Lecture 17 - Glomerular Filtration, Daily Filtered Load and Maximum Transport

Question 1

What is tubular reabsorption?

Answer 1

The reabsorption of filtrate back into the blood (plasma)

Question 2

How is tubular reabsorption achieved?

Answer 2

Tubular reabsorption is achieved through action of specific channels and transporters in the membranes of the epithelial cells of the tubule, and through the tight junctions of some of the segments of the nephron.

Question 3

How much water (L) is filtered, excreted, and reabsorbed per day?

Answer 3

Filtered: 180L
Excreted: 1.8L
Percent reabsorption: 99%

Question 4

How much sodium (g) is filtered, excreted, and reabsorbed per day?

Answer 4

Filtered: 630g
Excreted: 3.2g
Percent reabsorption: 99.5%

Question 5

How much glucose (g) is filtered, excreted, and reabsorbed per day?

Answer 5

Filtered: 180g
Excreted: 0g
Percent reabsorption: 100%

Question 6

How much urea (g) is filtered, excreted, and reabsorbed per day?

Answer 6

Filtered: 54g
Excreted: 30g
Percent reabsorption: 44%

Question 7

What is the role of urea in the medullary section of the nephron?

Answer 7

Maintaining osmotic gradient

Question 8

What is tubular secretion?

Answer 8

The secretion of substances from the peritubular capillaries to the filtrate.
Secretion at peritubular capillaries enables disposal of substances at a higher rate than depending on filtration alone

Question 9

What substances can be moved during tubular secretion?

Answer 9

H+, K+, penicillin, metabolites from pesticides and many substances

Question 10

Which of the following statements is TRUE?
A. 100% of the filtered urea is reabsorbed by the nephron.
B. Reabsorption is the only renal process that ‘recovers’ filtered substances and water.
C. Secretion is not an energy intensive process.
D. Normally, 95% of the filtered glucose is reabsorbed by your kidneys, unless you are a diabetic.

Answer 10

B. Reabsorption is the only renal process that ‘recovers’ filtered substances and water.

Question 11

What is GFR?

Answer 11

The amount of filtrate produced per unit of time - Equal to sum of filtration rate of all functioning nephrons in the two kidneys.

Question 12

What is the average GFR?

Answer 12

Normal value = 125 ml / min (180 L / day)

Question 13

What can GFR be an indicator of?

Answer 13

Renal functioning

Question 14

What is renal clearance (RC)?

Answer 14

The volume of plasma from which a substance is completely cleared by the kidneys per unit time

Question 15

What is needed to determine RC?

Answer 15

One must know the concentration of the substance in the urine and plasma and rate of urine produced to determine RC.

Question 16

What is the formula for the clearance of substance?

Answer 16

Clearance (Cs) = Us x V / Ps
The equation describes the rate at which substance S is cleared by the kidneys (unit per time)
Us = concentration of S in urine (mg/L or mmol/L)
V = volume of urine per unit time (ml/min or L/hour)
Ps = concentration of S in plasma (mg/L or mmol/L)

Question 17

What criteria must a substance have in order for its RC to be able to estimate GFR?

Answer 17

Not be reabsorbed from the tubule
Not be secreted into the tubule
Not be metabolised

Question 18

What substances are used to calculate RC and GFR?

Answer 18

Inulin (not to be confused with insulin!!)
Creatinine

Question 19

What is inulin?

Answer 19

Polysaccharide not metabolised by body. Not found in body and must be constantly infused - used for more specific diagnosis

Question 20

What is creatinine?

Answer 20

Waste product produced by muscles
Filtered but not reabsorbed
Small amount of secretion (~10% secreted)
Already in body so most commonly used clinically

Question 21

What is the relationship between GFR and creatinine?

Answer 21

The higher the GFR, the less creatinine found in the plasma. - as creatinine is secreted, not reabsorbed

Question 22

Which of the following statements is FALSE?
A. The typical GFR for a healthy person is 125 ml/min.
B. Renal clearance is the volume of plasma from which a substance is cleared by the kidneys per unit time.
C. GFR is not a useful indicator of renal function.
D. Creatinine is a substance that can be used to determine GFR

Answer 22

C. GFR is not a useful indicator of renal function.

Question 23

What is the daily filtered load?

Answer 23

How much (volume) of a substance is being filtered per day - overall rate

Question 24

How can daily filtered load be calculated?

Answer 24

Daily filtered load = GFR x [substance]plasma
Provided we know the GFR and the plasma concentration of a substance we can calculate the daily filtered load of that substance.

Question 25

What is transport maximum?

Answer 25

Transport maximum is how much in total can be transported - limited amount can be transported due to transport proteins - not everything can be cleared, they work at a given rate

Question 26

What is the only way TM can be changed?

Answer 26

By the addition of transport proteins

Question 27

Where and how is glucose reabsorbed?

Answer 27

Glucose is reabsorbed in proximal tubule across leaky absorptive epithelia using Na+-glucose cotransporters

Question 28

What is the TM of [glucose]plasma?

Answer 28

[glucose]plasma does not normally exceed 150 mg / 100 ml

Question 29

What happens in diabetes mellitus?

Answer 29

In diabetes mellitus there is a lack of control by insulin leading to hyperglycaemia (too much glucose in the blood)
- [glucose]plasma can exceed 200 mg / 100 ml

Question 30

What happens to the reabsorption of glucose during diabetes mellitus?

Answer 30

In diabetes mellitus, there is a large increase in the filtered load of glucose. At this [glucose]plasma, tubular Na+-glucose co-transporters become saturated and can not reabsorb all of the filtered glucose, resulting in glucosuria.

Question 31

What is glucosuria?

Answer 31

Glucose appears in urine = Glucosuria