Normally, 7-8% of human body weight is from blood. This essential fluid carries out the critical functions of transporting oxygen and nutrients to our cells and getting rid of carbon dioxide and other waste products. In addition, it plays a vital role in our immune system and in maintaining a relatively constant body temperature. Blood is a highly specialized tissue composed of many different kinds of components. Four of the most important ones are red cells, white cells, platelets, and plasma. All humans produce these blood components
Red cells, or erythrocytes, are relatively large microscopic cells without nuclei. In this latter trait, they are similar to the primitive prokaryotic cells of bacteria. Red cells normally make up 40-50% of the total blood volume. They transport oxygen from the lungs to all of the living tissues of the body and carry away carbon dioxide. The red cells are produced continuously in our bone marrow from stem cells. Hemoglobin is the gas transporting protein molecule that makes up 95% of a red cell. Each red cell has about 270,000,000 iron-rich hemoglobin molecules. People who are anemic generally have a deficiency in red cells. The red color of blood is primarily due to oxygenated red cells
White cells, or leukocytes, exist in variable numbers and types but make up a very small part of blood's volume--normally only about 1%. Most are produced in our bone marrow from the same kind of stem cells that produce red cells. Some white cells (called lymphocytes) are a major part of the immune system. Other white cells (called granulocytes and macrophages protect our bodies from infection by surrounding and destroying bacteria, viruses, fungi, or other parasites. They also have the function of getting rid of old, unneeded blood cells as well as foreign matter such as dust and asbestos. The red cells remain viable for only about 120 days before they are removed from the blood and their components recycled in the spleen. Individual white cells usually only last 18-36 hours before they also are removed.
Platelets, or thrombocytes, are cells that clot blood at the site of wounds. They do this by adhering to the walls of blood vessels, thereby plugging the rupture in the vascular wall. They also can release coagulating chemicals which cause clots to form in the blood that can plug up narrowed blood vessels. There are more than a dozen types of blood clotting factors and platelets that need to interact in the blood clotting process. Recent research has shown that platelets help fight infections by releasing proteins that kill invading bacteria and some other microorganisms. In addition, platelets stimulate the immune system. Individual platelets are about 1/3 the size of red cells. They have a lifespan of 9-10 days. Like the red and white blood cells, platelets are produced in bone marrow from stem cells.
Plasma is the relatively clear liquid protein and salt solution which carries the red cells, white cells, and platelets. Normally, 55% of our blood's volume is made up of plasma. About 95% of it consists of water. As the heart pumps blood to cells throughout the body, plasma brings nourishment to them and removes the waste products of metabolism. Plasma also contains blood clotting factors, sugars, lipids, vitamins, minerals, hormones, enzymes, antibodies, and other proteins. It is likely that plasma contains some of every protein produced by the body--approximately 500 have been identified in human plasma so far.
Sometimes when the blood of two people is mixed together, it clumps or forms visible islands in the liquid plasma--the red cells become attached to one another. This is agglutination.
Unagglutinated blood smear
When different types of blood are mixed within the body, the reaction can be a bursting of the red cells as well as agglutination. Different types of blood are recognized on the molecular level and sometimes rejected by being destroyed and ultimately filtered out by the kidneys in order to expel them from the body along with urine. In the case of a transfusion mistake, there can be so much of the wrong type of blood in the system that it can result in kidney failure and death. This is due to the fact that when the kidneys try to filter the blood, they essentially become clogged as they are overwhelmed and cease being effective filters.
The compositional difference between blood types is in the specific kinds of antigens found on the surface of the red cells. Antigens are relatively large protein molecules that provide the biological signature of an individual's blood type.
(not actual shape or size of antigens)
Within blood, there are substances called antibodies which distinguish particular antigens from others, causing bursting or agglutination of the red cells when alien antigens are found. The antibodies bind to the antigens. In the case of agglutination, the antibodies "glue" together the antigens from different red cells thereby sticking the red cells together (as shown below on the right).
Antibodies seeking specific antigens
Antibodies agglutinating red cells (not actual shape or size of antigens and antibodies)
This is not the same thing as clotting. When agglutination occurs, the blood mostly remains liquid. With clotting, however, it does not.
