Major Urinary Solutes

Question 1

Proximal Segment Absorption and Secretion

Answer 1

• the bulk of substances are reabsorbed in the proximal segment
  
  • includes the proximal tubule and the loop of Henle
• under normal circumstances, protein, glucose and amino acids never leave the proximal tubule.
• Secretion:
  - P-aminohippurate (PAH)
  - Choline
  - Creatinine
  - 3,5-diiodotyrosine
  - Nitrofurantoin
  - Phenosulfonphthalein
  - Tetraethyl ammonium
  - Urate
  - Hydrogen
• Reabsorption:
  - Amino acids
  - Ascorbate
  - Bicarbonate
  
  • Glucose
    - Phosphate
    - Potassium
    - Protein
    - Sodium and Chloride
    - Sulfate
    - Urate
    - Water

Question 2

Distal Segment and Collecting Duct Reabsorption and Secretion

Answer 2

• Fine tuning of the reabsorption of many substances accomplished in the distal segments of the tubular system.
  
  • sodium, potassium, hydrogen (acid) and water
• It is this fine tuning that determines the eventual excretion rate of solutes and water.
• Secretion
  - Ammonia, Hydrogen, Potassium
• Reabsorption
  - Calcium, Magnesium, Potassium
  - Sodium and Chloride
  
  • Water

Question 3

Renal Tubular Reabsorption

Answer 3

-Tubular reabsorption different from glomerular filtration.
=Filtration occurs by bulk flow, but little bulk flow across the tubular cells from lumen to interstitium
-b/c the tubular epithelium is relatively “nonporous”
-hydraulic and oncotic pressure gradients are minimal.
-reabsorption of different substances may be linked
-reabsorption of many AAs, glucose, and other solutes are linked to the reabsorption of sodium),
-A single reabsorption system may be transporting several distinct but structurally similar substances
-4 of the simple carbs are reabsorbed by a single system
-Only a few substances reabsorbed by simple diffusion across tight junctions b/t cells (such as Mg+)
-All others must cross two membranes from tubular lumen to interstitial fluid
-the luminal membrane (separating luminal fluid from cell cytoplasm)
-and the basolateral membrane (separating cytoplasm from interstitial fluid).
-“Active” transport: at least one of the membrane crossings is by a primary or secondary active process (i.e., uphill transport against the electrochemical gradient of the substance has occurred somewhere between lumen and interstitial fluid).

Question 4

Transport Maximum (Tm) -Titration Curves

Answer 4

-Many reabsorptive systems can transport only limited amounts of material/unit time,
-primarily because the membrane proteins responsible for the transport become saturated.
-Classical example is the proximal transport process for glucose.
-Normally no glucose is excreted from the kidney.
-As the plasma [glucose] is increased above the renal threshold, glucose appears in the urine.
-The higher the plasma [glucose], the greater the quantity excreted in the urine.
-Quantity of glucose filtered/minute is a linear function of the [plasma] when measurements made under conditions of a stable filtration rate
- Quantity of glucose excreted/minute is a linear function of the [plasma] once threshold is exceeded.
-The difference between quantity filtered/minute and quantity excreted/minute is the quantity reabsorbed/minute.
-This represents the tubular maximal reabsorptive capacity for glucose and is abbreviated Tm.​
​Tm = FG - EG when EG is greater than 0
​FG = filtered glucose (mg/min)
EG = excreted glucose (mg/min)

Question 5

Renal Tubular Secretion

Answer 5

• Resembles tubular reabsorption except in the orientation of the transport mechanism.
• Occurs from peritubular fluid to tubular lumen;
• For Tm-limited mechanisms:
  
  • the Tm for secreted substances is equivalent to the amount excreted minus that filtered i.e.
• TmX = EX - FX when EX is greater than FX
• ​EX​ = excreted substance (mg/min)