Blood components and their functions

Blood is a tissue composed of a mixture of components:

  1. Plasma
  2. Red blood cells
  3. White blood cells
  4. Platelets

Plasma is the liquid portion of the blood composed mostly of water. It makes up approximately 54% of blood volume. It functions in transporting cells and substances all around the body.

Red blood cells
Red blood cells are also called erythrocytes. They are made in the bone marrow of long bones. They function in transporting oxygen around the body. They have a red pigment called haemoglobin that enables them to carry out this important role. Haemoglobin is a protein with an iron atom at its centre (which is why the mineral iron is an important part of a balanced diet). Red blood cells have a biconcave shape (see below). This enables them to transfer oxygen more efficiently as it gives a greater surface area for diffusion. Red bloods cells do not have a nucleus. This enables them to carry more haemoglobin and hence oxygen and enables them to be smaller cells capable of squeezing through very narrow capillaries.

White blood cells
There are many types and sub-types of white blood cells. They are also produced in the bone marrow and mature in various organs and tissues throughout the body. They function in keeping the body free of pathogens (disease-causing organisms).

For this chapter you only have to know about two types:

Monocytes – develop into more specific white blood cells called macrophages and phagocytes.

Lymphocytes – develop into a variety of sub-types of lymphocytes, some of which are responsible for producing antibodies.

Platelets are also known as thrombocytes. They are also produced in the bone marrow and function in blood clotting.
Deep vein thrombosis can result if blood clots form in a vein – usually in a large vein in the leg.

Blood groups

The ABO blood group system is the most important blood grouping system used.
There are four blood groups:

  • A
  • B
  • AB
  • o

Another blood grouping system used is called the Rhesus system.
Everyone is either Rhesus positive (Rh+) or Rhesus negative (Rh-). If a person is Rhesus positive, then they possess the Rhesus factor on the surface of their red blood cells; if a person is Rhesus negative, then they do not possess the Rhesus factor on their red blood cells.

It is possible for a foetus to have a different blood type to its mother. It is important that the blood of the foetus does not mix with the blood of the mother. The placenta ensure this (see Chapter 41). If they do mix, then a haemolytic reaction can occur where antibodies are produced against either the mother’s red blood cells and/or the baby’s red blood cells.