The proximal tubule and loop of Henle Flashcards
Where does most tubular reabsorption occur?
Proximal tubule
Is tubular reabsorption active or passive?
Active
Is filtration specific or non specific?
Non-specific
Is tubular reabsorption specific or non specific?
Specific
Why is fluid reabsorbed in the proximal tubule iso-osmotic with filtrate?
Equal amounts of salt and water are reabsorbed
Reabsorption of which substance occurs completely within the proximal tubule?
Glucose
What separates the apical and basolateral membranes of tubular epithelial cells?
Tight junctions
During transcellular reabsorption which structures must be crossed?
Apical membrane Cytoplasma Basolateral membrane Interstitium Endothelial cells
During paracellular reabsorption which structures must be crossed?
Tight junctions
Interstitium
Endothelium
What is primary active transport?
Energy required to fuel the carrier protein to transport the substrate against its concentration gradient
What is secondary active transport?
The carrier protein transports the substrate coupled to the concentration gradient of an ion (usually sodium)
What is facilitated diffusion?
Passive carrier mediated transport of a substance down its concentration gradient
Why is sodium commonly utilised during active transport?
Because there are more sodium ions in the ECF and ICF and most transport is occuring into the cell
Diffusion can occur passively through ion channels. T/F
True
How does sodium reabsorption occur at the kidney?
Sodium enters tubular cells >
Sodium-potassium ATPase transports 3 sodium into the interstitium and 2 potassium into the cell at the basolateral membrane>
Sodium diffuses across the endothelium
Is the sodium-potassium pump energy dependent or independent?
Dependent
How does sodium enter tubular cells?
Secondary active transport of sodium and:
- Glucose
- Amino acids
Countertransport of sodium into cell and hydrogen ions out
How does chlorine reabsorption occur at the kidney?
The positively charged sodium ions in the interstitium set up and electrochemical gradient which attracts chloride ions paracellularly
How does water reabsorption occur at the kidney?
Salt absorbed attracts water (paracellular)
How is glucose reabsorbed in the kidney?
Secondary active transport with sodium into tubular cells then facilitated diffusion out of tubular cells
What is transport maximum for glucose?
Active transport mechanisms (i.e those which use conformational changes in membrane proteins) can only move a set number of molecules per unit time. Transport maximum occurs when these transport mechanisms become saturated (max glucose molecules per unit time)
Clearance of reabsorbed or secreted substances is constant once the transport maximum of that substance is reached. T/F
False - it is NOT constant once transport maximum is reached
Do secretory or reabsorptive mechanisms reach a transport maximum?
Both do
What is the function of the loop of Henle? How does it achieve this?
Producing concentrated urine. Creating a cortico-medullary solute concentration gradient
What is countercurrent flow?
Opposing flow in the two limbs of the loop of Henle
The loop of Henle and vasa recta are responsible for creating a hyper-osmotic medullary interstitium. Therefore which type of nephron is involved?
Juxto-medullary
What is being reabsorbed at the descending loop of Henle? What is it impermeable to?
Water
Salts
What is being reabsorbed at the ascending loop of Henle? What is it impermeable to?
Salts
Water
How does reabsorption of salts differ within the ascending loop of Henle?
Thick upper portion - active transport
Thin lower portion - passive
What is the effect of the differing permeabilities between the limbs of the loop of Henle?
Enable osmotic gradient to be established within the medulla
How are ions reabsorbed within the loop of Henle?
Triple cotransporter
Explain the triple cotransporter
Ions are pumped into the tubular cells: 1 sodium 1 potassium 2 choride (equal charges)
At which membrane is the triple cotransporter always found?
Luminal
How do loop diuretics work?
By blocking the triple cotransporter at the loop of Henle thereby limiting salt reabsorption and thus water reabsorption (water follows salt)
How is potassium important in salt reabsorption at the loop of Henle?
Potassium is involved in the active transport of chloride and sodium ions and is continually recycled (i.e continually moves in and out of the cell)
What happens to the fluid as it passes through the loop of Henle?
Isotonic fluid enters descending limb >
Hypertonic fluid enters the ascending limb >
Hypotonic fluid leaves the ascending limb
What is countercurrent multiplication?
The effect the loop of Henle has to transform a horizontal concentration gradient into a vertical one within the medulla
Describe the concentration gradient produced by the loop of Henle
Osmolarity should be higher deeper within the medulla
Apart from the loop of Henle, what is important in creating the corticomedullary concentration gradient?
Urea cycle
Hows ADH promote or oppose urea secretion into the proximal tubule?
Promotes
Is the distal tubule permeable or impermeable to urea?
Impermeable
Why is the cortico-medullary gradient important for urine production?
It allows different concentrations and volumes of urine to be produced
What is the countercurrent exchanger?
The vasa recta acts as a countercurrent exchanger for juxtamedullary nephrons allowing blood to equilibrate with the juxtamedullary interstitial gradient
What is the countercurrent system?
The combined efforts of the vasa recta and the loop of henle
How is the loss of salt and urea from the medulla minimised?
Vasa recta runs in a hairpin loop
Vasa recta capillaries freely permeable to salt and water
Blood flow to vasa recta is low (minimal juxtamedullary nephrons)
Passive exchange of salt and water across the vasa recta helps to maintain the juxtamedullary gradient. T/F
True - since the blood equilibrates
What is the function of the vasa recta?
To ensure solute is not washed away from the medulla
The high medullary gradient allows the production of hypertonic urine in the presence of ADH. T/F
True
