The blood is pumped by the heart through the distribution system of the arteries and arterioles, and into the " exchange vessels" that consist of the capillaries and small, non-muscular venules. It delivers and pick up substances that are essential for survival of the human organism. The blood provides for the following.


 Whole blood consists of red and white blood cells and blood platelets ( fragments of megakaryocytes ) suspended in a fluid called plasma.

 The difference between plasma and serum: if no anticoagulant is added, and the whole blood is allowed to clot before centrifuging it, the clear fluid that is found above the clot is not plasma- it is called serum. Serum differs from plasma in lacking the protein fibrinogen ( which is converted to insoluble fibrin in the clot), prothrombin and other factors involved in clotting.

 Plasma consists of 91% water and 9% of other constituents, of which the major component are protein, salts ( sodium, chloride and bicarbonate ions, potassium, calcium, magnesium and phosphate ions) and small organic molecules: include glucose, fatty acids, amino acids.



 Three major classes of cells in blood - red cells ( erythrocytes ), white cells ( leukocytes ) and platelets.

 Red blood cells - their major function is to carry oxygen from the lungs to the tissue


There are five types of white blood cell

1. suppressor T-lymphocytes

2. helper T-lymphocytes

3. cytotoxic T-lymphocytes


In the embryo, blood cells are formed first in the yolk sac by some of the endothelial cells of the vascular system, then this job is taken over mainly by the liver and spleen. In the adult, the bone marrow becomes the predominant site where blood cells are manufactured.

In adults, red blood cells, white blood cells and platelets are formed in the red bone marrow.

Blood cells arise from common progenitor cells within the bone marrow. These common progenitor cells can differentiate into two lines of stem cell.



The differentiation of bone marrow cells needs an adequate supply of two B vitamins: Vitamin B 12 and folic acid. These compounds are needed for DNA synthesis. Lack of vitamin B 12 leads to reduced numbers of erythrocytes.

In addition to the B vitamins, iron is necessary because it is a component of the hemoglobin molecule.


Erythrocytes represent 99% of all cells in the blood, and there are 5 million per cubic millimeter ( this number is called the red blood cell count). Also the erythrocytes have no nucleus.

Red cells are normally quite flexible. They can squeeze through capillaries only half their diameter, then return to their original shape.

Red blood cells look red because they contain a high concentration of the red protein hemoglobin. Hemoglobin consists of an iron-containing organic molecule called heme attached to a protein called globin.

The bone marrow form about 2.5 million red cells per second. On average, each cells has a lifetime of 120 days and will cover a distance of about 250 miles.



The membranes of red blood cells contain a variety of glycolipids called agglutinogens, so called because they are involved in red blood cell agglutination. The most important and best known of these are the A and B agglutinogens. The agglutinogen determines blood type. People have the following blood types, which are inherited:

Blood type A- erthrocytes have only A agglutinogens

Blood type B- erthrocytes have only B agglutinogens

Blood type AB- erthrocytes have both A and B agglutinogens

Blood type O- erthrocytes have neither A nor B agglutinogens


To complicate matters, individuals have antibodies in their plasma against those agglutinogens that are absent from their own erythrocytes. These particular antibodies are called agglutinins. They have many binding sites for agglutinogens, and this promotes clumping or agglutination of red blood cells.

Agglutinins in various blood types

Blood type A - anti -B agglutinins (= antibodies)

Blood type B - anti -A agglutinins (= antibodies)

Blood type AB - no agglutinins (= antibodies)

Blood type O - anti -A and anti -B agglutinins (= antibodies)


Blood types are very important during a blood transfusion. If an individual with blood type A receives a transfusion of blood from a donor of blood type B, then the Anti-B agglutinin(=antibody) in the plasma of the recipient will first agglutinate and cause hemolysis ( via macrophage action) of the B-type red blood cells in the transfused blood. This as dangerous, because products liberated from the hemolyzed red cells can cause kidney damage and death.

Note that the plasma of the b-type donor blood will contain anti-a agglutinins ( = antibodies).

These are capable of agglutinating the recipientís A-type cells. However, the donor plasma

usually becomes so diluted in the recipientís blood that this is unlikely to occur.

Therefore, for a transfusion to be successful, the recipient must lack antibodies (agglutinins) directed against the transfused cells.

Blood type _ _ _ Agglutinins _ _ _ Percent in U.S. _ _ _ plasma agglutinates

_ _ O _ _ _ _ __anti-A & B_ _ _ _ _ 45 _ _ _ _ _ _ _ _ _ _ _ A,B and AB

_ _ A _ _ _ _ __anti-B _ _ _ _ _ _ _ 41 _ _ _ _ _ _ _ _ _ _ _ B and AB

_ _ B _ _ _ _ _ _anti-A _ _ _ _ _ _ _10_ _ _ _ _ _ _ _ _ _ _  A and AB

_ _AB_ _ _ _ _  none_ _ _ _ _ _ _ _  4_ _ _ _ _ _ _ _ _ _ _ _ -------

Type O people are sometimes called universal donors because their red cells have no agglutinogens on their membranes, and therefore cannot be agglutinated by any type of agglutinin when they are transfused into people with other blood types.


Apart from the ABO system, the Rh system is important clinically. it is named after the rhesus monkey, where it was first studied. it is a system composed of many antigens, D being the most important.

If a person is Rh-positive, the red cells are likely to have agglutinogen D. eighty five percent of Caucasians, 93 percent of African-Americans, and over ninety nine percent of Asians are Rh - positive.

For the comparatively few people who are Rh-negative, the red cells will not have agglutinogen D. Rh-negative individuals have anti-D (anti-Rh) agglutinins only if they have ever been transfused or otherwise exposed to D-positive blood.