What is blood?
Where is blood found?
What is normal blood made up of?
- Blood cells, which can be seen under a microscope, make up about 40% of the blood's volume. Blood cells are divided into three main types:
- Red cells (erythrocytes). These make blood a red colour. One drop of blood contains about five million red cells. A constant new supply of red blood cells is needed to replace old cells that break down. Millions of red blood cells are made each day. Red cells contain a chemical called haemoglobin. This binds to oxygen, and takes oxygen from the lungs to all parts of the body.
- White cells (leucocytes). There are different types of white cells which are called neutrophils (polymorphs), lymphocytes, eosinophils, monocytes, and basophils. They are part of the immune system. Their main role is to defend the body against infection. Neutrophils engulf bacteria and destroy them with special chemicals. Eosinophils and monocytes also work by swallowing up foreign particles in the body. Basophils help to intensify inflammation. Inflammation makes blood vessels leaky. This helps specialised white blood cells get to where they are needed. Lymphocytes have a variety of different functions. They attack viruses and other pathogens. They also make antibodies which help to destroy bacteria.
- Platelets. These are tiny and help the blood to clot if we cut ourselves.
- Plasma is the liquid part of blood and makes up about 60% of the blood's volume. Plasma is mainly made from water, but also contains many different proteins and other chemicals such as hormones, antibodies, enzymes, glucose, fat particles, salts, etc.
What does blood do?
- Transport. Blood takes oxygen from the lungs to the cells of the body. It takes carbon dioxide from the body's cells to the lungs for exhalation. It carries nutrients, hormones and waste products.
- Regulation. Blood helps to keep the acid-alkali balance of the body in check. It also plays a part in regulating body temperature. Increasing the amount of blood flowing close to the skin helps the body to lose heat.
- Protection. White blood cells attack and destroy invading bacteria and other pathogens. Blood clots which protects the body from losing too much blood after injury.
- Lymphocyte white blood cells develop from lymphoid stem cells. There are three types of mature lymphocytes:
- B lymphocytes make antibodies which attack infecting bacteria, viruses, etc.
- T lymphocytes help the B lymphocytes to make antibodies.
- Natural killer cells which also help to protect against infection.
- All the other different blood cells (red blood cells, platelets, neutrophils, basophils, eosinophils and monocytes) develop from myeloid stem cells.
Blood, oxygen and other chemicals
A special chemical called haemoglobin is found inside red blood cells. Haemoglobin has a strong attraction to oxygen. Red blood cells pass through the lungs within the bloodstream. Here in the lungs the oxygen you breathe in passes into red blood cells, and binds to haemoglobin. Blood then flows from the lungs to the heart. The heart pumps blood around the body. When red blood cells come into contact with tissues that need oxygen, haemoglobin releases the oxygen it is carrying.
Carbon dioxide produced by your body's tissues is also carried by blood. When it reaches the lungs it passes out of the blood vessels and into your airways. This allows carbon dioxide to leave your body when you breathe out.
As well as transporting oxygen and carbon dioxide, blood carries many of the chemicals and nutrients essential to life. This includes the nutrients produced by the digestion of food, enzymes (chemicals produced by the body), hormones and waste products. Blood also helps to buffer all the different chemicals in the body. By doing this it stops your body fluids from becoming too acidic or too alkali.
Blood and blood vessels
Arteries carry blood away from the heart to other organs. They can vary in size.
Arterioles are the smallest arteries in the body. They deliver blood to capillaries. Arterioles are also capable of constricting or dilating and by doing this they control how much blood enters the capillaries.
Capillaries are tiny vessels that connect arterioles to venules. They have very thin walls which allow nutrients from the blood to pass into the body tissues. Waste products from body tissues can also pass into the capillaries. For this reason capillaries are known as exchange vessels.
Groups of capillaries within a tissue reunite to form small veins called venules. Venules collect blood from capillaries and drain into veins.
Veins are the blood vessels that carry blood back to the heart. They may contain valves which stop blood flowing away from the heart.
What is a blood group?
- If you have type A antigens on the surface of your red blood cells, you also have anti-B antibodies in your plasma.
- If you have type B antigens on the surface of your red blood cells, you also have anti-A antibodies in your plasma.
