Hemostasis

Not to be confused with homeostasis.

Hemostasis or haemostasis (from the Ancient Greek: αἱμόστασις haimóstasis "styptic (drug)") is a process which causes bleeding to stop, meaning to keep blood within a damaged blood vessel (the opposite of hemostasis is hemorrhage). Most of the time this includes blood changing from a liquid to a solid state. Intact blood vessels are central to moderating blood's tendency to clot. The endothelial cells of intact vessels prevent blood clotting with a heparin-like molecule and thrombomodulin and prevent platelet aggregation with nitric oxide and prostacyclin. When endothelial injury occurs, the endothelial cells stop secretion of coagulation and aggregation inhibitors and instead secrete von Willebrand factor which initiate the maintenance of hemostasis after injury. Hemostasis has three major steps: 1) vasoconstriction, 2) temporary blockage of a break by a platelet plug, and 3) blood coagulation, or formation of a clot that seals the hole until tissues are repaired.

History of Hemostasis

The process of preventing blood loss from a vessel or organ of the body is referred to as Hemostasis. The term comes from the Greek roots "heme" meaning blood, and "stasis" meaning halting; Put together means the "halting of the blood". The origin of Hemostasis dates back as far as ancient Greece; first referenced to being used in the battle of Troy. It started with the realization that excessive bleeding inevitably equaled death. Vegetable and mineral styptics were used on large wounds by the Greeks and Romans until the takeover of Egypt around 332BC by Greece. At this time many more advances in the general medical field were developed based off the study of Egyptian mummification practice, which led to greater knowledge of the hemostatic process. It was during this time that many of the veins and arteries ran throughout the human body were found and this directions in which they traveled. Doctors of this time realized if these were plugged, blood could not continue to flow out of the body. Nevertheless it took until the invention of the printing press during the fifteenth century for medical notes and ideas to travel westward, allowing for the idea and practice of Hemostasis to be expanded.

Overview

Hemostasis occurs when blood is present outside of the body or blood vessels. It is the instinctive response for the body to stop bleeding and loss of blood. During Hemostasis three steps occur in a rapid sequence. Vascular spasm is the first response as the blood vessels constrict to allow less blood to be lost. In the second step, platelet plug formation, platelets stick together to form a temporary seal to cover the break in the vessel wall. The third and last step is called coagulation or blood clotting. Coagulation reinforces the platelet plug with fibrin threads that act as a “molecular glue”.

Platelets are a large factor in the hemostatic process. They allow for the creation the “platelet plug” that forms almost directly after a blood vessel has been ruptured. Within seconds of a blood vessel’s epithelial wall being disrupted platelets begin to adhere to the sub-endothelium surface. It takes approximately sixty seconds until the first fibrin strands begin to intersperse among the wound. After several minutes the platelet plug is completely formed by fibrin. Hemostasis can be broken down into three main steps occurring in rapid sequence after disruption to a blood vessel.

Mechanisms

Hemostasis is maintained in the body via three mechanisms:

• Vascular spasm - Damaged blood vessels constrict. Vascular spasm is the blood vessels first response to injury. The damaged vessels will constrict (vasoconstrict) which reduces the amount of blood flow through the area and limits the amount of blood loss. This response is triggered by factors such as a direct injury to vascular smooth muscle, chemicals released by endothelial cells and platelets, and reflexes initiated by local pain receptors. The spasm response becomes more effective as the amount of damage is increased. Vascular spasm is much more effective in smaller blood vessels.

• Platelet plug formation - Platelets adhere to damaged endothelium to form platelet plug (primary hemostasis) and then degranulate. Platelet Plug Formation: Platelets play one of the biggest factors in the hemostatic process. Being the second step in the sequence they stick together (aggregation) to form a plug that temporarily seals the break in the vessel wall. As platelets adhere to the collagen fibers of a wound they become spiked and much stickier. They then release chemical messengers such as adenosine diphosphate (ADP), serotonin and thromboxane A2. These chemicals are released to cause more platelets to stick to the area and release their contents and enhance vascular spasms. As more chemicals are released more platelets stick and release their chemicals; creating a platelet plug and continuing the process in a positive feedback loop. Platelets alone are responsible for stopping the bleeding of unnoticed wear and tear of our skin on a daily basis.

