Tubular Reabsorption & Secretion

Question 1

What substances are reabsorbed?

Answer 1

Glucose
a.a’s
Na+, Cl-
H20

Question 2

Which hormones can control reabsorption?

Answer 2

Aldosterone (Na+)

Vasopressin (H20)

Question 3

Which substances can be secreted?

Answer 3

K+
H+
Some drugs

Question 4

Which hormones can control secretion?

Answer 4

Aldosterone (K+)

Question 5

What is the overall percentage of reabsorption in the PCT?

Answer 5

65% Na+

65% H20

Question 6

Discuss reabsorption in the PCT

Answer 6

• MICROVILLI increase SA, thus increase reabsorption
• Increase # mitochondria
• Glucose, a.a’s, 65% Na+, Cl-, K+ & H20 are REABSORBED UNREGULATED back into peritubular capillaries

Question 7

What is the overall percentage of reabsorption in the LOH?

Answer 7

25% Na+

15% H20

Question 8

Discuss reabsorption in the DESCENDING LOH

Answer 8

WATER OUT! (Freely permeable to H20)
-impermeable to salts

THUS, filtrate more concentrated as it moves deeper into medulla.

Question 9

Discuss reabsorption in the ASCENDING LOH

Answer 9

Freely permeable to Na+ & Cl-
-impermeable to H20

THUS, filtrate more concentrated as it moves towards DCT

Question 10

What is the overall percentage of reabsorption in the DCT?

Answer 10

5% Na+

5% H20

Question 11

Discuss reabsorption in the DCT

Answer 11

MAINLY SECRETION
(rarely contains microvilli, THUS little reabsorption)

REGULATES pH
(reabsorbs HCO3-, secretes H+)

FINE TUNING OF Na+ & K+ WITH ALDOSTERONE

SECRETION OF DRUGS (penicillin), METAOLITES AND TOXINS

Question 12

Discuss reabsorption in the COLLECTING DUCT

Answer 12

[Only 1-2L left] –> from 180L filtered

FINE TUNING OF H20 REABSORPTION (controlled by vasopressin)

CONTINUED FINE TUNING OF Na+ & K+ WITH ALDOSTERONE

Absorbs approx. 4-5% Na+ & 5% H20
-but can increase reabsorption to 24% in times of dehydration

Question 13

What is the overall percentage of reabsorption in the COLLECTING DUCT

Answer 13

4-5% Na

5% H20

Question 14

What is the Countercurrent Multiplier Mechanism?

Answer 14

The process of using energy to generate an osmotic gradient, enabling the reabsorption of water from tubular fluid to produce concentrated urine.

Prevents you from producing litres and litres of dilute urine every day

Question 15

What is the Countercurrent Multiplier Mechanism in the DESCENDING LOH?

Answer 15

High permeability yo H20

No absorption of Na+

Question 16

What is the Countercurrent Multiplier Mechanism in the ASCENDING LOH?

Answer 16

Actively transports Na+ (& thus Cl-)

ALWAYS impermeable to H20

Question 17

What advantage results from the 2 limbs of the LOH being close together?

Answer 17

The conc. gradient and THEREFORE the osmotic gradient is formed down the nephron into the medulla.

A ‘Symbiotic functional relationship’

Question 18

Define Tubular Maximum (Tm) reabsorption

Answer 18

The point at which increases in concentration of a substance DO NOT result in an increase in movement of a substance across a cell membrane

(There is only a finite #/activity of protein transporters embedded in the membrane,
THUS, a maximum achievable rate of reabsorption)

Question 19

What happens when Tm is reached?

Answer 19

No additional reabsorption can occur!

The surplus is excreted in the urine

Question 20

What happens with Tm during hyperglycaemia in Diabetes Mellitus?

Answer 20

High glucose in PCT SURPASSES the Tm for glucose.

THUS, glucose remains in filtrate!
(causes an osmotic P against reabsorption)

THUS, increase urine vol. & glucouria

THUS, increase thirst because loss of H20 in urine!