Anatomy And Physiology

Anatomy Physiology

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Blood accounts for approximately 8% total body weight in humans, with an average of 5 liters in women and 5.5 liters in men. It consists of three different cellular elements, erythrocytes (red blood cells), platelets and leukocytes (white blood cells), suspended in a complex liquid matrix called plasma. Over 99% of all blood cells are red blood cells, which are responsible for oxygen transport. Platelets are small cell fragments (from large bone marrow-bound cells called megakaryocytes) that are responsible for forming a plug at a blood vessel injury, to stop blood flow.

So, that leaves us with the leukocytes, also called white blood cells. White blood cells make up less than 1% of total blood cells and are the mobile units of the body's immune system. The body's immune system is responsible for resisting and eliminating potentially harmful foreign substances or abnormal cells. There are five different types of white blood cells, all with specific functions and characteristic structures. Neutrophils, eosinophils and basophils are categorized as polymorphonuclear (many-shaped nucleus) granulocytes (granule-containing) and are named on the basis for their varying affinities of their granules for certain dyes. Monocytes and lymphocytes are known as mononuclear (single nucleus) agranulocytes (lacking granules).

Neutrophils are the phagocytic specialists of white blood cells. They are the first cells at the site of bacterial invasion and play an important role in the inflammatory process. Additionally, they scavenge and clean up debris. An increase in circulating neutrophils usually accompanies acute bacterial infections. Neutrophils are "neutral" and show no dye preference.

Eosinophils are a different type of specialist and usually accompany allergic conditions and internal parasite infestations, such as worms. Eosinophils are too small to engulf a much larger parasitic worm, but instead attach to it and secrete chemical mediators to attack it. Eosinophils are stained red with the dye eosin.

Basophils are the least numerous of all the leukocytes and are quite similar structurally and functionally to mast cells. Basophils synthesize and store histamine and heparin, which are powerful chemical substances that are released upon specific stimulation. Histamine is released during allergic reactions, whereas heparin hastens the removal of fat from the blood after a fatty meal.

The number of circulating granulocytes is adjusted in two ways. The majority of mature granulocytes are stored within the bone marrow, providing an available pool of these cells that are released into the blood in response to specific signals. Once released into the blood, granulocytes usually remain in transit in the blood for less than a day before entering the tissues. They usually survive 3 4 days unless they die sooner due to their immunological actions.

Monocytes, like neutrophils, become phagocytic specialist white blood cells. Monocytes emerge from the bone marrow as immature cells and circulate in the blood for 1 2 days before settling in different tissues of the body. It is here that monocytes grow in size and develop into the large tissue phagocytic macrophages. The life span of a macrophage can last from months to years unless it is destroyed through the phagocytic process. A phagocytic cell can ingest only a limited amount of material before dying.

Lymphocytes are extremely specialized white blood cells and provide immune defense against targets for which they are specifically programmed. There are two types of lymphocytes, B lymphocytes and T lymphocytes. B lymphocytes produce antibodies which circulate in the blood and bind to specific targets, marking them for destruction via phagocytosis or other means. T lymphocytes do not produce antibodies, but instead directly destroy their target cells via a process called "cell-mediated immune response". The target cells of T lymphocytes include viral-infected cells and cancer cells. Lymphocytes have a lifespan of approximately 100 -300 days and continually recycle among the lymphoid tissues, lymph and blood.

To conclude, leukocytes are extremely important in the immune system of humans. They primarily function outside of the blood and the five different types are produced at varying rates depending on the changing defense needs of the body.

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