Tubular Function

Question 1

After glomerular filtration, how is it ensured that all waste is excreted and that useful substances are retained

Answer 1

Controlled reabsorption (99% of the filtrate) and secretion

Question 2

What is osmolarity

Answer 2

Measure of the osmotic pressure exerted by a solution across a perfect semi-permeable membrane
Dependent on the number of particles in a. solution
units = mosmol/L

Question 3

What is the normal plasma and urine osmolarity

Answer 3

plasma = 285-295 (140 Na)
urine = 50-1200

Question 4

Describe passive diffusion

Answer 4

Protein independent

Rate increases as solute concentration increases (no limitation)

Question 5

Describe facilitated diffusion

Answer 5

Protein dependent
Hydrophilic
Rate increases then plateaus with solute concentration (limited by no. of transporters)

Question 6

Describe active transport

Answer 6

Either directly coupled to ATP hydrolysis or indirectly coupled

Question 7

Describe osmosis

Answer 7

Transport of water through tight junctions (para) or through aquaporins (trans) to areas of high osmolarity

Question 8

How are glucose, amino acids and ions re absorbed from the filtrate in the nephron

Answer 8

Specialised protein transporters are located on the membrane
Molecules become trapped as they flow by them
Each transporter only traps one or two types of molecule
Transporters are located in different parts of the nephron

Question 9

Describe the distribution of sodium transporters

Answer 9

Most are located in the proximal tubule, fewer in the other segments

Question 10

What is secretion in the kidneys

Answer 10

Moves substances form the peritubular capillaries to the tubular lumen
Diffusion or transcellular mediated transport
Active secretion from the blood into the cell (basolateral membrane) and from cell to lumen (luminal membrane)

Question 11

What substances are secreted into the tubular lumen

Answer 11

Most importantly H+ and K+

Choline, creatinine, penicillin and other drugs

Question 12

Describe the absorption in the proximal convoluted tubule

Answer 12

60-70% of all solute
100% glucose
65% sodium
90% bicarbonate
Water and anions follow sodium

Question 13

Describe the absorption in the loop of Henle

Answer 13

Concentration of urine occurs here

25% of sodium

Question 14

Describe the absorption in the distal convoluted tubule

Answer 14

8% of sodium

Sodium reabsorption to “fine” tune the filtrate

Question 15

Describe absorption in the collecting duct

Answer 15

Variable

Regulated by aldosterone and vasopressin

Question 16

Which substances are not reabsorbed from the filtrate

Answer 16

Urea
Creatinine
Sulfates
Phosphates
Nitrates

Question 17

Describe the processes occurring at the basolateral membrane

Answer 17

sodium potassium pump keeps intracellular sodium low and potassium high
This favours sodium movement into the cell

Question 18

Describe the processes occurring at the early proximal tubule membrane

Answer 18

Sodium enters down its concentration gradient, therefore facilitating in the diffusion of amino acids and glucose and the outflow of H+

Question 19

What is the effect of carbonic anhydrase in the early proximal tubule

Answer 19

Leads to sodium reabsorption and increases urinary acidity (H+ is secreted)

Question 20

What is passively reabsorbed in the proximal convoluted tubule

Answer 20

urea and water

Question 21

What is actively reabsorbed in the proximal convoluted tubule

Answer 21

Glucose
Amino acids
Sodium
Potassium
Calcium
Vitamin C 
Uric acid

Question 22

Describe the epithelium of the loop of Henle

Answer 22

Descending limb = squamous epithelium, few mitochondria

Ascending limb - cuboidal epithelium, dew microvilli, lots of mitochondria, impermeable to water

Question 23

Describe absorption in the descending and ascending limbs of the loop of Henle

Answer 23

Descending = water passively absorbed, sodium + potassium (due to interstitial fluid ions)

Ascending = chloride actively absorbed, sodium passively absorbed with it, bicarbonate absorbed

Question 24

Describe the fluid leaving the loop of Henle

Answer 24

85% water and 90% sodium + potassium have been reabsorbed

Hypo-osmolar with respect to plasma

Question 25

Describe the epithelium of the distal convoluted tubule

Answer 25

Cuboidal epithelium Few microvilli Complex lateral membrane interdigitations with Na+ pumps Lots of large mitochondria Sodium chloride co-transporters at the lumen, sodium calcium anti porters at the capillary side

Question 26

How are changes in sodium concentration of the filtrate detected

Answer 26

Macula densa in the juxtaglomerular apparatus

Question 27

What is sodium reabsorption dependent on in the distal convoluted tubule

Answer 27

Aldosterone

Question 28

What is water reabsorption dependent on in the distal convoluted tubule

Answer 28

ADH | distal part is impermeable without ADH

Question 29

What is involved in the collecting duct and distal part of the distal tubule

Answer 29

apical sodium channel sensitive to aldosterone Linked potassium channel pH control

Question 30

What is a principal cell

Answer 30

Distal convoluted tubule and collecting duct Important in sodium, potassium and water balance (mediated via Na/K ATP pump) very tight -> very little paracellular transport Aldosterone receptors

Question 31

What is a intercalated cell

Answer 31

Distal convoluted tubule and collecting duct | important in acid-base balance (mediate via H-ATP pump)

Question 32

What are some single gene defects that affect tubular function

Answer 32

``` Renal tubule acidosis Bartter syndrome Fanconi syndrome (Dent's) ```

Question 33

What is the effect of renal tubular acidosis

Answer 33

hyperchloremic metabolic acidosis of the blood Impaired growth Hypokalaemia

Question 34

Describe Bartter syndrome

Answer 34

Excessive electrolyte secretion Prenatal - premature birth, polyhydramnios, severe salt loss, moderate metabolic alkalosis, hypokalaemia, renin and aldosterone hypersecretion

Question 35

Describe Fanconi syndrome

Answer 35

Increase excretion of uric acid, glucose, phosphate, bicarbonate increase excretion of low MW protein Disease of the proximal tubules associated with renal tubular acidosis, protein cannot be separated from the carrier as the endosome is not acidified Cl-/H+ transporter inhibited

Question 36

What is transport maxima

Answer 36

Threshold at which the rate cannot exceed and above which reabsorption cannot occur e.g. glucose cannot be reabsorbed over a certain level

Question 37

What is the effect of thiazides

Answer 37

Inhibition of the Na/Cl channel Increase in plasma sodium concentration as there is increased reabsorption in the DCT increased calcium concentration

Question 38

What are some causes of renal tubular acidosis

Answer 38

Failure of proton excretion -> metabolic acidosis | Failure of carbonic anhydrase -> sodium reabsorption and increased urinary activity

Question 39

Describe protein reabsorption

Answer 39

1. Receptors which have a low specificity but high capacity for binding proteins 2. The proteins bind to the receptors 3. Endocytosis 4. pH in the endosome decreases 5. Protein dissociates from receptor and can be reabsorbed into the blood Cl-/H+ transporter pumps chloride in to the endosome to reduce the +ve charge