Invasive blood pressure measuring and hydrostatics I don't understand why the solution-manometric liquid must be at the catheter level.
I'm studying biophysics, in particular fluid mechanics.
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Why A' has to be in the same level of A?
 A: I'm unable to see your picture but believe I know what you are asking. An arterial line is placed to directly measure arterial blood pressure by accessing an artery with a catheter and measuring the pressure in a continuous manner with a pressure transducer. Access to the artery requires that the catheter remain patent, that is that the line does not clot, so some means is required for low flow rate, continuous flushing of saline into the artery. This requires a saline bag with a pressurized cuff that is inflated to roughly match the mean arterial pressure and a metering valve to adjust the infusion rate. The pressure transducer is 'T'eed off this line thus measuring the mean pressure and the dynamic excursions.
Without a cuff, the saline bag would have to be raised to a much higher level than the heart to at least balance the pressure that the heart maintains (about 65 - 110 mm Hg or about 50 to 60 inches of water pressure) and additionally higher to maintain the flush rate.
So the manometric fluid must be kept to at least the catheter (pressure) level to prevent arterial blood from flowing out of the patient, and marginally higher to maintain a flush rate to keep the catheter patent. Additionally heparin is often used in the saline to prevent clotting.