The specific types of antigens on our red blood cells determine our blood types. There are 27 known human blood systems, or groups, for which each of us can be typed. As a result, there is one or more antigens for each of these blood groups. Since many of these blood systems also are found in apes and monkeys, it is likely that they evolved prior to the time that we became a separate species.
White Cell Antibodies
The blood type antigen-antibody interaction is one of many similar recognition-rejection phenomena in our bodies. Infectious microorganisms, such as viruses, also carry foreign antigens which stimulate the production of white cell antibodies (lymphocytes) that attack the antigens by binding to them as a way of getting rid of the invading parasites. Once stem cells in our bone marrow produce antibodies to identify a specific alien antigen, we have the ability to produce them more quickly and in larger numbers. This results in the development of a long-term active immunity to future invasions of the same kind of alien antigen. This is the key to successful vaccination for viruses and some other microorganisms that invade our bodies.
White cell antibodies are also responsible for recognizing and rejecting alien body tissues, or, more accurately, the antigens on their cells. This is the main reason that organ transplants were most often unsuccessful in the past until the creation of drugs that can suppress the immune system and thereby prevent organ rejection. The immune system that is responsible is called the human leukocyte antigen (HLA) system. This is by far the most polymorphic of all known human genetic systems--there are more than 100 antigens on tissue cells in humans resulting in approximately 30,000,000 possible HLA genotypes. The chances of two unrelated people having the same HLA genotypes is very slim. Subsequently, HLA incompatibility between organ donors and recipients are common.
WHOLE BLOOD AND BLOOD COMPONENTS
Blood may be transfused as whole blood or as one of its components. Because patients seldom require all of the components of whole blood, it makes sense to transfuse only that portion needed by the patient for a specific condition or disease. This treatment, referred to as “blood component therapy,” allows several patients to benefit from one unit of donated whole blood. Blood components include red blood cells, plasma, platelets, and cryoprecipitated antihemophilic factor (AHF). Up to four components may be derived from one unit of blood.
Whole blood is a living tissue that circulates through the heart, arteries, veins, and capillaries carrying nourishment, electrolytes, hormones, vitamins, antibodies, heat, and oxygen to the body's tissues. Whole blood contains red blood cells, white blood cells, and platelets suspended in a fluid called plasma.
If blood is treated to prevent clotting and permitted to stand in a container, the red blood cells, which weigh more than the other components, will settle to the bottom; the plasma will stay on top; and the white blood cells and platelets will remain suspended between the plasma and the red blood cells. A centrifuge may be used to hasten this separation process. The platelet-rich plasma is then removed and placed into a sterile bag, and it can be used to prepare platelets and plasma or cryoprecipitated AHF. To obtain platelets, the platelet-rich plasma is centrifuged, causing the platelets to settle at the bottom of the bag. Plasma and platelets are then separated and made available for transfusion. The plasma also may be pooled with plasma from other donors and further processed, or fractionated, to provide purified plasma proteins such as albumin, immunoglobulin (IVIG), and clotting factors.
Red blood cells are perhaps the most recognizable component of whole blood. Red blood cells contain hemoglobin, a complex iron-containing protein that carries oxygen throughout the body and gives blood its red color. The percentage of blood volume composed of red blood cells is called the “hematocrit.” The average hematocrit in an adult male is 47 percent. There are about one billion red blood cells in two to three drops of blood, and, for every 600 red blood cells, there are about 40 platelets and one white cell. Manufactured in the bone marrow, red blood cells are continuously being produced and broken down. They live for approximately 120 days in the circulatory system and are eventually removed by the spleen.
Red blood cells are prepared from whole blood by removing the plasma, or the liquid portion of the blood. They can raise the patient's hematocrit and hemoglobin levels while minimizing an increase in volume.
Patients who benefit most from transfusions of red blood cells include those with chronic anemia resulting from disorders such as kidney failure, malignancy, or gastrointestinal bleeding and those with acute blood loss resulting from trauma or surgery. Since red blood cells have reduced amounts of plasma, they are well suited for treating anemia patients who have congestive heart failure or who are elderly or debilitated; these patients might not tolerate the increased volume provided by whole blood.
