An Introduction to Blood Clots


Published: 18 May 2021

Blood clots affect 30 000 Australians every year. They are potentially fatal if left untreated and account for about 10% of hospital deaths (ACSQHC 2018; Healthdirect 2019a).

As a healthcare professional, being able to recognise and manage blood clots is an essential skill that may save a patient’s life.

Note: While this article is a general overview of blood clots, it will mainly focus on VTE (DVT and PE).

What is a Blood Clot?

A blood clot is a semisolid mass of blood components (platelets, proteins and cells) that clumps together in a process known as coagulation (i.e. the clotting mechanism) (Axtell & Murrell 2018).

The blood components that form clots are referred to as ‘clotting factors’ (Cancer.Net 2019).

Blood is essential for life as it delivers oxygen to the tissues and cells, and a significant loss of blood may be fatal. Coagulation, a component of haemostasis, is the body’s crucial mechanism to prevent significant blood loss when we are injured (Garmo, Bajwa & Burns 2020; Moake 2020).

Coagulation works by forming a clot to plug the blood vessel wall injury. Once the injured blood vessel has healed, the clot will generally dissolve on its own (Garmo, Bajwa & Burns 2020; Moake 2020).

What is the Clotting Cascade?

blot clot clotting cascade diagram

The clotting cascade is a sequence of actions that occur upon the injury of a blood vessel in order to rapidly achieve haemostasis. It causes clotting factors to activate one after the other like a waterfall, resulting in the creation of a blood clot to plug the injury site until it has healed (Lab Tests Online 2021; Chaudhry, Usama & Babiker 2020).

There are two stages of the clotting cascade:

  1. Primary haemostasis, where a weak platelet plug is formed in order to temporarily prevent haemorrhage until stability can be achieved through secondary haemostasis. It comprises four phases: vasoconstriction, platelet adhesion, platelet activation, and platelet aggregation.
  2. Secondary haemostasis triggers the clotting cascade in order to stabilise the weak platelet plug that has been formed. There are two possible pathways for the cascade - intrinsic and extrinsic - both of which meet at the common pathway to finish the process.

(Garmo, Bajwa & Burns 2020)

The intrinsic and extrinsic pathways have different triggers (Reading 2018).

The extrinsic pathway is the shorter and more dominant of the two pathways (Chaudhry, Usama & Babiker 2020; Reading 2018). It is activated in response to:

  • Direct damage to the blood vessel
  • Tissue damage outside of the blood vessel
  • Hypoxia
  • Sepsis
  • Inflammation
  • Malignancy.

(Reading 2018)

The intrinsic pathway is longer (Chaudhry, Usama & Babiker 2020). It is activated in response to:

  • Direct damage to the blood vessel
  • Collagen being exposed to platelets circulating in the blood.

(Reading 2018)

The extrinsic and intrinsic pathways then converge into the common pathway, stabilising the platelet plug (Chaudhry, Usama & Babiker 2020).

For more information, watch the Ausmed lecture The Clotting Cascade - Understanding Coagulation.

Disruption to Haemostasis

While haemostasis is an essential physiological process, there is a delicate balance that must be maintained between clotting factors that promote bleeding and those that promote coagulation (Cancer.Net 2019).

If there is too much bleeding, the individual is at risk of losing excessive blood from a minor injury. On the other hand, if there is too much clotting, uninjured blood vessels can become plugged. Any abnormality in haemostasis may disrupt this balance, with potentially dangerous consequences (Moake 2020).

If the clotting factors that promote coagulation are missing or damaged, there is a risk that the body will form blood clots that block normal blood flow (Cancer.Net 2019).

Types of Blood Clots

Note: A thrombus is a blood clot that forms in a vein or artery (Healthdirect 2020).

Venous thromboembolism (VTE)

Venous thromboembolism (VTE) occurs when a thrombus forms inside healthy veins, obstructing normal blood flow (CEC 2014).

The term VTE is used to describe two conditions:

  1. Deep vein thrombosis (DVT)
  2. Pulmonary embolism (PE).

(ACSQHC 2020)

DVT occurs when a thrombus forms in a deep vein. This can occur anywhere in the body but is most common in the lower leg, thigh, pelvis or arm (CDC 2020). The most common symptoms are pain and swelling in the affected area. In some cases, a DVT will dissolve on its own (Healthdirect 2019a; ACSQHC 2020).

blood clot deep vein thrombosis

PE is a complication of DVT that occurs when the thrombus partially or completely dislodges and travels to the lungs. This is a serious and potentially life-threatening complication (ACSQHC 2020; Healthdirect 2019a).

blood clot pulmonary embolism

Other Types of Blood Clots

  • Blood clot in the brain (ischaemic stroke)
  • Blood clot in the heart (heart attack)
  • Blood clot in the abdomen
  • Blood clot in a vein near the skin (superficial thrombosis).

