Secretion and Reabsorption

how much filtrate> how much urine?

180 L, 1-2 liters of urine, most filtrate is reabsorbed by kidney tubule

what types of substances are reabsorbed?

• water

• good solutes (ions Na+, k+, Cl-, Ca++)

• nutrients

• waste produces (creatine or urea)

• physiological control:

  • water: yes up to 99%

  • good solutes: yes up to 99% Na

• not under control

  • nutrients: 100%

  • waste: no creatine, half urea

diffusion is ___ while mediated transport is __

passive, active

paracellular goes thru?

transcellular goes thru?

• para: tight junction

• trans: thru cell:

  • luminal membrane, basolateral membrane

diffusion:

• down, water soluble

• down: lipid soluble, urea

if the solute conc is the same between filtrate and interstitial fluid, how can there be transepithelial movement of substances by diffusion

• conc gradient has to be established

• concentration gradient is established by the readorption of water

aquaporin

makes cell highly permeable to water

where does reabsorption of glucose via cotransport with Na occur?

proximal tubule

steps in the mediated transport

• 1. maintain Na+ concentration gradient (intracellular conc. of na in tubular cells is low compared to fluid)

  • ATPase, using K+,produces continous conc. gradient of Na+ on basolateral side

• 2. reabsorption of Na+ and another molecule (glucose)

  • Na-glucose cotransporter

  • secondary active transport

• 3/4: reabsorption of water and movement of solutes and water into capillary

  • aquaporin channels on both luminal and basolateral side

  • water moves by osmosis either thru the channels or leaky tight junction

why is there net movement of water and solute from interstitial space to peritubular capillary via bulk flow?

Startling forces favor absorption

what does reabsorption of Na accomplish (4)

• reabsorption of other solutes by facilitated transport (transcellular)

  • glucose + amino: all that is filtered

  • phosphate + cl-: some that is filtered

• Reabsorption of water

• reabsorption of urea (bec of water)

• Reabsorption of K+, Ca++, Cl-

  • because of paracellular movement of water - solvent drag

reabsorption of glucose and amino acid is carrier mediated… so?

limit of the rate → how people w diabetes have glucose in urine

What is the osmolairty of filtrate that enters prox tubule? that exits?

iso, iso

take home message for reabsorption in PCT: (5)

• 2/3 of Na+/K+/H20

• all of glucose, amino acids

• most bicarbonate

• start to concentrate wastes

• isomotic amount of water

  • beg and end osmolarity is 300

what happens in the loop of henle?

reabsorb electrolytes and water (na+, Cl, K, Ca, Mg)

thin descending limb:

thin/thick ascending limb:

• thin descending: aquaporins: permeable to water

  • reabsorbs water

• ascending: impermeable to water transport NaCl

  • thin diffusion (na+ channels)

  • thick ascending: active (reabsorbs Na+ and other ions)

reabsorption in the thick ascending loop

• Na+, K+, Cl- enter cell via Na-K-2Cl cotransporter on luminal membrane using energy of Na+ electrochemical gradient

• Na+ exits the cell via, ATPase, keeping intracellular conc of Na low

• Cl- exits via Cl- on basolateral membrane, K+ is receycle back into lumen via K+ channel on luminal membrane

paracellular reabsoprtion of Na+ and other + ions in loop of henle

• lose + and 2- from henle → +6 mV charge

  • na, mg, k, ca paraceullar reabsorb to cancel charge

take home message for reabsorption of loop of henle

• 20% NaCl is reabsorbed

• more solute than water is reabsorbed

• Beg of loop: 300 osmo

• End: 100 osmo

Reabsorption in early distal convoluted tubule

• Na-Cl cotransporter: cl enters the cell then leaves the cell on basolateral side

• Ca+ enters cell regulated by parathyroid hormone

• impermeable to water

what happens to filtrate as it moves down distal convuluated tubule

becomes more dilute

take home message for reabsorption in distal convoluted tubule

• 10% of NaCl is reabsorbed

• No more water is reabsorbed

• Osmolarity:

  • beg: 100 osmolarity

  • end: 80: osmolarity

what happens in the collecting duct

• Na+ enters the cell via Na+ channel on luminal membrane moving down its electrochemical gradient

• Na+ exits the cell via Na-K ATPase on basolateral side, keeping intracellular conc of Na low

• Loss of Na+ from tubular fluid produces lumen negative electrochemical which drives K+

• (basically:Na+ is reabsorbed for exchange in secretion of K+): controlled by aldosterone

• Aquarporin 2 on luminal side, 3,4 on basolateral side: variable permeability to water: controlled by ADH

Take home message for reabsorption in collecting duct

• 2-5% of Na+ is reasborbed, K+ is secreted

  • variable based on aldosterone

  • helps regulate total body Na+ and K+

• Water is reabsorbed

  • variable on ADH

  • Osmo

    • beg: 80

    • end: variable

    • helps regulate total body water