Reabsorption – Tubular Reabsorption

This section was adapted primarily from [http://faculty.etsu.edu/currie/filtreab.htm ] to provide an overview of the various reabsorption processes along the nephron.  The bullet outline format is used to collect points of interest.  Video links follow to provide more details.

• Approximately 99% of filtrate is reabsorbed.

• Urine output is about 1.5 L per day but can fall as low as 400 mL/day to conserve water, or increase dramatically to rid the body of excess water.

• When urine output falls into the 400-600 mL/day range, it is nearly impossible to maintain a normal electrolyte and pH balance.

• Approximately 65% of salt and water re-absorption occurs in the proximal tubule.  Since the proximal tubule is freely permeable to salt and water, this reabsorption does not significantly alter the concentration of the filtrate.

  • Water follows sodium transfer in the proximal tubule, but osmolarity stays the same. 

  • Both Cl- and H2O passively follow the Na+ (the H2O follows Na+ in the form of a sphere of hydration while Cl-follows Na+ because of the charge attraction). 

  • Glucose and proteins/amino acids are absorbed only in the proximal tubule (there are no proteins or glucose in the loop of Henle in the normal kidney).

• Approximately 20% of the renal filtrate is reabsorbed in the loop of Henle.

• The distal convoluted tubule is permeable to water, but the permeability if regulated by ADH control of aquaporin sites.

  • Antidiuretic hormone (ADH) is synthesized in the hypothalamus, but is stored and released from the posterior pituitary.  ADH also causes vasoconstriction and is part of the RAAS.

• In distal tubule, K+ and  creatinine are excreted, and calcium is reabsorbed.

• The collecting duct passes through the molarity gradient in the medulla (described in the following paragraph).  If ADH has caused an increase in the number of functional water channels in the collecting duct, additional water will be reabsorbed into the bloodstream and the filtrate will be further concentrated.

The renal medulla essentially functions as a “salt bed” (hypertonic) with increasing ionic concentration toward the center of the kidney which reaches a peak of approximately 1200 mOsm.  Key features of this structure include:

• The descending limb of the loop of Henle concentrates solutes in so it will be easier for the ascending limb to pump those solutes into the interstitial fluid of the medulla.

  • Permeable to water

• The ascending limb concentrates solutes in the medulla.

  • Impermeable to water

  • Permeable to Na+, Cl-, and K+

• The "symbiotic functional relationship" between the ascending and descending limb is known as the countercurrent multiplier system.

• The vasa recta capillary beds mimic the prevailing concentration gradient in the kidney tubules to help trap solutes in the medullary region and maintain the high solute concentration. 

  • The vasa recta are freely permeable to salt and water.

  • Solutes diffusing out of the ascending limb on the way up diffuse back into the descending limb and are carried back down.

  • Blood flow through the vasa recta is slow to allow time for equilibration of the steep concentration gradients.

• In order to produce concentrated urine, the medulla must have a high solute concentration.

• The longer the loops and the greater the number of loops; then the more time, distance and surface area available to create gradients and result in greater ability to produce concentrated urine.

 

Video:  Khan Academy, Secondary Active Transport in Nephron

www.youtube.com/watch?v=czY5nyvZ7cU

Outline of video content:

• Proximal tubule has “brush border” facing the lumen (apical side)

• Peritubular capillaries surround the tubule

• Proximal convoluted tubule reabsorbs glucose, amino acids, Na+ (ATP-driven active transport)

• Loop of Henle reabsorbs Na+, K+, Cl- (NKC co-transporter, a secondary active transporter driven by Na+ gradient)

• Distal tubule reabsorbs Ca2+ (Na-Ca anti-porter, a secondary transporter driven by Na+ gradient)

 

Video:  Andrew Wolf, Renal Physiology:  Reabsorption and Excretion

www.youtube.com/watch?v=uKj4PwMic7E

Outline of video content:

• Vasa recta lined with endothelial cells, basement membrane

• Basal side of cuboidal cells on capillaries, apical surface pointing into lumen of tubule

• In cuboidal cells:

  • Basal surface – Na-K-ATPase pump:  3 Na+ out, 3 K+ in (requires ATP, O2, glucose)

    • Active pump sets up Na+ gradient that drives Na+ transfer at apical surface

  • Apical surface – Na-Glucose co-transporter:  Na+ in, Glucose in

  • Apical surface – Na-K-Cl- co-transporter:  Na+ in, K+ in, 2 Cl- in

  • Apical surface – Na-H counter-transport:  Na+ in, H+ out (acid-base balance)

  • Apical surface – protein fragments phagocytosis 

Video:  Andrew Wolf, Concentration of Urine
www.youtube.com/watch?v=8Ssh4c9cJf0

Outline of video content:

• Osmolarity gradient created by loop of Henle

• Filtrate enters isotonic at 300 mOsm

• Filtrate leaves hypotonic at 200 mOsm

• Maximum at the bottom of the loop of Henle 1200 mOsm

Armando Hasudungan, Pharmacology - Diuretics

www.youtube.com/watch?v=NzdvoGZquIk

Outline of video content:

• Reduce blood pressure by increasing excretion of Na and water in urine

• Proximal tubule

  • 65-70% of Na reabsorbed

  • Bicarbonate reabsorbed as part of the pH control system

• Distal tubule

  • 5% of Na reabsorbed, Cl

• Thick descending limb

  • Water

• Thick ascending limb

  • 25% Na reabsorbed, K,Cl

• Collecting duct

  • 1-2% of Na reabsorbed

  • Water can also be reabsorbed

• Osmotic diuretics

  • In Proximal and Loop of Henle

  • Increases solute concentration within the tubules, and thus causes more water to be retained

  • Decrease plasma volume, decrease blood pressure

• Loop diuretics

  • Works at thick ascending limb

  • Prevent Na reabsorption (inhibits active transport of Na-K-ATPase)

• Thiazide diuretics

  • Works at distal convoluted tubule

  • Inhibits Na-Cl transporter

• Potassium Sparing diuretics

  • Works at distal end of distal tubule and collecting duct