Definition of Blood

• Blood is the life maintaining fluid that circulates throughout our bodies. The circulation of blood begins in the heart; it then passes through the arteries to the capillaries, then enters the veins to be sent back to the heart for resending. It is sent to the lungs where it is purified and oxygenated before it starts its journey through the body again. There are four main blood types: A, B, O and AB. These could be either rhesus positive or negative.
  
  

Components of Blood

• Human blood is made of 78 percent liquid and 22 percent solids. There are many components that come together to make up blood. There is the plasma, which is made of red and white blood cells and the platelets. The erythrocytes (red blood cells) move oxygen from the lungs into the body and carbon dioxide from the body to the lungs. The leukocytes (white blood cells) help in combating infections and assist the immune system. These cells are made up of five other components. Platelets (thrombocytes) are the substances that help control bleeding whenever it occurs.
  
  

Other Components

• Apart from the above, which are the normal well-known parts of blood, there are other components that are also found in the blood. These are essentially nutrients and waste products from the metabolic processes of the body. There are fat globules, chemical substances such as carbohydrates, protein and hormones which are being transported to other parts of the body. In addition, there are some gases in the blood. These are oxygen, carbon dioxide and nitrogen. Carbon dioxide is the waste that is generated when the body burns carbohydrates by using oxygen to produce energy.
  
  

Where Blood Cells Are Made

• Blood is produced in the bone marrow which is the soft, spongy material found inside the bones. About 95 percent of all blood cells are produced in the bone marrow. When blood cells are formed in the marrow, they come out as stem cells, which are the initial stage of blood cells. Then the various parts of blood--the red and white blood cells, as well as the platelets develop from there. Some immature blood cells (blasts) remain in the bone marrow until they are mature, while others migrate to the other parts of the body to mature.
  
  

Importance of Blood

• Blood can be donated and transfused into people who need it. This is a very important aspect of medicine today. People who are involved in accidents where they lose lots of blood, those who lose blood during surgeries and generally anemic patients need blood transfusions. About 4.5 million people admitted to the hospital in the United States and Canada need blood transfusions.