Improvements in cell preservative solutions over the last 15 years have increased the shelf life of red blood cells from 21 to 42 days. Red blood cells may be treated and frozen for extended storage (up to 10 years).
Plasma is the liquid portion of the blood — a protein-salt solution in which red and white blood cells and platelets are suspended. Plasma, which is 90 percent water, constitutes about 55 percent of blood volume. Plasma contains albumin (the chief protein constituent), fibrinogen (responsible, in part, for the clotting of blood), globulins (including antibodies), and other clotting proteins. Plasma serves a variety of functions, from maintaining a satisfactory blood pressure and volume to supplying critical proteins for blood clotting and immunity. It also serves as the medium of exchange for vital minerals such as sodium and potassium, thus helping maintain a proper balance in the body, which is critical to cell function. Plasma is obtained by separating the liquid portion of blood from the cells. Plasma is usually not used for transfusion purpose but is fractionated (separated) into specific products such as albumin, specific clotting factor concentrates and IVIG (intravenous immune globulin).
Fresh frozen plasma is plasma frozen within hours after donation in order to preserve clotting factors, stored for one to seven years, and thawed before it is transfused. It is most often used to treat certain bleeding disorders, when a clotting factor or multiple factors are deficient and no factor-specific concentrate is available. It also can be used for plasma replacement via a process called plasma exchange.
Cryoprecipitated AHF is the portion of plasma that is rich in certain clotting factors, including Factor VIII, fibrinogen, von Willebrand factor, and Factor XIII. Cryoprecipitated AHF is removed from plasma by freezing and then slowly thawing the plasma. It is used to prevent or control bleeding in individuals with hemophilia and von Willebrand’s disease, which are common, inherited major coagulation abnormalities. Its use in these conditions is reserved for times when viral-inactivated concentrates containing Factor VIII and von Willebrand factor are unavailable and plasma components must be used. It may also be used as hemostatic preparation [fibrin sealant or fibrin glue] in surgery.
Platelets (or thrombocytes) are very small cellular components of blood that help the clotting process by sticking to the lining of blood vessels. Platelets are made in the bone marrow and survive in the circulatory system for an average of 9–10 days before being removed from the body by the spleen. The platelet is vital to life, because it helps prevent massive blood loss resulting from trauma, as well as blood vessel leakage that would otherwise occur in the course of normal, day-to-day activity. Units of platelets are prepared by using a centrifuge to separate the platelet-rich plasma from the donated unit of whole blood. The platelet-rich plasma is then centrifuged again to concentrate the platelets further.
Platelets also may be obtained from a donor by a process known as apheresis, or plateletpheresis. In this process, blood is drawn from the donor into an apheresis instrument, which, using centrifugation, separates the blood into its components, retains the platelets, and returns the remainder of the blood to the donor. The resulting component contains about six times as many platelets as a unit of platelets obtained from whole blood. Platelets are used to treat a condition called thrombocytopenia, in which there is a shortage of platelets, and in patients with abnormal platelet function. Platelets are stored at room temperature for up to five days.
White blood cells are responsible for protecting the body from invasion by foreign substances such as bacteria, fungi, and viruses. The majority of white blood cells are produced in the bone marrow, where they outnumber red blood cells by two to one. However, in the blood stream, there are about 600 red blood cells for every white blood cell. There are several types of white blood cells; Granulocytes and macrophages protect against infection by surrounding and destroying invading bacteria and viruses, and lymphocytes aid in immune defense.
Granulocytes can be collected by apheresis or by centrifugation of whole blood. They are transfused within 24 hours after collection and are used for infections that are unresponsive to antibiotic therapy. The effectiveness of white blood cell transfusion is still being investigated.
Plasma derivatives are concentrates of specific plasma proteins that are prepared from pools (many units) of plasma. Plasma derivatives are obtained through a process, known as fractionation, developed during World War II, and are heat-treated and/or solvent detergent-treated to kill certain viruses, including HIV and hepatitis B and C. Plasma derivatives include:
Factor VIII Concentrate
Factor IX Concentrate
Anti-Inhibitor Coagulation Complex (AICC)
Immune Globulins, including Rh Immune Globulin
Anti-Thrombin III Concentrate
Alpha 1-Proteinase Inhibitor Concentrate