Unit 3 Lecture 3: Trans-epithelial Transport Flashcards
Define Trans-epithelial Transport
The filtrate once it has passed the glomerulus and has been filtered being reasbsorbed into the peritubular capillary
Note: Highly Selective
What is renal clearance? How is it related to selective reabsorption?
- It is the volume of plasma with substances removed from the filtrate itself; if there is no glucose in the filtrate that means renal clearance is perfect
- Related to selective reabsorption as some substances have a partial renal clearance or complete renal clearance depending on how much substance is reabsorbed
What are substances that are not reabsorbed but fully secreted?
H+, K+, penicillin;
Renal clearance is 0 and it actually increases renal clearance
For substances like penicillin where it would be used for infections, how would we counteract the effects of it being fully excreted?
We would need a higher concentration of penicillin to effectively work on the body
Why is inulin = glomerular filtration rate?
Inulin is not reabsorbed at all so it is excreted out with the 125mL/min of plasma which is the GFR
Substances that are partially reabsorbed include?
Urea - Concentrated with nitrogen which may be beneficial to the body
Sodium - 99.5% of it is completely absorbed by the body as 80% of the kidneys function on the usage of sodium!
Explain the basic idea behind tubular reabsoption process? (Not sodium or water)
- Sodium is initially actively absorbed (mostly in proximal tubule)
- Flow of cations causes an electrochemical gradient which anions are attracted to +ve charge in peritubular capillary
- Ions accumulating in interstitial fluid will create an osmotic gradient (pulls fluid towards solutes)
- Water moves by osmosis towards this osmotic gradient of high solute concentration
How does urea which mostly stays in the tubule become highly concentrated?
Because most of the substances are reabsorbed other than the entirety of urea, which makes it highly concentrated in the tubule and therefore in the urine
Note: Essentially because there’s a lot of urea in the tubule while most of the other things are reabsorbed, there is a high [Urea] compared to [Glucose] which is 0
Describe the process and barriers that a substance must pass through the filtrate to get to the plasma
- Luminal Cell Membrane
- Cytosol
- Basolateral cell membrane
- Interstitial fluid
- Capillary wall
What makes the inner part, outer part & between both for the epithelial cells of the tubule?
Inner part - Luminar membrane (in the lumen of the tubule)
Outer part - Basolateral membrane (facing interstitial fluid)
Between epithelial cells - Lateral space
Epithelial cells make up the tubular wall
What is the purpose of the tight junction in between tubular lumen and tubular epithelial cell?
To not allow substances to slip between the tubular epithelial cell and make the substances go through the tubular epithelial cell
Explain the steps for sodium reabsorption?
- Na+ passively enters the tubular epithelial cell by going through Na+ channels (concentration + electrochemical gradient)
- Na+ is actively pumped to the basolateral side by Na+/K+ ATPase pump
- Na+ diffuses into peritubular capillary
What is creating the concentration gradient and electrochemical gradient in sodium reabsorption? How is it doing this?
Na+
* Higher concentration of Na+ in the tubular epithelial cells as it passively moves into that
* More cations (Na) moving out but less K+ returning so more negative electrochemical gradient in tubular cell
Negative gradient in tubular cell will pull the Na+ passively towards it
What is the purpose of the Na+/K+ ATPase pump in the tubular cell?
- Maintain concentration gradient for passive Na+ influx into tubular cell
- Maintain electrochemical gradient for passive Na+ influx into tubular cell
- Actively pump Na+ to aid in Na+ efflux from tubular cell
Most of the energy used in the kidneys is dedicated to what?
Moving Sodium into the capillary (ATPase pump); 80% of the energy used by the kidney is for Na+ movement
Sodium reabsorbing mainly in the proximal tubule also does what to the other substances?
It creates an osmotic gradient to allow other substances like water, glucose, amino acids, urea, Cl-
Explain the steps for water reabsorption
- H20 moves through selective channels (aquaporins)
- [Na+] is high in lateral space due to ATPase pump moving it out, it creates an osmotic pressure which essentially pulls water out of the tubular cell
- H20 moves into peritubular capillaries (Hydrostatic + osmotic pressure)
Hydrostatic pressure is involved because interstitial fluid in these capillaries are more confined so pressure is high and it moves towards lower pressure
