Facts About Blood and Blood Cells

This information explains the different kinds of blood cells and their functions.

Blood carries oxygen and nutrients to all of the cells in your body. Blood cells also fight infection and control bleeding.

Most blood cells are made in the bone marrow. They are constantly being made and replaced, based on their lifespan.

Blood is made up of 4 components: red blood cells, white blood cells, platelets, and plasma.

Blood Components

Red blood cells (erythrocytes)

Red blood cells carry oxygen from your lungs to your tissues and bring carbon dioxide back to your lungs. They make up about 44% of your blood. The lifespan of a red blood cell is around 120 days.

White blood cells (leukocytes)

White blood cells fight infection and help you develop immunity. They make up less than 1% of your blood. There are 3 types of white blood cells: granulocytes, monocytes, and lymphocytes. Each type has specific functions.

  • Granulocytes have 3 subtypes:
    • Neutrophils, which help fight bacterial and fungal infections.
    • Basophils, which are associated with immune responses, although their exact function is not well known.
    • Eosinophils, which help fight parasitic infections.
  • Monocytes break down and remove foreign organisms and dying cells from your body.
  • Lymphocytes make up the immune system.

White blood cells have a wide range of lifespans, from hours to years.

Platelets (thrombocytes)

Platelets are colorless fragments of cells whose main function is to control bleeding. They make up less than 1% of your blood. The lifespan of platelets is about 9 to 12 days.


Plasma is the pale-yellow liquid part of your blood that holds all of the blood cells. It makes up about 55% of your total blood volume.

Plasma transports water, nutrients, minerals, medications, and hormones (throughout your body), and carries waste products to your kidneys for removal from your body. It is made up of water, protein, lipids (fats), and hormones.