This is a wonderful site with some good graphics. http://library.thinkquest.org/C0115080/?c=intro_bld It says:The average human being has 5 liters of blood in his body. 55% of the blood is made up of plasma constituting the fluid part of the blood. The cells and platelets that are present in our blood make up the other 45%. Here is another very good site: http://simscience.org/membranes/advanced/essay/blood_comp_and_func1.html 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 Approximately 90% of plasma is water- blood's solvent with the rest composed of dissolved substances, primarily proteins (e.g. albumin, globulin, fibronogen). Plasma typically accounts for 55% by volume of blood and of the remaining 45% the greatest contribution is from the red blood cells. 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. Hope this helps.

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 Approximately 90% of plasma is water- blood's solvent with the rest composed of dissolved substances, primarily proteins (e.g. albumin, globulin, fibronogen). Plasma typically accounts for 55% by volume of blood and of the remaining 45% the greatest contribution is from the red blood cells. Reference: http://simscience.org/membranes/advanced/essay/blood_comp_and_func1.html