Blood, heart and circulation: components and functions of blood

Learning objectives

By the end of this lesson, you should be able to:

• identify the main components of mammalian blood
• describe the structure and function of red blood cells, white blood cells and platelets
• explain the roles of plasma
• relate blood groups and clotting to safe blood transport
• distinguish the functions of the different blood components

Blood as a transport tissue

Blood is a fluid connective tissue. It transports substances around the body, helps regulate internal conditions and protects the body against disease and blood loss. Mammalian blood consists of plasma and formed elements: red blood cells, white blood cells and platelets.

Plasma

Plasma is the pale-yellow liquid part of blood. It is mostly water and carries dissolved substances, including glucose, amino acids, mineral ions, hormones, carbon dioxide (mainly as hydrogencarbonate ions), urea and plasma proteins. It distributes heat around the body. Red blood cells are suspended in plasma, but plasma itself does not carry most oxygen; haemoglobin inside red blood cells performs that role.

Red blood cells

Red blood cells, or erythrocytes, transport oxygen from the lungs to body tissues. They contain haemoglobin, a pigment that combines reversibly with oxygen to form oxyhaemoglobin. In mammals, mature red blood cells have no nucleus and have a biconcave disc shape. These features provide more room for haemoglobin and a large surface area relative to volume. Their flexible membranes allow them to pass through narrow capillaries.

Red blood cells are produced in red bone marrow. They have a limited life span and are removed mainly by the liver and spleen. Iron is needed to make haemoglobin. A shortage of iron can reduce the blood's oxygen-carrying capacity and cause iron-deficiency anaemia.

White blood cells

White blood cells, or leucocytes, defend the body against pathogens. They have nuclei and are generally larger and fewer than red blood cells. Different types have different functions.

Phagocytes engulf and digest microorganisms by phagocytosis. Lymphocytes are involved in specific immune responses. Some produce antibodies that bind to particular antigens; others develop into cells that help provide immune memory. Antibodies are specific, so an antibody against one antigen does not automatically protect against every pathogen.

Platelets and blood clotting

Platelets are small cell fragments that help stop bleeding. When a blood vessel is damaged, platelets gather at the site and help initiate a sequence of reactions that leads to formation of a fibrin mesh. Fibrin traps blood cells and forms a clot. The clot reduces blood loss and helps prevent pathogens from entering the body through the wound.

Clotting requires several factors, including calcium ions. Vitamin K is needed by the liver to produce certain clotting factors. A clot inside an uninjured blood vessel can be dangerous because it may obstruct blood flow; this is different from protective clotting at a wound.

Blood groups and transfusion

The ABO blood group system depends on antigens on the surface of red blood cells and antibodies in plasma. Group A red cells carry A antigen; group B carry B antigen; group AB carry both; group O carry neither A nor B antigen. A transfusion must be compatible because recipient antibodies can react with donor red-cell antigens, causing agglutination and potentially serious haemolysis.

The Rhesus (Rh) system is another important blood-group system. In basic school-level descriptions, a person is Rh-positive if the relevant D antigen is present and Rh-negative if it is absent. Safe transfusion requires proper blood grouping, cross-matching and clinical procedures; the simplified ABO chart alone is not enough to decide a real transfusion.

WASSCE examination focus

A common question asks for the components of blood and one function of each. State plasma, red blood cells, white blood cells and platelets, then give precise roles. Do not say that platelets transport oxygen or that plasma contains haemoglobin. For immunity, distinguish phagocytosis from antibody production.

Quick check

1. Which blood component transports most oxygen?
2. State two adaptations of a mammalian red blood cell.
3. How do phagocytes protect the body?
4. Why must blood be grouped and cross-matched before transfusion?

Answers: 1. Red blood cells, using haemoglobin. 2. Biconcave shape, no nucleus when mature, and flexibility. 3. They engulf and digest pathogens by phagocytosis. 4. To reduce the risk of incompatible antigen-antibody reactions, agglutination and haemolysis.