net filtration pressure

Examples of net filtration pressure in the following topics:

• Capillary Dynamics

  • The movement of materials across the wall is dependent on pressure and is bi-directional depending on the net filtration pressure derived from the four Starling forces that modulate capillary dynamics.
  • The net filtration pressure derived from the sum of the four forces described above determines the fluid flow into or out of the capillary.
  • Due to the pressure of the blood in the capillaries, blood hydrostatic pressure is greater than interstitial fluid hydrostatic pressure, promoting a net flow of fluid from the blood vessels into the interstitium.
  • In conditions where plasma proteins are reduced (e.g. from being lost in the urine or from malnutrition), or blood pressure is significantly increased, a change in net filtration pressure and an increase in fluid movement across the capillary result in excess fluid build-up in the tissues (edema).
  • Describe hydrostatic pressure and osmotic pressure, the factors of capillary dynamics

• 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.
  • Modern evidence shows that in most cases, venular blood pressure exceeds the opposing pressure, thus maintaining a positive outward force.
  • This indicates that capillaries are normally in a state of filtration along their entire length.

• Movement of Fluid Among Compartments

  • 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.
  • The solution to the equation is known as the net filtration or net fluid movement.
  • If positive, fluid will tend to leave the capillary (filtration).

• Glomerular Filtration

  • Glomerular filtration is the renal process whereby fluid in the blood is filtered across the capillaries of the glomerulus.
  • The process by which glomerular filtration occurs is called renal ultrafiltration.
  • The force of hydrostatic pressure in the glomerulus (ie. the force of pressure exerted from the pressure of the blood vessel itself) is the driving force that pushes filtrate out of the capillaries and into the slits in the nephron.
  • Osmotic pressure (ie. the pulling force exerted by albumins) works against the greater force of hydrostatic pressure, and the difference between the two determines the "effective pressure" of the glomerulus which determines the force by which molecules are filtered.
  • These factors will influence the glomeruluar filtration rate, along with a few other factors.

• Regulation of Glomerular Filtration Rate

  • Glomerular filtration rate (GFR) is the measure that describes the total amount of filtrate formed by all the renal corpuscles in both kidneys per minute.
  • The glomerular filtration rate is directly proportional to the pressure gradient in the glomerulus, so changes in pressure will change GFR. 
  • The Starling equation for GFR is GFR=Filtration Constant X (Hydrostatic Glomerulus Pressure-Hydrostatic Bowman's Capsule Pressure)-(Osmotic Glomerulus Pressure+Osmotic Bowman's Capsule Pressure).
  • Osmotic pressure is the force exerted by proteins and works against filtration because the proteins draw water in.
  • GFR is the rate at which is this filtration occurs.

• Kidney Function and Physiology

  • The process of glomerular filtration filters out most of the solutes due to the high blood pressure and specialized membranes in the afferent arteriole.
  • The blood pressure in the glomerulus is maintained independent of factors that affect systemic blood pressure.
  • Glomerular filtration rate (GFR) is the volume of glomerular filtrate formed per minute by the kidneys.
  • Because Na+ is actively transported out of the tubule, water follows to even out the osmotic pressure.
  • This occurs due to the low blood pressure and high osmotic pressure in the peritubular capillaries.

• Diffusion

  • This lack of a concentration gradient in which there is no net movement of a substance is known as dynamic equilibrium.
  • A variation of diffusion is the process of filtration.
  • In filtration, material moves according to its concentration gradient through a membrane; sometimes the rate of diffusion is enhanced by pressure, causing the substances to filter more rapidly.
  • The rate of diffusion in this instance is almost totally dependent on pressure.
  • One of the effects of high blood pressure is the appearance of protein in the urine, which is "squeezed through" by the abnormally high pressure.

• Tubular Reabsorption

  • As the fluid filtered from blood, called filtrate, passes through the nephron, much of the filtrate and its contents are reabsorbed into the body.
  • Reabsorption is a finely tuned process that is altered in maintaining homeostasis of blood volume, blood pressure, plasma osmolarity, and blood pH.
  • As filtrate passes through the nephron, its osmolarity (ion concentration) changes as ions and water are reabsorbed.
  • The filtrate entering the proximal convoluted tubule is 300 mOsm/L, which is the same osmolarity as normal plasma osmolarity.
  • In the proximal convoluted tubules, all the glucose in the filtrate is reabsorbed, along with an equal concentration of ions and water (through cotransport), so that the filtrate is still 300 mOsm/L as it leaves the tubule.

• Osmotic Pressure

  • Osmosis is defined as the net flow or movement of solvent molecules through a semipermeable membrane through which solute molecules cannot pass.
  • If a solution consisting of both solute and solvent molecules is placed on one side of a membrane and pure solvent is placed on the other side, there is a net flow of solvent into the solution side of the membrane.
  • The height of the solution will continue to increase due to a net flow of solvent until the added pressure of the height will cause the flow of solution to stop.
  • Osmotic pressure can also be explained as the pressure necessary to nullify osmosis.
  • The osmotic pressure is the pressure required to achieve osmotic equilibrium.

• Other Hormonal Controls for Osmoregulation

  • Defective renin production can cause a continued decrease in blood pressure and cardiac output.
  • It acts directly on the nephrons, decreasing glomerular filtration rate.
  • Thus, via the RAAS, the kidneys control blood pressure and volume directly.
  • Medically, blood pressure can be controlled by drugs that inhibit ACE (called ACE inhibitors).
  • The renin-angiotensin-aldosterone system increases blood pressure and volume.