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History of Blood
A Roman doctor named Galen believed that blood was formed in the liver and was unaware that the heart pumped blood through the arteries and veins. It wasn’t until the 1600’s that it was discovered blood circulates throughout the body, pumped by the heart.
White Blood Cells Defend the Body Against Disease & Infection
White blood cells are the body’s defense system. There are three types of white blood cells: granulocytes, lymphocytes and monocytes. They all fight infection from bacteria, viruses and microbes that can cause diseases.
Blood Circulation & Respiration
One complete round trip will take, on average only 30 to 45 seconds. And, even less during exercise. The human body has so many miles of blood vessels inside of it that they could encircle the earth twice, then a little bit more.
Platelets & Blood Vessel Repair
Platelets are small pieces of cell material, or cytoplasm, whose job it is to plug holes in vessel walls. They look like two plates stuck together, but when they are helping to form a clot, they change shape. In the event of a tear inside a blood vessel wall millions of platelets respond to the injury, throwing themselves over the cut. They stick to the wound's edges and to each other to form a plug that slows the loss of blood within three to five minutes.
Blood Types & Activity
There are four different human blood groups: O, A, B and AB. Groups A and B have certain inherited antigens, or markers, on their red blood cells. The most common blood types in the United States are A Rh positive and O Rh positive… about 72 percent of the population is one or the other. The rarest blood type is AB Rh negative.
Stem Cells
All of the blood cells in your body are produced in your bones, inside the bone marrow.
Bone marrow looks like a network of tiny little connected caves, similar to a honeycomb. Inside, are some very special parent cells called stem cells. A stem cell can divide itself and produce a twin. This process of cell division called mitosis. Through mitosis, the stem cell can keep on creating more and more stem cells just like itself.
Process of Cell Division or "Mitosis"
A stem cell can develop into all the other different blood cells as well. The stem cell can actually differentiate into red cells, white cells and platelets. Inside the cell, a structure called the nucleus acts very much like a computer program.
It directs the cell to produce a special protein called hemoglobin. And it's this hemoglobin that makes red cells look red and gives them the ability to attract and transport oxygen.
After the red cell is full of hemoglobin, the job of the nucleus is done and it gets kicked out. Then, the mature red cell has a little dip in its middle on both sides.
At this point, the red blood cell leaves the bone marrow and begins to circulate in the bloodstream.
Through mitosis, stem cells can also become many different kinds of white cells.
White cells are an extremely important part of the body's immune system because they guard the body against infections and diseases.
Stem cells can also become many platelets. Platelets are extremely important in helping blood clot when a cut is made. The stem cell turns into a factory cell called a megakaryocyte. This is a very large cell with several nuclei.
The megakaryocyte never leaves the bone marrow, but does produce many, many fragments. These fragments are actually the platelets: small pieces of cell material or cytoplasm. And they do leave the bone marrow and circulate freely in the bloodstream.
In addition to continuing to produce all the cells in your own body, stem cells are also extremely important in medicine and research.
People who need a bone marrow transplant because they are ill with a disease such as leukemia or cancer, may receive new stem cells from the healthy bone marrow of a volunteer donor through a transfusion. When a bone marrow transplant does take place, it is only the liquid part of the marrow that is donated, not the bone. It is hoped that with this treatment, their own bodies will begin producing a healthy variety of blood cells again.
Red Blood Cells
The scientific name for red blood cells is Erythrocytes. They are formed in the bone marrow and are created by a Stem Cell. Red cells are the most numerous of all blood cells in the blood. They are produced at a rate of 4-5 billion every hour in an adult human.
When a red cell matures, it ejects its nucleus before entering the bloodstream. It looks like a doughnut, but without a hole in the middle. Red cells are 7 to 8 microns in diameter. Yet, they are the heaviest particles in the blood.
Red blood cells contain hemoglobin, a protein that carries oxygen. Oxygen is known as O2. Each time we take a breath in, we are inhaling oxygen in the air.