• Blood coagulation - Clots form upon the conversion of fibrinogen to fibrin, and its addition to the platelet plug (secondary hemostasis). Coagulation: The third and final step in this rapid response reinforces the platelet plug. Coagulation or blood clotting uses fibrin threads that act as a glue for the sticky platelets. As the fibrin mesh begins to form the blood is also transformed from a liquid to a gel like substance through involvement of clotting factors and pro-coagulants. The coagulation process is useful in closing up and maintaining the platelet plug on larger wounds. The release of Prothrombin also plays an essential part in the coagulation process because it allows for the formation of a thrombus, or clot, to form. This final step forces blood cells and platelets to stay trapped in the wounded area. Though this is often a good step for wound healing, it has the ability to cause severe health problems if the thrombus becomes detached from the vessel wall and travels through the circulatory system; If it reaches the heart or brain it could lead to stroke, heart attack, or pulmonary embolism. However, without this process the healing of a wound would not be possible.

Steps

Aggregation of thrombocytes (platelets). Platelet rich human blood plasma (left vial) is a turbid liquid. Upon addition of ADP, platelets are activated and start to aggregate, forming white flakes (right vial)

• The first step is immediate constriction of damaged vessels caused by vasoconstrictive paracrine released by the endothelium. Vasoconstriction temporarily decreases blood flow and pressure within the vessel. When you put pressure on a bleeding wound, you also decrease flow within the damaged vessel.
• Vasoconstriction is rapidly followed by the second step, The second stage of Hemostasis involves platelets that move throughout the blood. When the platelets find an exposed area or an injury, they begin to form what is called a platelet plug. The platelet plug formation is activated by a glycoprotein called the Von Willebrand factors (VWF), which are found in the body’s blood plasma. When the platelets in the blood are activated, they then become very sticky so allowing them to stick to other platelets and adhere to the injured area.
• There are a dozen proteins that travel along the blood plasma in an inactive state and are known as clotting factors. Once the platelet plug has been formed by the platelets, the clotting factors begin creating the platelet plug. When this occurs the clotting factors begin to form a collagen fiber called fibrin. Fibrin mesh is then produced all around the platelet plug, which helps hold the fibrin in place. Once this begins, red and white blood cells caught up in the fibrin mesh which causes the clot to become even stronger.

Types of Hemostasis

Hemostasis can be achieved in various other ways if the body cannot do it naturally (or needs help) during surgery or medical treatment. When the body is under shock and stress Hemostasis is harder to achieve. Though natural Hemostasis is most desired, having other means of achieving this is vital for survival in many emergency settings. Without the ability to stimulate Hemostasis the risk of hemorrhaging is great. During surgical procedures the types of Hemostasis listed below can be used to control bleeding while avoiding and reducing the risk of tissue destruction. Hemostasis can be achieved by chemical agents as well as mechanical or physical agents. Which Hemostasis type used is determined based on the situation.

