# Fluids - pressure

1. Nov 2, 2008

### maniacp08

Fluids -- pressure

Blood plasma flows from a bag through a tube into a patient's vein, where the blood pressure is 10 mmHg. The specific gravity of blood plasma at 37°C is 1.03. What is the minimum elevation the bag must have so the plasma flows into the vein?

Prof gave this problem for us to solve, and I went through my notes and didn't see much that would help me get started.
I got this equation P = P0 + density * g * h
density = mass / volume

Is this equation relevant to this problem?
Im given the pressure 10 mmHg, gravity 1.03, but what is the density?
I need to solve for height.

Can someone help me get started?

2. Nov 2, 2008

### mgb_phys

Re: Fluids -- pressure

'specific gravity' is the density compared to water, in this case 1.03* the density of fresh water (which is 1g/cc)

3. Nov 2, 2008

### maniacp08

Re: Fluids -- pressure

Oh so the density is 1.03, then g would be 9.81m/s then.
P = P0 + density * g * h
10mmHg = 1.03 * 9.81m/s * h
and solve for h?

4. Nov 2, 2008

### mgb_phys

Re: Fluids -- pressure

Yes, make sure you have consistent units.
(You also need the density of mercury.)

5. Nov 2, 2008

### maniacp08

Re: Fluids -- pressure

where/how would the density of mercury come in?
They want the height in cm.

10mmHg = 1.03 kg/m^3 * 9.81N/kg * h

6. Nov 2, 2008

### mgb_phys

Re: Fluids -- pressure

You have a pressure of X cm of blood pushing down from the drip.
You have a pressure equal to a column of mercury 10mm high pushing up.
These pressures have to balance, you need to find X