Lecture 5: Renal Tubular Transport (Bolser) Flashcards
what are the 4 different methods of membrane transport systems
- diffusion
- osmosis
- facilitated transport (aka carrier mediated transport)
- active transport - primary Na, K, ATPase
and secondary transport (co-transport) glucose and amino acids
why is there more creatinine in the urine than what is filtered?
because more is added after the filtration apparatus. aka its secreted.
what are the characteristics of Na in tubular transport? (3 key points)
- Na is reabsorbed - mostly in the proximal tubule, some in the thin and thick ascending limbs and tiny bit in the collecting duct
- Na is NEVER secreted by the nephron
- Na reabsorption is active and is powered by Na-K-ATPase at the basolateral membrane
what are the characteristics of water in tubular transport (4 key points)
- water follows Na
- solute coupled in proximal tubules - solute reabsorption creates a small difference in osmolarity between the luminal fluid and the interstitium.
- water is mostly absorbed in the proximal tubule, a bit (10%) in the descending thin limb and a wide range (1 - 24%) in the collecting ducts.
- amount of water absorbed in collecting ducts is determined by ADH levels. if ADH is increased then water reabsorption will be increased.
what are the characteristics of K+ in tubular transport (3 key points)
- most of the filtered K is reabsorbed in the proximal tubule (55%), 30% is cotransported with Na in the ascending limb of the loop of Henle. only 10% of filtered K enters the distal tubule where active reabsorption occurs.
- secretion of K is regulated, primarily at the distal tubule and collecting duct to control overall K excretion.
- K is actively transported from the peritubular fluid into the cell and reaches the lumen by diffusion and active transport out of the cell.
at what location within the nephron are glucose, amino acids and proteins reabsorbed by the nephron?
in the early proximal tubule
why does inulin appear concentrated in the late proximal tubule?
because water has already left making inulin concentrations 3x higher
when is HCO3 reabsorbed by the nephron?
its favored to be reabsorbed in the early proximal tubule because it is reabsorbed in association with Na.
what are the 8 important features of action of the proximal tubule on the tubular fluid?
- reabsorption is isosmotic
- overall Na reabsorption is active
- overall Cl reabsorption is passive
- water reabsorption is passive
- 2/3 filtrate reabsorbed
- K passively reabsorbed
- glucose, proteins, acetoacetate, ions, vitamins, AA all actively reabsorbed
- H+ are secreted by a counter-transport involving sodium against concentration gradient
what are the two mechanisms by which sodium moves across the apical membrane in the first half of the proximal tubule
- co-transported with several other freely filtered molecules
- transpored by a Na-H antiporter
what drives Na transport in the first half of the proximal tubule?
the concentration gradient created by the Na/K ATPase as well as the reabsorption of the co-transported molecules
what is the role carbonic anhydrase plays in the first half of the proximal tubule? (2 things)
- it facilitates the production of H ions to exchange with Na ions in the first half of the proximal tubule
- bicarbonate is also reabsorbed in the first half of the proximal tubule which promotes the transport of Na in association with Cl mostly in the second half of the proximal tubule
how are Na and Cl reabsorbed in the second half of the proximal tubule (2 ways)
- paracellular diffusion
2. a complex co-transporter system involving two antiporters and weak acids
anions are moved from the lumen in exchange for secreted organic acids including ___?
chloride, urate and hydroxyl ions.
tertiary active transport
- occurs in organic anion secretion
- the transport of an organic acid is linked to Na transport by the Na/K/ATPase through common intermediaries
how do organic cations enter the cell from the blood?
by carrier-mediated facilitated diffusion
what are some prominent organic cations?
acetylcholine, epinephrine, dopamine, atropine and morphine
what is characteristic of tubular transport in the thin descending limb? (2 things)
- cells have minimal metabolic activity - aka little active transport
- HIGH permeability to water, moderate permeability to urea, sodium and other ions
what is characteristic of tubular transport in the thin ascending limb? (3 things)
- little active transport in these cells
- little to no permeability to water
- high permeability to Na
what is characteristic of tubular transport in the thick ascending limb (4 things)
- impermeable to water
- net reabsorption of Na, Cl and K. active Cl transport via Na, K, and 2Cl co-transporter
- high ion concentration gradient between lumen and peritublar fluid
- high osmolar concentration gradient between lumen and peritubular fluid
what is the effect of diuretic lasix in the thick ascending limb?
diuretic lasix binds to Cl on the Na/Cl/K symporter suppressing its activity. the reduction in Na transport affects the ability of more distal segments of the nephron to reabsorb water
are cells in the thick ascending limb permeable or impermeable to water?
impermeable - this portion of the nephron is known as the diluting segment
what is function of the Na/K/ATPase symporter in the thick ascending limb?
it creates a concentration gradient for the function of the apical membrane transporters
is the early distal tubule permeable or impermeable to water? what is the significance of this in relation to Na transport here?
impermeable
b/c this segment is impermeable to water but Na is transported out of the lumen in the early distal tubule, dilution of the tubular fluid occurs here
what is the effect of reabsorption of ions in the early distal tubule (2 things)
- it can establish high ionic concentration gradients between urine and plasma
- can establish a high osmolar concentration gradient between urine and plasma
what happens to transport of Na in the early distal tubule
the Na/Cl symporter is responsible for Na reabsorption
Na concentration can be reduced to zero in the tubular fluid
Cl reabsorption is mostly passive
what happens to the transport of water in the distal tubule?
variable permeability to water (controlled by ADH) both here and in the collecting ducts
what happens to the transport of K in the distal tubule
principal cells are responsible for K secretion into the tubular fluid
varies from no net K transport to net secretion and is controlled by cell K content which is regulated by active transport into the cell across the peritubular membrane.
what happens to the transport of H in the distal tubule
actively secreted into peritubular lumen by intercalated cells via H/ATPase
pump can produce TF/p gradient (tubular fluid/plasma) 1000:1
what is the function of principal cells
reabsorption of Na and secretion of K in the late distal tubule and collecting ducts.
they are also responsible for the action of ADH
what is the function of type B intercalated cells
bicarbonate secretion
what is the function of type a intercalated cells
acid secretion
