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Blood is the circulatory fluid of humans. It is the body main transport network. Blood has been called the river of life The heart pumps blood around the body. The blood remains within a closed cardiovascular system, which comprises, the heart, blood vessels and blood itself. Arteries, which have thick muscular walls, carry blood away from the heart to all parts of the body and veins, which have thin floppy walls, return blood to the heart.

blood cells

The Functions of Blood

1. Blood carries digested food, oxygen and hormones to parts of the body that need it.
2. The carrying away of waste products and carbon dioxide.
2. Defending the body from infection.
4. Control and regulation.

The sequence of blood flow from the heart is:

Heart - Artery - small artery - capillaries

Blood can de divided into four major components.

1. Red Blood cells (erythrocytes): Their function is to carry oxygen from the lungs to the tissues and cells of the body that need it for respiration. Red blood cells are red because they are full of a protein called haemoglobin. This is bound to oxygen from the lungs and becomes a bright red colour. No one has blue blood, not even the Royal Family.

2. White blood cells (leukocytes): These are part of the immune system and are mainly involved in combating infection.

3. Platelets (thrombocytes): These are microscopic cell fragments that help the blood to clot if the skin is cut.

4. Plasma: This is the liquid part of the blood and contains proteins and other chemicals. It is 91% water and is a pale straw colour. Plasma helps maintain blood pressure, distributes body heat and maintains the pH level in the body.

Amazing fact
The human body contains 5.5 litres of blood and in one day blood is transported around the body approximately 1000 times.

How Blood Cells Are Made

Blood cells begin life in the spongy bone marrow that is found in some bones. In childhood most of our bones produce blood but as we age this is restricted to the vertebrae, sternum, pelvis, ribs and the small bones in the upper legs and arms. This blood making process is called haematopoiesis. Bone marrow that produces blood cells is called red marrow and the marrow that no longer produces cells is called yellow marrow. All blood cells (red, white and platelets) come from the same type of cell called the pluripotential haematopoietic stem cell. This group of cells is able to form any of the different types of blood cells. It can also reproduce itself. This cell forms committed stem cells that will form specific types of blood cells. During formation the red blood cell eventually loses its nucleus and leaves the bone marrow as a reticulocyte. Remaining in the reticulocyte are the remnants of the organelles but eventually these too leave the cell and a mature erythrocyte is thus formed.

Amazing fact
Red blood cells are replaced continually as they become less efficient after 120 days.

Blood Groups

Since the early seventeenth century doctors, physicians and scientists have conducted investigations into haematology. These experiments have often involved transferring blood from person to person and even animal to person! These early investigations met with varying degrees of success and it was left to Viennese doctor and subsequent Nobel Prize Winner in Medicine Karl Landsteiner to discover that when the serum of one person's blood was mixed with the red cells of another the cells clumped together. In other instances they would not be affected. He was subsequently able to divide humans into distinctive blood groups based on his findings.

Blood is classified into 4 groups A, B, O and AB. The classification of blood groups depend on two criteria

The particular proteins (antigens) that are present on the surface of the red blood cells. Antigens are determined by the genetic make-up that we inherit from our parents.

Antibodies (agglutinins) that the plasma contains.

When an antigen is present on a red blood cell (RBC) then the opposite antibody is present in the plasma.

e.g. Blood group A has anti-type B antibodies
Blood group B has anti-type A antibodies
Blood group AB has no antibodies in the plasma
Blood group O has both anti -type A and anti -type B antibodies in the plasma.

These antibodies are not present at birth but are formed during a child's infancy. They last for life. When red blood cells carrying one or both antigens are exposed to the corresponding antibodies they clump together (agglutinate).

Did you know?
Just as communities of people, customs and languages change throughout the countries of the world so do blood groups. Despite the movement of people from country to country and marriage between people of different ethnic groups it is still possible to see how blood groups vary in different populations.

The O blood group is the oldest of all the blood groups dating back to the Stone Age. Today in the UK it is the most common group particularly in the North of England.

Group A is the second oldest blood group developing where large farming settlements first appeared. It is the most prevalent group in Norway, Denmark, Austria, Armenia and Japan.

Group B developed as tribes emigrated from Africa to Europe, Asia and the Americas. It is represented by just 9% of the UK's population overall but one quarter of people share this blood group if they are from an Asian background.

