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Blood
This article gives a brief overview of blood, blood cells and how they work.
What is blood?
Blood is made up of liquid (plasma) and various different types of cells. An average-sized man has about 5-6
litres of blood in his body; a woman has slightly less. Blood has many different functions - detailed below.
Where is blood found?
Blood is found in blood vessels. Blood vessels (arteries, arterioles, capillaries, venules and veins) take blood to
and from every part of your body. Blood is pumped through blood vessels by your heart.
What is normal blood made up of?
Blood cells
These can be seen under a microscope and 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 (leukocytes). 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 germs (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 germs (pathogens). They also
make antibodies which help to destroy pathogens.
Platelets. These are tiny and help the blood to clot if we cut ourselves.
Plasma
This 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.
When blood spills from your body (or a blood sample is taken into a plain glass tube) the cells and certain plasma
proteins clump together to form a clot. The remaining clear fluid is called serum.
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What does blood do?
Blood has a variety of different functions. These include:
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 where it is breathed out. Blood carries nutrients, hormones and waste
products around the body.
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 germs (bacteria) and other pathogens.
Blood clots, which protects the body from losing too much blood after injury.
The bone marrow, stem cells and blood cell production
Bone marrow
Blood cells are made in the bone marrow by stem cells. The bone marrow is the soft spongy-like material in the
centre of bones. The large flat bones such as the pelvis and breastbone (sternum) contain the most bone
marrow. To make blood cells continuously you need a healthy bone marrow. You also need nutrients from your
diet, including iron and certain vitamins.
Stem cells
Stem cells are immature (primitive) cells. There are two main types in the bone marrow - myeloid and lymphoid
stem cells. These derive from even more primitive common stem cells called pluripotent stem cells. They are
called this because they are able to form many different cell types. Stem cells constantly divide and produce new
cells. Some new cells remain as stem cells and others go through a series of maturing stages (precursor or
blast cells) before forming into mature blood cells. Mature blood cells are released from the bone marrow into the
bloodstream.
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.
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Blood production
You make millions of blood cells every day. Each type of cell has an expected lifespan. For example, red blood
cells normally last about 120 days. Some white blood cells last just hours or days - some last longer. Every day
millions of blood cells die and are broken down at the end of their lifespan. There is normally a fine balance
between the number of blood cells that you make and the number that die and are broken down. Various factors
help to maintain this balance. For example, certain hormones in the bloodstream, and chemicals in the bone
marrow, called growth factors, help to regulate the number of blood cells that are made. Erythropoietin is a
hormone made in the kidneys which increases production of red blood cells and is used to treat some blood
disorders. It is also known as EPO and is one of the most widely known drugs used to enhance athletic
performance; it is banned by the World Anti-Doping Agency.
Blood, oxygen and other chemicals
The cells that make up the organs and tissues of your body need oxygen to live. They also produce carbon
dioxide which needs to be removed from the body. One of the main functions of blood is to transport oxygen and
carbon dioxide around the body.
A chemical called haemoglobin is present 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.
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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, chemicals produced by the body (hormones
and enzymes) 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
The main function of blood vessels is to transport blood around the body. Blood vessels are found throughout the
body. There are five main types of blood vessels: arteries, arterioles, capillaries, venules and veins.
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.
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What is a blood group?
Red blood cells have certain proteins on their surface, called antigens. Also, your plasma contains antibodies
which will attack certain antigens if they are present. There are various types of red blood cell antigens - the ABO
and rhesus types are the most important.
ABO types
These were the first type discovered.
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 nor type B antigens on the surface of your red blood cells, you have anti-A
and anti-B antibodies in your plasma.
Rhesus types
Most people are rhesus positive, as they have rhesus antigens on their red blood cells. But, about 3 in 20 people
do not have rhesus antibodies and are said to be rhesus negative.
Blood group names
Your blood group depends on which antigens occur on the surface of your red blood cells. Your blood group is
said to be:
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.
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O+ (O positive) if you have neither A nor B antigens but you have rhesus antigens.
