4. Tubular Function

Question 1

How much of the ultra filtrate needs to be reabsorbed?

Answer 1

99%

Question 2

Define osmolarity

Answer 2

A measure of the osmotic pressure exerted by a solution across a perfect semi-permeable membrane

Question 3

What is osmolarity dependent on?

Answer 3

The number of particles in the a solution and NOT the nature of the particles

Question 4

Osmolarity =

Answer 4

All the concentrations of the different solutes (measured in mmol/l) added together. Each ion is counted separately

Question 5

What is normal plasma osmolarity?

Answer 5

285-295 mosmol/l

Mainly made up of sodium

Question 6

What is urine osmolarity?

Answer 6

Can vary between 50-1200 mosmol/l

This is because change the amount of salt we reabsorb

Question 7

What pathway does reabsorption occur?

Answer 7

Transcellular or paracellular

Question 8

What pathway does secretion occur?

Answer 8

Transcellular or paracellular

Question 9

Describe how the rate would change with increasing solute concentration provided the solute moves via protein independent passive movement (hydrophobic molecules)?

Answer 9

The rate would increase with increasing solute concentration

Question 10

Water always move from…

Answer 10

Low osmolarity to high osmolarity either via a transcellular pathway (aquaporins) or paracellular pathway

Question 11

What happens when maximum of glucose reabsorption is hit?

Answer 11

The excess glucose is excreted - diabetes mellitus

There is not enough capacity to reabsorb all the glucose

Question 12

What is secretion?

Answer 12

Moves substances from peritubular capillaries into tubular lumen

Question 13

How are proteins reabsorbed?

Answer 13

With receptors that get endocytosed with the protein before the receptor is moved back into the membrane again

Question 14

Where is most of the sodium reabsorbed?

Answer 14

Proximal tubule

Question 15

What are important substances secreted?

Answer 15

H+ and K+

also choline, creatinine, penicilin & other drugs also secreted

Question 16

What substances are suitable for measuring GFR?

Answer 16

Freely filtered, non secreted, non reabsorbed

Question 17

Where is 60-70% of all solute reabsorbed?

Answer 17

Proximal convoluted tubule

Question 18

What is the function of the loop of henle?

Answer 18

concentration of urine

Question 19

What does the distal convoluted tubule absorb?

Answer 19

8% of Na

Question 20

Describe how the rate would change with increasing solute concentration provided the solute moves via protein dependent passive movement (hydrophilic molecules)?

Answer 20

The rate would increase then plateau limited by the number of transport proteins in the membranes

Question 21

What is the percentage of Na reabsorbed in the PCT?

Answer 21

65%

Question 22

Where is glucose reabsorbed?

Answer 22

All of it is reabsorbed in the PCT

Question 23

Where is the bicarbonate reabsorbed?

Answer 23

90% of it is reabsorbed in the PCT

Question 24

How much and where is the sodium reabsorbed in the loop of henle?

Answer 24

25% of sodium is reabsorbed in the ascending limb of the loop of henle

Question 25

What is the function of the collecting duct?

Answer 25

Variable absorption regulated by aldosterone and vasopressin

Question 26

What do areas heavily involved in reabsorption have?

Answer 26

Lots of mitochondria for ATP for active transport

Question 27

Where are the majority of the Na+/K+ ATPase?

Answer 27

They lie on the basolateral membrane and maintain a low intracellular Na+ concentration and high extracellular K+ concentration. The energy from this process couples the cotransport of other substances into the cell from the lumen. Secondary active transport

Question 28

How is bicarbonate (HCO3-) recovered from the lumen?

Answer 28

A Na+ antiporter transports H+ ions into the lumen which associates with bicarbonate to give carbonic acid. Carbonic anhydrase converts carbonic acid to water and CO2 which can both diffuse through the membrane into the epithelial cell. Another carbonic anhydrase converts the water and CO2 back into bicarbonate which dissociates into bicarbonate and H+. The H+ can then be pumped out again into the lumen while the HCO3- diffuses through the basolateral membrane.

Question 29

What is passively reabsorbed in the PCT?

Answer 29

Urea and Water

Question 30

What is actively reabsorbed in the PCT?

Answer 30

Glucose, AA, Na+, K+, Ca2+, Vitamin C and uric acid

Question 31

Compare the descending and ascending limb of the loop of henle

Answer 31

Descending limb: Squamous epithelium with few mitochondria Ascending limb: Cuboidal epithelium with many mitochondria. Few microvili Cl- actively absorbed

Question 32

What does the descending limb of the loop of henle absorb?

Answer 32

Passive reabsorption of water - draws in some Na+ and K+

Question 33

What does the ascending limb of the loop of henle absorb?

Answer 33

Cl- actively reabsorbed, Na+ is passively reabsorbed, Bicarbonate is reabsorbed. But it is impermeable to water

Question 34

What transporter lies on the ascending limb of the loop of henle?

Answer 34

Na+,2Cl- and K+ triple transporter all into the cuboidal cells of the epithelium.

Question 35

How much water and sodium and potassium has been reabsorbed when you reach the DCT?

Answer 35

85% water and 90% sodium and potassium

Question 36

What is the epithelium of the DCT?

Answer 36

cuboidal epithelium

Question 37

What transporter is in the lumen of the DCT?

Answer 37

Na+ and Cl- cotransporter dependent on the Na+, K+ ATPase on the basalateral membrane

Question 38

What do thiazides do?

Answer 38

They increase plasma calcium. (look at DCT slide on tubule lecture - follow the transporters) They inhibit Na+Cl- chennels in the DCT but do affect the Na+/K+ATPase

Question 39

What part of the tubule detects changes in Na+ concentration of the filtrate?

Answer 39

Macular densa - part of the juxtaglomerular apparatus

Question 40

What are the two types of cells in the collecting duct & distal part of the DCT?

Answer 40

Principle cells and Intercalated cells

Question 41

What is the function of prinicple cells?

Answer 41

Important in maintaining sodium, potassium and water balance

Question 42

What is the function of intercalated cells?

Answer 42

Important in acid-base balance (mediated via H- ATP pump)

Question 43

What is a key feature of the cortical collecting duct?

Answer 43

They have a very tight epithelium because of regulation of substances

Question 44

What is water transport regulated by?

Answer 44

Vassopressin

Question 45

What is renal tubular acidosis?

Answer 45

Metabolic acidosis - leading to impaired growth and hypokalemia

Question 46

What are causes of renal tubular acidosis?

Answer 46

The proton pump facing the lumen is dysfunctional and protons are not being pumped into the lumen resulting in a low plasma pH If there is no active carbonic anhydrase - no release of protons

Question 47

What is Bartter syndrome

Answer 47

Excessive electrolyte secretion - Premature birth, polyhydramnios, severe salt loss, moderate metabolic alkalosis, hypkalemia and renin and aldosterone hypersecretion

Question 48

What is cause of Bartter syndrome?

Answer 48

Mutation in the Sodium, chlorine and potassium triple transporter. This means that you are removing a lot more salt which mean more water is being lost as well

Question 49

What is Fanconi Syndrome?

Answer 49

In Dents disease there is an increase excretion of uric acid, glucose, phosphate, bicarbonate. In dents disease the receptor is not separated from the protein in vesicles in the cell by pumping protons into the cell. Protons are needed to dissociate the receptor and protein. This is due to a faulty transporter in the chloride channel. The chloride channel essentially allows more H+. This results in fewer receptors in the lumen resulting in more excretion of the lumen.