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Overview of Blood


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Last full review/revision Apr 2020| Content last modified Apr 2020
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Blood is the red fluid in your arteries and veins. It provides the oxygen, water, and nutrients that your tissues and organs need to survive. You have about 5 liters of blood in your body. Your heart constantly pumps blood throughout your body.

What does blood do?

Blood is your body's delivery service. Your blood:

  • Carries oxygen, water, and nutrients to all the tissues in your body

  • Collects waste products from your tissues and takes them to be removed

  • Carries cells and proteins that help defend your body from foreign substances

All the cells in your body need oxygen and water to live. Your cells also need nutrients such as sugar, proteins, and fats. Your blood picks up oxygen in your lungs, and water and nutrients from your stomach and intestines.

Your cells create waste products when they process nutrients. Oxygen and nutrients turn into carbon dioxide and the chemical waste products that become part of urine. Your blood carries carbon dioxide to the lungs, where it is breathed out. Waste products are carried to your kidneys, where they are filtered out of blood and into urine. Your blood carries other waste products to your liver for further processing and removal.

Your immune system is your body's defense system against invaders such as germs and cancer cells. Your blood carries the special cells and proteins of your immune system to where they're needed.

What is in blood?

Blood is made up of:

  • Liquid (plasma)

  • Red blood cells

  • White blood cells

  • Platelets

Red and white blood cells and platelets are constantly being made in your bone marrow, which is inside your bones.


Plasma is mostly water. Plasma also carries important minerals and salts (electrolytes) and many useful proteins. Some of the proteins help form blood clots. Other proteins attack invaders such as germs.

Red blood cells

Red blood cells contain a red material called hemoglobin. When blood flows through your lungs, hemoglobin picks up oxygen to go to your tissues. Hemoglobin also carries carbon dioxide back to your lungs so you can breathe it out.

White blood cells

White blood cells are part of your immune system. White blood cells travel through your blood to find and fight foreign substances such as germs and cancer cells. Once they fight a certain substance, white blood cells usually remember it so they can fight it more quickly the next time it appears.


Platelets are tiny particles smaller than red or white blood cells. They work with proteins in your blood to help your blood clot so you can stop bleeding.

What problems can develop in your blood?

Common blood problems usually involve:

  • Having too few or too many red blood cells

  • Having too few or too many white blood cells

  • Having too few or too many platelets

Having too few red blood cells (anemia) causes weakness and tiredness because the tissues of your body can't get enough oxygen. Having too many red blood cells can cause a health problem called polycythemia vera.

Having too few white blood cells makes you more likely to get an infection. Having too many white blood cells can be a sign of a type of blood cancer called leukemia.

Too few platelets increases your risk of bleeding after injuries, surgery, or even for no reason at all. Having too many platelets makes it hard for your blood to clot normally. This can cause too much clotting or not enough clotting.

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