Ultrafiltration

• What is ultrafiltration?

  Filtration under pressure which separates small molecules (water, glucose, urea, amino acids) from the plasma of the blood
• Where does ultrafiltration occur?

  the glomerulus and Bowman's capsule
• How is high hydrostatic pressure maintained in the glomerulus?

  -contractions of the heart

  -afferent arteriole has a wider diameter than the efferent arteriole

  -large surface area of the capillaries of the glomerulus
• What does glomerular filtrate contain?

  -water

  -glucose

  -salts

  -urea

  -amino acids
• Proteins and red blood cells cannot pass into the tubule as they are too big (RMM is greater than 68,000)
• Molecules with RMM less than 30,000 pass through the basement membrane easily
• The rate at which fluid passes from the blood to the Bowman's capsule is called the glomerular filtrate rate
• What is the glomerular filtrate rate determined by?

  The difference in water potential between the Bowman's capsule and the blood in the capillary
• The net effect of higher pressure in the capillary and low solute concentration in the Bowman's capsule is that fluid moves out of the capillary and into the lumen of the capsule
• What is step 1 of ultrafiltration?

  • blood enters the glomerulus through the afferent arteriole
  • the afferent arteriole is wider than the efferent arteriole, which increases hydrostatic pressure of the plasma
  • this forces small, soluble molecules (water, glucose, urea and amino acids) out of the plasma
  • the endothelium of the capillaries contain pores called fenestrae
  • these molecules pass through the fenestrae
• What is step 2 of ultrafiltration?

  • the next layer is the basement membrane which is made of collagen
  • it is an extracellular layer that acts as a molecular filter
  • the pores in the basement membrane are the main site of filtration, as they are smaller than the fenestrae and filtration slits
  • molecules with an RMM smaller than 30,000 pass through the pores in the basement membrane
• What is step 3 of ultrafiltration?

  • the cells of the inner wall of the Bowman's capsule are called podocytes
  • podocytes have feet-like extensions called pedicels
  • pedicels wrap around the outside of the capillary
  • gaps between the pedicels allow small molecules to pass into the Bowman's capsule
• How do podocytes increase the efficiency of filtration?

  • pedicels increase the surface area for filtration
  • there is a short diffusion distance between the capillary and the podocytes
  • channels between pedicels increase the concentration gradient between the tissue fluid by increasing the concentration of small molecules
• Why does water and small molecules pass out of the plasma?

  • osmotic pressure of the plasma is lower than that of the filtrate due to the presence of plasma proteins
  • fluid pressure in the Bowman's capsule increases as the volume of fluid increases