Public and Patient information

Exploring research into bleeding and abnormal clotting after traumatic injury 

In this page, you will find:

Key research into bleeding after traumatic injury

Key research produced by the Centre for Trauma Sciences (C4TS) 

The way that bleeding patients have been treated after a traumatic injury (such as vehicle crashes, falls, or assaults) has developed a lot over recent years.


Through research!

Thanks to the participation of patients and the ongoing work of research teams, we have learned more about how a traumatic injury can affect blood, particularly in the hours following a ‘major trauma’ injury.  This type of injury is an injury where the patient has sustained an injury that has led to a large volume of blood loss.

From this understanding, new treatments have been developed that improve recovery, rehabilitation, and the patient’s quality of life.

An introduction to blood and bleeding

We have collected some of the most frequently asked questions about blood and bleeding here.

Why is blood so important?

On average, a person has around 5 litres of blood circulating around their body. Blood travels through a network of blood vessels that run to every part of our body, from our major organs such as our brain and liver, to our muscles and skin.

One of the most important jobs of the blood is to transport oxygen and nutrients to every cell in our body.

Another important job is to form blood clots where there is damage to blood vessels. This reduces the amount of blood we lose, for example when we get a cut.

What are blood vessels?

Blood vessels are the tubes that blood flows through to travel around the body. There are three different kinds of blood vessels, arteries, veins and capillaries. They each have different roles.

Arteries: Arteries carry oxygen-rich blood to the organs; they are found deep inside our skin and have high blood pressure.

Trauma affecting arteries is life-threatening as lots of blood is lost very quickly.

Veins: Once the oxygen has been used by cells in the body, veins bring the blood back to the heart. Veins are found both deep inside the skin and near the skin’s surface. Blood flows through veins at a lower pressure than in arteries.

Trauma affecting veins can range from being moderate to life-threatening, depending on the location and size of the injury.

Capillaries: Capillaries are the smallest and thinnest type of blood vessel. They deliver oxygen and nutrients to cells and collect waste. They act as connectors between arteries and veins.

Damage to capillaries can cause minor bleeding and bruising.

Which cells are in our blood and what do they do?

Different cells within the blood have different jobs.

Red blood cells: These are cells that carry oxygen around the body.

Platelets: These are cells that help form blood clots when we bleed. When these cells are circulating around the body, they are inactive. Platelets become activated when there is bleeding.

This is why our blood only clots when we have an injury.

White blood cells: These are cells that respond to infection and fight it.

Plasma: This is the liquid part of blood that contains sugar, salt, fat and other important minerals.

What happens if blood can’t get to parts of the body?

Our organs (heart, brain, lungs etc.) are made up of lots of cells. If blood can’t get to a cell, then that cell won’t get any oxygen.

A cell can survive for a small amount of time without oxygen using its own energy stores- like a battery. When the cell has used up its energy storage, it will no longer be able to function.

When a cell can’t function, it dies. Cells swell up, burst, and release their contents into the blood. These cell contents are harmful and can cause damage to other parts of the body.

If we lose lots of cells from within a particular organ, then this organ will stop working. This is called ‘organ failure’ and it is very dangerous. This is why losing a lot of blood is life-threatening.

Because of this, our body has lots of ways to make sure that when we bleed from an injury, the bleeding is stopped as quickly as possible.

What normally happens when we bleed?

Bleeding happens when there is damage to a blood vessel. When the blood vessel wall becomes damaged, platelets circulating in the blood become ‘switched on’ (activated) and start to stick together at the site of the damage. This is the first stage of a blood clot being formed.

When a platelet is activated, a protein called fibrinogen can stick to it. Fibrinogen is a rope-like protein that forms a mesh around the platelets, giving the blood clot its structure.

With the help of many important proteins, fibrinogen is then converted into the protein fibrin. Fibrin is very strong and will not dissolve in water, this means that the clot is strong and able to ‘plug’ the gap made by the damage. 

When the blood clot is formed, blood doesn’t leak out of the vessel anymore. This stops the bleeding until the damage to the blood vessel can be repaired by the body.

The formation of a blood clot is called ‘coagulation’

A problem with the formation of a blood clot is called a ‘coagulopathy’

What happens to the blood clot when the damage is repaired?

Normally, when the damage to the blood vessel has been healed by the body, blood clots are broken down by the enzyme plasmin. Plasmin breaks down the protein fibrin, found in blood clots which give them their strength. 

This is an important process to make sure that blood clots do not stay in blood vessels for a long time, as this could lead to the vessel getting blocked and other complications. 

What can happen to the blood when trauma patients bleed? 

Often, patients who have suffered a traumatic injury lose a lot of blood.

In 2003, Professor Karim Brohi found that when a trauma patient loses a lot of blood, immediately after their injury, their blood becomes less able to stick together and form a strong blood clot.

With the blood unable to form a strong clot ‘plug’ to block off the injured blood vessel, even more blood is lost. This means that essential organs such as the brain and heart don’t get enough oxygen and patients often die within minutes of their injuries.

