Osmotic pressure is the difference between blood in the capillaries and interstitial fluid between the cells, according to Kimball's Biology Pages. As blood moves through the capillaries, it filters into the tissue space, delivering nutrients to the cells. Since this interstitial fluid has a lower concentration of plasma than proteins and a relatively higher concentration of water, this results in higher pressure and causes the fluid to re-enter the capillaries.
Osmotic blood pressure can be affected by salt. An increased amount of salt in the blood causes the cells to release more water due to osmotic pressure. Blood pressure that is higher within the capillaries than in the surrounding fluid increases total blood volume as well as pressure on the walls of the blood vessels. Salt is considered to be a contributing factor in approximately one-third of patients with "essential" hypertension, defined as cases where no specific medical cause is known. About 60 percent of patients with hypertension find their condition improves with a reduction in salt intake. New research shows that too little calcium or potassium also has an impact on blood pressure, according to The Doctors' Medical Library. Patients with even moderately high blood pressure have a shorter life expectancy.