Chapter 72 – Haemostasis

Abstract

‘Haemostasis’ is a collective term for the mechanisms that stop blood loss. Macroscopically, the most obvious haemostatic mechanism is the conversion of liquid blood to a solid gel – a process called coagulation. The process of clot formation is known as thrombosis.

Chapter 72 Haemostasis

What is haemostasis?

Haemostasis can be life-saving: when blood vessels have been damaged, it is important that haemostasis occurs rapidly to prevent excessive blood loss. However, it is equally important that the haemostatic response is controlled and localised to the area of vessel damage. Widespread coagulation could prevent blood flow completely, damage red blood cells (RBCs) through microangiopathic haemolytic anaemia or lead to paradoxical bleeding due to depletion of clotting factors through disseminated intravascular coagulation (DIC). Therefore, the mechanisms that promote and inhibit haemostasis are finely balanced.

The three main components involved in haemostasis are:

Platelets;
Endothelium;
Coagulation proteins.

All three must be intact for haemostasis to be effective.

How does the vascular endothelium prevent haemostasis?

The endothelium is extremely important in the balance between haemostasis and anti-haemostasis. Its normal function is to prevent haemostasis and promote blood flow, but when damaged, it rapidly initiates a haemostatic response. The endothelium prevents haemostasis by:

Inhibition of platelet adhesion through the secretion of nitric oxide (NO) and prostacyclin (PCI₂). The endothelium also produces the enzyme adenosine diphosphatase, which degrades ADP, an essential compound for platelet activation.
Anticoagulant effects result from two endothelial membrane-bound proteins:
1. – Heparan sulphate, which has a similar structure to heparin, activates the plasma protein antithrombin III, which in turn inactivates thrombin (also known as factor IIa) and factor Xa.
2. – Thrombomodulin has two roles: it directly binds thrombin, effectively removing thrombin from the circulation. The thrombin–thrombomodulin complex also activates protein C. Together with a cofactor (protein S), activated protein C is a potent anticoagulant, inactivating factor Va and VIIIa.
Fibrinolytic effects – endothelial cells also secrete the enzyme tissue plasminogen activator (t-PA). This potent enzyme cleaves the proenzyme plasminogen to form plasmin. Plasmin degrades fibrin clots from the endothelial cell surface in a process called fibrinolysis (see p. 348).

Outline the steps involved in haemostasis

Clot formation has three key steps:

Vasoconstriction;
Platelet aggregation;
Coagulation.

When a vessel is disrupted, platelets must aggregate and plug the hole. To prevent the platelet plug being washed away as it is being formed, the vessel first vasoconstricts, resulting in decreased blood flow – this also has the effect of minimising blood loss. The plasma coagulation proteins then trigger a fibrin mesh to form around the platelet plug, resulting in a stable clot.

How is haemostasis initiated?

When a vessel is damaged, plasma becomes exposed to a number of substances:

von Willebrand factor (vWF). Endothelial cells are the main site of the synthesis and storage of vWF. Normally, a limited amount of vWF is secreted into the vessel lumen, where it binds to factor VIII, protecting the clotting factor from degradation. A damaged vessel releases a large amount of vWF from its endothelial cells; vWF then binds platelets to subendothelial collagen fibres.
Collagen fibres. Vessel damage exposes subendothelial collagen fibres. Platelets bind to collagen (through a bridging vWF molecule) and become activated.
Tissue factor (TF) is expressed by subendothelial cells, such as smooth muscle cells, but not normally by endothelial cells, unless they become damaged. TF activates plasma coagulation proteins (through the extrinsic pathway), culminating in the production of thrombin.

Describe the steps involved in platelet activation and aggregation

Platelets are disc-shaped (hence the name) anuclear cell fragments. They have a short lifespan in the circulation, typically of 7 days. Blood vessel damage exposes TF, collagen and vWF. Passing platelets are activated by collagen and vWF, and also by thrombin itself, produced by TF activation of the coagulation cascade. When platelets are activated, they change shape from disc-like to stellate and release a number of molecules from storage granules (Figure 72.1):

Serotonin (5-HT);
Thromboxane A₂;
ADP;
Platelet activating factor (PAF);
vWF;
Fibrinogen;
Thrombin;
Ca²⁺ ions;
Platelet-derived growth factor (PDGF).

