Technische Universität München

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The blood circulatory system is the main route through the human body in which oxygen, carried by body-cells, and nutrients are distributed among the organelles. There are between 5 and 6 liters of blood in the body of a male adult. The average woman holds between 4 and 5 liters. The blood mainly consists of plasma which contains 90% water. Additionally, plasma also contains salts, glucose and further required substances. It comprises proteins that carry important nutrients to the body-cells and strengthen the immune system.

Blood tissue is viscous because it is made up of a variety of cells, each having a different function. There are three main types of blood cells that circulate with the plasma:
  • Platelets help the blood to clot. Clotting prevents the loss of blood due to internal or external hemorrhage. Platelets are also called thrombocytes.
  • Red blood cells, also called erythrocytes, carry oxygen. Normally, a healthy adult has about 35 trillion. The body creates these cells at a rate of about 2.4 million a second and they have a life span of about 120 days.
  • White blood cells, also called leukocytes, defend infections. These cells are found in many shapes and sizes and are vital to the immune system. They are produced numerously, when the organism fights against an infection. Compared to the number of erythrocytes the number of leukocytes in the body is low. Most healthy adults have about 700 times as many red blood cells as white ones.
Amongst these three main cell types, blood transports various hormones, fats, carbohydrates, proteins and gases.

One major function of blood is the transport of oxygen from the lungs and nutrients from the digestive tract to the body-cells and removing carbon dioxide and metabolites from there. Furthermore, it regulates the temperature of the body through the modulation of the circulation, carries hormones to the tissues and sends antibodies for immune response and contains clotting factors.

Amongst humans, there can be assigned four different blood types: A, B AB and O. These are based on genes that are inherited from the parents. Since blood is originated by the bone marrow, the body can usually replace any blood loss caused by small hemorrhages. If massive blood loss occurs, it has to be replaced through a blood transfusion. In that case, the donor and recipient blood types must be identical.
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