Haemostasis Physiology

Haemostasis is the term to describe the control of bleeding in the body. The main players involved in haemostasis are the wall of the blood vessel, platelets, and clotting factors.

Blood Vessel Wall

The vessel wall is made of endothelial cells, which produce various substances. When the vessel wall is intact, it prevents clot formation. When the vessel wall is damaged, it releases substances and exposes underlying connective tissue, which subsequently activates the processes involved in forming a clot.1

Platelets

Platelets are produced by megakaryocytes in the bone marrow. Their life span in the blood is 8-12 days. Thrombopoietin (TPO) is a hormone produced by the liver, which stimulates increases platelet production in the bone marrow. Platelets have a phospholipid membrane which contains glycoprotein receptors, including GPIb and GPIIbIIIa. Fibrinogen links platelets to each other via GPIIbIIIa receptors. Platelets bind to von Willebrand factor in the endothelium via their GPIb receptors. The platelet cytoplasm contains storage granules of 3 different types: dense granules, alpha granules, and glycogen granules.1

Clotting Factors

Clotting factors are proteins made by the liver. They are involved at different stages of the clotting cascade. There are some clotting factors that require fat-soluble vitamin K in order to be synthesized. These include:

Factor 2 (Prothrombin)
Factor 7
Factor 9
Factor 10

Factors are released in an inactive form, and are only converted to their active form when activated in the clotting cascade.1

Haemostasis

Haemostasis can be broken down into the following stages: 1) Primary haemostasis (formation of platelet plug), 2) Secondary haemostasis (fibrin formation to stabilise the plug), 3) Anticoagulation, and 4) Fibrinolysis.

Primary Haemostasis

When vessel wall injury occurs, the response by the blood vessel is to vasoconstrict, so that blood loss is limited. Platelets bind to vWF via their GPIb receptors, which helps binds them to the subendothelial collagen. Platelets aggregate together using their GPIIbIIIa receptors. They release the contents of their cytoplasmic granules, which help with vasoconstriction, and help promote the formation of clotting factors by providing their phospholipid surfaces on which clotting factors are converted from their inactive to active form.1

Secondary Haemostasis (Clotting Cascade)

In secondary haemostasis, the platelet plug is made stronger with a protein called fibrin. This process is mediated by clotting factors that get activated one by one in a coagulation cascade process. Tissue factor released from the damaged endothelium activates clotting factors in a series of steps. This ultimately leads to the activation of the enzyme thrombin, which catalyses the conversion of soluble plasma fibrinogen to insoluble fibrin molecules. These fibrin molecules can then polymerise to form fibrin strands. These strands stabilise platelet aggregates and red cells at the site of injury to form a strong fibrin plug.1

The clotting cascade has been described as the intrinsic and extrinsic pathway, which then converge in the common pathway. This is not what happens in vivo, but is used for convenienvce of describing the process and based on the clotting tests used in the laboratory. The intrinsic pathway is measured by the APTT, and the extrinsic pathway is measured by the PT.1

TOP TIP: One way to remember this is to think of the 2 T's in APTT forming an I for Intrinsic!

Hover your mouse over the boxes to see the different parts of the clotting cascade

Intrinsic Pathway

This pathway augments thrombin production initiated by the intrinsic pathway so that sufficient quantities are produced for clot formation

Extrinsic Pathway

Tissue factor exposed in damaged vessels initiates the production of thrombin.

Common Pathway

Both pathways converge to produce thrombin, which converts fibrinogen to fibrin

Clinical Application: Interpreting Clotting Tests

The PT measures factors involved in both the extrinsic and common pathway. Factors involved in PT include Factors 2,5, 7, and 10. Factor 7 is the only factor in the extrinsic pathway, all others are in the common pathway. Warfarin prolongs the PT because it is a vitamin K antagonist and therefore affects production of vitamin-K dependent clotting factors 2,7,9, and 10. Warfarin has more of an impact on the PT but will also prolong the APTT.

