2.13 - Tubule Reabsorption & Secretion Flashcards
What are the different pathways for filtered substances to be reabsorbed?
Paracellularly
Transcellularly
Passive Diffusion
Active Diffusion
Osmosis (water)
Describe renal handling of Sodium in the basolateral membrane
Sodium is transported across the basolateral membrane via the Na+/K+ ATPase antiporter.
By actively removing the Na+ from the cell, this creates a concentration gradient across the apical membrane that allows the Na+ in the tubule to cross the apical membrane passively.
How much of the filtered sodium is reabsorbed in the PCT?
65-67%
What type of apical transporter is responsible for the reabsorption of sodium in the PCT?
A Na+/H+ antiporter. H+ in to the lumen, Na+ in to the tubular cells
What type of apical transporter is responsible for the reabsorption of sodium in the Thick Ascending Limb?
Na+/2Cl-/K+ (NaCK). All ions in to the tubular cell. Inhibited by Furosemide
As the potassium ion can re-enter the tubule via an ROMK channel, the net effect is the removal of one sodium and two chloride ions, leaving the tubular lumen positively charged. This positive potential drives the paracellular transport of positively charged ions, including sodium
What type of apical transporter is responsible for the reabsorption of sodium in the distal tubule?
A Na+/Cl- co-transporter. All ions in to the tubular cell. Inhibited by Thiazide diuretics
What type of apical transporter is responsible for the reabsorption of sodium in the collecting tubules?
Epithelial sodium channel (ENaC). Inihibted by Amiloride
Describe the reabsoorption of glucose in the kindeys
98% reabsorbed in the proximal convoluted tubule. The remaining 2% in the remaining nephron segments.
SGLT2 (a glucose, Na+ co-transporter) is responsible for bringing in glucose against its concentration gradint, using the energy of Na+ moving down its concentration gradient.
Describe renal potassium handling in the proximal tubule
65% of K+ is reabsorbed in the proximal tubule. This reabsorption largely tied with the reabsorption of sodium and water. Reabsorption of water increases the concentration gradient of K+ in the tubule to the interstitium –> paracellular reabsorption of K+
Removal of K+ from the interstitium in to the tubular cells by the basolateral Na+/K+ ATPase may enhance this paracellular reabsorption
Describe renal potassium handling in the Thick Ascending Limb
Around 30% of the filtered potassium is reabsorbed in the thick ascending limb of the loop of Henle. As in the proximal tubule, this potassium reabsorption is linked to sodium reabsorption. This is mediated by the NKCC2 transporter, but there is also significant paracellular reabsorption, encouraged by the posi- tive potential in the tubular lumen.
Describe renal potassium handling in the Collecting Duct
Principal cells. The Na+/K+ ATPase drives potassium secretion in principal cells by pumping potassium into the cells at the basolateral surface. The basolateral surface is not very per- meable to potassium, but at the apical surface, potassium ions can leave the cell through potassium channels or in co-transport with chloride via KCC channels. The negative potential in the tubular lumen due to sodium reabsorption also promotes potas- sium secretion.
Describe renal urea handling
Urea is filtered, reabsorbed and then secreted (to raise medullary interstitial osmolarity to aid the concentrating of urine)
Changes in urine flow affect renal urea handling. A decreased GFR –> decreased urine flow –> urea retention –> increased BUN
Describe renal handling of Paraaminohippuric acid
An organic acid not normally present in the body. Filterted, so reabsorbed and the remainder is secreted. Is therefore useful for measuring renal plasma flow
