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The blood cells and blood count 

The blood cells and blood count
Chapter:
The blood cells and blood count
Author(s):

Tyler J. Albert

and Erik R. Swenson

DOI:
10.1093/med/9780199600830.003.0265
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date: 29 February 2020

Blood is a dynamic fluid consisting of cellular and plasma components undergoing constant regeneration and recycling. Like most physiological systems, the concentrations of these components are tightly regulated within narrow limits under normal conditions. In the critically-ill population, however, haematological abnormalities frequently occur and are largely due to non-haematological single- or multiple-organ pathology. Haematopoiesis originates from the pluripotent stem cell, which undergoes replication, proliferation, and differentiation, giving rise to cells of the erythroid, myeloid, and lymphoid series, as well as megakaryocytes, the precursors to platelets. The haemostatic system is responsible for maintaining blood fluidity and, at the same time, prevents blood loss by initiating rapid, localized, and appropriate blood clotting at sites of vascular damage. This system is complex, comprising both cellular and plasma elements, i.e. platelets, coagulation and fibrinolytic cascades, the natural intrinsic and extrinsic pathways of anticoagulation, and the vascular endothelium. A rapid, reliable, and inexpensive method of examining haematological disorders is the peripheral blood smear, which allows practitioners to assess the functional status of the bone marrow during cytopenic states. Red blood cells, which are primarily concerned with oxygen and carbon dioxide transport, have a normal lifespan of only 120 days and require constant erythropoiesis. White blood cells represent a summation of several circulating cell types, each deriving from the hematopoietic stem cell, together forming the critical components of both the innate and adaptive immune systems. Platelets are integral to haemostasis, and also aid our inflammatory and immune responses, help maintain vascular integrity, and contribute to wound healing.

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