The role of red cells is to absorb oxygen through the little alveoli in the lungs and deliver it to all the muscles, tissues and organs in the body.
To do this, they travel through large arteries and tiny capillaries. Sometimes the capillaries are so small, the red cells have to squeeze and bend themselves in half to get through in order to release their load of oxygen.
That's only half the trip. After they deliver the oxygen, the red blood cells pick up a waste product called carbon dioxide, known as CO2. Then they make the return trip back to the lungs through the veins where the CO2 can finally be released. The body eliminates carbon dioxide every time webreathe out! Then, the red blood cells start the trip all over again. It takes, on average only 30-45 seconds for blood cells to make one complete circuit through the body. Red blood cells have a life span of about 120 days. Then they die and are removed from circulation by an organ called the spleen.
Platelets
Platelets are another important part or component of your blood. Platelets are sticky little pieces that help prevent bleeding and make a blood clot when a cut is made in the vessel wall. The picture above shows a great view of platelets using an electron microscope.
When a stem cell decides to make platelets, it turns into a factory cell called a megakaryocyte. This is a very large cell with several nuclei. The megakaryocyte never leaves the bone marrow, but it does produce many tiny fragments. These fragments are actually the platelets - small pieces of cell material or cytoplasm.
Platelets do leave the bone marrow and circulate freely in the bloodstream. Normally, platelets look round and smooth, but when they begin to work to plug up they become spiky and ragged around the edges. When an injury occurs to a blood vessel wall, the platelets respond by attaching themselves over the cut to form a temporary plug within minutes slowing the loss of blood.
The platelets also attract a protein found in plasma called fibrin and use it to form a dense netting that traps red blood cells and quickly becomes a clot.
From the outside of a cut, you can see the scab that forms over the wound. It looks hard and crusty. As long as there is still an unhealed hole in the blood vessel wall, the clot is constantly being formed, dissolved and reformed by fresh platelets so that bleeding is prevented. When the wound is finally healed by new cells growing over it, the clot will be cleared away and blood will begin to flow through the vessel again.
Plasma
All of the blood cells in the body are mixed together in a slightly yellowish liquid called Plasma. Plasma is mostly made up of water, but also contains proteins, sugars and salt. In addition to carrying blood cells throughout your body, plasma also carries hormones, nutrients and chemicals, such as iron. Plasma has the important function of maintaining the pH of the blood at approximately 7.4.
Blook Types
In 1901, scientist Karl Landsteiner, reported that blood could be classified into blood "types." By matching these blood types, a successful blood transfusion could be made between a healthy donor and a patient in need of blood replacement due to an injury, disease or surgery. A blood transfusion can either be whole blood, or a blood component, such as red cells or platelets or plasma.
Blood types are based on specific proteins called antigens that are found on the surface of red blood cells, and antibodies found in plasma. Antibodies can recognize markers on foreign cells (those that are not the body's own cells). When the blood of two people mixes during a transfusion, the antibodies will act against any cells bearing the wrong marker.
There are four basic blood groups:
Group A with A antigen on the red cells and anti-B antibodies in the plasma.
Group B with B antigen on the red cells and anti-A antibodies in the plasma.
Group AB with both A and B antigens on the red cells and neither anti-A nor anti-B in the plasma.
Group O with no A or B antigens on the red cells and both anti-A and anti-B antibodies in the plasma.
If you are a Group A person, you do not carry antibodies against A markers. But you do have antibodies against Group B blood.
If you are a Group B person, you have antibodies against Group A cells.
If you are a Group O, you have antibodies against both Group A and B!
The antibody reaction that occurs when two different blood groups are mixed, causes the foreign red cells to be destroyed (hemolysis). This can lead to kidney damage and death. That is why matching blood groups between donor and patient is so important before a transfusion is given.
参考资料：http://www.mybloodyourblood.org/hs_biology.htm