hydrostatic pressure

Meaning: Hydrostatic pressure is the pressure in a fluid at rest due to the weight of the fluid above. Pressure rises linearly with depth and acts equally in all directions at a point. Points at the same depth in one connected fluid share the same pressure, whatever the container's shape. Biology uses the same phrase for capillary blood pressure that pushes fluid out of vessels, opposed by oncotic pressure.

Equation: P = P₀ + ρgh, where P is absolute pressure (Pa), P₀ is pressure at the fluid surface (Pa), ρ is fluid density (kg/m³), g is gravitational acceleration, 9.8 m/s² (m/s²), and h is depth below the surface (m). The term ρgh alone is the gauge pressure.

On the MCAT: Typical setups are a diver or submarine, a U-tube or manometer holding two fluids, or an IV bag raised higher. You predict that pressure grows with depth and density, or set pressures equal at the same level in the same fluid. The pressure difference between the top and bottom of a submerged object also produces the buoyant force.

Common confusion: Students think a wider tank or more total fluid means more pressure. Only vertical depth, density and g matter. Also, forgetting P₀ gives gauge pressure when the question asks for absolute pressure.