The blood's importance to human life has been recognised since pre-history. Early man knew that if enough blood leaves the body then life ceases to function. As a result of this the blood has acquired mystic and religious significance throughout the ages. Blood was thought to carry rejuvenating properties or was used to represent desirable human characteristics such as purity or courage.Even today we talk of blood brothers or blood lines although we know that scientifically it is the genes which carry traits from generation to generation. The phrase "blood is thicker than water" is still commonly heard. We talk of our "blood boiling" when we get angry. Many people faint simply as a result of seeing blood.
Biologically the blood is of extreme importance in maintaining homeostasis by transporting needed materials to the appropriate part of the body and by protecting the body against disease.
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Functions of the blood | |
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Delivers nutrients from the digestive system to all parts of the body. | |
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Transports oxygen from the lungs to all parts of the body. | |
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Transports carbon dioxide from all parts of the body to the lungs. | |
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Transports waste products from cells to the external environment mainly via the kidneys. | |
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Transports hormones from the endocrine system to target cells or organs within the body. | |
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Through continuous exchange of it's components with tissue fluids promotes fluid and electrolyte balance. | |
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Defends the body against attack from foreign organisms via the white blood cells and antibodies. | |
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Defends the body against injury or infection via the inflammatory response. | |
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Prevents serious haemorrhage by the clotting process. | |
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Maintains the body's temperature by circulating heat. |
Blood is of huge importance clinically as a diagnostic aid because of it's many functions outlined above.
Samples of blood tissue are easily obtained and analysis of them will reveal much about the state of the body.
Composition of the Blood
The blood is a mixture of cells, fluid, proteins and metabolites.
Blood has four major elements
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red blood cells (RBC's or erythrocytes) |
transport oxygen from the lungs to organs and peripheral sites.
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white blood cells (or leukocytes) |
have a defensive role in destroying invading organisms e.g. bacteria and viruses
assist in the removal of dead or damaged tissue cells.
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platelets (or thrombocytes) |
the first line of defence against damage to blood vessels.
they adhere to any defects and assist in the clotting process.
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plasma |
the proteinaceous substance in which the other three cellular elements circulate.
it carries nutrients, metabolites antibodies,
proteins involved in blood clotting, as well as
hormones and other molecules to all parts of the body.
In normal human blood the red blood cells account for about 45% of the total volume. Plasma accounts for just below 55% of the total volume with the other two cellular elements (platelets and white blood cells) making up less than 1% of total blood volume.
In the laboratory blood samples can be separated out in a centrifuge. This spins the blood sample at high speed in a specially calibrated tube. The plasma floats to the top and the red blood cells sink to the bottom leaving a very thin middle layer composing of the other components in the middle. This layer is known as the buffy coat. This is a good test for determining the percentage of red blood cells in the total blood count in order to determine if an individual is anaemic. The test is called a haematocrit.
This picture shows two haematocrit tests, normal blood sample on the left and a sample from an anaemic individual on the right
This picture shows an ideal blood smear. The pale coloured background represents the plasma. Two types of white blood cells (neutrophils and lymphocytes) are evident.
