What Is the Relationship between Hydrostatic and Colloid Osmotic Pressure?

Hydrostatic pressure is a measure of exertion of a fluid, such as water, while at rest. There are two factors of influence: local gravity and liquid density. Colloid osmotic pressure (COP) is the amount of exertion of blood plasma proteins that have the ability to pull water into the circulatory system. The plasma proteins within capillaries maintain a high pressure balance to keep fluids from leaking from capillaries, normally. If there is a loss of colloid osmotic pressure, this balance can become disturbed, allowing fluids to cross membrane barriers into the tissues and causing a fluid buildup known as edema.

In studies of tumors to determine if COP and hydrostatic pressure show any difference between cancerous tissues and fluids and normal tissues and fluids, breast tumors there were found to have a marked drop in hydrostatic pressure and an increase in colloid osmotic pressure, causing leaky tumor vessels. The studies of three other types of tumors showed a difference between pressure readings in tumor tissues versus the pressure readings of normal tissues. The differences were not as marked, however, as in the tumors of breast cancer. It is believed that cell membrane integrity losses and tissue inflammation within tumors cause the elevated COP readings.

Fluid balances usually equalize in normal blood vessels and surrounding interstitial tissues; however, as high sodium in the blood assists proteins to pull water into the blood supply, it can lead to a saturated vessel condition known as hypervolemia and a consequent dehydration of surrounding tissues. The reverse is true when sodium levels decrease and hydrostatic pressures hold fluids in the tissues. Since hydrostatic pressure is a mechanical force, when there is a drop in blood pressures, circulatory shock will cause mild symptoms of tachycardia and mild peripheral vasoconstriction of vessels, or severe symptoms including systolic rates less than 60, marked pallor, and mental stupor. Neurogenic, anaphylactic or septic shock states show marked increases and decreases in hydrostatic pressures and colloid osmotic pressures and exhibit signs of hypervolemia or hypovolemia.

A condition known as ascites is an abnormal accumulation of fluids in the tissues of the abdominal area. Diseases such as cirrhosis and hypertension cause ascites and are usually treated with low sodium diets and diuretics; however, some patients require more relief. A treatment known as transjugular intrahepatic portosystemic shunt (TIPS) has in recent years begun to deliver significant improvements in 50% to 70% of cases, with few complications. These successes show an elevation of colloid osmotic pressures and balancing decrease in hydrostatic pressure, inhibiting further water leakage from blood vessels.