Blood is the body’s internal transport system, constantly flowing around the body delivering oxygen, nutrients, and other substances to the tissues and removing waste products from them. Running almost parallel with the blood’s circulation is the lymphatic system, which collects excess fluid from the tissues and returns it to the blood. Both the blood and the lymphatic system form part of the body’s immune system.
In this section
Body tissues, such as muscle, brain, heart, and other internal organs, need to have a constant supply of energy to function. The energy is obtained from glucose and oxygen, which are carried to body tissues by the blood in the circulation. Blood circulates around the body in about 1 minute at rest and 20 seconds during vigorous exercise.
Glucose, a simple sugar that is derived from the breakdown of many foods, is dissolved in the blood and carried in the circulation to every cell within the body. To release its energy, glucose must be “burned” inside the cells, which requires oxygen (a process known as oxidation). The oxygen is transported by red blood cells from the lungs to the body cells, where it is released.
In addition to glucose, the body cells require proteins, fats, vitamins and minerals, and lipids such as cholesterol. These substances are carried in the plasma, the fluid part of the blood, to every cell within the body.
As body cells carry out their various functions, grow, reproduce, and repair damage, they release waste products into the bloodstream. These waste products include carbon dioxide produced from the oxidation of glucose, protein wastes such as urea, and bilirubin from the breakdown of the pigment haemoglobin. The carbon dioxide is eliminated from the blood in the lungs, while the other waste products are mostly processed in the liver before being excreted in the faeces or transported to the kidneys for excretion in the urine.
The body’s defences
Blood contains colourless cells known as white cells. These are the main components of the immune system, which protects the body against infection, against toxins produced by bacteria, and against some cancers.
Other essential components of the immune system transported in the blood are proteins called antibodies, which help to destroy microorganisms. Blood also contains billions of small cells called platelets, which seal injured blood vessels through blood clotting.
The lymphatic system also forms part of the immune system and helps to protect the body by filtering out and destroying “foreign” particles, such as infectious organisms and cancer cells, that have contaminated body fluids.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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