Bleeding is the escape of blood from damaged blood vessels and can occur externally through a wound or internally into tissues or body cavities. It ranges from minor superficial bleeding to life-threatening haemorrhage. Arterial bleeding can be associated with high-pressure blood flow, venous bleeding generally arises from lower-pressure vessels, and capillary bleeding usually involves smaller superficial vessels.
The appearance and rate of bleeding do not always reliably indicate its seriousness because internal bleeding can be concealed and venous or arterial injuries can vary substantially in severity. Severe haemorrhage can reduce circulating blood volume, impair tissue perfusion and cause hypovolaemic shock. In first aid, life-threatening bleeding is treated as an immediate emergency because a person can deteriorate rapidly even before obvious signs of shock develop.
Blood clotting is a complex physiological process involving vasoconstriction, platelet adhesion and activation, coagulation factors and formation of a stable fibrin clot. Platelets adhere to damaged vessel surfaces and help create an initial haemostatic plug, while the coagulation cascade generates thrombin and fibrin that reinforce the developing clot. The process is regulated so that clotting occurs at the site of injury without producing unrestricted thrombosis throughout the circulation.
Conditions affecting platelets, clotting factors, blood vessels or anticoagulant medicines can alter bleeding risk. People taking anticoagulants may experience more prolonged or severe bleeding after some injuries, although anticoagulation should not be stopped without appropriate medical advice. Understanding normal haemostasis explains why pressure and time can be important in allowing an injured vessel to seal.
First aid for significant external bleeding prioritises direct pressure. The rescuer should apply firm pressure over the bleeding site using an appropriate dressing or clean material and maintain pressure rather than repeatedly lifting the dressing to inspect the wound. If blood soaks through, additional material can generally be placed over the existing dressing while pressure continues. A severe limb haemorrhage that cannot be controlled with direct pressure may require a tourniquet according to established emergency guidance and appropriate training.
Tourniquets are designed to stop arterial blood flow to the affected limb and should be applied according to recognised technique, with the application time communicated to emergency professionals. The priority in catastrophic bleeding is rapid haemorrhage control and emergency medical activation rather than extensive cleaning of the wound before bleeding is controlled.
Internal bleeding can occur after blunt or penetrating trauma and may not be immediately visible. Warning signs can include increasing weakness, dizziness, pallor, sweating, rapid breathing, confusion, abdominal or chest pain, swelling, bruising, vomiting blood, coughing blood, blood in urine or stool, or deterioration in consciousness, although presentation varies with the site and amount of bleeding. Internal haemorrhage can arise in the chest, abdomen, pelvis, skull or soft tissues and may continue without an external wound.
A person with suspected significant internal bleeding requires urgent professional assessment because first aid alone cannot stop bleeding from many internal vessels. Keeping the person under observation and avoiding unnecessary movement can be appropriate while awaiting emergency care, subject to the injury and immediate threats to life.
Nosebleeds are a common form of external bleeding and are generally managed differently from major traumatic haemorrhage. The person is usually encouraged to sit upright and lean slightly forward rather than tilt the head backwards, while the soft part of the nose is compressed continuously for an appropriate period. This helps reduce swallowing of blood and applies pressure to the likely bleeding site.
Persistent, recurrent or unusually heavy nosebleeds can require medical evaluation, particularly when associated with anticoagulant use, trauma or other symptoms. Bleeding from the mouth, gastrointestinal tract or urinary system can also have many causes and should not be assumed to be minor simply because the amount initially observed appears small. The underlying cause determines the required assessment and treatment.
Shock can develop when blood loss becomes sufficient to compromise circulation. Early signs can include pale or clammy skin, rapid pulse, rapid breathing, thirst, weakness, anxiety or altered behaviour, followed by worsening confusion, reduced consciousness and cardiovascular collapse in severe cases. A person with major bleeding should receive emergency medical assistance immediately. Preventing unnecessary heat loss can be useful while awaiting help, but first aid should not be allowed to distract from controlling the haemorrhage. Giving food or drink to a person with serious traumatic bleeding is generally inappropriate because urgent surgery or anaesthesia may later be required. Continuous observation is important because a person's condition can deteriorate even after external bleeding appears controlled.
The source and mechanism of bleeding affect its management. Cuts can damage superficial vessels, while lacerations, punctures, crush injuries and amputations can involve deeper structures. Penetrating injuries can damage major vessels and internal organs even when the skin wound is relatively small. An embedded object should generally not be removed in first aid because it may be tamponading a damaged vessel, and removing it can cause catastrophic bleeding.
Severe limb injuries can sometimes involve fractures, nerve damage and vascular compromise simultaneously, while scalp wounds can bleed heavily despite appearing relatively superficial. Chemical or thermal injuries can also damage vessels and tissues in ways that complicate bleeding control. Assessment must therefore consider the mechanism of injury as well as visible blood loss.
Bleeding has major significance in trauma care because uncontrolled haemorrhage is a preventable cause of death in some emergencies. Modern emergency systems therefore emphasise rapid recognition and control of catastrophic bleeding alongside airway, breathing and circulation assessment. Training in haemorrhage control can teach people to apply pressure, recognise when a tourniquet may be indicated and summon emergency assistance promptly.
At a healthcare level, treatment may involve intravenous or intraosseous access, blood products, surgical haemostasis, interventional radiology, medications and management of the underlying cause, depending on the situation. First aid cannot replace these interventions, but rapid control of external bleeding can preserve circulating blood volume during the critical interval before definitive treatment.
