Hypovolaemic shock: shock caused by blood or fluid loss

The circulatory system supplies the body's tissues with oxygen and nutrients and removes waste products. NHS inform explains that shock happens when this system stops working properly, so that important organs like the brain and heart do not get enough oxygen. It stresses that this is not the same as the emotional response of feeling shocked.

There are several kinds of shock, named after what goes wrong. NHS inform lists the causes as severe bleeding, fluid loss, heart problems, severe allergic reaction, severe infection and spinal cord injury. Hypovolaemic shock is the form linked to losing blood or other body fluid, so that there is too little volume for the heart to pump effectively. St John Ambulance describes the same group of causes as anything that reduces the flow of blood.

The speed matters. Shock can get worse because of pain or fear, NHS inform notes, which is why reassurance and keeping a person calm are part of first aid.

The sources group the causes that reduce the amount of blood or fluid in circulation as follows.

NHS inform and St John Ambulance set out how shock tends to progress. A person does not necessarily show all of the signs, and the order may vary, but a combination of these in someone who has bled, is badly dehydrated or has lost a lot of fluid is a reason to call 999.

• Early: a fast pulse, pale, cold and clammy skin, and sweating.

• As shock develops: rapid, shallow breathing, a weak pulse, grey-blue skin (especially inside the lips; St John Ambulance says to look at the palms and the base of the feet in people with dark skin), weakness and dizziness, nausea and vomiting, and thirst.

• As the brain's oxygen supply weakens further: restlessness and aggressive behaviour, yawning and gasping for air, and the person may become unresponsive.

NHS inform and St John Ambulance give the same sequence. Call 999 as part of it, and do not delay for the other steps if you are on your own: tell the call handler that you think the person has signs of shock.

• Treat the cause if you can see it, for example severe bleeding, and reassure the person that you will help them.

• Help them lie down, on a blanket or rug if there is one, to protect them from the cold.

• Raise and support their legs above the level of their heart, for example on a chair. This improves the blood supply to the vital organs.

• Phone 999 or 112 and say you think the person is in shock, explaining what you think caused it.

• Loosen clothing at the neck, chest and waist.

• Keep them warm with coats or blankets, and monitor them until help arrives. If they become unresponsive, treat them as an unresponsive casualty.

Most dehydration is mild and can be managed with fluids. The NHS says dehydration means your body loses more fluid than you take in, and that babies, children and older adults are at more risk. It lists urgent advice from a GP or NHS 111 if you are unusually tired, dizzy on standing, pass dark yellow or less pee, are breathing quickly or have a fast heart rate, or a baby has few tears or a sunken soft spot.

The NHS says to call 999 or go to A&E if someone with signs of dehydration has blue, grey, pale or blotchy skin, lips or tongue, skin that feels cold, difficulty breathing or lots of quick breaths, confusion, or is sleepier than normal or difficult to wake, because these could be signs of shock. It adds that you should not drive to A&E yourself and should bring any medicines you take.

The sources used for this page are first aid and symptom pages, and they do not describe hospital treatment for hypovolaemic shock in detail. In general terms, care is directed at finding and stopping the cause and at restoring the circulation, and it is delivered by emergency and critical care teams. The ambulance crew will be told what you saw, so give them the details: what happened, when, how much blood or fluid was lost, and any medicines the person takes.

Shock is a 999 emergency and is managed through the NHS ambulance and emergency department pathway, regardless of whether someone has private insurance. Because shock cannot be predicted or arranged in advance, the practical step available to most people is learning first aid before an emergency.