Blood |
Adults have an average of 5,5 liters of blood. Blood, even if it looks that way, is not just a liquid. In reality it is a specialised tissue which contains lots of cell types embedded in a substance called blood plasma. These cells, which make up for the 45% of the bloods volume and are produced in the red bone marrow, are the red blood cells, white blood cells, and the platelets. The blood plasma, about 90% of which is water, also contains salt, enzymes, clotting factors, and the proteins albumin and fibrinogen.
Red blood cells are numerous; even a drop of blood contains millions of them. What they do is transfer oxygen to tissues and distance carbon dioxide from them. Mature red blood cells are thicker at their circumference and thinner at the center. Their shape is that way because of the absence of a nucleus. Their cytoplasm contains mainly hemoglobin, a specialised protein responsible for the tranfering of oxygen, which is what makes the blood red. Hemoglobin A, commonly found in adults, consists of two pairs of polypeptide chains.
Red blood cells live for around four months. After that they leave the bloodstream and go to the liver and the spleen to destroy themselves. In the same time though, since their number has to remain approximately the same, more are produced in the red bone marrow. In some cases they are produced in quicker rates, for example in higher altitudes.
White blood cells have a nucleus, are significantly less in number than red blood cells, and play an important in the body's defense mechanism. They are produced in the red bone marrow and types of them include: granulocytes (neutrophils, eosinophils, and basophils), monocytes, and lymphocytes (B cells - responsible for the production of antibodies).
White blood cellls live from a few days up to a few weeks and are 5,000-10,000 per mm3 of blood. In the case of infection the number of white blood cells becomes significantly bigger.
The blood platelets are scattered pieces of cells with a diameter of 2-4μm. They are produced in the red bone marrow, have a life-span of about 5-9 days, and their number usually is from 250,000 to 400,000 per mm3 of blood. Their shape is irregular, they are colorless, and they do not have a nucleus. Lastly, they play an important role in the coagulation of the blood.
ABO blood group system
Blood groups are determined by the presence or not of a specialised antigen on the surface of the red blood cells. Based on these antigens there are four blood groups; A, B, AB, and O.
In the plasma of the atoms of a certain blood type moves the opposite antibody. If the same type of antigen comes close to the same type of antibody this would result in the agglutination of red blood cells, in which case the circulation of blood stops. That follows hemolysis which ultimately results in death.
Rhesus blood group system
For the characterisation of a person's blood type the ABO system isn't the only factor taken into consideration. The rhesus agent is a protein which makes an atom either Rhesus positive (Rh+) or Rhesus negative (Rh-), depending on whether it is present or absent respectively. If this protein gets injected in a Rhesus negative person's blood, then it would cause the production of anti-Rh antibodies.