Functions of Blood

The Functions of Blood listed on the structures and functions of blood appear again below for reference.

Each of the items listed above can be classified into one the three categories of functions of blood as follows:

The functions of blood can be listed in various ways and described in different levels of detail. Introductory biology courses often include at least basic information about the structure and functions of blood.

One way to remember the main functions of blood is to divide them into the three categories:

  1. Homeostatic functions of blood
    (How does blood help the body to maintain homeostasis?)
  2. Transport functions of blood
    (What does blood move around the body - how, where, why and what are the benefits of this?)
  3. Immune functions of blood
    (How does blood help to defend the body against infection, especially by disease-causing organisms?)

Before considering each of these three categories of functions of blood in more detail, notice how the functions of blood mentioned on the page about the structure and functions of blood (reproduced on the right for ease of reference), fall into one of the three categories listed above.


More about each of the above three categories of functions of blood:

1. Homeostatic functions of Blood

Homeostasis is the maintenance (via the body's physiological mechanisms) of relatively stable conditions within the body's internal environment despite changes occurring both inside and outside the body e.g. due to eating, exercise, pregnancy, variations in external conditions, etc..

The composition of blood plasma provides the cells in the body with a suitable and stable chemical environment.

2. Transport functions of Blood

  1. Transports oxygen from the lungs to tissues around the body
  2. Transports carbon dioxide from tissues around the body to the lungs (for removal from the body)
  3. Transports products of digestion (i.e. nutrients) from the intestine to tissues around the body.
    Soluble products of digestion include glucose, salts, some vitamins and some proteins. They are carried in solution by the blood plasma.
  4. Transports nitrogenous waste from the liver to the kidneys
  5. Transports hormones from hormone producing glands to the target organs of specific hormones. (What is a hormone?)
  6. Transports heat released by chemical processes in the body, i.e. processes of metabolism, to cooler regions of the body or to the skin e.g. on the limbs and head, to be released from the body if all areas of the body already have sufficient heat energy.

Blood does not move from one specific place (part of the body) or organ to another, e.g. "from the liver to the kidneys" (as mentioned in (4.) above. Blood flows through the body's blood circulation system as shown on the diagram of systemic circulation. Therefore a specific molecule might move around the whole body several times before it is removed by a relevant gland or organ e.g. in the case of a hormone for which there is a specific "target organ" or before it is absorbed into tissue that uses it e.g. to release oxygen or a product of digestion.

3. Immune functions of Blood

Of the main constituents of blood (blood plasma, red blood cells, white blood cells and blood platelets), the white blood cells which are called leucocytes, perform important immune functions and are therefore described as a major part of the immune system. However, other parts of blood also help to protect the body against invasion and disease caused by pathogens e.g. red blood cells that form blood clots to seal damaged blood vessels - see (1.) below.

  1. Blood clotting helps to prevent harmful microorganisms e.g. bacteria entering the body via open wounds
    The response of blood and blood vessels to damage to blood vessels (including blood clotting) not only prevents further loss of blood from the site of an injury but also prevents foreign matter, such as harmful bacteria, from entering the body via the site of the injury. (Injuries from which blood is released are called open wounds.)
  2. Phagocytosis
    Monocytes (also known as phagocytes) can engulf and digest small particles such as bacteria, protozoa, cells, cell debris, some of which might otherwise cause harm to the body. This process is called phagocytosis. Phagocytes can protect the body in this way at the site of wounds, within blood vessels or lymph nodes and even within tissues outside of blood vessels because phagocytes are small enough to pass through capillaries to attack bacteria present in the surrounding tissue.
  3. Production of antibodies
    Lymphocytes produce proteins called antibodies that attack the antigens (chemical markers) on the surfaces of potentially harmful cells e.g. of bacteria and other cells not recognized as part of the person's own body.
    Antibodies can work in various different ways, e.g. by
    • attaching to the surface of the alien cells, e.g. bacteria, making it easier for the body's phagocytes to ingest those alien cells, e.g. bacteria.
    • 'clumping' alien cells, e.g. bacteria, together
    • neutralizing toxins (poisonous proteins) released by alien cells, e.g. bacteria.

More information about the different types of blood cells, including the different types of leucocytes (white blood cells) is included on the page about the structure and functions of blood.

Note: Some of the information on this page over-laps with the page about the structures and functions of blood
This page lists the main functions of blood in three general categories with little explanation of the processes and the specific cells involved. See the links in the text for further information about specific functions.

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