The key steps of platelet aggregation are:

Serotonin and thromboxane A₂ are potent vasoconstrictors that reduce blood flow at the site of injury.
Platelets are attracted to the site of injury. Initially, platelets are activated by and bind to subendothelial collagen (via vWF); the exposed collagen becomes coated in a layer of platelets. The next cohort of platelets cannot make contact with collagen. Instead, they are activated by the ADP molecules released from the first cohort of platelets. Binding of ADP causes the platelets to change shape and release more chemicals from storage granules.
Activated platelets exhibit a glycoprotein Ilb/IIIa receptor on their surface. Fibrinogen and vWF ‘glue’ platelets together through this receptor (Figure 72.1). More and more platelets become activated and bind to the site of injury, forming a soft platelet plug.
The soft platelet plug formed is often not enough to achieve haemostasis – the plug must be reinforced to make a strong clot. Conveniently, the strands of fibrinogen that are interwoven in the soft platelet plug are converted to fibrin (a strong insoluble protein) by thrombin, the end point of the coagulation cascade.

Figure 72.1 Platelet activation and aggregation.

Describe the steps of the coagulation cascade

The coagulation cascade is a complex biochemical pathway involving a number of plasma proteins, culminating in the formation of thrombin. The coagulation cascade is an example of a biological amplification system: starting from a small number of activated molecules, the sequential activation of circulating coagulation proteins results in a magnified response.

Classically, the coagulation cascade is initiated by one of two distinct pathways: intrinsic or extrinsic (Figure 72.2). The two pathways converge on a final common pathway, resulting in thrombin formation. These classical pathways explain the mechanism of coagulation in vitro and reflect the laboratory clotting screen. However, there are a number of flaws with the classical model that have more recently led to the development of a cell-based coagulation model, which is thought to better reflect the mechanism of in vivo coagulation.

Clinical relevance: antiplatelet drugs

Antiplatelet drugs are commonly prescribed for patients at risk of arterial thrombosis, such as those with myocardial infarction or stroke. Antiplatelet drugs act through different mechanisms, each targeting a different part of the platelet activation and aggregation mechanism:

Cyclo-oxygenase (COX) inhibitors; for example, aspirin. Thromboxane A₂ is a potent platelet activator and vasoconstrictor produced from platelet membrane phospholipid, a process that is catalysed by the enzyme COX. At the same time, endothelial cells produce PGI₂, a platelet inhibitor also catalysed by COX. The balance between thromboxane A₂ and PGI₂ determines whether a clot is formed. Aspirin is a non-specific, irreversible inhibitor of COX that prevents platelets from producing thromboxane A₂ and endothelial cells from producing PGI₂. However, as platelets contain no nucleus, COX remains inhibited for the entirety of their lifespan (approximately 7 days). Conversely, endothelial cells may produce new COX within hours. The result is a net increase in platelet inhibition.
ADP receptor antagonists; for example, clopidogrel, ticagrelor and prasugrel. This class of drug specifically blocks the platelet ADP receptor, which prevents further platelet activation and inhibits the expression of the glycoprotein llb/llla complex, thus inhibiting platelet aggregation.
Glycoprotein llb/llla inhibitors; for example, tirofiban and abciximab. Platelet aggregation is inhibited by preventing platelets from binding to fibrinogen.
Phosphodiesterase inhibitors; for example, dipyridamole, which acts through a number of possible mechanisms. One mechanism is the inhibition of the platelet phosphodiesterase enzyme, whose usual role is to break down cyclic AMP (cAMP). Increased platelet cAMP inhibits ADP release, resulting in impaired platelet aggregation and reduced thromboxane A₂ synthesis.