Group AB, the rarest and incidentally the newest blood group, occurred when existing blood groups were mixed on a large scale. It is believed to be 500 - 1000 years old. In addition to the ABO blood group system there is the Rhesus factor. Proteins can be present on the surface of the red blood cell. The D antigen is the most common antigen. If it is present then the blood is categorised as Rh positive. If it is missing then the blood is said to be Rh negative. Unlike in the ABO system the corresponding antibody develops only when the Rh- person is exposed to the Rh antigen by blood transfusion or during pregnancy. Blood groups can, therefore, be labelled as positive or negative.

In transfusions it is vital that blood of the donor is compatible with that of the recipient. Unless the blood is cross- matched the recipient will develop clots in response to the donor's blood. This in turn will lead to heart attacks, embolisms and strokes.

Amazing Fact
Type O- is called the universal donor because it can be given to anybody. It has no protein to cause clumps. Type AB+ is the universal receiver because the recipient has all of the proteins and so will not form clumps.

The National Blood Service

blood transfusion

The two World Wars saw the demand for blood soar and thousands of civilians donated blood to help in the treatment of war related injuries. In 1946 the National Blood Service was established as The Blood Transfusion Service. Today 1.6 million people are active blood donors in England and Wales, but this is just 6% of the population and unfortunately these donors do not always represent many of the ethnic groups resident in these two countries. The National Blood Service needs 9000 blood donations everyday just to conserve their stocks.

Donating Blood

The donation of just under a pint of blood takes about 10 minutes. The donation is used in surgery, for the victims of accidents and in the treatment of cancer, leukaemia and haemophilia.

Your Blood Donation

Every unit of donated blood is spun in a machine called a Centrifuge. This separates the blood into different components so that it can be given to people with specific medical needs. Sometimes the component needed e.g. platelets is immediately separated. The remaining blood cells are returned to the donor. This means that the donor can safely give platelets again earlier than the average 16 week gap between donor sessions.

Amazing fact
In 1975 The National Blood Service replaced glass bottles with plastic bags for blood donations so that greater use could be made of blood components than previously.

Red Cells
These are transfused to increase the oxygen carrying capacity in patients who are anaemic or who have lost red cells as a result of childbirth, accidents or surgery.

These are given to people with abnormally functioning platelets or a very low platelet count.

Fresh frozen plasma once thawed can treat many bleeding disorders where clotting factors are either missing or at a very low level.

This is used in cases of liver failure, severe bleeding and severe burns.

These are antibodies from blood donors, some of which are likely to stop the activity of toxin molecules in the body.

Factor VIII
This is a protein substance, a globulin. It is given to haemophiliacs to control their tendency to bleed.

Blood Disorders

Leukaemia is the name for a number of cancers of the white blood cells. In leukaemia white blood cells multiply in an uncontrolled way and gradually take the place of the other components of the blood. Leukaemia is the most common cancer in children but there are far more cases in adults.

This is the general name for a range of disorders affecting the red blood cells. In anaemia there is a shortage of red blood cells and consequently the blood doesn't carry enough oxygen around the body. Iron deficiency also occurs. It is not an inherited condition.

This is an inherited disorder whereby the blood does not have the ability or only partial ability to produce one of the chemicals needed for clotting. This leads to very slow clotting of the blood and a life long tendency to excessive bleeding. It affects females as carriers but it is the males who inherit the condition.

Sickle Cell Disease
This is an inherited form of anaemia where the haemoglobin molecules are not the correct shape. These misshapen molecules pull the red blood cells into a sickle shape, hence the name of the disease. As a consequence of their shape insufficient oxygen is carried around the body and anaemia occurs. This disorder once only seen in the Middle East, India, parts of the Mediterranean and in the African Continent now occurs in many European countries.

This is another inherited blood disease which is prevalent in parts of the world where malaria occurs but because of migration and intermarriage it is being seen more and more in the United Kingdom. It is caused by a defect in one of the genes that make up a haemoglobin molecule. There are a number of different types of Thalassemia.

HIV stands for human immunodeficiency virus. To become infected with HIV some infected blood or bodily fluids have to enter your body. It cannot be caught through ordinary contact e.g. shaking hands, hugging, sharing towels etc with someone who has the virus. When HIV enters the body it seeks out a type of blood cell called Helper T- lymphocytes, which usually defends the body against infection. The virus takes over the T-lymphocytes causing them to die and releases billions of copies of the virus into the blood. The new viruses attach themselves to the new T-lymphocytes and the infection spreads. Consequently the immune system starts to fail and the person becomes susceptible to other infections, which would not normally make them ill. The term AIDS is used to describe the most advanced stages of the HIV infection.

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