Urine production

• Urine is formed in each nephron by ultrafiltration of blood plasma and filtration pressure of some molecules filtered from the blood, required by body cells (urine is filtered urea, some water, and some blood plasma salts)
• Ultrafiltration Process

  • Involves high blood pressure (rises as afferent arteriole has wider diameter than efferent) in glomerulus capillaries, forces water and small solute molecules out of blood plasma and Bowman’s capsule
  • Only molecules with a relative molecular mass of <69,000 filter (blood cells and proteins remain - too big), filter forms glomerular filtrate, contains water, glucose, amino acids, urea, and ions
• Filtration Pressure: High blood pressure in glomerular capillaries, forcing glomerular filtrate out of blood into Bowman's capsule; also higher hydrostatic pressure than Bowman's capsule, so net pressure favors glomerular filtrate formation
• Filtration Pressure

  A) Efferent

  B) Afferent

  C) Glomerulus

  D) Bowman's capsule

  E) Net filtration

  F) 10

  G) Net filtration pressure

  H) 55

  I) 30

  J) 15

  K) 55

  L) 30

  M) 15

  N) 10

  O) hydrostatic pressure

  P) blood

  Q) Colloid

  R) osmotic

  S) gradient

  T) proteins

  U) fluid pressure

  V) fluid

  
• Filtrate from glomerular capillaries must cross 3 barriers before entering Bowman’s capsule

  A) Filtered Material

  B) Basal Lamina (basement membrane)

  C) Filtration slit

  D) Foot processes of podocytes

  E) Lumen

  F) Bowman's capsule

  G) Pores (or fenestration)

  H) endothelium

  I) Capillary lumen

  
• 1st layer; capillary wall of glomerular capillaries, hassingle endothelial cell layer with above average pores/fenestrations between cells, filtrate is forced through these by blood pressure
• 2nd Layer; basement membrane, made of collagen fibres and glycoproteins, underlies/supports endothelium, true filter and determines molecules forced into Bowman’s capsule (molecules with mass of <69,000)
• 3rd layer; specialised epithelial cells (podocytes) form Bowman's capsule wall, long finger-like membrane extensions (primary and secondary processes), there are slit pores between secondary processes
• Glomerulus/Bowman’s Capsule Filter System 1

  A) Filtration slit

  B) minor

  C) podocytes

  D) Nucleus

  E) Major

  F) Minor

  G) foot

  H) podocyte

  I) Glomerular

  
• Glomerulus/Bowman’s Capsule Filter System 2

  A) Basement

  B) Capillary

  C) Secondary

  D) Primary

  E) Gaps

  F) secondary

  G) slit pores

  H) Nucleus

  I) podocytes

  J) Glomerular capillary

  K) Basement

  L) Primary processes

  M) podocyte

  N) Slit pore

  O) Lumen

  P) Bowman's capsule

  Q) Secondary processes

  R) podocyte

  S) Nucleus

  T) Endothelial

  U) Pore

  V) Lumen

  W) Capillary