Tubular processing Flashcards

1
Q

Where in the nephron does the majority of reabsorption occur?

A

Proximal convoluted tubule

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2
Q

What things are mostly reabsorbed in the proximal convoluted tubule?

A

Sodium, water, glucose and amino acids

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3
Q

Is the proximal convoluted tubule permeable to water?

A

Yes.

Tubule fluid leaving PCT is isosmotic as epithelium is freely permeable to water.

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4
Q

Does secretion occur in the proximal convoluted tubule?

A

Yes, site of secretion of metabolic acids, bases, drugs etc.

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5
Q

Which reabsorption transporters are found in the proximal convoluted tubule?

A

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.

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6
Q

Which secretion transporters are found in the proximal convoluted tubule?

A

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)

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7
Q

What are the three parts of the loop of Henle?

A

Thin descending limb
Thin ascending limb
Thick ascending limb

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8
Q

What happens in the thin descending limb of loop of Henle?

A

Permeable to water.

No active reabsorption or secretion of solutes.

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9
Q

What happens in the thin ascending limb of loop of Henle?

A

Impermeable to water.

Essentially no active reabsorption or secretion of solutes.

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10
Q

What happens in the thick ascending limb of loop of Henle?

A

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.

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11
Q

Describe how transport occurs in the thick ascending limb of loop of Henle

A

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

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12
Q

What happens in the early distal tubule?

A

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

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13
Q

Are the late distal tubule and cortical collecting duct water permeable?

A

Water permeability of this part of nephron under hormonal control by antidiuretic hormone (ADH):

Water permeable when ADH present
Water impermeable when ADH absent

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14
Q

What are the two cell types in the late distal tubule and cortical collecting tubule?

A

Principal cells involved in sodium reabsorption and potassium secretion

Intercalated cells involved in potassium reabsorption and hydrogen ion secretion.

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15
Q

Describe the transporters in the principal cells

A

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

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16
Q

Describe the water and urea permeability of the medullary collecting duct

A

Final site for urine processing

Water permeability of this part of nephron under hormonal control by antidiuretic hormone (ADH)

Surrounded by a medullary interstitium with a high concentration of solutes

Key role in regulating degree of urine concentration

Urea permeability allows medullary interstitium to remain concentrated

17
Q

In the proximal convoluted tubule, is HCO3- reabsorption sodium dependent or chloride dependent?

A

Sodium dependent

18
Q

In the distal convoluted tubule and collecting duct, is HCO3- reabsorption sodium dependent or chloride dependent?

A

Chloride dependent

19
Q

What does aldosterone do?

A

Secreted by adrenal glands.
Increases sodium reabsorption and therefore water reabsorption in the collecting duct.
It also causes and increase in potassium secretion/decrease in potassium reabsorption (could lead to hypokalaemia)

20
Q

What does ADH do?

A

Secreted by posterior pituitary.

Increase amount of aquaporins in the collecting duct and therefore increases water reabsorption.