Proximal tubule- reabsorption Flashcards
Label?
Label
What epithelial cells present in proximal tubule?
Simple cuboidal
What does proximal tubule recieve filtrate from?
Bowmans space
What is blood supply of proximal tubule?
Efferent arteriole
State function of proximal tubule? (2)
What involved?
1) Reabsorption of: Na, Cl, H2O, Glucose, Amino acids,
HCO3-, Phosphate
2) Secretion organic ions
State 3 things reabsorbed at proximal tubule?
Na, HcO3-, Glucose
What facilitates reabsorption Na+?
Location?
Na/K ATPase on basolateral membrane
How are substrate reabsorbed in proximal tubule? (4)
1) Na+- Na/K ATPase on basolateral membrane
2) Secondary active transport
3) Tight junctions
4) Passive diffusion
How is proximal tubule adapted?
1) Lots mitochondria
2) Good vascular supply- Na/K ATPase
3) High permeability H2O- osmosis
Where are Cl- ions usually absorbed?
Proximal tubule
Latter
Through what channel is water absorbed?
Where?
Aquaporin 1
Apical and basolateral membrane
Osmosis
Name two ways water passes through?
1) Aquaporin 1 (AQP1) channels on apical and basolateral
membranes
2) Tight junctions leaky to water- paracellularly
How is glucose reabsorbed in proximal tubule?
SGLT1 (carrier protein) 2Na+ and 1 glucose
Latter parts proximal tubule
High affinity for glucose as low concentration
SGLT2 (carrier protein) 1Na+ and 1 glucose
Earlier parts tubule
How does glucose cross basolateral membrane?
Passively
GLUT
What should glucose content be in urine?
0 (almost)
What function SGLT1/2
Reabsorb glucose
How is sodium reabsorbed?
1) Na+ actively transported out of proximal CT epithelial cell
Via Na/K ATPase pump (3Na+ out, 2 K+ in) on basolateral membrane
Into the interstitial fluid
2) Lowers conc Na+ intracellularly
3) Na+ move from the tubular lumen into the cell
4) Na+ transported into cell: in exchange for H+ or
co-transported (glucose or phosphate)
5) Once in cell pumped into the interstitium with Na/K ATPase or co-
transported with HCO3-
6) Water follows Na+ passively through osmosis
7) Differences water concentration between lumen and interstitial fluid results in net diffusion of water from lumen across the cell membrane and tight junctions
What percentage of Na+ is reabsorbed for protons?
80%
What pump used for reabsorption Na?
Na/K ATPase pump
What 2 ways N+ moves into cell?
1) Exchange for H+
2) Co-transported e.g. with glucose or phosphate- secondary active transport
When in cell how is Na+ pumped into interstitium?
1) Na/K ATPase
2) co-transported with HCO3-
What follows Na+?
Water
What follows Na+?
Water
How is bicarbonate reabsorbed?
1) Tubular secretion of H+ in exchange Na+
2) H+ combines with HCO3– to produce H2CO3 (carbonic acid)
3) H2CO3 dissociates into H2O and CO2
4) CO2 diffuses through the cell wall as it small and non-polar
5) H2O is reabsorbed through osmosis through AQP1 channels
6) Inside cell H2O and CO2 join together to form H2CO3
7) Dissociates into HCO3– and H+
8) H+ is recycled through this sequence again
2 methods of HCO3– transport across the basolateral membrane:
1) HCO3- (bicarbonate) is co-transported with Na+ into the interstitium
and then into the blood; 1Na+ with 3HCO3-
2) Pumped passively in exchange for an anion
What are two methods of transporting HCO3- across basolateral membrane?
1) HCO3- (bicarbonate) is co-transported with Na+ into the interstitium
and then into the blood; 1Na+ with 3HCO3-
2) Pumped passively in exchange for an anion
What does the combination of HCO3- and H+ form?
What does it require?
H2CO3
Carbonc anhydrase
What does CO2 and H20 form?
What is needed?
H2C03
Carbonic anhydrase
Write important equation needed move bicarbonate?
How is chloride reabsorbed?
How transported on basolateral membrane?
Paracellularly (between cells)
Exchanged base on apical membrane (formate)
Transported in NKCC2 channel on the basolateral membrane into blood
What base is exchanged with chloride on apical membrane?
Formate
Becomes formic acid- diffuse- reused
Give 2 ways amino acids transported?
Flow past basolateral membrane?
1) Amino acid transporters
2) Co-transported Na+
Can flow out passively on the basolateral membrane through their own channels
Should protein be present in proximal tubule?
No
If proteins manages get in what occurs?
1) Microvilli specialised sensors- bind any protein
2) Endocytosis occurs in endosomes
3) Protein degraded by lysosomes to produce amino acids
