Lecture 30 - Urine formation pt 2

two types of transepithelial transport

• tubular reabsorption — movement of substance from the lumen into the peritubular capillary

• tubular secretion — movement of substances from the peritubular capillary into the lumen

PCT made of

simple cuboidal and microvilli

LOH (thin limbs) made of

simple squamous

thick ascending limb and DCT made of

simple cuboidal + reduced microvilli

collecting duct made of

simple cuboidal

what do the renal epithelial cells consist of?

• a luminal membrane — faces filtrate

• a basolateral membrane — base and sides - facing interstitium

how can transport occur across the epithelial cells?

• through: transcellular transport

  • primary route

  • typically requires proteins on both side

• between: paracellular transport

  • diffusion

peritubular capillary

• paracellular - between

• transcellular — through the cell

• luminal side to the interstitial space

what are the transport mechanisms?

• active transport - requires ATP

• osmosis - movement of water through aquaporins (H2O channels)

  • some are constitutively (permanently) expressed, others are not

• passive transport - requires favorable electrochemical gradient

  • simple diffusion = slow/non-selective

  • ion channels = highly selective; open or gated

  • facilitated diffusion = transport proteins

    • uniporters - move 1 solute across mem

    • symporters - move 2 solutes in same direction

    • antiporters - move 2 solutes in opposite direction

transport mechanisms in secretion and reabsorption in proximal tubule

• transport proteins function passively, but they can be maintained thru active transport

  • secondary active transport

so why does each part of nephron have different job?

• because they vary in:

  • types of channels/transporters present

  • hormone receptor presence/absence

  • type of epithelial cells

  • interstitial fluid tonicity

describe reabsorption and secretion in the PCT

• 1st point of filtrate modification

• more substances move across mems of PCT than any other part of nephron

• 2/3 of glomerular filtrate is reabsorbed in PCT

what are the reabsorption rates in the PCT

• glucose = 100% filtered load

• Na = 65%

• K+ = 65%

• water = 67%

reabsorption in LOH

• thin asc. and thick asc. have different jobs

  • descending limb = permeable to water ONLY

  • ascending limb = permeable to solutes ONLY

• interstitial osmolarity progressively INC from cortex → inner medulla

interstitial fluid osmolarity

thick ascending limb - interstitial fluid osmolarity. what channel involved?

NKCC2

solutes only

thin ascending

water only

aquaporins

the descending limb fxn and how

• function: reabsorb H2O

  • 15% filtered load of h2o

• how: aquaporin-1

• the concentrating segment

the ascending limb function and how

• function: reabsorb NaCl

  • 25% filtered load of Na+

  • 20% filtered load of K+

• how: Na-K-2Cl cotransporter

• the diluting segment

aquaporin-1 — NKCC2 — Na/K-atpase

NKCC2 → Bartter syndrome