In response to a fall in blood pressure, specilised cells in the kidneys release Angiotensin 1 into the blood stream, this acts upon a gammaglobulin in the plasma to form Angiotensin 2, the most powerful vasoconstrictor known.
This has the effect of shutting down pereferral capillaries and concentrating blood supply at the body core.
This is generally a good thing when the blood loss is from, say, a severed artery in the arm or leg but is extremely bad if
the blood loss is internal to the body cavity, ina major vessel like the aorta or the vena cavae, since it ramps up blood pressure in the major torsal vessels and increases blood loss.
Pressure in the aorta is about as high as it gets in the body and exsanguination will proceed at a brisk pace with severe hypovolimic shock setting in in seconds and unconciousness, coma and permanent cessation of respiration and heartbeat in a very short space of time, say a maximum of 5 minutes.