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LSS 2 - Urinary - Laz > Tubular Function > Flashcards

Tubular Function Flashcards

1
Q

What components of urine do we not have a method of transporting on their own?

A

Urea and Water - so these move in by passive transport

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2
Q

What percentage of the ultrafiltrate is reabsorbed?

A

99%

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3
Q

Define osmolarity.

A

The measure of osmotic pressure exerted by a solution across a semi-permeable membrane.
This is dependent on the number of particles not the nature.

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4
Q

What is the range for normal plasma osmolarity? What makes up the majority of this?

A

285-295 mosmol/L

Mainly consists of sodium (140 mmol/L)

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5
Q

What is the range for normal urine osmolarity?

A

50-1200 mosmol/L

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6
Q

What are the two pathways through the urinary epithelium?

A

Transcellular and Paracellular (through tight junctions)

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7
Q

What is the difference between lipophilic passive transport and hydrophilic passive transport?

A

Lipophilic passive transport has a linear relationship with solute concentration
Hydrophilic passive transport is saturable because it is dependent on the availability of channel proteins.

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8
Q

What are the two routes for water to pass through the renal tubular wall?

A

Transcellular and Paracellular

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9
Q

How can hydrophilic passive transport be upregulated or downregulated?

A

By changing the number of transporters available e.g. ADH increases the amount of Aquaporin 2 on the apical membrane

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10
Q

How does protein reabsorption normally happen?

A

Receptor mediated endocytosis - the protein binds to a receptor and is endocytosed
The acidity of the endosome allows the complex to dissociate and the receptors are recycled

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11
Q

What happens if the concentration of a solute in the urine exceeds the transport maxima?

A

It is excreted in the urine

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12
Q

What are the most important substances that are secreted?

A

H+

K+

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13
Q

Describe the differences in sodium reabsorption throughout the nephron.

A

65% reabsorbed in PCT
25% reabsorbed in ascending loop of Henle
8% reabsorbed in DCT

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14
Q

Where is most bicarbonate reabsorbed?

A

90% is reabsorbed in the PCT

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15
Q

Where, along the nephron, do you find cells that don’t have that many mitochondria?

A

Descending loop of Henle and collecting duct

These areas are mainly involved in the passive transport of water

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16
Q

Describe the features of a cell in the proximal convoluted tubule.

A

Numerous mitochondria
Brush border to increase surface area
Designed for lots of reabsorption

17
Q

What is the most important protein of the cells lining the tubules throughout the nephron?

A

Na+/K+ ATPase - responsible for the sodium gradient that drives the movement of most substances

18
Q

Which substances move in or out with Na+ in the early proximal tubule?

A

H+ moves out (Na+/H+ countertransport)
Glucose in (Na+/glucose cotransport)
Amino acids in (Na+/amino acid cotransport)

19
Q

How is proton excretion linked to bicarbonate reabsorption?

A

Protons are pumped into the tubule via (Na+/H+ exchanger)
Protons react with HCO3- to form H2CO3
H2CO3 converted by carbonic anhydrase to CO2 + H2O
CO2 + H2O moves into the cell and carbonic anhydrase converts it back to H2CO3, which dissociates to form H+ and HCO3-.
HCO3- is passes into the blood, H+ moves out again via the Na+/H+ exchanger

20
Q

Describe the differences between the ascending and descending loop of Henle.

A

Descending loop of Henle - permeable to water - SQUAMOUS epithelium
Ascending loop of Henle - impermeable to water, Na+, K+ and Cl- reabsorbed here - CUBOIDAL epithelium
Water leaves the top of the loop of Henle being hypoosmolar

21
Q

What channels are found in the cells lining the ascending loop of Henle?

A

Na+/K+/Cl- triple transporter

22
Q

What type of diuretics blocks this channel?

A

Loop diuretics

23
Q

Describe the epithelium of the distal convoluted tubule.

A

Cuboidal epithelium + few microvilli
There are lateral membrane interdigitations with Na+ pumps
Numerous large mitochondria

24
Q

Which transporter is found on the apical membrane in cells in the DCT?

A

Na+/Cl- cotransporter

25
Q

What other substance is reabsorbed here?

A

Ca2+ - there is an Na+/Ca2+ exchanger on the basolateral membrane

26
Q

What type of diuretic acts on this transporter and what are the consequences?

A

Thiazide diuretics - leads to increase in plasma calcium concentration

27
Q

What does the macula densa cells do?

A

Detect Na+ concentration of the fluid in the tubule - can stimulate release of renin

28
Q

What is the reabsorption of sodium

A

in the distal part of the DCT and the collecting duct dependent on?
Aldosterone

29
Q

What is needed for reabsorption of water in the collecting duct?

A

Vasopressin

30
Q

What are the two types of cells in the collecting duct and how do their functions differ?

A

Principal cells - regulate movement of Na+/K+/water

Intercalated cells - regulate acid-base balance

31
Q

State three single gene defects that affect tubular function.

A 
Renal tubule acidosis
Bartter syndrome
Fanconi syndrome (Dent's disease)
32
Q

What is renal tubule acidosis? State some clinical features.

A

Metabolic acidosis caused by failure of the renal tubules.
Hyperchloremia
Hypokalemia
Impaired growth

33
Q

What is Bartter syndrome? State some clinical features.

A 
Excessive electrolyte secretion
Hypokalemia
Premature birth
Polyhydramnios
Renin and aldosterone hypersecretion 
Moderate metabolic alkalosis
34
Q

What causes Bartter syndrome?

A

Mutation in the Na+/K+/Cl- triple transporter

NOTE: at this point in the loop of Henle around 25% of sodium is reabsorbed

35
Q

What is Fanconi syndrome (Dent’s disease) and what causes it? Refer to endosomes.

A

Failure in endosomal recirculation.
Caused by failure of the chloride transporter
Protein-Receptor complex dissociates due to the acidity of the endosome, which is caused by influx of H+ ions
H+ influx must be balanced by influx of Cl- to ensure a charge gradient isn’t established (which would make it more difficult to pump more H+ in)
Failure of the chloride channel means that the endosome pH never becomes low enough for the protein-receptor complex to dissociate.
Clinical features of Fanconi syndrome: Increased excretion of low molecular weight proteins, increased excretion of uric acid, glucose, phosphate and bicarbonate

LSS 2 - Urinary - Laz (8 decks)

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