Blood: composition and functions

Blood is the medium in which dissolved gases, nutrients, hormones and waste products are transported. Blood along with the heart and the blood vessels (e.g. veins and arteries) comprises the circulatory system of the body. The circulatory system helps in maintaining balanced conditions within the body (i.e. homeostasis). For example oxygen is picked up by blood as it passes through the lungs- this blood in turn flows through successively narrower blood vessels: from arteries to arterioles and finally the capillaries, where the oxygen rich blood delivers its oxygen to the cells.

Blood is composed of a straw-colored liquid called plasma which contains suspended cells. The different specialized cells found in blood are

• red blood cells
• white blood cells
• platelets

Red blood cells (also called erythrocytes):

These are the familiar discotic shaped cells which make up 99% of the cells in the blood. They are the principal carriers of the red colored hemoglobin molecules. Hemoglobin is an iron containing protein and binds about 97% of all oxygen in the body. We know that oxygen is not very soluble in water hence if oxygen were simply dissolved in the fluid of blood not much could be carried by the bloodstream. Each hemoglobin molecule binds four oxygen molecules to itself- consequently hemoglobin permits human blood to carry more than 70 times the amount of oxygen that it could have carried otherwise. The unique surface shape of red blood cells is nature's design to maximize surface area in order to facilitate absorbtion and release of oxygen. (Given a spherical and a discotic cell of the same volume which do you think has a greater surface area and hence more ability to absorb?)

Not all blood is equal: this fact was learnt the hard way in the mid 1600's when blood from a lamb was transfused into a human with fatal results. The cell membrane of red blood cells contains different proteins, which are responsible for different types of blood. There are primarily two types of proteins found in the cell membrane of red blood cells- protein A and B. Different combinations of these proteins and their antibodies results in four types of blood

• type A : have protein A and antibodies to B protein
• type B : have protein B and antibodies to A protein
• type AB : have both protein A and B but neither of the antibodies
• type O : have neither proteins but have both the antibodies

Type AB is called 'universal acceptor' and type O is called 'universal donor' because of the ability of people with this blood group to accept blood or donate blood to all other blood groups respectively.

White blood cells (also called leukocytes):

The role of blood is not confined to the transportation of materila within the body. The white blood cells are a vital source of defense against external organisms. White blood cells also serve as 'sanitary engineers' cleaning up dead cells and tissue debris that would otherwise accumulate to and lead to problems. There are five classes of leukocytes: neutrophil, eosinophil, basophil, monocyte and lymphocytes.
Many infections stimulate the body to release into the bloodstream large number of protective leukocytes thate are normally held in reserve, causing the white cell number to rise. The increased number of white cells is easily detected in a blood test and is an a very useful first approximation in diagnosis. Some white blood cells may die in the process of fighting against an infection and their dead bodies accumulate and contribute to the white substance that is commonly seen at the sight of infections, usually called 'pus'. Not all infections lead to an increase in the white blood count- the virus that is responsible for AIDS results in a reduced white blood count and hence reduced ability to fight other infections.

Plasma and platelets in clotting:

If damage occurs to a blood vessel, circulating platelets immediately get trapped at the injury site. On accumulating the platelets 'plug' the leak in the vessel providing a first step in damage control. This mechanism is supplemented by 'blood coagulation', or clotting, which is the most important means of defense against bleeding. As mentioned plasma contains several dissolved proteins. Fibrinogen is a rod shaped soluble protein which in the presence of a catalyst thrombin gets converted to an insoluble protein fibrin . Fibrin molecules make a tangled net of fibers by adhering end-to-end and side-to-side which immobilizes the fluid portion of blood (causing it to solidify) and also traps the red blood cells.

Here we see a red blood cell (which actually looks blue in the picture!) snared in the fibrin mesh (thin lines) of a blood clot.

The combined action of the platelets and 'fibrin web' is sufficient to prevent a dangerous loss of blood. In cases where the formation of fibrin and hence formation of a clot is impaired due to some reason (e.g. a genetic disorder as in hemophilia) a person is at great risk of bleeding to death.