The Open Door Web Site
Part XXI: The Heart and Blood System
THE HEART AND BLOOD SYSTEM
Blood moves around the body in special tubes called blood vessels. It is kept moving by the pumping action of an organ called the heart.
There are three different kinds of blood vessels. The arteries (singular: artery) always carry blood away from the heart and towards the capillaries.
The veins always carry blood which has come from the capillaries and is moving back to the heart.
The capillaries (singular: capillary) are extremely thin blood vessels. They form networks which reach every living cell of the body.
Blood is made of a liquid plasma which has red and white blood cells in it. The plasma is mostly water with many different chemicals dissolved in it. Since the blood moves around the body it is used to transport chemicals to the parts of the body that need them. Plasma contains chemicals such as proteins, hormones, soluble food molecules and urea.
Plasma also has special cells suspended it. These cells are of two types:
Red Blood Cells
Red blood cells are made in the bone marrow which is found in the centre of the long bones in the limbs. Each red blood cell has a biconcave shape. These cells do not contain a nucleus, in fact they are the only cells in the body without nuclei. The shape of each red blood cell is designed to give the cell maximum efficiency for picking up and releasing oxygen. The presence of a nucleus would change this shape.
Each red blood cell is packed with an iron-containing protein called haemoglobin. It is the haemoglobin which carries the oxygen. There are five million red blood cells in every millimetre cubed of blood! Each of these cells have a life span of around 120 days, after which it is broken down by the liver. So red blood cells are constantly produced by the bone marrow to replace those which have been destroyed.
It is in the capillary system of the lungs where oxygen moves into the red blood cells. The oxygen remains attached to the haemoglobin until the blood reaches a capillary network in the body. Once in a capillary the oxygen moves from the red blood cells into the living cells which surround the capillaries.
White Blood Cells
The lymphocytes produce anti-bodies and the phagocytes 'eat' the broken up bacteria cells.
There are many different types of white blood cells. All of them help the body to fight disease caused by organisms such as bacteria. Some types of white blood cell produce chemicals called antibodies which surround the invading bacteria and cause them to break up. Other types literally ‘eat’ the bits of bacteria after they have been destroyed. There are around 7,000 white blood cells in one millimetre cubed of blood. Of course this number will be higher if a person is suffering from an infection.
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