It goes without saying that children carry much less blood than adults so they will lose less blood volume quickly and could die from blood loss much quicker.
An individual carries 0.5 litres of blood per 7kgs of body weight – that is about one pint of blood per stone but it is important to note that this increases if someone gains weight, the larger the body, the more blood is needed and the harder the heart has to work.
The average adult carries approximately 10 pints / 6 litres of blood, the consequences of them losing about a 5th of their blood volume (20%) is their body can shut down and go into shock, this is a life-threatening medical condition and is a medical emergency.
To put this into perspective, if a baby loses the equivalent of a teacup full of blood, this would certainly be catastrophic
What if a patient loses a massive amount of blood, say more than 40% of their blood volume?
The human body cannot compensate for such a huge amount of blood loss; and they will go into hypovolaemic shock This is a low volume of blood in their body causing failure of the circulatory system. It is a catastrophic bleed and they will die without treatment.
Anyone with a catastrophic bleed from either a major artery or vein may be indicated in a manner that the bleeding is pulsating to the rhythm of the heart. In most cases this with an arterial bleed, but it can also ocurr with a venous bleed.
In terms of time, an extremely heavy or indeed a catastrophic bleed from an adult could quite easily lose 40% of their blood volume in just 3 or 4 minutes; much quicker in relation to infants.