Renal Transport of Ions and Organic Solvents

Question 1

How does chronic renal failure affect the BUN, plasma phosphate, and plasma calcium?

Answer 1

Increased BUN, phosphate and decreased calcium

Question 2

How does chronic renal failure cause decreased plasma calcium?

Answer 2

Decreased formation of Vitamin D, decreasing the absorption of calcium from the gut

Question 3

In order for homeostasis to be maintained, what percentage of daily urea produced must be excreted daily?

Answer 3

Equal amounts- 100%

Question 4

Where is urea produced?

Answer 4

Liver

Question 5

What percentage of the total urine osmolality is due to urea?

Answer 5

40%

Question 6

What are BUN levels? What is the normal range? At what value should a patient go on dialysis?

Answer 6

Blood Urea Nitrogen- mg elemental nitrogen/ dL plasma; 7-18 mg/dL; >100 mg/dL

Question 7

What is the relationship between BUN and GFR?

Answer 7

Inversely proportional

Question 8

What are elevated BUN levels possibly indicative of?

Answer 8

Reduced kidney function

Question 9

What is medullary recycling of urea? What is the physiologic importance of this occurring?

Answer 9

The bidirectional transport mechanism by which urea is reabsorbed by the PT and IMCD, and it is secreted by the loops of Henle; it establishesand prevents dissipation of high [urea] in the intermedulla so kidney conserves H2O

Question 10

With normal urine flow, what percentage of the original filtered urea load is excreted?

Answer 10

About 40%

Question 11

What is the number one waste product that must be excreted by the kidneys?

Answer 11

Urea

Question 12

What drives the paracellular diffusion of urea?

Answer 12

Water reabsorption increases the luminal urea concentration

Question 13

What are the mechanisms by which urea is reabsorbed?

Answer 13

Paracellular diffusion and transcellular via urea transporters

Question 14

What causes glucose in the urine?

Answer 14

Uncontrolled diabetes or glucosuria (glucose transporter mutation); When the plasma glucose overwhelms the nephrons ability to reabsorb it

Question 15

Where it the most glucose reabsorbed? What percentage of filtered glucose is reabsorbed here? What are the other sites of reabsorption?

Answer 15

PCT- 98%; Proximal straight tubule

Question 16

How is glucose reabsorbed by the proximal convoluted tubule?

Answer 16

Through SGLT2 transporter in apical membrane and then GLUT2 in BL membrane

Question 17

What are the relative capacities and affinities of the SGLT1 and SGLT2 receptors?

Answer 17

SGLT1- low capacity, high affinity; 2- high capacity, low affinity

Question 18

What is the ratio of sodium to glucose transported through SGLT1? SGLT2?

Answer 18

SGLT1- 2:1; SGLT2- 1:1

Question 19

How can the filtered load of glucose be calculated?

Answer 19

Plasma glucose x GFR

Question 20

What is Tm(g)? What is its typical value? At what plasma glucose are carriers saturated?

Answer 20

The maximal reabsorptive rate of glucose; 400 mg/dL; ~200 mg/dL

Question 21

Why is there a splay seen in the glucose titration curve?

Answer 21

Nephrons have different filtered loads of glucose and expression of SGLTs

Question 22

Which organic solutes are reabsorbed almost completely by the proximal tubule?

Answer 22

Amino acids, peptides, mono-,di- and tricarboxylates, and urate

Question 23

What is the primary way phosphate homeostasis is maintained?

Answer 23

Renal excretion

Question 24

What structure reabsorbs most of the filtered phosphate? How much?

Answer 24

PT reabsorbs 80%

Question 25

Approximately how much of filtered phosphate is excreted under normal conditions?

Answer 25

10%

Question 26

What is the mechanism by which phosphate is reabsorbed?

Answer 26

Through Na+/Pi cotransporter

Question 27

How does PTH affect renal phosphate handling?

Answer 27

INcreased serum phosphate stimulates the release of PTH, which inhibits renal phosphate reabsorption

Question 28

Typically what is the level of PTH secretion in patients with chronic renal failure? Why?

Answer 28

Increased-- A decreased GFR decreases the excreted phosphate and increases plasma phosphate; decreased formation of Vitamin D decreased GI absorption of Ca2+, decreasing the plasma concentration

Question 29

What forms of calcium can be filtered? Which forms cannot be? What is the relative amount of both?

Answer 29

Calcium bound to protein (40%) cannot be; Calcium that is complexed with sulfate, citrate, or phosphate (15%) or ionized (45%) can be

Question 30

What percent of filtered Ca2+ is reabsorbed? Where is it mostly done?

Answer 30

99%; 70% in the proximal tubule

Question 31

Where are calcium-sensing receptors located? What is their function?

Answer 31

In the TAL; increased ECF calcium decreases reabsorption

Question 32

What is the mechanism by which PTH stimulates calcium reabsorption?

Answer 32

TRPV5 Ca++ channels in the DCT

Question 33

How do the kidneys contribute to the formation of Vitamin D?

Answer 33

Kidneys synthesize 1-alpha-hydroxylase

Question 34

How can chronic renal failure cause osteodystrophy?

Answer 34

Increased serum phosphate increases PTH secretion which increases bone resorption