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The blood and bone marrow

The blood and bone marrow are closely related. The blood is made up of blood cells in a liquid called plasma. Blood cells are made in the bone marrow, which is the soft, spongy area inside of 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 a 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 (also called 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. It also carries carbon dioxide away from cells so it can be exhaled by the lungs. Almost all of the cells in the blood are red blood cells.

White blood cells (also called 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.

There are different types of white blood cells.

  • Neutrophils and monocytes fight infection and eat (ingest) germs and dead cell parts.
  • Lymphocytes are an important part of the immune system. They help make antibodies and fight infection.
  • Eosinophils and basophils are involved in allergic reactions and respond to inflammation in the body.

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

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.

Stem cells

Stem cells are found in the bone marrow, blood and umbilical cord blood. In adults, most stem cells are found in the bone marrow. The bone marrow has about 10 to 100 times more stem cells than the blood. Stem cells develop into blood cells in the bone marrow. When blood cells are mature and able to function, they leave the bone marrow and move into the blood.

Stem cells in the blood 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

Blood cells develop along 2 cell lines.

The lymphoid cell line has stem cells that develop into lymphocytes. Lymphocytes are a type of white blood cell. Lymphocytes help fight infection and destroy abnormal cells.

The myeloid cell line has stem cells that develop into red blood cells, platelets and other types of white blood cells (neutrophils, eosinophils, basophils and monocytes). Red blood cells carry oxygen to all tissues of the body. Platelets form clots in damaged blood vessels to stop bleeding. Granulocytes (neutrophils, eosinophils and basophils) and monocytes are white blood cells that destroy bacteria and other foreign invaders and help fight infection.

Blood Cell Development

The blood and cancer

Cancer can spread through the blood or lymphatic system to other parts of the body. This happens when cancer cells break off from the main tumour and travel through the blood or lymph. The cancer cells sometimes get stuck inside tiny blood vessels where they can break through the wall of the blood vessel and start dividing to form another mass of cancer cells that forms a new tumour. Tumours that develop in other parts of the body far from the main tumour are called metastases.

Researchers are looking for ways to find cancer in the blood with a simple blood test. This method is called a liquid biopsy.

Bone marrow

Bone marrow is the soft spongy area inside of most bones. The bone marrow makes blood cells.

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, breast bone (sternum) and skull.

There are 2 main types of tissue in the bone marrow. One type is where stem cells develop into red blood cells, white blood cells and platelets (called red bone marrow). The other type stores fatty tissue (called yellow bone marrow).

The bone marrow and cancer

Cancer and cancer treatments can affect the bone marrow.

Several types of cancer can start in bone marrow. These include leukemia, lymphoma and multiple myeloma. These cancers are also called blood cancers since they change the way blood cells are made and how they work. People who have a type of blood cancer have many abnormal blood cells in the bone marrow. These cells crowd out normal blood cells and prevent them from developing into mature blood cells. This can lead to anemia, to a weakened immune system that makes it hard for the body to fight off infection and to bleeding problems.

Cancer treatments like chemotherapy and radiation therapy can affect the bone marrow so it doesn’t make normal numbers of blood cells. This can cause low blood cell counts, a condition called bone marrow suppression. People with low blood cell counts can also develop anemia, a weakened immune system and bleeding problems.

immune system

The complex group of cells and organs that defend the body against infection, disease and foreign substances.


The lower part of the abdomen between the hip bones that contains the urinary system and reproductive system.

Pelvic means referring to or having to do with the pelvis, as in pelvic exam.


A reduction in the number of healthy red blood cells.


The invasion by and multiplication of organisms that cause disease (such as viruses, bacteria, yeast or fungi) in the body. Symptoms depend on where the infection occurs in the body and may include fever, inflammation and other problems.

An infection that occurs because of a weakened immune system is referred to as an opportunistic infection.


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