Urinary System 4 - Tubular Function Flashcards
Define osmolarity
All the concentrations of the different solutes (measured in mmol/l) added together. separately
Compare passive and active movement in reabsorption
- As the concentration of lipophilic molecules increases, rate increases
- Protein dependent transport reaches a peak
- ATP dependent processes reach a peak
How is water transported into filtrate?
- Through tight junctions (paracellular)
- Through aquaporins
How are passive uptake systems regulated?
- Controlled by the number of protein channels present in the membrane
- Channels can be stored in the cytoplasm
Describe the process of protein reabsorption
- Uptaken by endocytosis
- pH causes a conformational change and the protein and receptor split
- Receptor retaken to the membrane
What is the transport maxima?
- The maximum rate of reabsorption
- Can vary between basal rate and stimulated rate depending on protein expression
- Once surpassed, a substance will enter the urine
Compare reabsorption in the different parts of the nephron
- PCT 60-70% of all solute, all glucose, 65% Na, 90% bicarbonate
- Loop of henle reabsorbs 25% Na
- Distal convoluted tubule absorbs 8% Na
- Collecting duct water only
Describe the process of reabsorption in the proximal convoluted tubule
- Sodium is coupled with chloride, glucose and amino acids
- Protein is reabsorbed by endocytosis
- Na/proton pump in the apical membrane
- Urea, potassium and magnesium has paracellular pathways
- Basolateral membrane as glucose and amino acid transporters, sodium potassium pump and cl-/HCO3- pump (Cl- into blood)
Describe the reabsorption of bicarbonate ions in the early PCT
- In the tubule, H+ from the sodium proton pump reacts with bicarbonate ions
- Carbonic anhydrase converts carbonic acid into H2O and CO2
- These diffuse into the cells of the proximal tubule, were carbonic anhydrase is made again and dissociates
- Bicarbonate ions are reabsorbed into the blood, while protons are pumped back out through the same initial sodium, proton exchanger
List the substances that undergo active reabsorption in the proximal convoluted tubule
- Glucose
- Amino acids
- Sodium
- Potassium
- Calcium
- Vitamin C
- Uric acid
List the substances that undergo passive reabsorption in the proximal convoluted tubule
- Urea
- Water
Why is secretion in the PCT important?
- Route of excretion of some substances
- Some drugs enter the tubular fluid here to act further down the nephron
What cells line the descending and ascending limbs of the loop of henle?
- Descending limb is thin - squamous epithelium with a few mitochondria
- Ascending limb is thick - Cuboidal with some microvilli and many mitochondria
Compare the functions of the ascending and descending limbs of the loop of henle
- Descending limb passively absorbs water, and sodium and potassium follow down the osmotic gradient
- Ascending limb actively reabsorbs chloride, with sodium passively following, as well as bicarbonate. It is impermiable to water
Describe the tubular fluid leaving the loop of henle
- 85% water and 90% sodium has been reabsorbed
- The solution is hypo-osmolar compared to plasma
Describe the ion movement in the ascending loop of henle
- Sodium enters with chloride and potassium
- Potassium diffuses into plasma through open channels
- On the basal membrane, sodium potassium pump, potassium channels, and a potassium chloride transporter (out)
- Chloride also has separate channels
- Sodium, potassium, calcium and magnesium ions also diffuse via paracellular pathway
Describe the cells lining the distal convoluted tubule
- Cuboidal epithelim with some microvilli
- Complex lateral membrane interdigitations with sodium pumps
- Multiple mitochondria
Describe the ion movement in the distal convoluted tubule
- Sodium chloride pump at the apical membrane
- Calcium channel too
- Basolateral membrane, sodium calcium pump (3Na in 1Ca out), chloride and potassium channels + sodium potassium pump
How do thiazides affect absorption in the distal convoluted tubule?
- Thiazides are diuretics
- They block the sodium-chloride channels on the apical membrane
- This results in plasma calcium rising as sodium is still pumped out of the cell by Na/K ATPase, so the only other entry into the cell is through the Na/Ca pump on the basal membrane, which pumps calcium out
What are principal cells?
- Present in the distal tubule and collecting duct
- They are affected by aldosterone
- Important in sodium, potassium and water balance
- Mediated by Na/K ATPase
- Water reabsorption controlled by vasopressin
What are intercalated cells?
- Important in acid-base balance
- Mediated by H+ ATP pump (pumps protons into tubular fluid)
- Present in distal tubule and collecting duct
- Basal membrane contains Cl-/HCO3- pump
Why is there very little paracellular transport in the collecting duct?
Tight epithelium with strong tight junctions
List the three single gene defects affecting tubular function
- Renal tubule acidosis
- Bartter syndrome
- Faconi syndrome (dents disease)
What is renal tubular acidosis?
- Hyperchloremic metabolic acidosis
- Impaired growth
- Hypokalemia
- Protons are not excreted in the kidney, or there may be issues with carbonic anhydrase (linked with proton production for excretion)
What is bartter syndrome?
- Excessive electrolyte secretion
- Premature birth
- Polyydramnios
- Salt loss
- Metabolic alkalosis
- Hypokalemia
- Renin and aldosterone hyper secretion
What is Faconi syndrome?
- Increased excretion of uric acid, glucose, phosphate and bicarbonate
- Excretion of small proteins, associated with renal tubular acidosis as the protein isn’t separated from its membrane protein in the proximal tubules
- Protons are pumped in to the cells, and the H+/Cl- pump is faulty and so a low enough pH can not be reached in the endosome for dissociation
