system
Given the following post, respond as if you're commenting on Reddit:
human
How does amputation affect the amount of blood contained within the body?

So for example, if somebody had both their legs amputated. Would the body very quickly adjust to this and ramp down production immediately? Or would it instead change the pressure of the blood within the system in the short term, then decrease the total amount over time?
  
I read a little about baroreceptors to try and find out, but I don't know.
gpt
Blood pressure is mostly regulated by two processes: cardiac output (how much blood the heart is pumping per minute) and mean arterial pressure (basically the average tightness of the tubes). Both of these things are regulated by certain things as well. Cardiac output is related to how much blood the heart gets in and how hard and fast it pumps. Mean Arterial Pressure (MAP) is related to hormones such as adrenaline as well as chemicals like CO2 and lactic acid.

When a limb is amputated, there are two options to deal with the blood vessels. One is to seal them off (most common) and the other is to connect them to each other as a shunt. If you assume minimal blood loss from the surgery, then as you mentioned there is now more blood per unit are than there was before. The immediate reaction to this is for the pressure-sensitive baroreceptors to signal for the release of hormones which loosen the arterioles. At the microscopic level, the small vessels called capillaries are not strictly pipes and can leak fluid into the tissues which leads to swelling known as edema. 

The heart will also be signalled by these chemicals and initially will pump less rapidly and effectively. Soon however, since the loosening of the arteries leads to more space for the blood to be in, the heart starts getting less blood coming in and it will soon resume its normal function. Regulation of blood pressure is actually quite a fascinating topic and the problems when that regulation fails lead to many different consequences that affect billions worldwide.