Blood is a mixture of cells and a watery liquid, called plasma, that the cells float in. It also contains other things like nutrients (such as sugar), hormones, clotting agents, and waste products to be flushed out of the body.
There are three kinds of cells in the blood: red blood cells, white blood cells, and platelets. Red blood cells carry oxygen from the lungs throughout the body, white blood cells help fight infection, and platelets help in clotting.
Red blood cells (also called erythrocytes) are the most numerous, making up 40-45 percent of one's blood, and they give blood its characteristic color. Red blood cells are shaped like tiny doughnuts, with an indentation in the center instead of a hole. They contain a special molecule called hemoglobin, which carries the oxygen. In the lungs, where there is a lot of oxygen, the hemoglobin molecules loosely bind with oxygen. Each molecule of hemoglobin contains four iron atoms, and each iron atom can bind with one molecule of oxygen, allowing each hemoglobin molecule to carry four molecules of oxygen. In the capillaries, where there is little oxygen, the hemoglobin readily sheds the oxygen it is carrying and allows it to be absorbed by the body's cells. The iron in hemoglobin is what makes blood red.
A diagram of blood cells.
White blood cells (leukocytes) are the body's mobile warriors in the battle against infection and invasion. There are three types of white blood cell: granulocytes, lymphocytes, and monocytes. There are, in turn, three kinds of granulocyte: neutrophils, eosinophils, and basophils. (Granulocytes are called that because they contain granules that hold digestive enzymes.) Neutrophils kill invading bacteria by ingesting and then digesting them. Eosinophils kill parasites, and are involved in allergic reactions. Basophils also function in allergic reactions, but are not well understood.
Lymphocytes are key parts of the body's immune system. There are two kinds of lymphocyte: T cells and B lymphocytes. T cells direct the activity of the immune system. B lymphocytes produce antibodies, which destroy foreign bodies. Monocytes, the largest kind of white blood cells, enter the tissues of the body and turn into even larger cells called macrophages. These eat foreign bacteria and destroy damaged, old, and dead cells of the body itself.
The blood cells called platelets (thrombocytes) help blood to clot, in several different ways. When bleeding occurs, platelets clump together to help form a clot. Also, when they are exposed to air (as they would be by a wound), platelets start breaking down and release a substance into the bloodstream. This substance starts a chain of chemical events that eventually causes a protein in the blood, fibrinogen, to turn into a different substance, fibrin, which forms long threads. These threads tangle up red blood cells to help form a clot, or scab, over the wound.
In their "resting" state, platelets look like two plates stuck together (hence the name). When "activated" and helping to form a clot, they change shape and look like tiny roundish blobs with tentacles. At only two to three microns, they are the smallest kind of blood cell.
Plasma is a clear, straw-colored liquid that carries the blood cells and various hormones, nutrients, and so on through the body. It makes up a little more than half of the total blood volume. Plasma is about 90 percent water. Much of the other ten percent comprises various kinds of protein molecules, including enzymes, clotting agents, immunoglobulins (part of the immune system), and proteins that carry hormones, vitamins, cholesterol, and other things the body needs. Plasma also contains sugar (glucose) and electrolytes like sodium, potassium, and calcium, as well as other things like the aforementioned hormones, vitamins, and cholesterol.
How is blood Produced
Blood cells are produced in the bone marrow, a jellylike substance inside the bones that is composed of, among other things, fat, blood, and special cells that turn into the various kinds of blood cells. In children, the marrow of most of the bones produces blood. But in adults, only the marrow of certain bones -- the spine, ribs, pelvis, and some others -- continues to make blood. Bone marrow that actively produces blood cells is called red marrow, and bone marrow that no longer produces blood cells is called yellow marrow.
All blood cells come from the same kind of stem cell, which has the potential to turn into any kind of blood cell. These stem cells are called pluripotential hematopoietic stem cells.
As the blood cells develop from the stem cells in the marrow, they seep into the blood that passes through the bones and on into the bloodstream. The different kinds of blood cells have different "life spans" -- red blood cells last about 120 days in the bloodstream; platelets about 10 days; and the various kinds of white blood cells can last anywhere from days to years.
