Hemoglobin, a globular protein  
green = haem (or heme) groups  
red & blue = protein subunits
A scanning electron microscope (SEM) image of a normal red blood cell (left), a platelet (middle), and a white blood cell (right)
**Blood** is a liquid in humans and many animals. Blood is pushed through the organism by the heart. It brings nutrients and oxygen to the tissues of the body. It also takes away waste and carbon dioxide from tissues.

Blood is made up of blood plasma and various cells. These include red blood cells, white blood cells and platelets. Platelets help blood to clot. Hemoglobin is in red blood cells. White blood cells help fight infections and heal wounds.

Plasma
------

Blood plasma is the yellow liquid in which blood cells float. Plasma is made up of nutrients, electrolytes (salts), gases, non-protein hormones, waste, lipids, and proteins. These proteins are albumin, antibodies (also called *immunoglobulins*), clotting factors, and protein hormones. Plasma that does not have the protein fibrinogen is called serum and cannot clot. Adults have about 3 liters of plasma. Plasma is a liquid, mostly water (90%). Plasma takes up 55% of blood volume.

Red blood cells
---------------

Red blood cells
Another name for red blood cell is *erythrocyte*. 'Erythro' means red; 'cyte' means cell. **RBC** is an acronym for **r**ed **b**lood **c**ell.

RBCs carry oxygen and carbon dioxide around the body. Cells in the body need oxygen to live. Cells also make carbon dioxide as a waste. 

RBCs are filled with hemoglobin. This is a protein. It is made to carry a large amount of oxygen. Hemoglobin has iron in it. The iron and oxygen gives hemoglobin its red color. Erythropoietin promotes the creation of RBCs. Blood type antigens are carried on the surface of red cells.

RBCs also help the blood stay at a normal pH. The blood needs to be at a pH of 7.4. If it is much more or less than 7.4, a person can get very sick or die. RBCs stops changes in blood pH. The proteins and the carbon dioxide in the RBC do this.

White blood cells
-----------------

White blood cells are a big part of the immune system. They attack things that do not belong in the body. They kill germs such as bacteria and viruses. They kill cancer cells. White blood cells also help to fight other toxic substances.

White blood cells find where the germs are, and start to destroy them. WBCs arrive in the blood. They also go out of the blood in places where there is infection. WBCs do this to fight the germs that make the infection. If they go out of the blood to fight an infection, they may return in the lymphatic system. So WBCs are in lymph nodes. 

Another name for white blood cell is *leukocyte*. Leuko means white. -cyte means cell. **WBC** is an acronym for **w**hite **b**lood **c**ell. There are three main kinds of WBCs. They are lymphocytes, granulocytes and monocytes. Some of the WBCs mature into cells which do similar work in the tissues.

The different WBCs work in different ways. Some WBCs kill and eat germs and cancer cells. Some WBCs make antibodies. These are proteins that stick to a cell and tell other WBCs to kill it. Some WBCs make chemicals. They release these chemicals to fight things that do not belong in the body. These chemicals cause inflammation in a part of the body. When a germ makes someone sick, the body shows it. If a bacteria gets under someone's skin and causes an infection, the skin gets red, hot, and painful. This redness, heat, and pain are signs of *inflammation*. This shows that WBCs are fighting the infection and killing the bacteria.

Platelets
---------

Platelets help make blood clot. A clot is when the liquid blood becomes solid. The body makes blood clot when the skin is cut. This stops blood from going out of the skin too much. 

For blood to be able to clot is essential. But, rarely, some blood clots are bad. If a blood clot happens in a blood vessel going to the brain, it can cause a stroke. If it happens in a blood vessel going to the heart, it can cause a heart attack. This does not usually happen to young, healthy people with the exception of clotting conditions. 

Platelets are not the only things that make clots. There are proteins in the blood that help make clots. Both platelets and *clotting proteins* are needed to make good clots.

Where blood comes from
----------------------

Illustration of bone marrow cells from *Gray's Anatomy*
Blood cells are made in the bone marrow. The spleen (a smaller, vital organ situated inside the ribcage) manages the amount of white blood cells that flow throughout your blood, it also can manage many functions of the liver. The bone marrow is the soft material in the middle of bones. Special cells in the bone marrow make most of the blood cells in the body.

Plasma proteins are made mostly by the liver. The water and electrolytes in plasma come from the food and water that a person eats.

Although blood is a fluid, in some respects it is a kind of connective tissue. Its cells originate in bone marrow and the spleen, and in the blood there are potential molecular fibres in the form of fibrinogen. These are activated when a blood clot forms.

Games
-----

* Bloody Bunny: The Game (2021)
* Bloody Bunny 2: Dark Rabbit's Revenge (2024)

1. ↑  The Franklin Institute. "Blood – the human heart". Archived from the original on 5 March 2009. Retrieved 19 March 2009.
2. ↑  Platelets are also called 'thrombocytes'.