L18 Flashcards
The proximal tubule is concerned with….
the uptake and secretion of several solutes and water, but the fluid in the tubule remains
isotonic throughout.
The apical ( .... ) surface of the cells have numerous .... which enhance their transport function.
(tubule side)
microvilli
The proximal tubule has several important functions: (3)
- Reabsorption of the bulk of filtered NaCl as isotonic NaCl solution essential solutes, e.g. glucose and all amino acids
- Regulation of body fluid pH (by bicarbonate reabsorption).
- Secretion of some organic molecules.
Reabsorption is the net movement
from …
apical to baso-lateral
membrane
Secretion is the net movement
from….
baso-lateral to apical membrane
About ….% of the filtered load of NaCl is reabsorbed, but tubular fluid remains isotonic
80
Describe NaCl and Water Reabsorption
- Na+ are transported into the epithelial cell by secondary active transport
- coupled co-transport with other molecules
- coupled counter-transport
- the Na+ are removed from the baso-lateral surface by active transport-the Na pump
- Cl-and H2O follow passively.
- Therefore isotonic NaCl reabsorption has occurred.
Przypomnienie:
In normal individuals, all the filtered load of glucose and other vital metabolites (amino acids, etc.) are reabsorbed in the proximal tubule.
Describe Glucose Clearance
Glucose is:
- freely filtered
- absorbed by the tubule from lumen to blood
- not metabolised
- does not affect GFR itself
If the Pglu is not high all the glucose in the glomerular filtrate is reabsorbed and the urine excretion is zero:
Cglu=0!!!!
This is a physiologically useful function of the tubule: to reabsorb useful substrates from the filtrate that otherwise would be lost in the urine.
Glucose re-absorption is a carrier-mediated process, and therefore has a maximum transport rate called:
The tubular transport maximum or Tmax
The kidney can reabsorb completely the filtered glucose load at normal plasma (and therefore filtrate levels) ~=…
4-6mM or 70-110 mg/dl
Explain what happens in diabetes mellitus
If the concentration exceeds a critical level CARRIERS ARE SATURATED AND THE EXCESS GLUCOSE APPEARS IN THE URINE
This plasma level is ~= 300 mg/dl or ~= 16mM. It is usually lower ~= 11 mM as nephrons have various Tmax
Describe the mechanisms used in Reabsorption of Bicarbonate (HCO3-) in Proximal Tubule
- The mechanisms used are secondary active transport
* Are dependent on carbonic anhydrase
Describe the process of Reabsorption of Bicarbonate (HCO3-) in Proximal Tubule
- Secretion of H+
- Buffering of H+ within the tubular lumen be reabsorbing HCO3-
- Buffering of H+ within the tubular lumen with HPO42- or NH3
- Resultant urine pH 4.5-8.2 due to balance of H+ and HCO3- secretion.
- Net result all HCO3- is reabsorbed in the PCT.
The kidney plays an important role in the regulation of pH through several mechanisms:
- Secretion of H+
- Buffering of H+ within the tubular lumen be reabsorbing HCO3-
- Buffering of H+ within the tubular lumen with HPO42- or NH3
