Blood components

The human body, on average contains about 5 liters of blood, which circulates around the vessels performing multiple functions:

  • Homeostasis - blood regulates elements of the body’s internal environment such as pH, fluid balance and temperature.

  • Transport - blood moves gases such as oxygen and carbon dioxide between the lungs and all the cells in the body as well as distributing nutrients, transporting hormones, and removing waste products.

  • Protection - the many kinds of white blood cells in the blood form the body’s immune system, which protects against disease and infection from bacteria, viruses, fungi, and parasites. Platelets also help to protect against excessive blood loss at sites of injury.

The majority of blood is fluid plasma (see Figure 1), which is mostly water but contains solutes such as proteins (e.g. antibodies), electrolytes, gases, and nutrients.

Suspended in the plasma are the ‘formed elements’ of the blood (see Figure 1). These are the red blood cells, white blood cells, and platelets, which all uniquely contribute to the specialized functions of the blood.

Illustration of a test tube where the composition of blood can be seen. Blood composition is divided into four. From top to bottom of the test tube: plasma, making between fifty-two and sixty-two percent of blood composition; white blood cells, making up less than one percent; platelets, also making approximately one percent of the blood; and red blood cells, which make up between thirty-eight and forty-eight percent of the blood. To the right of the tube there is an illustration of the main blood cells: red blood cells, platelets, lymphocytes, monocytes, neutrophils, basophils, and eosinophils.

Figure 1. The components of blood