The haematological system: made incredibly easy

The haematological system consists of the blood and bone marrow. Blood delivers oxygen and nutrients to all tissues, removes wastes, and transports gases, blood cells, immune cells, antibodies and hormones throughout the body.

Living up to their potential

The haematological system manufactures new blood cells through a process called haematopoiesis.

Multipotential stem cells in bone marrow give rise to five distinct cell types, called unipotential stem cells. Unipotential cells differentiate into one of the following four types of blood cells:

• eryhrocytes (the most common type)
• granulocytes
• agranulocytes
• platelets.

Blood components

Blood consists of various formed elements, or blood cells, suspended in a fluid called plasma. The RBCs of blood - and the WBCs and platelets, too Formed elements in the blood include:

• red blood cells (RBCs), or erythrocytes
• white blood cells (WBCs), or leucocytes
• platelets, or thrombocytes.

RBCs and platelets function entirely within blood vessels; some WBCs remain in the blood while others can enter tissues.

Red blood cells

RBCs mainly transport oxygen to from the lungs to the body tissues. They contain haemoglobin, the oxygen-carrying substance that gives blood its red colour. When the red cells have given up the oxygen, they are capable of transporting some carbon dioxide back to the lungs for removal.

However, carbon dioxide is mainly transported in the blood as bicarbonate.

The life and times of the RBC

RBCs have an average life span of 120 days. Bone marrow releases RBCs into circulation in immature form as reticulocytes. The reticulocytes mature into RBCs in about 1 day. The spleen removes old, worn-out RBCs from circulation.

A balance between removal and renewal

The rate of reticulocyte release usually equals the rate of old RBC removal. When RBC depletion occurs (e.g. with haemorrhage), the bone marrow increases reticulocyte production to maintain the normal RBC count.

White blood cells

Five types of WBCs participate in the body’s defence and immune systems. These five types of cells are classifi ed as granulocytes (neutrophils, eosinophils and basophils) and agranulocytes (monocytes and lymphocytes).

Granulocytes

Granulocytes are a group of WBCs that contain granules in their cytoplasm. They can be subclassified into neutrophils, eosinophils and basophils. Neutrophils contain a multilobed nucleus, while nucleus of eosinophils and basophils are bilobed. Each cell type exhibits different properties and each is activated by different stimuli.

Haematological changes with aging

As a person ages, fatty bone marrow replaces some of the body’s active blood-forming marrow first in the long bones and later in the flat bones. The altered bone marrow can’t increase erythrocyte production as readily in response to such stimuli as hormones, anoxia, haemorrhage and haemolysis.

Vitamin B12 absorption may also diminish with age, resulting in reduced erythrocyte mass and decreased haemoglobin levels and haematocrit (packed cell volume).