This is called an ‘acute traumatic coagulopathy’- a sudden problem with the blood being unable to clot like normal, caused by losing a lot of blood in a traumatic injury.  

What happens to blood clots in trauma? 

As we have seen, losing a lot of blood from a traumatic injury can cause changes to a patient’s blood, which affects how it clots. 

In trauma, it has been found that patients' blood breaks down the fibrin in clots much more quicker than normal. Because of this, clots become less effective at ‘plugging’ holes in blood vessels and so the patient continues to lose blood. 

Blood loss remains one of the leading causes of death in trauma patients. This is why research into bleeding in trauma is so important. 



Prior to 2007

When a trauma patient had lost a lot of blood, we would give them lots of salty water (saline).

The idea behind this was that it would help to replace the volume of liquid circulating around the body that had been lost through bleeding. 

What was the problem with this?

The salty water (saline) that we gave the patients did not contain any of the blood cells needed to carry oxygen or make blood clots.

As we have seen, for blood to clot, we need platelets, and, to carry oxygen to the tissues we need red blood cells.

So, what the salty water just diluted the blood left in the body.

This is called a ‘dilutional coagulopathy’- with the salty water diluting the blood, leading to a problem with the blood being able to clot.


Research studies are observational or interventional.

Observational studies collect data and compare outcomes in different settings to look for patterns and identify the best treatments.

Interventional studies are clinical trials that test a new medical intervention such as a new drug or test, in a clinical setting such as a hospital. 

A randomised control trial is a type of research study used to test a new medical intervention, such as a medication. Here, a number of similar people are randomly assigned to two different groups.

In one group, the patient will get the medication and in the other, the patient will get the current treatment. The results from these groups will be compared to see if there is a benefit for patients who receive the new treatment.

Code Red (2007)

Code red is the name of the new Major Haemorrhage Protocol introduced in 2007. 

What is a Major Haemorrhage protocol?

A Major Haemorrhage Protocol is a set of steps used by the medical team to guide treatments when a patient is bleeding severely.

It makes sure that all the medical team are in the right place, at the right time with the right medications and treatments for the patient.

How have trauma patients benefited from the ‘code red’ major haemorrhage protocol? 

Timing is key when looking after a trauma patient who is bleeding a lot. 

The priority for helping these patients is to stop the bleeding and replace the blood that they have lost by giving them blood products.

Activation of the Major Haemorrhage Protocol allows a more coordinated response to save the bleeding patient and reduces the time in which blood products are delivered.

It also involves all the healthcare professionals (surgeons, anaesthetists etc.) that are crucial for the care of the bleeding patient. 

Code red is based on new data and as well as, and replenishing the fluids improves the blood clotting of a bleeding patient.

The international CRASH-2 trial found that giving patients a medication called Tranexamic Acid shortly after an injury helps trauma patient's blood to clot. 

In 2010, tranexamic acid was introduced into the Code Red Major Haemorrhage Protocol and by 2017, 98% of bleeding trauma patients received 1g of tranexamic acid during resuscitation. 

What is Tranexamic Acid?

Tranexamic acid is a drug that stops blood clots from being broken down. 

It does this by stopping the enzyme plasmin from breaking down the protein fibrin, found in blood clots which gives them their strength. 

As we have seen, losing a lot of blood from a traumatic injury can cause changes to a patient’s blood, which affects how it clots. Keeping the clots that are formed in place is obviously valuable in reducing blood loss.

Because the study found it was important to give tranexamic acid quickly after injury, it was recommended that this drug should be given at the scene of the injury, before the patient arrives at the hospital. 

What is ACIT?

Activation of Coagulation and Inflammation in Trauma (ACIT) is the main on-going study at the Centre for Trauma Sciences (C4TS). All trauma patients, young and old, who arrive in participating trauma centres are eligible for enrolment. 

This allows clinical data and blood samples to be collected from trauma patients to support lots of different studies investigating the body’s response to blood and transfusion therapy. 

ACIT is designed to investigate how the body's inflammation and coagulation (clotting) pathways are activated immediately following major trauma - so we can find ways to target and treat them if they go wrong! 

Patient participation in ACIT 

Trauma patient participation in ACIT is completely voluntary. ACIT operates on an opt-out basis. All patients who wish to withdraw from the study can do so at any point in their care, and the information gathered will be removed from records. 

Would you like to read the study?

Find it here

As you have seen, the last decade of the Centre for Trauma Science’s research into bleeding and trauma has changed many of the ways we approach bleeding in traumatic injuries today. 

From new procedures to new medications, research has led to new ideas and treatments that have saved lives. Most importantly, none of this would have been possible without the participation of our patients.

But our understanding of bleeding in trauma is by no means complete!

There are many more mysteries tucked away within each and every one of your thirty trillion cells.

Our research and the search for answers continues! 

How can you get involved?

C4TS has a number of ways in which patients, volunteers, and professionals can participate in research. Click on the link below to find out more!

This page has been written by Emily Long and Ioannis Hannadjas (QMUL TSCIPP students) with review by Mr James Piercy and Mr Peter Blackman (PAIR group chair and member)


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