The body has a feedback system that tells it when to make new red blood cells. If bodily oxygen levels are low (as they would be if there are too few red blood cells circulating), the kidneys produce a hormone called erythropoietin, which stimulates the stem cells in the marrow to produce more red blood cells.
How does circulatory systems works ?
It says in the Book of Genesis that the life lies in the blood, and one can see why. Blood carries oxygen and nutrients to every cell of the body, carries away waste products, and plays a central role in the body's defense against intruders.
The circulatory system keeps approximately five liters (for the average adult human) of blood circulating constantly through the body. Its most important organ is the heart, that double pump that forces the blood through the blood vessels. The body's circulatory system really has three distinct parts: circulation through the lungs (pulmonary circulation), the heart (coronary circulation), and the rest of the system (systemic circulation).
Pulmonary circulation, in which blood goes from the heart to the lungs and back to the heart, is when the blood exchanges carbon dioxide, a waste product it has picked up from the rest of the body, for oxygen, which it will carry to the rest of the body. The de-oxygenated, carbon-dioxide-laden blood returning to the heart from the body enters the right side of the heart through two veins (one for blood coming from the lower half of the body, the other for blood from the upper half). The heart pumps it to the lungs, where it drops off carbon dioxide to be excreted (exhaled), and picks up oxygen. The blood then returns to the left side of the heart, which pumps it out to the rest of the body. The flow of the blood from the heart to the rest of the body and back to the heart is systemic circulation. The heart pumps the now oxygen-rich blood out through the arteries, which are wide, thick-walled blood vessels. Then the blood moves into smaller vessels called capillaries, where it releases the oxygen and nutrients it carries to the body's cells. At the same time, it picks up waste products like carbon dioxide. The blood then flows into the veins, which carry it back to the heart.
During systemic circulation, blood flows through the kidneys, in a phase called renal circulation. The kidneys filter out waste products like urea and excess salt, which will be excreted in the urine.
Blood also passes through the small intestine, in a phase called portal circulation. It picks up sugars from digested food and then collects in the portal vein, which passes through the liver. The liver filters sugars from the blood to store for the body to use as energy later.
Like any other organ, the heart itself needs blood to supply it with nourishment and carry away wastes. The movement of blood through the capillaries of the heart for these purposes is called coronary circulation.
What to expect when donating ?
Every two seconds someone needs blood. Most of us only have to look as far as our circle of friends and neighbors to find someone whose life was touched by the generous gift of a volunteer blood donor. Donating blood for the first time, though, can be intimidating. What is the process? What should you expect? How will you feel afterward?
When you arrive at the blood center, you will fill out a form answering a series of questions designed to determine whether it is safe for you to donate, and for someone to receive your blood. Questions relate to health issues and behavioral risks, including intravenous drug use. We understand that some of these questions may be of a sensitive nature, so you will meet privately with a trained staff member to review these questions and discuss any concerns you may have. We treat this information in a highly confidential manner.
You also will be given a "mini-physical." A trained health historian will check your blood pressure, pulse, temperature and test your red cell count by taking a small sample of blood from your finger.
A blood donation in progress.
After you have completed the health history, you will move on to the actual donation. During your whole blood donation, you'll give about one pint of blood. Your body will replace the lost fluids within 24 hours. You'll feel just a slight pinch when the needle is inserted. A collections staff member will stay with you during the entire donation process. Usually the needle is in your arm only 8 to 10 minutes. All of the needles and supplies are used only once and then discarded. It is not possible to contract HIV or any other disease through donating blood.
Most people experience no side effects from donation, and aside from avoiding strenuous exercise and heavy lifting, you can go about your normal routine once you leave.
Be sure to drink plenty of fluids before and after donating blood. It's also important to remember to eat foods high in iron content between donations. You will be eligible to donate whole blood again in 56 days. The entire process takes about an hour, and each donation has the potential to save three lives. Can you think of a better way to spend an hour?
Despite the fact that giving blood is easy, safe, and takes less than one hour, only five percent of eligible Americans donate blood. To donate blood, one must be healthy, at least 17 years old, and weigh 110 pounds or more.
