Tubular processing Flashcards
Where in the nephron does the majority of reabsorption occur?
Proximal convoluted tubule
What things are mostly reabsorbed in the proximal convoluted tubule?
Sodium, water, glucose and amino acids
Is the proximal convoluted tubule permeable to water?
Yes.
Tubule fluid leaving PCT is isosmotic as epithelium is freely permeable to water.
Does secretion occur in the proximal convoluted tubule?
Yes, site of secretion of metabolic acids, bases, drugs etc.
Which reabsorption transporters are found in the proximal convoluted tubule?
Secondary active co-transporters linked to sodium reabsorption.
Sodium glucose co-transporters (SGLT2) on luminal side move glucose against concentration gradient.
Glucose transporters (GLUT) on basal side allow facilitated diffusion into interstitial fluid.
Similar process for amino acids.
Which secretion transporters are found in the proximal convoluted tubule?
Sodium reabsorption also linked to the secondary active transport of hydrogen ions into the lumen (secretion).
Important for bicarbonate reabsorption.
Transporter is a Na+/H+ exchanger, NHE (antiporter)
What are the three parts of the loop of Henle?
Thin descending limb
Thin ascending limb
Thick ascending limb
What happens in the thin descending limb of loop of Henle?
Permeable to water.
No active reabsorption or secretion of solutes.
What happens in the thin ascending limb of loop of Henle?
Impermeable to water.
Essentially no active reabsorption or secretion of solutes.
What happens in the thick ascending limb of loop of Henle?
Impermeable to water.
Active reabsorption of sodium (~25% filtered load) and other solutes.
‘Dilutes’ the luminal fluid (hypo-osmotic) as solutes are removed/reabsorbed but water cannot follow.
Describe how transport occurs in the thick ascending limb of loop of Henle
Reabsorption from tubule lumen mediated primarily by sodium, potassium 2-chloride co-transporter (Na+K+2Cl-)
Positive charge in lumen encourages paracellular reabsorption of cations (including Ca2+ and Mg2+)
As water cannot follow the solutes, the remaining tubular lumen fluid is diluted
What happens in the early distal tubule?
First portion contains the macula densa (sensitive to [NaCl]) – part of the juxtaglomerular apparatus involved with feedback control of GFR and blood pressure
Early distal tubule impermeable to water and contributes to dilution of the filtrate
Active reabsorption of sodium (~5% filtered load) via a sodium-chloride co-transporter
Are the late distal tubule and cortical collecting duct water permeable?
Water permeability of this part of nephron under hormonal control by antidiuretic hormone (ADH):
Water permeable when ADH present
Water impermeable when ADH absent
What are the two cell types in the late distal tubule and cortical collecting tubule?
Principal cells involved in sodium reabsorption and potassium secretion
Intercalated cells involved in potassium reabsorption and hydrogen ion secretion.
Describe the transporters in the principal cells
Sodium enters principal cells through epithelial sodium channels (ENaC) on luminal side
Transported out of cells by Na+/K+ ATPase to help maintain concentration gradient
Number of ENaC channels and activity of ATPase under hormonal control by aldosterone - adjustable
Therefore an important site of regulation and fine tuning of sodium reabsorption and potassium secretion