O− (O negative) if you do not have A, B or rhesus antigens.
Other blood types
There are many other types of antigens which may occur on the surface of red blood cells. However, most are
classed as minor and are not as important as ABO and rhesus.
How does blood clot?
Within seconds of cutting a blood vessel, the damaged tissue causes platelets to become sticky and clump
together around the cut. These activated platelets and the damaged tissue release chemicals which react with
other chemicals and proteins in the plasma, called clotting factors. There are 13 known clotting factors which are
called by their Roman numbers - factor I to factor XIII. A complex cascade of chemical reactions involving these
clotting factors quickly occurs next to a cut. The final step of this cascade of chemical reactions is to convert
factor I (a soluble protein, also called fibrinogen) into thin strands of a solid protein called fibrin. The strands of
fibrin form a meshwork, and trap blood cells and platelets so that a solid clot is formed.
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. Therefore, 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 fewer red blood cells than normal, or have less haemoglobin than
normal in each red blood cell. A normal haemoglobin (Hb) concentration is 130-180 g/L in adult males
and 115-165 g/L in adult, non-pregnant females.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, which is called polycythaemia and can be due to various causes.
Too few white cells, which is called leukopenia. 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, which is called leukocytosis. Depending on which type of white cell is
increased it is called neutrophilia, lymphocytosis, eosinophilia, monocytosis, or basophilia. There are
various causes - for example:
Various infections can cause an increase of white blood cells.
Certain allergies can cause an eosinophilia.
Leukaemia is a type of blood cancer where there is a large number of abnormal blood cells,
usually 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.
Bleeding disorders
There are various conditions where you tend to bleed excessively if you damage or cut a blood vessel - for
example:
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.
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Liver disorders can sometimes cause bleeding problems, as your liver makes most of the clotting
factors.
Clotting disorders (thrombophilia)
Sometimes a blood clot forms within a blood vessel which has not been injured or cut - for example:
A blood clot which forms within a heart (coronary) 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 fatty
material (atheroma) 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
If you have a blood transfusion, it is vital that the blood you receive is compatible with your own. For example, if
you receive blood from a person who is A positive and you are B positive then the anti-A antibodies in your
plasma will attack the red blood cells of the donated blood. This causes the red cells of the donated blood to
clump together. This can cause a serious or even fatal reaction in your body.
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, it is safe to transfuse the blood.
Donating blood
Why not consider donating some blood? See below for details of the NHS Blood and Transplant service.
Some disorders of blood
Anaemia (various types).
Idiopathic thrombocytopenia.
Leukaemia.
Myeloma.
Sickle cell disease and sickle cell anaemia.
Sickle cell trait.
Thalassaemia.
Thrombophilia.
Further more detailed information available free online
Human Physiology/Blood physiology
From Wikibooks, the open-content textbooks collection.
Web: http://en.wikibooks.org/wiki/Human_Physiology/Blood_physiology
Further help & information
NHS Blood and Transplant
Oak House, Reeds Crescent, Watford, Hertfordshire, WD24 4QN
Tel: 0300 123 23 23
Web: www.nhsbt.nhs.uk/
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Anthony Nolan
2 Heathgate Place, 75-87 Agincourt Road, London, NW3 2NU
Tel: 0303 303 0303
Web: www.anthonynolan.org
Further reading & references
Weinman J, Yusuf G, Berks R, et al; How accurate is patients' anatomical knowledge: a cross-sectional, questionnaire
study of six patient groups and a general public sample. BMC Fam Pract. 2009 Jun 12;10:43.
Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical
conditions. EMIS has used all reasonable care in compiling the information but makes no warranty as to its
accuracy. Consult a doctor or other healthcare professional for diagnosis and treatment of medical conditions.
For details see our conditions.
Original Author:
Dr Tim Kenny
Current Version:
Dr Jacqueline Payne
Peer Reviewer:
Dr John Cox
Document ID:
4881 (v41)
Last Checked:
12/10/2015
Next Review:
11/10/2018
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