Renal Physiology -- Tubular Reabsorption and Secretion

what type of process is tubular reabsorption

active and passive process

tubular reabsorption, where to substances go

filtrate → body, occurs at epithelial cells of renal tubules, allows body to get nutrients it needs

types of tubular reabsorption

• PCT reabsorption

• loop of Henle reabsorption

• DCT and collecting duct reabsorption

what is reabsorbed by PCT

Na, K, glucose, vitamins, amino acids

Na, K, glucose inside PCT cell vs outside PCT cell

inside: low Na, high K, high glucose

outside: high Na, low K, low glucose

apical side of PCT cell

near lumen

basolateral side of PCT cell

near ECF — plasma, interstitial fluid, peritubular capillaries

channels on apical side of PCT

• sodium channel

  • Na outside → inside (along gradient)

  • Glucose outside → inside (against gradient)

  • secondary transport!

channels on basolateral side of PCT

• sodium potassium pump

  • 3 Na inside → outside

  • 2 K outside → inside

• potassium leak channel

  • K inside → outside → peritubular capillary or back inside

• glucose carrier protein

  • glucose inside → outside

h2o transport in pct cells

aquaporins, lumen → ECF

urea / ions transport in pct cells

through gap junctions, lumen → ECF

bicarbonate / HCO3- ion reabsorption in PCT cells

increased HCO3- reabsorption = increased H+ secretion = blood H+ decrease, pH increase

what is absorbed by loop of Henle

h2o, Nacl

descending limb — what gets reabsorbed what doesn’t

water leaves limb → reabsorbed

nacl cannot leave

ascending limb — what gets reabsorbed what doesn’t

nacl leaves limb → reabsorbed

water cannot leave

what controls what can leave the loop of Henle

osmotic gradient

fluid: low osmolarity → high osmolarity

osmotic gradient in loop of henle

more concentrated as you go down, less concentrated as you go up

which limb is filtrated more concentrated in and why?

descending loop because solutes / Nacl cannot leave

what is reabsorbed by DCT and collecting tube

water, Na++, Ca++

what hormone causes water reabsorption by DCT / collecting tube

ADH

ADH — trigger and response

trigger: high plasma Na / osmolarity, low blood volume

response: increase aquaporins + water reabsorption

which hormones cause and prevent (respectively) Na reabsorption by DCT / collecting tubes

aldosterone and ANP

aldosterone — trigger and response

trigger: low plasma Na and BP, high plasma K, renin release

response: plasma Na and BP increase and K decrease because sodium potassium pumps increased

ANP leads to …

decreased Na absorption at collecting duct = decreased plasma Na

what causes Ca to be absorbed by DCT

PTH

tubular secretion where do substances move

blood → lumen of PCT, DCT, collecting ducts to get rid of substances we do not need!

what is secreted during tubular secretion

H+, K+, foreign and nonforeign substances

what happens when plasma pH decreases and ECF H+ increases

• H+ secretion increases, HCO3- reabsorption increases

  • decreased H+ and increased pH in plasma

what happens when ECF H+ decreases and plasma pH increases

• H+ secretion and HCO3- reabsorption decreases

  • blood H+ increases and plasma pH decreases

how is K secreted

via aldosterone if levels are elevated

dilute urine — cause and response

cause: overhydration → low ADH

response: reabsorption decreases, volume of urine increases due to increased water

concentrated urine — cause and response

cause: dehydration → high ADH

response: reabsorption increases, volume of urine decreases due to decreased water