Glossary of 4 - Tubular Transport I
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- What proteins are thought to be involved with paracellular transport?
- Compare filtered load and excreted load in reabsorption and in secretion.
- *reabsorption: filtered load > excreted load
*secretion: excreted load > filtered load
- Why is movement of water from the interstitium to the peritubular capillaries favored?
- The oncotic pressure in the capillaries exceeds hydrostatic pressure, thus movement into the capillary is favored. Also, high flow aids in uptake.
- What tubular segment does not show transcellular reabsorption of NaCl?
- The descending limb of the loop of Henle.
- Is movement of Na+ from the lumen across the apical membrane with or against the electrochemical gradient? What is the benefit of this?
- It is a passive process that follows the electrochemical gradient, and therefore it can be coupled to drive other secondary transport mechanisms (Glc, H+, Cl-, etc.).
- Is movement of Na+ across the basolateral membrane into the interstitium a passive or active process?
- It is active and requires a carrier, usually Na+/K+ ATPase.
- Is paracellular movement of Na+ a passive or active process?
- It is usually passive.
- What two segments of the nephron feature an especially leaky epithelia? Compare the paracellular movement in these two segments.
- The proximal convoluted tubule (reabsorption) and descending loop of Henle (secretion of Na+).
- Which nephron segments feature a tight epithelia? Describe the movement of water through these segments.
- Ascending limb of the loop of Henle, distal tubule, and the collecting ducts. Water movement is transcellular and through aquaporins other than AQP-1.
- Describe AQP-1: where is it found? How is it controlled?
- AQP-1 is found in the PCT and vasa recta...it is constitutively expressed and not regulated by ADH.
- Describe the transport of water across the distal tubule and collecting ducts.
- In these segments water moves transcellular through AQP-2 which is regulated by ADH.
- Where is AQP-3 found? How is it regulated?
- AQP-3 is found on the basolateral membrane of distal tubule and collecting duct epithelia. It is non-regulated.
- Describe the osmolarity of the reabsorbate and the exiting filtrate of the proximal tubule.
- Because the proximal tubule reabsorbs large amounts of both water and solute, both the reabsorbate and the exiting fluid is isosmolar to plasma.
- Does the loop of Henle reabsorb more water or more solute? Describe the osmolarity of the reabsorbate and the exiting filtrate of this segment.
- The loop of Henle reabsorbs more solute than water. As a result, the reabsorbate is hyperosmolar to plasma, whereas the exiting filtrate is hyposmolar.
- Do the distal tubule and collecting ducts reabsorb more water or more solute? What determines this?
- These segments reabsorb more water than solute, but only when ADH is present. In its absence the converse is true.
- Describe the osmolarity of the reabsorbate and the exiting filtrate of the distal tubule and collecting ducts when ADH is present.
- Because more water than solute is reabsorbed in this situation, the reabsorbate is hyposmolar to plasma and exiting filtrate is hyperosmolar to plasma.
- Name three solutes that are co-transported with the reabsorption of Na+ in the proximal convoluted tubule.
- Reabsorption of Cl-, glucose, and amino acids are coupled to the passive uptake of Na+.
- What is the maximum activity for the Na+/glucose transporter in the proximal tubule?
- How is Na+ moved across the basolateral membrane in the PCT? How is it then removed from the interstitium?
- Transport across the basolateral membrane is an active process facilitated by Na+/K+ ATPase. From the interstitium, Na+ is carried with water into the peritubular capillaries. Also there is some paracellular leakage back into the lumen.
- In the PCT there are open channels for water, but what powers its movement?
- Water moves down small, local, transient osmotic gradients.
- What is osmotic diuresis? How does it occur in diabetes?
- Failure to reabsorb an osmotic particle causes osmotic diuresis by creating an osmotic pressure that draws fluid into the lumen. In DM, glucose acts as this particle.
- Describe the movement of H+, bicarbonate, and CO2 across the apical membrane of the proximal tubule.
- H+ is secreted into the lumen via a Na+/H+ antiporter. It combines with bicarbonate (which does not pass the membrane on its own) to form CO2 and water. CO2 diffuses rapidly across the apical membrane into the cell.
- How is bicarbonate moved across the basolateral membrane?
- Movement is via a Cl-/bicarbonate ATPase which moves HCO3- out and Cl- in.
- What percentage of filtered urea is reabsorbed by the proximal tubule? How is this regulated?
- The proximal tubule reabsorbs 50% of filtered urea via unregulated passive diffusion.
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