The following is a diagram showing the mammalian circulation of blood.
Plasma is what the blood cells are suspended in. It is a yellowish fluid in which all the cells circulate. It contains other substances like salts, various proteins and blood clotting factors. Plasma is a straw coloured liquid that is 90% water. Microbe-fighting antibodies travel to diseases by attaching themselves to the plasma, which transports the antibodies. Plasma is not constant in composition.
The main types of blood cell are Erythrocytes, red blood cells. Their function is to transport the respiratory gases oxygen and carbon dioxide around the body. The red colour of the erythrocytes is due to the presence of the haemoglobin pigment, which also carries the oxygen. The typical lifespan of the red blood cell is 90-120 days, before they are removed in the liver and spleen by macrophages. “The spleen filters the blood looking for foreign cells and old red blood cells that are in need of replacement. If a person does not have a spleen, they would get sick more often than someone with a spleen.” (www.howstuffworks.com)
Platlets are not true cells but are fragments of cells called megakaryocytes. They are involved in blood clotting. They disintegrate to release thromboplasts, otherwise they survive on an average of 10 days, when they are removed by the liver and spleen.
Leucocytes are white blood cells. They help fight infections and aid in the immune process. Types of white blood cells include Lymphocytes, Monocytes and Granulocytes. Granulocytes comprise of Neutrophils, Eosinophils and Basophils. All white blood cells start in the bone marrow as stem cells. Stem cells are generic cells that can form into many different types of leukocytes as they mature. The stem cells will divide and differentiate into different types of white blood cells. The following are diagrams of leucocytes:
Lymphocytes make up around 30% of the Leukocytes. The cells destined to become B cells develop in the bone marrow before entering the bloodstream. T cells start in the bone marrow but migrate through the bloodstream to the thymus and mature there. T cells and B cells are often found in the blood stream but tend to concentrate in lymph tissues such as the lymph node, thymus and spleen. B cells and T cells both have different functions. When the B cells are stimulated they mature into plasma cells that produce antibodies. When there is a germ present in the body, the B cells can clone themselves to produce millions of antibodies designed to eliminate the germ. The T cells coordinate the immune response, detect viruses in the body and kill them.
Monocytes are the largest of the leucocytes and have a large bean shaped nucleus. They spend a short time, 2-3 days, in the circulatory system before moving into the tissues where they mature into phagocytic macrophages. Macrophages can swim freely and one of their jobs is to clean up dead Neutrophils. They clean up pus as a part of the healing process.
Neutrophils are the most common type of leucocytes. They have short lives of 12-75 hours and are responsible for phagocytosis of microorganisms. They migrate from the blood tissues and as they are so active they are replaced at a rate of 100,000,000,000 per day. Once they are in the bloodstream Neutrophils can move through capillary walls into tissue. They are attracted to unknown material, inflammation and bacteria. For example, if a person gets a splinter or a cut, Neutrophils will be attracted by a process called Chemotaxis. Chemotaxis lets motile cells move toward higher concentrations of a chemical. Once a Neutrophil finds a foreign particle or bacteria, it will engulf it, releasing enzymes from its granules to kill the bacteria. When Neutrophils die they form pus and other cellular debris.
Eosinophils have a double lobed nucleus and granules. They help control the allergic response. Their numbers increase during allergic reactions and in response to some parasitic infections. Basophils have an s-shaped nucleus and secrete large amounts of histamine, which increases inflammation, this helps to keep a balance between blood clotting and not blood clotting.
Blood circulation around the body is continuous and is vital as it provides the body with oxygen so that muscles and organs are able to work. Eventually the components of the blood are used up for protection against bacteria, removed by the liver and spleen or die.
References:
A New Introduction To Biology: Indge, Rowland. Baker
Letts Revise AS Biology: John Parker
