L-45 Tubular Transport Mechanisms

Front 1

How many neprons are in a kidney?

Back 1

Each human kidney has 1 million nephrons. Kidney size is dependent on numbe rof nephrons.

Front 2

AV nicking

Back 2

Symptom of hypertension - pinching between arterial and venous side in eye.

Front 3

Basic functions of proximal tubule

Back 3

Workhouse of nephron

Reabsorbs 66% of glomerular filtrate

Also reabsorbs HCO3-, glucose, amino acids

Secretes H+, organic acids

Front 4

Early proximal tubule

Back 4

Apical Na+ driven, secondary active transporters

Basolateral Na/K ATPase moves Na+ across

basal X transporters

Front 5

Reabsorption of solutes over distance of proximal tubule

Back 5

As substance reabsorbed, concentration decreases. By halfway through proximal tubule, almost all of nutrients have been absorbed.

Front 6

Reabsorption of HCO3- in early proximal tubule

Back 6

1. Filtered bicarbonate combines with secreted proton outside cell - H+ + HCO3- -> H2CO3 -CA-> CO2 + H2O

2. CO2 gas diffuses into cell. CA catalyzes CO + H2O to create H+ and HCO3-

3- HCO3- moves out of basolateral membrane via HCO3-/Na+ transporter.

For every H+ filtered through glomerulus, one HCO3- is reabsorbed. 85% of bicarbonate filtered load reabsorbed (1/3 kg per day)

Front 7

Filtered Load

Back 7

GFR * P(bicarbonate)

Front 8

Late proximal tubule - transcellular absorption

Back 8

2/3 of NaCl absorption in late proximal tubule.

NaCl absorption coupled by nonionic diffusion.

Secreted H+ combines with secretes organic acid to form HA - nonionic species that diffuses into cell.

HA dissociates in cell and H+ and A- are recycled to maintain Na+ and Cl- absorption.

Na+ passed through basolateral membrane via Na/K ATPase. Cl- moves through basolateral membrane through selective channel or K+/Cl- cotransporter

Front 9

Late proximal tubule - paracellular absorption

Back 9

For every HCO3- absorbed in PT, we leave behind a Cl- so Cl- increases along tubule.

Transepithelial potential is lumen negative in early proximal tubule because reabsorbing Na+ and leaving behind Cl-.

As Cl- builds up in lumen, transepithelial potential becomes positive because Cl- is paracellularly reabsorbed down concentration gradient.

Positive transepithelial potential causes Na+ to be reabsorbed paracellularly via electral gradient.

Front 10

Isoomotic Water Reabsorption

Back 10

PT is very leaky - generates little to no detectable osmotic gradient.
Isoosmotic water reabsorption occurs through water channels, driven by tiny gradients.

140L of water reabsorbed by proximal tubule each day. Cell volume turns over once per minute.

Front 11

Change in sodium osmolarity over course of PT

Back 11

Does not change - reabsorbed isotonically with water.

Front 12

Inulation osmolarity over course of PT

Back 12

Inulin is not reabsorbed so change is index of water being absorbed.

[Inulin] increases linearly through length of PT as water is reabsorbed, leaving inulin behind.

Front 13

Absorption in thin limb

Back 13

Passive, driven by gradients created in thick limb

Front 14

NaCl absorption in ascending thick limb

Back 14

Reabsorbs 20-25% of filtered NaCl transcellularly.

Apical Na/2Cl/K brings Cl- into cell.

Cl- diffuses passively through basolateral channel.

Na+ is pumped out by Na/K pump.

K+ is needed for Cl- import. Must be recycled by apical K+ channel to keep transporter going.

Front 15

Na absorption in ascending thick limb

Back 15

Na/HCO3- pathway - same as in proximal tubule

Front 16

Paracellular transport in ascending thick limb

Back 16

K channel is only electrogenic transporter on apical side.

Creates positive transepithelial potential which repels Na+, K, Ca2+, and Mg2+ to be reabsorbed paracellularly.

Front 17

Water transport in thick ascending limb

Back 17

Thick ascending limb is water impermeable.

Absorbs salt and leaves water behind

Luminal fluid becomes more dilute - thick ascending limb is also called diluting segment.

Front 18

Early distal convoluted tubule

Back 18

Reabsorbs 5-7% of filtered load of Na+

Water impermeable - tubular fluid diluted as salt reabsorbed.

Front 19

Late distal convoluted tubule and CCD - Principal cell

Back 19

Majority cell type.

Exchanges Na+ with K+.

Uses apical Na+ channel ENaC. Na+ moves down EC gradient into cell and pumped out via Na/K ATPase.

K+ recycled out via apical or basal K+ channels.

ENaC creates negative transepithelial potential so apical K+ channel is more active.

Negative transepithelial potential also causes Cl- reabsorption paracellularly.

Front 20

Influence of ADH on principal cells (late distal tubule)

Back 20

Principal cell is impermeable to water

ADH/vasopressin induces water channels to be incorporated into membrane so water can be reabsorbed. Regulated.

Front 21

Intercalated cell of late distal tubule

Back 21

Responsible for acid base transport

Alpha-intercalated cell secretes protons into lumen.

Beta-intercalated cell secretes base/HCO3- into lumen.

Balance of alpha and beta intercalated cell regulates acid-base balance

Front 22

Medullary collecting duct

Back 22

Populated by principal-like cells except no apical K+ channel, no K secretion.

Carries out final salt reabsorption.

ADH controls both water and urea transport in this segment.