Anatomy & Physiology

Anatomy & Physiology Study Guide that activates a second proenzyme, and so on in a chain reaction, or cascade. Enzymes and proenzymes interact during the coagulation phase. Feedback Control of Blood Clotting Thrombin generated in the common pathway stimulates blood clotting by stimulating the formation of tissue factor and stimulating the release of PF-3 by platelets. This positive feedback loop accelerates the clotting process, and speed can be very important in reducing blood loss after a severe injury. Blood clotting is restricted by substances that either deactivate or remove clotting factors and other stimulatory agents from the blood. The clotting process involves a complex chain of events, and disorders that affect any individual clotting factor can disrupt the entire process. As a result, managing many clinical conditions involves controlling or manipulating the clotting response. Calcium ions and vitamin K affect almost every aspect of the clotting process. Adequate amounts of vitamin K must be present for the liver to be able to synthesize four of the clotting factors. Roughly half of the daily requirement is obtained from the diet, and the other half is manufactured by bacteria in the large intestine. Some circumstances may lead to Vitamin K deficiency: an inadequate diet that is deficient in fats or vitamin K, a disorder that affects fat digestion and absorption (such as problems with bile production), or prolonged use of antibiotics that kill normal intestinal bacteria may lead to a vitamin K deficiency. Once the fibrin meshwork has formed, platelets and red blood cells stick to the fibrin strands. The platelets then contract, and the entire clot begins to undergo clot retraction, or syneresis . Occurring over a period of 30-60 minutes, clot retraction produces two critical actions: (1) pulling the torn edges of the vessel closer together, thereby reducing bleeding and stabilizing the injury site, and (2) reducing the size of the damaged area, making it easier for fibrocytes, smooth muscle cells, and endothelial cells to complete repairs. As the repairs proceed, the clot gradually dissolves. This process, called fibrinolysis , begins with the activation of the proenzyme plasminogen by two enzymes: thrombin and tissue plasminogen activator, released by damaged tissues at the site of injury. The activation of plasminogen produces the enzyme plasmin, which begins digesting the fibrin strands and eroding the foundation of the clot.

©2018

Achieve Page 232

of 368