- If you have type A and type B antigens on the surface of your red blood cells, you do not have antibodies to A or B antigens in your plasma.
- If you have neither type A or type B antigens on the surface of your red blood cells, you have anti-A and anti-B antibodies in your plasma.
Blood group names
- A+ (A positive) if you have A and rhesus antigens.
- A– (A negative) if you have A antigens, but not rhesus antigens.
- B+ (B positive) if you have B and rhesus antigens.
- B– (B negative) if you have B antigens, but not rhesus antigens.
- AB+ (AB positive) if you have A, B and rhesus antigens.
- AB– (AB negative) if you have A and B antigens, but not rhesus antigens.
- O+ (O positive) if you have neither A nor B antigens, but you have rhesus antigens.
- O– (O negative) if you have do not have A, B or rhesus antigens.
Other blood types
How does blood clot?
If a blood clot forms within a healthy blood vessel it can cause serious problems. So, there are also chemicals in the blood which prevent clots from forming, and chemicals which 'dissolve' clots. So, there is a balance between forming clots and preventing clots. Normally, unless a blood vessel is damaged or cut, the 'balance' tips in favour of preventing clots forming within blood vessels.
Some types of blood disorders
Problems with blood cells
- Anaemia means that you have less red blood cells than normal, or have less haemoglobin than normal in each red blood cell. There are many causes of anaemia. For example, the most common cause of anaemia in the UK is a lack of iron. (Iron is needed to make haemoglobin.) Other causes include lack of vitamins B12 or folate which are needed to make red blood cells. Abnormalities of red blood cell production can cause anaemia. For example, various hereditary conditions such as sickle cell disease and thalassaemia.
- Too many red cells is called polycythaemia and can be due to various causes.
- Too few white cells is called leucopenia. Depending on which type of white cell is reduced it can be called neutropenia, lymphopenia, or eosinopenia. There are various causes.
- Too many white blood cells is called leucocytosis. Depending on which type of white cell is increased it is called neutrophilia, lymphocytosis, eosinophilia, monocytosis, basophilia. There are various causes. For example:
- Various infections can cause an increase of white blood cells.
- Certain allergies can cause an eosinophilia.
- Leukaemia causes a large increase in the number of white blood cells. The type of leukaemia depends on the type of white cell affected.
- Too few platelets which is called thrombocytopenia. This may make you bruise or bleed easily. There are various causes.
- Too many platelets which is called thrombocythaemia (or thrombocytosis). This is due to disorders which affect cells in the bone marrow which make platelets.
- Too few platelets (thrombocytopenia) - due to various causes.
- Genetic conditions where you do not make one or more clotting factors. The most well known is haemophilia A which occurs in people who do not make factor VIII.
- Lack of vitamin K can cause bleeding problems as you need this vitamin to make certain clotting factors.
- Liver disorders can sometimes cause bleeding problems as your liver makes most of the clotting factors.
Clotting disorders (thrombophilia)
- A blood clot which forms within a coronary (heart) artery or in an artery within the brain is the common cause of heart attack and stroke. The platelets become sticky and clump next to patches of atheroma (fatty material) in blood vessels and activate the clotting mechanism.
- Sluggish blood flow can make blood clot more readily than usual. This is a factor in deep vein thrombosis (DVT) which is a blood clot that sometimes forms in a leg vein.
- Certain genetic conditions can make the blood clot more easily than usual.
- Certain medicines can affect the blood clotting mechanism, or increase the amount of some clotting factors, which may result in the blood clotting more readily.
- Liver disorders can sometimes cause clotting problems as your liver makes some of the chemicals involved in preventing and dissolving clots.
Problems with blood groups
So, before a blood transfusion is done, a donor bag of blood is selected with the same ABO and rhesus blood group as yourself. Then, to make sure there is no incompatibility a sample of your blood is mixed with a sample of the donor blood. After a short time the mixed blood is looked at under a microscope to see if there has been any clumping of blood. If there is no clumping, then it is safe to transfuse the blood.
Some disorders of blood
- Anaemia (various types)
- Idiopathic Thrombocytopenic Purpura (ITP)
- Sickle Cell Disease and Sickle Cell Anaemia
- Sickle Cell Trait and Sickle Cell Screening Tests