Blood is an important fluid that keeps us alive. We cannot live without it. The heart pumps blood to all parts of the body and brings them oxygen and food. At the same time blood carries all the substances we don’t need away from us. Blood fights infections, keeps our body temperature the same and carries chemicals that control body functions. Finally, blood has substances that repair broken blood vessels so that we don’t bleed to death.


What blood is made of

Blood is a mixture of fluid and solid matter.

Plasma is the liquid part of our blood. It makes up about 50 – 60 % of it. Plasma consists mostly of water but many other substances are in it. It contains dissolved food, chemicals that control our growth and do other jobs, proteins, minerals and waste products.

Red blood cells look like flat round discs. They contain haemoglobin, a protein that carries oxygen to the body and gives blood its red colour. Each drop of blood has about 300 million of these red cells.

White blood cells, also called leukocytes, fight infections and harmful substances that invade the body. Most of these cells are round and colourless. They have different sizes and shapes. White blood cells are not as numerous as red ones. For every 700 red blood cells there is only one white blood cell.

Platelets are tiny bodies that are much smaller than red blood cells. They stick to the edges of a cut and form blood clots to stop bleeding. The blood of a normal adult has about 2 trillion platelets.



How blood works in the body


The circulatory system carries blood to all parts of your body. The heart pumps blood through big blood vessels called arteries and veins. In our body there are also millions of small blood vessels called capillaries. Oxygen, food and other substances pass through the thin walls of these capillaries into the tissue.

When you inhale air oxygen passes through your lungs and and is picked up by haemoglobin which transports it to your whole body. It is released into cells which produce energy. In return cells produce carbon dioxide which enters your blood stream and is transported back to your lungs where it is exhaled.



Food also reaches your body by means of blood. It is digested in your stomach and important substances like fat, sugar, proteins, vitamins and minerals are separated. These nutrients enter your blood stream and are moved to the cells and muscles where they are needed in order to give you energy or fuel. The work of the muscles and other tissue creates heat. Blood is the transporting system which carries heat throughout your body and warms you. The things that you don’t need are transported to your intestines and kidneys and leave your body again.

White blood cells play an important role in your immune system. When harmful substances invade your body an alarm goes off and white blood cells are activated. Then they work to destroy the invaders. They fight off viruses, harmful bacteria and begin anti-body production.

Blood also carries hormones to places where they are needed. When a hormone reaches a part of the body it controls growth, how the body uses food and other things.

You would bleed to death from a small cut if your blood didn’t clot. When a blood vessel breaks platelets rush to the damaged area and stick to one another , forming a plug.


The blood supply


Blood cells come from bone marrow. They begin as stem cells and then develop into red or white blood cells, or platelets. They don’t live forever and must be replaced by new ones. Red blood cells live an average of 120 days before wearing out. Then they are captured and destroyed in the liver and spleen. Platelets live only for about 10 days.

The amount of blood in your body depends on your size, weight and the altitude at which you live. An adult who weighs 80 kg has about 5 litres of blood, a 40 kg child about half the amount. People who live in high areas where the air is thinner need more blood to deliver more oxygen to the body.


Blood groups

Blood groups are very important in order to find out if a person can donate blood or receive blood in case of an accident or another disease. Almost everyone’s plasma has antibodies that that may not work together with another person’s blood.

There are four main blood groups :

  • Type 0 is the most common blood group. In an emergency type 0 blood can be transfused to anybody.
  • Type AB is the most seldom group. People with this blood group can receive any other blood in case of an emergency.
  • Type A can only be received by donors with A or 0.
  • Type B can only be received by donors with B or 0.




Blood transfusion

If an adult suddenly loses a litre or more of blood he may die unless the blood in his body can be replaced. Over the years blood transfusions have saved countless lives. Transfusions can also help patients who cannot produce enough blood cells to survive. They also help during operations when patients lose some blood.

Blood banks collect blood from donors and put it in sterile bags. It is cooled down and can be stored for up to 50 days. Laboratory workers screen blood for infectious diseases like AIDS and hepatitis. Only clean and safe blood can be given to patients.