(Healthdirect 2019a; 2020)

What is Virchow’s Triad?

Virchow’s triad describes the three primary risk factor categories for venous thromboembolism:

  1. Hypercoagulability (blood disorders that increase the risk of clotting), for example, an inherited Factor V Leiden mutation
  2. Abnormal blood flow, for example, stasis due to obesity or immobility
  3. Vessel wall injury (aka endothelial Injury), for example, damage from smoking or sepsis.

(Garmo, Bajwa & Burns 2020; Kushner, West & Pillarisetty 2020)

Risk Factors for Blood Clots

Specific risk factors include:

  • Older age (over 65), partially due to increased risk of comorbidity
  • Immobility due to prolonged travel (e.g. long-haul flights), prolonged surgery, post-surgical immobilisation or trauma
  • Surgery (particularly major orthopedic and neurovascular surgeries)
  • Previous blood clot
  • Smoking
  • Obesity
  • Pregnancy (particularly during the postpartum period), which causes a hypercoagulable state (increased risk for women with multiple pregnancies and women of a late maternal age)
  • Cancer (which is associated with hypercoagulability), particularly pancreatic, ovarian, stomach, renal, adenocarcinoma, glioblastoma, metastatic melanoma, lymphoma and advanced breast cancers; acute leukemias; and myeloma
  • Chronic conditions including atherosclerosis, heart failure, hypertension, chronic kidney disease, rheumatoid arthritis, inflammatory bowel disease, sepsis, COVID-19, tuberculosis, asthma, obstructive sleep apnoea, diabetes mellitus and others
  • Chemotherapy
  • Hormone therapy (including oral contraceptives)
  • Dehydration
  • Having a central venous catheter in-situ
  • Inherited thrombophilias such as Factor V Leiden mutation
  • Antiphospholipid syndrome.

(Healthdirect 2019a; McLendon et al. 2021)

Between 50 and 75% of hospitalised patients have at least one risk factor for VTE and 40% have three or more (ACSQHC 2021).

Recognising a Blood Clot

The following resources provide a more comprehensive explanation of how to recognise a clot:

Deep Vein Thrombosis

DVT symptoms may include:

  • Pain and tenderness
  • Pain when extending the affected area
  • Swelling
  • Warm skin
  • Changes in skin colour in the affected area (e.g. red, pale or bluish skin).

(Better Health Channel 2020)

If the DVT occurs in a leg vein, these symptoms generally only affect one leg (NHS 2019).

Note that some people may not experience any symptoms (Healthdirect 2019a).

blood clot deep vein thrombosis photo
'Deep vein thrombosis of the right leg' by James Heilman is licensed under CC BY-SA 3.0.

Pulmonary Embolism

The symptoms of PE can range from absent or mild to extremely severe, with the possibility of shock or sudden death (Reading 2018).

In fact, sudden death is the first clinical sign in one-quarter of patients (ACSQHC 2021).

This condition therefore requires urgent treatment.

Possible symptoms include:

  • Shortness of breath, especially if it is new or sudden
  • Increased respiratory rate and decreased oxygen saturation
  • Chest pain
  • Cough or coughing up blood
  • Loss of consciousness
  • Hypotension
  • Increased or irregular heart rate
  • Sweating
  • Fever
  • Dizziness
  • Clamminess.

(Healthdirect 2019b)

Note that a PE can occur even without the presentation of DVT symptoms (Salzman 2018).

One-third of people who have had a PE will experience another within their lifetime (Healthdirect 2019b).

Treating a Blood Clot

Treatment will depend on the type of blood clot. Refer to the following resources for more information:

Treatment for superficial thrombosis may include topical treatments, systemic medicines, surgery, walking and resting in the Trendelenburg position (Almeida et al. 2019).

Preventing a Blood Clot

Blood clots are a leading cause of preventable death in Australia (ACSQHC 2021).

Depending on the individual, prevention strategies may include:

  • Smoking cessation
  • Reducing alcohol consumption
  • Losing weight
  • Regular exercise
  • Dietary changes
  • Wearing compression stockings (must be properly fitted if worn and directed by your healthcare professional)
  • Medicines such as anticoagulants
  • Mobilisation after prolonged immobility (e.g. surgery, injury or travelling)
  • Wearing loose-fitting clothes.

(Healthdirect 2020; CDC 2020)

Additional Resources


Test Your Knowledge

Question 1 of 3

Mason has just taken an 11-hour flight. Which risk factor from Virchow’s triad does this fall under?


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