells membrane, but it also favors the deposition of cholesterol in arterial walls and thus appears to play a role in disease of the blood vessels and heart. HDL and LDL therefore play important role in the regulation of cholesterol and hence have a large impact on cardiovascular disease.

      3. Fibrinogen: This is a soluble precursor of a sticky protein called FIBRIN, which forms the framework of blood clot. Fibrin plays a role in coagulation of blood.

       a-Amino acid: These are formed from the breakdown of tissue proteins or from the digestion of proteins.
       b-Nitrogenous waste: Nitrogenous waste is toxic, it is obtained from the breakdown of substances in the body. These are usually cleared from the blood stream and are excreted by the kidney at a rate that balances their production.

     c-Nutrients: Nutrients absorbed by digestive tracts are transported by the plasma and these include glucose, amino acids, fats, cholesterol, phospholipids, vitamins and minerals.

     d-Gases: Some oxygen and carbon dioxide are transported by plasma. Plasma also contains a substantial amount of dissolved nitrogen.

      e-The most abundant of the electrolyte in the blood is sodium ions which account for more of the blood’s osmolarity than any other solute.

  b.The formed elements: These are so named because they are enclosed in a plasma membrane and have a definite structure and shape. All formed elements are cells except platelets which are tiny fragments of bone marrow cells (Wikipedia).

  The formed elements include the following:

     a.Erythrocyte (red blood cells):  These are minute biconcave disc shaped cells that lacks nucleus at maturity (anucleate). They range from 7-8cm in diameter and have a large surface area.

  Erythrocytes have a life span of 120days (4months) after which they are destroyed in the spleen or the liver. Red blood cell contains the ion pigment (hemoglobin) which gives blood its red color. One cubic liter of blood contain ½ million RBCs (erythrocytes). Erythrocytes functions to pick up oxygen from the lungs and deliver it to tissues elsewhere and pickup carbon dioxide from other tissues and upload it to the lungs.The plasma membrane of matured erythrocytes (RBCs) has glycoprotein and glycolipids that determine a person’s blood type (Wikipedia).
     b. Leukocytes (white blood cells)- These are colorless nucleated cells found in the blood. They are larger than the erythrocyte and are capable of amoeboid movement; white blood cells are produced in the bone marrow and are part of the body immune system. White blood cells ingest bacteria, viruses and dead cells and prevent diseases. They move through blood vessels to the site of injury and form clot, thereby preventing further loss of blood and entry of foreign pathogens.

White blood cells can be divided into granulocytes and agranulocytes, the former have cytoplasm that contains organelles that appears as colored granules through light microscope, hence their name. Granulocytes consist of Eosinophil and Basophils, Neutrophil, in contrast, agranulocyte do not contain granules and consist of Monocytes and Lymphocytes.

    1. Neutrophils: Neutrophils, contains fine cytoplasmic granules. They play a role in the destructions of bacteria and release of chemicals that inhibit or kill the growth of bacterial infection, if the number is significantly high, it indicates early bacterial infection but a gross decrease indicates long lasting bacterial infection. The normal range of neutrophil is 40-75%.

     2. Basophils: Basophils secrete histamine which increases blood flow via dilating the blood vessels and also secrete heparin which is an anticoagulant that promotes motility of other WBCs by preventing clothing. Their nucleus is pale and usually hidden by granules.

     3. Eosinophil: It has a large granule and a prominent nucleus that is divided into lobes. It functions in the destruction of allergens and inflammatory chemicals and release enzymes that disable parasites.


         The granulocytes includes the following

 a- Lymphocytes: - these are usually classifies as small, medium or large. Medium and large Lymphocytes are generally seen in fibrous connective tissue and only occasionally in circulating blood stream.

 It functions in destroying cancer cell and cells infected with by viruses and foreign invading cells, they present antigens to activate other cells of the immune cells, secrete antibodies and serve in immune memory.

 b- Monocyte:These are the largest of the pre-formed element; they have abundant cytoplasm which is easily seen. Monocyte functions in differentiating into microphages which are large phagocytic cells.

 C- Thrombocytes (platelets:- These are tiny fragments of cells responsible for blood (coagulation). Thrombocytes convert fibrinogen into insoluble fibrin which creates a mesh unto which red blood cells are trapped and clothed, which then stops more blood loss and also prevent bacteria from entering the body.

There are about 250,000 platelets per mm3 of the blood: there diameters range from 2-4┬Ám, they survive for about two to five days before being destroyed in the spleen of the liver.

    Platelets secrete vasoconstriction which constricts blood vessels causing vascular spam in broken blood vessels thereby forming platelet plug to stop bleeding. They also digest and destroy bacteria and secrete chemical that attracts neutrophils and monocytes to site of inflammation and also secrete growth factors to maintain the lining of blood vessels (Jelin 2012).


The functions of blood are as follows;

     a- Transport media:- It transports the following sub  stances

   · Gases, namely Oxygen (O2) and carbon dioxide (CO2) from the lung to the rest of the body.

  ·Nutrients: Are transported from digestive tracts and storage sites to all cells where they are assimilated.

  ·Hormone: The blood acts as a vehicle that transports Hormone and enzymes from glands and site of origin to target site.

  ·Heat: It distributes heat produce in the muscles to all parts of the body thereby regulating temperature.

  ·Waste (Excretory products): The blood transports all waste products from cells to the liver and kidney where they are detoxified and eliminated, e.g Urea.

      b-Defense: The blood defends the body from microorganisms and their toxins due to phagocytic action of neutrophils which destroys invading microorganisms. (Jelin 2012).

     c-PH regulation: it regulates the ph of the body within narrow range of 7.35-7.35, making it slightly alkaline; blood with a PH of 7.35 is too acidic whereas, PH above 7.45 is too alkaline.  Blood PH partial pressure of oxygen (PO2) Partial pressure of Carbon dioxide (CO2) and partial pressure of  HCO3 are carefully regulated by a number of hemostatic mechanisms which exert their influences particularly through the respiratory and urinary systems in other to control the acid base balance and respiration (Wikipedia).

       d-  The blood also regulates water balance and blood pressure.

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