The APTT measures factors involved in the intrinsic and common pathway. Factors involved in APTT include factors 8,9, 11, 12 (only one of these are impacted by warfarin, which is why warfarin has less of an effect on APTT). Causes of isolated prolonged APTT include Haemophilias (Factor 8 and 9 deficiency), and lupus anticoagulant (anti-phospholipid antibody). If this antibody is present in the patient's plasma, it will react with the phospholipid used to conduct the APTT test. Even though it prolongs the APTT (suggesting increased risk of bleeding), the presence of this antibody actually increases the risk of CLOTTING. Unfractionated heparin is also a cause of a prolonged APTT. Unfractionated heparin works by enhancing the effect of anti-thrombin and inactivating factor 10a and 9a (which is part of the APTT!).

What happens once bleeding has stopped?

The final step of the coagulation process is to re-establish normal blood flow through the vessel. This is achieved via anticoagulation and fibrinolysis.

Anticoagulation

Anticoagulation is the process by which the body prevents more clots from forming. This is initiated by vasodilation that is caused by molecules such as prostacyclins and nitric oxide released by the endothelium. The next step in this process is to stop the formation of fibrin. There are three mechanisms of achieving this:

Anti-thrombin is a natural anti-coagulant protein. It inactivates thrombin and other clotting factors (mainly Factor Xa).
Thrombomodulin binds to thrombin to form a thrombin-thrombomodulin complex. This acts to alter the structure of thrombin and hence inactivate it.
Protein C and Protein S inactivate factors Va and VIIIa which prevent further coagulation.1

Fibrinolysis

Fibrinolysis is the process in which pre-existing clots are broken down. This involves the breakdown of fibrin and fibrinogen by the enzyme plasmin. Plasmin is not always found in the blood and needs to be formed from plasminogen. Inactive plasminogen can get activated by tissue plasminogen activators, that are released by endothelial cells, and get converted to plasmin. Plasmin then initiates this process of fibrinolysis. The breakdown products of the fibrinolysis pathway are called fibrin degradation products (FDPs) - an example of this is D-dimer, a test used to determine whether a person has had a venous thromboembolism, or in the workup of a patient with DIC.1

Clinical Application: Anticoagulant medications

Patients at high risk of clot formation (for example, patients with atrial fibrillation, previous strokes or transient ischaemic attacks, or patients with mechanical heart valves) are put on anticoagulant medications which work on the clotting cascade in various ways to prevent clot formation. Medications that work on the clotting cascade include warfarin, which is a vitamin K antagonist and therefore blocks the action of the vitamin K dependent clotting factors (2, 7, 9, and 10). Heparins are natural anti-coagulants in our body which enhance the action of anti-thrombin. Synthetic preparations of heparin include low molecular heparin (example: enoxaparin) or unfractionated heparin. Low molecular weight heparin (LMWH) is commonly used as prophylaxis against venous thromboembolism for surgical patients, or patients in hospital who have prolonged immobility or other risk factors for VTE. Direct Oral Anticoagulants (DOACs) are tablets which block either factor 10a (apixaban, rivaraoxaban, edoxaban) or block thrombin itself (dabigatran).2 Helpful hint: the factor 10a inhibitors have Xa in their names!

When a clot has already formed, there are drugs which can be used to break it down, which include alteplase, streptokinae, and urokinase. These drugs bind to fibrin and helps convert plasminogen into plasmin, which is responsible for fibrinolysis. These thrombolytic drugs are given in the cases of ischemic stroke once a haemorrhagic stroke has been ruled out, and if the patient has presented within 4.5 hours of symptom onset.3

The interactive diagrams below show which anticoagulant drugs work on which part of the clotting cascade!

Hover your mouse over the image to see the action of the drug

Warfarin inhibits the production of the Vitamin K-dependent Factors II, VII, IX and X (and Protein C and Protein S)

References

1. Mehta A, Hoffbrand A. Haematology at a glance. Chichester: Wiley Blackwell; 2014.

2.Harter K, Levine M, Henderson S. Anticoagulation Drug Therapy: A Review. Western Journal of Emergency Medicine. 2015;16(1):11-17.

3. National Institute for Health and Care Excellence. Stroke and transient ischaemic attack in over 16s: diagnosis and initial management. NICE; 2019.