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Biology of Blood
Blood is a complex mixture of plasma (the liquid component), white blood cells, red blood cells, and platelets. Blood together with the vascular system (arteries, veins and capillaries) and the heart form the circulatory system of the body. Blood performs various essential functions as it circulates through the body. The human body contains about 5 litres of circulating blood and during this circulation, oxygen and essential nutrients (such as fats, sugars, minerals, and vitamins) are delivered to the body's tissues.  Furthermore, it carries carbon dioxide to the lungs and other waste products to the kidneys for elimination from the body. It transports hormones and other chemical messengers, to allow various parts of the body to communicate with each other. Also, it carries components that fight infection and stop bleeding. Finally, blood helps regulate the body’s temperature (homeostasis).
Red blood cells, most white blood cells, and platelets are produced in the bone marrow, the soft fatty tissue inside bone cavities. Two types of white blood cells, the B lymphocytes are produced in the lymph nodes and spleen, and T lymphocytes are produced and mature in the thymus gland.
Within the bone marrow, all blood cells originate from a single type of unspecialized cell called a stem cell. When a stem cell divides, it first becomes an immature red blood cell, white blood cell, or platelet-producing cell. The immature cell then divides, matures further, and ultimately becomes a mature red blood cell, white blood cell, or platelet.
The rate of blood cell production is controlled by the body's needs. Normal blood cells last for a limited time (ranging from a few hours to a few days for white blood cells, to about 10 days for platelets, to about 120 days for red blood cells) and must be replaced constantly. Certain conditions may trigger additional production of blood cells. When the oxygen content of body tissues is low or the number of red blood cells decreases, the kidneys produce and release erythropoietin, a hormone that stimulates the bone marrow to produce more red blood cells. The bone marrow produces and releases more white blood cells in response to infections. It produces and releases more platelets in response to bleeding.
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Red Blood Cells Red blood cells (also called erythrocytes) make up about 40% of the blood's volume. Red blood cells contain haemoglobin, a protein that gives blood its red colour and enables it to carry oxygen from the lungs and deliver it to all body tissues. Oxygen is used by cells to produce energy that the body needs, leaving carbon dioxide as a waste product. Red blood cells carry carbon dioxide away from the tissues and back to the lungs. When the number of red blood cells is too low (anaemia), blood carries less oxygen, and fatigue and weakness develop. When the number of red blood cells is too high (polycythaemia), blood can become too thick, which may cause the blood to clot more easily and increase the risk of heart attacks and strokes.
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White Blood Cells
White blood cells (also called leukocytes) are fewer in number, than red blood cells, with a ratio of about 1 white blood cell to every 600 to 700 red blood cells. White blood cells are responsible primarily for defending the body against infection. Some white blood cells flow smoothly through the bloodstream, but many adhere to blood vessel walls or even penetrate the vessel walls to enter other tissues. When white blood cells reach the site of an infection or other problem, they release substances that attract more white blood cells. The white blood cells function like an army, dispersed throughout the body but ready at a moment's notice to gather and fight off an invading organism. White blood cells accomplish this by engulfing and digesting organisms and by producing antibodies that attach to organisms so that they can be more easily destroyed. When the number of white blood cells is too low (leukopenia), infections are more likely to occur. A higher than normal number of white blood cells (leukocytosis) may not directly cause symptoms, but the high number of cells can be an indication of a disease such as an infection or leukemia. There are five main types of white blood cells.
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Neutrophils
The most numerous type, help protect the body against infections by killing and ingesting bacteria and fungi and by ingesting foreign debris.
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Lymphocytes
Lymphocytes consist of three main types: T lymphocytes and natural killer cells, which both help protect against viral infections and can detect and destroy some cancer cells, and B lymphocytes, which develop into cells that produce antibodies.
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Monocytes
Monocytes ingest dead or damaged cells and help defend against many infectious organisms.
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Eosinophils
Eosinophils kill parasites, destroy cancer cells, and are involved in allergic responses.
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Basophils
Basophils also participate in allergic responses.
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Platelets
Platelets (also called thrombocytes) are cell-like particles that are smaller than red or white blood cells. Platelets are fewer in number than red blood cells, with a ratio of about 1 platelet to every 20 red blood cells. Platelets help in the clotting process by gathering at a bleeding site and clumping together to form a plug that helps seal the blood vessel. At the same time, they release substances that help promote further clotting. When the number of platelets is too low (thrombocytopenia), bruising and abnormal bleeding become more likely. When the number of platelets is too high (thrombocythemia), blood may clot excessively, producing a stroke or heart attack.
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