Algonquin College
11.7 - Hematopoiesis and Hemostasis
The circulatory system must maintain a constant blood volume and red blood cell concentration to function properly. Blood cells die and must be replaced in the process of hematopoiesis (blood cell production). If blood vessels are injured then hemostasis (blood clotting) occurs to prevent blood volume from decreasing to dangerous levels.

Hematopoiesis (Blood cell production)

Blood cells form in the red bone marrow from stem cells in the process of hematopoiesis. The stem cells differentiate into lymphocytes, erythrocytes, other leukocytes or platelets.

Oxygen levels can routinely drop well below normal upon starting a new higher intensity physical training program or by staying in a higher altitude area. Blood receptors detect the low oxygen levels and signal the kidneys to release erythropoietin into the blood. The erythropoietin travels to the bone marrow where it promotes the development of new red blood cells from stem cells. The concentration of red blood cells rise in the blood giving the body a higher oxygen carrying capacity and increasing blood oxygen levels.

Hemostasis (Blood clotting)

The blood clotting response (hemostasis) is needed to maintain blood volume and pressure after injury. Hemostasis has three main stages: Vascular, platelet and coagulation. It is a complex process involving several cell types and other substances in the blood.

Step 1: Vascular stage

The smooth muscle of the blood vessel(s) at the injury site begin to spasm and constrict. This restricts blood flow to the injury site to slow blood loss.

Step 2: Platelet stage

Endothelium cells in the injury site release chemicals to make their surface sticky, causing platelets to stick to exposed collagen fibers in the injury site. The platelets begin to plug the hole at the injury site in a process called platelet aggregation, which is stimulated by calcium ions. If the platelets seal the injury completely the aggregated platelets are called a platelet plug. The platelet plug is temporary and must be replaced by a more permanent clot in the coagulation stage.

Step 3: Coagulation stage

The coagulation stage requires the interaction of several different chemicals from the cells in the injury site and the blood plasma. Together the steps of coagulation are called the coagulation cascade and can be summarized as follows:
  1. Prothrombin activator forms when PF3 (from platelets), cytokines from damaged tissues and clotting factors in the blood plasma come together

  2. Prothrombin (factor X), a soluble factor in the plasma is converted to thrombin by the prothrombin activator

  3. Thrombin (fibrin-stabilizing factor) converts the soluble plasma protein fiber fibrinogen into the insoluble, much longer protein fiber fibrin

  4. A mesh of fibrin forms over the injury site, trapping red blood cells and forming a clot, which seals the injury site until the damaged tissues are healed