At Blood Center
Once you have donated blood, blood centers begin a rigorous process to ensure that your donation is made ready for a recipient in need.
If your blood has been collected by the American Red Cross, for example, it is sent to a Red Cross testing and processing facility. Each whole blood donation is typically separated into three components: red cells, platelets, and plasma.
Red cells, which carry oxygen, will help patients who are anemic due to blood loss. Usually these must be transfused within 42 days, unless frozen.
Platelets are cell fragments needed by cancer and other patients who may experience bleeding because they are unable to produce enough platelets on their own. These can only be stored for five days.
Plasma is the liquid portion of the blood. It carries nutrients, removes waste, transports antibodies, and provides coagulation factors to help the blood clot. Plasma may be frozen for transfusion, frozen for storage, or it can be used to extract cryoprecipitate, a substance rich in Factor VIII -- which is needed by patients with classic hemophilia.
Refrigerated blood.
Splitting your blood donation into these components means that your single donation has the potential to save as many as three lives!
As part of the donation process, four sample tubes of blood are collected, labeled, and sent to one of our nine state-of-the-art National Testing Laboratories. Our laboratory technicians determine your blood type (A, B, AB, or O) and Rh factor (positive or negative). They also perform up to 12 different tests on each sample of blood. Every unit of blood is screened for infectious diseases such as HIV, syphilis, and hepatitis B and C, as well as for unexpected antibodies that might cause a transfusion reaction in some people.
Each American Red Cross blood donation also undergoes nucleic acid testing (NAT), a highly sensitive testing method to detect both HIV-1 and hepatitis C before the body has begun to produce antibodies.
The test results are then sent via computer to the facility where your blood is being stored. Your unit of blood is tracked by the bar code placed on the bag and the Blood Donation Record when you came to donate blood. By electronically reading the barcode on the bag, the computer will print test results and a label with the blood type for that donation. This computerized process helps us diminish human error and enables us to track each unit of blood.
Each unit of blood is kept at the appropriate temperature until it needs to be shipped to a hospital. Many hospitals receive routine shipments of blood, but Red Cross distribution centers are staffed 24 hours a day in order to meet emergency needs. At the hospital, blood components are cross-matched with patients' blood. A patient may receive your blood donation only 48 hours after your donated it!
Every two seconds, someone in this country needs blood. Volunteer blood donations help patients being treated for accidents, routine surgeries, and serious diseases such as leukemia, lymphoma and other cancers, heart disease, sickle cell anemia, and hemophilia.
Blood Typing
Blood transfusions were not possible until Karl Landsteiner first identified the major human blood groups -- namely O, A, B, and AB -- in a series of experiments in 1901 that earned him the Nobel Prize. (At the time, Landsteiner identified only groups A, B, and O; further analysis, two years later, revealed group AB.)
The ABO blood groups are defined by specific inherited molecules, or antigens, that are present on the surface of red blood cells. Thus, one inherits either A or B antigens (group A or B), both A and B antigens (group AB), or neither antigen (group O). Conversely, a person develops a natural immunity, or antibody, in their plasma to the ABO antigens that are absent on their own red cells. Thus, a group A person has anti-B antibodies, and group O person has both anti-A and anti-B antibodies.
If group A red cells are mistakenly transfused to a group O recipient, for example, the anti-A antibody in the recipient's plasma destroys the transfused group A cells and a serious transfusion reaction occurs. Because group O has anti-A as well as anti-B antibodies, group O recipients can only accept blood from group O donors. Conversely, group AB recipients can receive blood from all groups.
There are many other antigens on the red cell surface. The most important is the Rh factor. A person is defined as either Rh positive or Rh negative depending on the presence of the primary Rh antigen on the red cell. In contrast to ABO antigens, however, a person only develops anti-Rh after exposure to Rh-positive red cells through transfusion or pregnancy. Modern blood-banking technology uses highly sensitive tests to properly identify and match blood between donor and recipient.
The most common blood types in the U.S. are A positive and O positive -- about 72 percent of the population has one or the other. AB negative is the rarest blood type (1 percent of the population
