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.

• 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.

• Glomerular Diseases

  • The Bowman's capsule empties the filtrate into a tubule that is also part of the nephron.
  • The resistance of these arterioles results in high pressure within the glomerulus.
  • A glomerulus and its surrounding Bowman's capsule constitute a renal corpuscle, the basic filtration unit of the kidney.
  • The glomerular filtration rate (GFR) is the rate at which blood is filtered through all of the glomeruli, and, thus, the measure of the overall renal function.

• 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.
  • As a result of partial pressure gradient changes, more oxygen is released to the tissues.
  • 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.
  • They decrease osmotic pressure by diluting the blood.

• Fetal Circulation

  • The circulatory system of the mother is not directly connected to that of the fetus, so the placenta functions as the respiratory center for the fetus as well as a site of filtration for plasma nutrients and wastes.
  • At birth, when the infant breathes for the first time, there is a decrease in the resistance in the pulmonary vasculature, which causes the pressure in the left atrium to increase relative to the pressure in the right atrium.

• Overview of the Urinary System

  • The purpose of the renal system is to eliminate wastes from the body, regulate blood volume and pressure, control levels of electrolytes and metabolites, and regulate blood pH.
  • Blood pressure homeostasis: the renal system alters water retention and thirst to slowly change blood volume and keep blood pressure in a normal range.
  • Kidneys play a very large role in human osmoregulation by regulating the amount of water reabsorbed from glomerular filtrate in kidney tubules, which is controlled by hormones such as antidiuretic hormone (ADH), renin, aldosterone, and angiotensin I and II.
  • The extent of blood volume and blood pressure regulation facilitated by the kidneys is a complex process.
  • Besides ADH secretion, the renin-angiotensin feedback system is critically important to maintain blood volume and blood pressure homeostasis.