Rao Objectives- Proximal Tubule Reabsorption and Secretion Flashcards
What percentage of the glomerular filtrate is reabsorbed by the proximal tubule?
Two thirds.
Describe reabsorption form the proximal tubule.
It is essential that it be rapid to maintain normal extracellular fluid volume.
It is also iso-osmotic here (meaning that fluid and filtrate are reabsorbed at the same level).
How is sodium reabsorbed in the basolateral membrane?
via active transport involving a Na/K ATPase pump
- *This is the only one that is directly linked to its energy source in the proximal tubule.
- **3 Na+ out to interstitial fluid for every 2 K+ into the cell
- **This uses 1 ATP
How is sodium absorbed in the lumenal membrane?
Here it is done via passive transport. This is due to the large decrease in intracellular sodium from the pump. This causes a large decrease in membrane potential and causes an influx of sodium moving down its electrochemical gradient.
Why are chloride and bicarbonate reabsorbed here?
They follow sodium to maintain electroneutrality.
How does anion (CL- and HCO3-) occur here?
Via the paracellular route predominantly due to leaky epithelium.
**As it progresses to the straight tubule, Cl- reabsorption is reduced and bicarbonate is absorbed more rapidly.
Why is water reabsorbed here?
It follows solute transport to maintain iso-osmolarity.
How is water reabsorbed here?
It is facilitated by leaky epithelium and high hydraulic conductivity. This is done using aquaporins (AQP-1= luminal membrane, AQP-4/5= basolateral membrane, AQP-2= distal tubule)
What drives the uptake of solutes and fluids?
Starling forces across peritubular capillary epithelium:
- Positive interstitial fluid pressure
- Low capillary hydrostatic pressure (reduces opposition)
- High capillary oncotic pressure
* *These all favor capillary uptake of solute and fluid
Describe solute transport in the proximal tubule:
It is selective (i.e. some active and some passive transport).
- Nutrients are reabsorbed completely.
- Ions are reabsorbed almost completely.
- Metabolic waste products are reabsorbed partially.
- Inulin is not reabsorbed- ratio increases to 3 because it is secreted into the lumen.
Describe bicarbonate reabsorption in the proximal tubule:
It requires active secretion of protons.
- *95% of bicarbonate is reabsorbed here.
- driven by H+ secretion into the lumen via the sodium/proton exchanger.
- *forms carbonic acid
Describe glucose reabsortion in the proximal tubule:
It is completely reabsorbed here until it reaches threshold level. Glucose filtration load above this is completely excreted in the urine.
**Glucose transport is sodium transport via a sodium/glucose cotransporter (SGLT) in the apical membrane.
How are amino acids and organic acids reabsorbed here?
cotransport with sodium in the apical membrane
- *electrochemical potential gradient is involved
- *almost completely reabsorbed
How are chloride, potassium, and urea reabsorbed?
passive transport mechanism
What type of substances can serve as diuretics?
poorly permanent solutes
**This is clinically significant because substance like this, such as mannitol, can be used to reduce intracranial and intraocular pressure and promote excretion of toxins and edema.
