Hydrostatic and osmotic pressure gradients facilitate filtration across a semipermeable membrane. Lubricants known as hydraulic fluid are used as the working fluid in hydrostatic power transmission. Physiologically, this occurs by a combination of hydrostatic pressure gradients and osmotic pressure gradients. Transudates are caused by disturbances of hydrostatic or colloid osmotic pressure, not by inflammation. Hulls are subject to various hydrostatic and hydrodynamic constraints. The pressurised fluids ascend towards shallower crustal levels that are under lower hydrostatic pressure. Steering is by gear differential with hydrostatic drive. Spherical bearings can be of a hydrostatic or mechanical construction. The cutterhead is filled with pressurised slurry which applies hydrostatic pressure to the excavation face. Piston pumps make up one half of a hydrostatic transmission. Pingos are generally classified as hydrostatic (closed-system) or hydraulic (open-system). Key properties are weldability, temperature resistance, flexibility and high hydrostatic pressure resistance. A rise in hydrostatic pressure occurs in cardiac failure. When diving, the pressure differences which cause the barotrauma are changes in hydrostatic pressure. The process must be carried out in a sealed cylinder to contain the hydrostatic medium.