osmotic pressure

Examples of osmotic pressure in the following topics:

• Capillary Dynamics

  • Hydrostatic and osmotic pressure are opposing factors that drive capillary dynamics.
  • Oncotic or colloid osmotic pressure is a form of osmotic pressure exerted by proteins in the blood plasma or interstitial fluid.
  • Hydrostatic pressure is the force generated by the pressure of fluid within or outside of capillary on the capillary wall.
  • However, because large plasma proteins, especially albumin, cannot easily cross through the capillary walls, their effect on the osmotic pressure of the capillary interiors will to some extent balance the tendency for fluid to leak from the capillaries.
  • Describe hydrostatic pressure and osmotic pressure, the factors of capillary dynamics

• Regulation of Glomerular Filtration Rate

  • The filtration constant is based on the surface area of the glomerular capillaries, and the hydrostatic pressure is a pushing force exerted from the flow of a fluid itself; osmotic pressure is the pulling force exerted by proteins.
  • Changes in either the hydrostatic or osmotic pressure in the glomerulus or Bowman's capsule will change GFR.
  • Increased osmotic pressure in the glomerulus is due to increased serum albumin in the bloodstream and decreases GFR, and vice versa.
  • Under normal conditions, albumins cannot be filtered into the Bowman's capsule, so the osmotic pressure in the Bowman's space is generally not present, and is removed from the GFR equation.
  • In certain kidney diseases, the basement membrane may be damaged (becoming leaky to proteins), which results in decreased GFR due to the increased Bowman's capsule osmotic pressure.

• Movement of Fluid Among Compartments

  • Hydrostatic pressure is generated by the contractions of the heart during systole.
  • The osmotic pressure drives water back into the vessels.
  • At the arterial end of a vessel, the hydrostatic pressure is greater than the osmotic pressure, so the net movement favors water and other solutes being passed into the tissue fluid.
  • At the venous end, the osmotic pressure is greater, so the net movement favors substances being passed back into the capillary.
  • Oncotic pressure exerted by proteins in blood plasma tends to pull water into the circulatory system.

• Bulk Flow: Filtration and Reabsorption

  • Capillary fluid movement occurs as a result of diffusion (colloid osmotic pressure), transcytosis, and filtration.
  • The movement of materials across the capillary wall is dependent on pressure and is bidirectional depending on the net filtration pressure derived from the four Starling forces.
  • When moving from the bloodstream into the interstitium, bulk flow is termed filtration, which is favored by blood hydrostatic pressure and interstitial fluid oncotic pressure.
  • When moving from the interstitium into the bloodstream, the process is termed reabsorption and is favored by blood oncotic pressure and interstitial fluid hydrostatic pressure.
  • Modern evidence shows that in most cases, venular blood pressure exceeds the opposing pressure, thus maintaining a positive outward force.

• Plasma and Blood Volume Expanders

  • For instance, the heart pumps more blood with each beat, which increases blood pressure.
  • Blood pressure is detected by the renal system, which increases blood volume and blood pressure by excreting less water during blood filtration.
  • Colloids: These solutions preserve a high-colloid osmotic pressure (protein-exerted pressure) in the blood, while this parameter is decreased by crystalloids due to hemodilution.
  • The higher osmotic pressure from colloids draws fluids inward, preventing it from leaking out into the tissues as easily, which increases intravascular blood volume.
  • They decrease osmotic pressure by diluting the blood.

• Distribution of Lymphatic Vessels

  • This fluid is mainly water from plasma that leaks into the intersitial space in the tissues due to pressure forces exerted by capillaries (hydrostatic pressure) or through osmotic forces from proteins (osmotic pressure).
  • When the pressure for interstitial fluid in the interstitial space becomes large enough it leaks into lymph capillaries, which are the site for lymph fluid collection.
  • Lymph vessels become larger, with better developed smooth muscle and valves to keep lymph moving forward despite the low pressure and adventia to support the lymph vessels.

• Blood Plasma

  • Albumins maintain the osmotic balance between the blood and tissue fluids.
  • These proteins exert a force that pulls water towards them, which is called oncotic or osmotic pressure. 
  • Albumins also assist in transport of different materials, such as vitamins and certain molecules and drugs (e.g. bilirubin, fatty acids, and penicillin) due to the force exerted by their oncotic pressure.
  • Plasma that is pulled into the tissues by albumin-exerted oncotic pressure becomes interstitial fluid.

• Edema

  • Cutaneous edema is referred to as pitting when, after pressure is applied to a small area, the indentation persists for some time after the release of the pressure.
  • Edema may also occur as a result of cardiac failure due to the rise in hydrostatic pressure.
  • A fall in osmotic pressure occurs in nephrotic syndrome and liver failure, and may cause edema.

• Developmental Changes in Fluids

  • The hypothalamic-neurohypophysial system plays a fundamental role in the maintenance of body fluid homeostasis by secreting arginine vasopressin (AVP) and oxytocin (OT) in response to a variety of signals, including osmotic and non-osmotic stimuli.
  • Recent research has demonstrated that intracerebroventricular (i.c.v.) injection of carbachol elicited an increase of blood pressure in near-term ovine fetuses.
  • Overview of the renin-angiotensin system that regulates blood pressure and water (fluid) balance.

• Velocity of Blood Flow

  • Movement of blood throughout the circulatory system is created by differences in pressure generated by the pumping of the heart.
  • Pressure is greatest immediately after exiting the heart and drops as it circulates around the body, particularly through the arterioles and capillary networks.
  • A greater difference in pressure results in a greater velocity assuming all else remains equal, so when increased blood flow is required the heart can pump more quickly and also in larger volume.
  • As resistance increases the difference in pressure which influences velocity decreases, which in turn reduces flow.
  • As such, its viscosity is changeable depending on osmotic balance and cell load.