Hydrostatic pressure is the force exerted by fluid molecules due to gravity. It affects fluid flow and is used in industry to test containers. Blood vessels use hydrostatic pressure to maintain adequate pressure, with osmotic pressure and the Gibbs-Donnan effect drawing fluids into the plasma. Abnormal pressure can cause swelling.
Hydrostatic pressure is the force that fluid molecules exert on each other due to the earth’s gravitational attraction. This force occurs whether the fluid is in motion or is completely stationary and pushes the fluids forward or outward when it encounters an area of least resistance. It is this energy that pushes water out of a hole in a paper cup, gas out of a leak in a pipe, and blood out of vessels into surrounding tissue.
Increasing elevation increases the amount of hydrostatic pressure. The fluid flowing down also increases the pressure, which causes the water traveling over the falls to flow faster than the water flowing down the creek to the fall. Temperature is another factor affecting pressure because as temperatures rise, molecules move faster, increasing pressure.
Industries commonly use hydrostatic pressure test methods to ensure that fluids remain indoors. The tests not only ensure that pipes and other types of containers do not leak, but also verify that the materials can withstand increased pressure from possible environmental changes. It’s not uncommon for companies to exert internal forces 150 times greater than normal while monitoring pressure changes with instrumentation.
Blood vessels have a unique way of maintaining adequate pressure throughout the body. Arterial capillary hydrostatic pressure normally measures 35 millimeters of mercury, or 35 mm Hg. Capillary venous pressure typically measures 15 mm Hg. The force behind the heart’s contractions along with gravity pulling blood away from the heart causes the pressure to rise. The porous nature of venous capillaries also reduces the pressure of flowing blood.
The liquid constituents of blood naturally flow through pores into the interstitial tissues due to this pressure, leaving behind lipids, proteins and particles too large to escape. This typically reduces venous pressure. Conversely, increased pressure within the tissues exerts a force back towards the capillaries, which is called hydrostatic osmotic pressure. As osmotic pressure pushes fluids into the pores of the capillaries, the electrical charges of the solids within the vessel cause the molecules to bind as they flow through the blood. This reaction is called the Gibbs-Donnan effect.
Osmotic pressure and the Gibbs-Donnan effect, working together, draw fluids from interstitial tissues into the plasma, which is referred to as colloidal osmotic pressure. When the body feels an abnormally low amount of venous pressure, the arteries usually compensate by constricting. When vessel damage occurs, the plasma contains too little solids, or blood pressure decreases, resulting in edema or swelling.
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