Blood diseases

When a person suffers from anaemia there are not enough red blood cells to supply the body with the oxygen he needs. Leukaemia is a kind of cancer of the bone marrow, in which not enough or abnormal white blood cells are produced. Without white blood cells diseases can enter your body without being controlled.

When your body does not have enough platelets blood cannot clot well. Even small injuries can lead to a loss of blood because bleeding doesn’t stop.


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Related Topics



  • abnormal = very different, unusual
  • activate = start to work
  • adult = a grown up person
  • altitude = the height that an object has above the sea
  • amount =quantity, how much of something
  • anaemia = when there are not enough red blood cells in your body
  • anti-body = something that your body produces to fight off diseases
  • artery = blood vessel that brings blood from the heart to parts of the body
  • average =normally, usually
  • bleed = to lose blood if you are hurt
  • blood bank = a store of human blood that is used in operations or when a patient is ill
  • blood clot = a thick solid mass of blood
  • blood stream =the flow of blood in your body
  • blood transfusion = putting blood from one person into the body of another
  • blood vessel = one of the tubes through which blood flows in your body
  • bone marrow = soft material in your bones
  • by means of = through
  • capture = to catch something and keep it
  • carbon dioxide = the gas that is produced when animals or people breathe out
  • carry = transport
  • circulatory = when blood moves around your body
  • clot = to become thicker
  • colourless = without a colour
  • common = it happens very often
  • consist = is made up of
  • contain = has
  • countless = so many that you cannot count them
  • create = make
  • deliver = transport
  • depend on =to be affected by something else
  • destroy =damage
  • develop = grow
  • digest = to change food that you have eaten into material that the body can use
  • disc = a round plate
  • disease = illness
  • dissolved = to mix with liquid and become a part of it
  • donate = to give something to a person in order to help him
  • donor = here: a person who gives blood
  • edge =the outside part of something
  • emergency = an unexpected situation that is dangerous
  • exhale = to breathe out
  • fluid =liquid, watery substance
  • fuel = energy
  • growth =the speed at which we become taller
  • harmful = dangerous
  • harmful = dangerous
  • hepatitis = a disease of the liver that gives you fever and makes your skin yellow
  • hormone = chemical substance produced by your body that controls growth and other things
  • in return =in exchange for
  • infectious = something that can be passed from one person to another, mostly by air
  • inhale = to breathe in
  • injury = if you are hurt
  • intestine = a long tube in your body through which food goes after it leaves your stomach
  • invade = go into, enter
  • kidney = one of the two organs in your back that takes away waste products from your blood and makes urine
  • lead =cause
  • liquid = same as “fluid”
  • liver = large organ in your body that cleans your blood
  • loss = if you lose something or don’t have it any more
  • mixture =combination of
  • numerous = many
  • nutrient = chemical or food that gives you what you need to grow
  • oxygen = a gas that is in the air and which we need to breathe
  • plug =here: an object that stops blood from getting out
  • protein = a substance that is in food like meat, eggs and beans and which your body needs to grow and stay strong
  • protein = a substance that is in food like meat, eggs and beans and which your body needs to grow and stay strong
  • reach = get to
  • receive = get
  • release = let go
  • repair = fix something that is broken
  • replace =to remove something and put something else in there instead
  • rush = hurry
  • screen = to test someone to find out if they have an illness
  • seldom = not very often
  • separate = divide
  • shape = form, what something looks like
  • size = how tall you are or how big something is
  • solid matter = something that is hard
  • spleen = an organ near your stomach that controls the quality of blood
  • stem cell = a special cell in your body that can divide in order to form other cells that do special things
  • sterile = completely clean ; something that does not have any bacteria
  • stick = to hang on to
  • store = keep in a place
  • substance =material
  • suddenly = very quickly, surprisingly
  • suffer from =to feel pain because of a certain illness
  • supply = here: give
  • survive = to continue to live
  • throughout = in your whole ...
  • tissue = the material that forms cells
  • transfuse =transfer from one person to another
  • trillion = a number that has 12 zeros
  • vein =blood vessel that brings blood from the parts of the body back to the heart
  • waste = something you don’t need any more
  • wear out = here: lose their power
  • weight = how heavy you are