The other night a nurse and I were bringing a critical patient to CT scan to determine the extent of his internal bleeding. When we arrived, he was hooked up to two liters of fluid being squeezed in by pressure bags to keep his BP in the 120s, with a HR of 60.
One line needed to be D/Ced for IV contrast dye, and as the scan went on, I watched as the cuff recycled every 2 minutes to display an increasingly lower BP. With less volume to pump out, the heart tried to compensate by increasing the rate of its contractions.
By the time we returned to the area, both the pressure and HR were in the 90s. Thankfully, we had some O-negative waiting for us, and a surgeon ready to accompany the patient to the OR.
Nothing like the nerdy satisfaction of seeing physiology in action.