Haematopoiesis refers to the commitment and differentiation processes that lead to the formation of all blood cells from haematopoietic stem cells. Haematopoiesis is also known as haemopoiesis or hemopoiesis and describes the process of blood cell formation. In adults, haematopoiesis occurs mainly in the bone marrow (medullary), but it can also occur in other tissues such as the liver, thymus and spleen (extramedullary). Haematopoiesis describes the process by which the cellular components of the blood are formed. In adults, the predominant site of haematopoiesis is the bone marrow. Here we find the multipotent hematopoietic stem cells (HSCs). The HSCs are able to differentiate into both myeloid or lymphoid cell lines.

Blood is composed of haematocrit and plasma (approx. 45% and 55% respectively). Haematocrit is composed of erythrocytes and it is the densest component of blood. In contrast, plasma is mainly composed of water - though it also contains protein (e.g. albumin, immunoglobulins, fibrinogen) and non-protein components (e.g. vitamins, lipids, hormones). When left stand within a test tube blood will separate out into its different layers. The more dense haematocrit will aggregate at the bottom, whilst plasma will float on the top. A thin layer, termed the 'buffy coat', sits in-between. It contains leucocytes and platelets. It accounts for <1% of blood volume.

The predominant site of haematopoiesis in the foetus during the second trimester is the liver. From seven months, the bone marrow takes over and continues to be the predominant site in adults. In the first four years of life, almost all marrow cavities contain haematopoietic tissue, termed red marrow. As we age, the quantity of this haematopoietic tissue decreases and is replaced by fat (yellow marrow). In adults red marrow is limited to the axial skeleton and long bones.