• Chemical/topical- This is a topical agent often used in surgery settings to stop bleeding. Microfibriller collagen is the most popular choice among surgeons because it attracts the patients natural platelets and starts the blood clotting process when it comes in contact with the platelets. This topical agent requires normal hemostatic pathway to be properly functional.
• Electrocoagulation-This type of Hemostasis uses radio waves to stimulate tissues near the sight that needs blood clotting. This process causes tissues to vibrate, causing heat to be given off which in turn promotes proteins to start the process of coagulation: A essential step in the events of Hemostasis. Electrocoagulation not only starts the process of coagulation, it also causes destruction of tissue to the area that was hit with the electrical current.
• Direct pressure or pressure dressing- This type of Hemostasis approach is most commonly used in situations where proper medical attention is not available. Putting pressure and/or dressing to a bleeding wound only slows the process of blood loss, allowing for more time to get to a emergency medical setting. Soldiers use this skill during combat when someone has been injured because this process allows for blood loss to be decreased, giving the system time to start coagulation.
• Sutures and ties- Sutures are often used to close an open wound, allowing for the injured area to stay free of pathogens and other unwanted debris to enter the site;however, it is also essential to the process of Hemostasis. Sutures and ties allow for skin to be joined back together allowing for platelets to start the process of Hemostasis at a quicker pace. Using sutures results in a quicker recovery period because the surface area of the wound has been decreased.
• Physical agents (gelatin sponge)- Gelatin sponges have been indicated as great Hemostatic devices. Once applied to a bleeding area, a gelatin sponge quickly stops or reduces the amount of bleeding present. These physical agents are mostly used in surgical settings as well as after surgery treatments. These sponges absorb blood, allow for coagulation to occur faster, and give off chemical responses that increase the time it takes for the Hemostasis pathway to start.

Hemostasis in emergency medicine

In medicine, hemostasis is achieved through multiple techniques. Hemostasis prevents shock.

Hemostasis is an important factor in emergency situations. Without this process the body would not be able to survive because bleeding would be too great, leading to hemorrhaging. Hemostatic agents have become a very significant part of emergency medicine due to their ability to aide in speeding up the blood clotting process.

• There are various types of hemostatic agents. The most commonly used is frozen plasma (FFP). Whereas, another agent called Cryoprecipitate is used for low fibrinogen. Both of these agents come in absorbable and non-absorbable topical agents. The absorbable style can be absorbed by the body via direct contact to the wound, which is a direct pathway into the blood stream. Though these topical agents give a direct route to injured area they can carry side effects such as, paralysis and nerve damage. The non-absorbable style, however, is applied directly to the skin as topical cream and then covered with a surgical dressing resulting in little to no side effects.

• The only known controversy about these Hemostatic agents is that there is no way to test or have clinical trials to see if there are long term effects of these blood clotting agents. All of the tests that have been conducted for these agents have only been tested on animals, causing a concern for the potential affects these products could have on humans in the future.

• Debates still continue to rise on the subject of Hemostasis and how to handle situations with large injuries. If an individual did acquire a large injury resulting in extreme blood loss, then a hemostatic agent alone would not be very affective. Medical professionals continue to debate on what the best ways to assist a patient in a chronic state are; however, it is universally accepted that hemostatic agents are the primary tool for smaller bleeding injuries.

Ladder of haemostasis

Disorders

Thrombocytopenia- is a mucosal bleeding disorder that causes Epistaxis, menorrhagia, and GI bleeding.

· Hemophilia- There are three types of hemophiliacs

• Hemophilia A-is a hereditary bleeding disorder, where the blood can’t clot properly because it has a lack of factor VIII.

• Hemophilia B-is a hereditary bleeding disorder, where the blood can’t clot properly because it has a lack of factor IX.

• Von Willebrand disease- Is the most common inherited bleeding disorder, Without the Von Willebrand factor the platelets are not able to adhere to the blood vessel walls.

These bleeding disorders are associated with the body’s lack of ability to perform the natural steps of hemostasis. When an area is injured and begins to bleed there is no way for the body to halt the blood loss from occurring.

For patients with these types of hemostasis conditions hemophilia therapy is used to try and help make of for the lack of ability to form blood clots. The best thing for these people is to try and prevent themselves from bleeding. Hemophilia therapy is where the clotting factors that are missing are actually replaced via medications. Most bleedings will stop after one dose but some require multiple treatments.

External links

• Hemostasis at the U.S. National Library of Medicine Medical Subject Headings (MeSH)