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 (a little more than 1 gallon) of blood in your body. Your heart Biology of the Heart The heart is a hollow organ made of muscle. The heart and blood vessels are part of your cardiovascular system. Your heart pumps blood through your blood vessels Blood carries oxygen and nutrients... read more 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 Cells Cells are the basic building blocks of the body. All tissues and organs are made of billions of different cells. Human cells vary in size, but all are quite small. Even the largest, a fertilized... read more 
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 Overview of the Respiratory System To sustain life, the body must produce sufficient energy. Energy is produced by burning molecules in food, which is done by the process of oxidation (whereby food molecules are combined with... read more , where it is breathed out. Waste products are carried to your kidneys Kidneys The kidneys are bean-shaped organs that figure prominently in the urinary tract. Each is about 4 to 5 inches (12 centimeters) long and weighs about one third of a pound (150 grams). One lies... read more , 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 Overview of the Immune System The immune system is your body's defense system. It helps protect you from illness and infection. The immune system's job is to attack things that don’t belong in your body, including: Germs... read more 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
Plasma is mostly water. Plasma also carries important minerals and salts (electrolytes Overview of Electrolytes Electrolytes are minerals that circulate in your blood. These minerals are also in your stomach juices, in your stool (poop), in your urine, and inside your body's tissues. Salt (sodium) is... read more ) and many useful proteins. Some of the proteins help form blood clots How Blood Clots A blood clot is a clump of material that your body makes to plug up a bleeding blood vessel. Blood clots look like dark purple jelly. Clots are made up of things in your bloodstream: Platelets... read more . 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 Overview of the Immune System The immune system is your body's defense system. It helps protect you from illness and infection. The immune system's job is to attack things that don’t belong in your body, including: Germs... read more . 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
Platelets are tiny particles smaller than red or white blood cells. They work with proteins in your blood to help your blood clot How Blood Clots A blood clot is a clump of material that your body makes to plug up a bleeding blood vessel. Blood clots look like dark purple jelly. Clots are made up of things in your bloodstream: Platelets... read more so you can stop bleeding Bruising and Bleeding Bruising or bleeding after an injury is normal. However, some people have blood clotting disorders that cause them to bruise or bleed too easily, such as after very minor injuries or even no... read more .
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 Overview of Anemia Anemia is not having enough red blood cells or hemoglobin. Red blood cells carry oxygen from your lungs to all of your other organs. Hemoglobin is the substance inside your red blood cells that... read more ) 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 Polycythemia Vera Polycythemia vera is a myeloproliferative neoplasm of the blood-producing cells of the bone marrow that results in overproduction of all types of blood cells. Polycythemia vera is due to mutations... read more .
Having too few white blood cells Low White Blood Cell Count White blood cells are part of your body's immune system defenses against infection and cancer. Your body has several types of white blood cells. Most white blood cells are made in your bone... read more makes you more likely to get an infection. Having too many white blood cells High White Blood Cell Count White blood cells (leukocytes) are an important part of the body’s defense against infectious organisms and foreign substances ( the immune system). The number of white blood cells (white cell... read more can be a sign of a type of blood cancer called leukemia Leukemias .
Too few platelets Overview of Low Platelet Count (Thrombocytopenia) Platelets are small cells that circulate in your bloodstream and help your blood clot. They're made in your bone marrow like other blood cells. Platelets are also called thrombocytes. Your blood... read more 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.
