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.

Image: von Clker-Free-Vector-Images via Pixabay

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.

Blood components

Image (altered) : Alan Sved, CC BY-SA 4.0,
via Wikimedia Commons

How blood works in our 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 body’s circulatory system
Image (modified) : Termininja, CC BY-SA 3.0, via Wikimedia Commons

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.

High-altitude training in the Swiss Alps
Professional athletes often carry out training in mountain regions. Such training produces more red blood cells, which can then carry more oxygen to other parts of the body.
Image: Christof Sonderegger von Photoplus.ch, CC BY-SA 3.0,
via Wikimedia Commons

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 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 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 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 they may die unless the blood in their 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.

Image:BruceBlaus. When using this image in external sources it can be cited as:Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436., CC BY 3.0,
via Wikimedia Commons

Blood diseases

When a person suffers from anaemia there are not enough red blood cells to supply the body with the oxygen it needs. Leukemia 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.

Image: (modified) Mikael Häggström,
Public domain, via Wikimedia Commons

fatigue = to become very tired
shortness of breath = to have problems breathing in a normal way
joint = part of your body that can bend because two bones meet there
swelling = part of your body that becomes larger than normal
lymph node = a small rounded swelling in your body through which a liquid passes before entering your blood system
enlargement = when something gets bigger
spleen = an organ near your stomach that controls the quality of your blood

Videos

Blood Components and Blood Cells
The Circulatory System
Blood Groups
Blood Transfusions

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