Googling, it looks like they are using the term differently than is my understanding for fluid dynamics. Here's the whole link:
http://www.cvphysiology.com/Microcirculation/M012.htm
The rest of the quote related to the issue:
The average capillary hydrostatic pressure is determined by arterial and venous pressures (PA and PV), and by the ratio of post-to-precapillary resistances (RV/RA). An increase in either arterial or venous pressure will increase capillary pressure; however, a given change in PA is only about one-fifth as effective in changing PC as the same absolute change in PV. Because venous resistance is relatively low, changes in PV are readily transmitted back to the capillary, and conversely, because arterial resistance is relatively high, changes in PA are poorly transmitted downstream to the capillary.
It looks to me like they are describing static pressure, not
hydrostatic pressure. The way I understand the definitions for fluid dynamics, hydrostatic pressure is a subset of static pressure. "Static pressure" is the pressure of a static fluid - meaning without direct influence from motion. But that pressure can be created in several ways:
1. By gravity.
2. By pressurization of a closed system (ie, a squeezed balloon).
3. By a pump.
4. By a device that converts velocity pressure to static pressure.
Here's a dictionary reference for "hydrostatic pressure" that seems to imply the engineering field and medical field use the same term to mean two different things:
http://www.answers.com/topic/hydrostatic-pressure
But anyway, since you asked about the gravity part of the definition...
Imagine a pair of reservoirs, one above the other. A pipe extends from the bottom of the upper one down to just below the surface of the bottom one. A valve is in the middle of the pipe. The pipe is full of water (and is less than 35 feet long).
Case 1: The valve is closed. when the valve is closed, you have two pressures at work. Hydrostatic pressure at the valve would be from the weight of the water sitting on the valve. You also have the static pressure due to the atmosphere pushing down on the reservoirs. This is often treated as a regular static pressure (ie, a pressurized vessel), but if the altitude changes enough, it is more like a hydrostatic pressure. This pressure increases the pressure at the valve - from both above and below. From above, it is obvious. From below, it is the air pressure pushing down on the lower reservoir and pushing water up the pipe to the valve.
Case 2: Open the valve completely. The hydrostatic pressure still exists, but now it acts all the way down the pipe. The static pressure at the bottom of the pipe won't be due to the weight of the water though: since the valve is open, much of that pressure is converted to velocity pressure in making the water move from the top reservoir to the bottom one. And since the valve is open and the atmospheric pressure pushes against both reservoirs equally, it cancels out and you can ignore it.
Case 3: Close the valve partway. Now the valve creates a restriction that again causes it to support the weight of the water above it, similar to case 1...except this time it only supports some of the weight. Some of that hydrostaticpressure also goes to making the water move. How does that relate to the question in the OP? Because of the restriction, there is a static pressure loss (drop) across the valve. The hydrostatic pressure below the valve is lower than it is above the valve because the valve is supporting some of the water column above it.
