The blood and bone marrow

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The blood is made up of blood cells in a liquid called plasma. Blood cells are made in the bone marrow. Bone marrow is the soft, spongy area inside most bones.


Blood is a mixture of plasma and blood cells. Plasma is mostly water but also has protein, salts, fats and sugar (glucose). Blood carries oxygen, carbon dioxide, nutrients, waste products, cells and hormones. It is part of the circulatory system that includes the heart and blood vessels. Testing the blood is an easy way for doctors to look for signs of disease and manage treatment for disease.

Types of blood cells

There are 3 types of blood cells – red blood cells, white blood cells and platelets.

Red blood cells (erythrocytes) carry oxygen to all cells in the body. The oxygen is carried to cells on a protein in red blood cells called hemoglobin. Hemoglobin is what makes the blood red. Almost all of the cells in the blood are red blood cells.

White blood cells (leukocytes) are an important part of the immune system. They fight infection by defending the body against bacteria, viruses and other germs. Most of the body’s white blood cells are outside the blood, in other tissues of the body. Normally, there are very few white blood cells in the blood.

Platelets (thrombocytes) make the blood clot and help stop bleeding. When a blood vessel is damaged, platelets travel to the area and clump together.

Types of white blood cells

There are different types of white blood cells. Each type has a different job.

Lymphocytes make antibodies to fight infection or they can fight invaders directly. They are found in the lymph nodes, thymus, spleen, tonsils, adenoids and bone marrow. They are also found in lymphatic tissue in other parts of the body, such as the appendix, the small intestine and other parts of the digestive system and respiratory system.

The 3 main types of lymphocytes are:

  • B cells, which make antibodies to fight bacteria, viruses and other foreign material, such as fungi
  • T cells, which fight infection, destroy abnormal cells and control the immune response
  • natural killer (NK) cells, which attack foreign cells

Granulocytes fight infection and become active in response to tissue inflammation. The 3 main types of granulocytes are:

  • neutrophils, which fight infection by surrounding and eating (ingesting) bacteria
  • eosinophils and basophils, which attack and destroy certain parasites and are activated during an allergic reaction

Monocytes help fight infection by changing into cells called macrophages, which eat foreign invaders, such as bacteria and waste from dying cells.

How blood cells are formed

Blood cells are formed through a process called hematopoiesis. All blood cells develop from young, immature cells called stem cells. Stem cells can change into the different types of cells, depending on what the body needs. Stem cells develop and mature into the various types of cells by a process called differentiation. When blood cells are mature and able to function, they leave the bone marrow and move into the blood.

Stem cells

In adults, most stem cells are found in the bone marrow. Stem cells can also be found in smaller amounts in the bloodstream. These are called peripheral blood stem cells. Umbilical cord blood also has stem cells but there are fewer stem cells than those in the bone marrow and blood.

Blood cell development

In the earliest stage of blood cell development, stem cells begin to develop into either the lymphoid cell line or the myeloid cell line. In both cell lines, the stem cells become blast cells, which are immature cells.

Lymphoid cell line

Lymphoid stem cells develop into lymphoblasts, which develop into lymphocytes. Lymphocytes are a type of white blood cell. Lymphocytes help fight infection and destroy abnormal cells.

Myeloid cell line

Myeloid stem cells develop into red blood cells, platelets and all of the non-lymphocyte types of white blood cells. Red blood cells carry oxygen to all tissues of the body. Platelets form clots in damaged blood vessels to stop bleeding. The white blood cells that are formed by the myeloid stem cells are called granulocytes and monocytes. Granulocytes can be further subdivided into neutrophils, eosinophils and basophils. These white blood cells destroy bacteria and other foreign invaders and help fight infection.

Diagram of blood cell development
Diagram of blood cell development

Bone marrow

Bone marrow is the soft, spongy area inside most bones. It makes blood cells.

There are 2 main types of bone marrow – red and yellow. Red bone marrow is where stem cells develop into red blood cells, white blood cells and platelets. Yellow bone marrow stores fatty tissue.

In adults, the bone marrow that makes the most blood cells is found in the hip bones (bones of the pelvis), shoulder bones (scapula), bones of the spine (vertebrae), ribs, breastbone (sternum) and skull.

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