Module 2 Lecture 5

Question 1

What concentration are body fluids maintained at?

Answer 1

Solute concentration of body fluids (ECF; plasma & interstitial
spaces) is maintained at ~300mOsm (concentration of solutes per
kg of water; isotonic)

Question 2

Why is keeping a constant solute concentration important?

Answer 2

This prevents cells from swelling (hypotonic) or shrinking

(hypertonic) due to osmotic movement of water

Question 3

How do the kidneys adjust to variations of fluid intake?

Answer 3

Kidneys adjust to variations in fluid intake & loss by changing
urine concentration & volume
– Dehydration –> kidneys produce a small volume of concentrated urine
– Overhydration –> kidneys produce a large volume of dilute urine

Question 4

What do nephrons do?

Answer 4

Nephrons create vertical osmotic
gradient in renal medulla interstitial
fluid –> to produce urine of varying
concentration as required

Question 5

How does fluid flow? LABEL DIAGRAM

Answer 5

Fluid flows in opposite directions of

adjacent segments of the same tube

Question 6

What is counter current multiplier

Answer 6

interaction
between flow of filtrate through
ascending & descending nephron limbs
establishes the gradient

Question 7

What is counter current exchanger?

Answer 7

flow of blood
through ascending & descending portions
of vasa recta preserves the gradient

Question 8

Why is countercurrent mechanisms so important?

Answer 8

Countercurrent mechanisms
establish & preserve the
osmotic gradient from the
cortex to medulla

Question 9

LABEL THE CYCLE for long nephron loops

Answer 9

memorize them Vivienne

Question 10

What is the point of concentrating filtrate and then diluting it?

Answer 10

It has 2 key benefits
1. Establishes a vertical osmotic gradient in the
interstitial fluid, which is then used by the collecting
duct to make more concentrated urine (dehydrated,
regulated by ADH)
2. Dilute filtrate entering distal tubule & collecting
duct allows kidneys to excrete dilute urine when
needed (overhydrated)

Question 11

What is the percentage of filtered water that enters distal and collecting tubules?

Answer 11

20% filtered water enters distal & collecting tubules for
‘variable’ water reabsorption

Question 12

What needs to happen in order for re absorption to occur?

Answer 12

For water reabsorption to occur:
1. An osmotic gradient needs to exist between tubule &
interstitial fluid
2. Distal & collecting tubules need to be permeable to water,
which is regulated by anti-diuretic hormone (ADH)

Question 13

What happens with maximal ADH?

Answer 13

With maximal ADH, 99% water in filtrate is

reabsorbed & returned to blood

Question 14

How is variable water re-absorption is regulated by ADH

Answer 14

ADH released by posterior pituitary upon ↑ ECF concentration
• Acts on distal & collecting tubule cells to promote insertion of
aquaporins in apical membrane

Question 15

What happens when dehydrated?

Answer 15

Dehydrated with maximal ADH: distal & collecting tubules permeable to H2O
- Small volume of
concentrated urine excreted
(up to 1200 mOsm/L);
reabsorbed water picked up
by peritubular capillaries

Question 16

What happens when hydrated?

Answer 16

overhydrated with NO ADH: distal and collecting tubules impermeable to water
 - Large volume of dilute urine
excreted (as low as 100
mOsm/L) no water reabsorbed
in distal nephron so excess
water excreted as urine

Question 17

What do nephrons do again?

Answer 17

nephrons create the vertical osmotic gradient - countercurrent multipliers

Question 18

What do vasa recta do?

Answer 18

Vasa recta preserve the vertical osmotic gradient

Question 19

What do the distal and collecting tubules do>

Answer 19

Distal and collecting tubules use the vertical osmotic gradient to adjust urine concentration

Question 20

What is urine excreation?

Answer 20

Urine contains high amounts of water (95%), waste
products (urea & creatinine) + variable amounts of other substances à
useful substances “conserved” by the body

Question 21

What is plasma clearance?

Answer 21

Plasma clearance: Kidneys clean or “clear” the plasma flowing through
them. This is the volume of plasma completely cleared of a substance by
the kidneys per minute

Question 22

What is the clearance rate of substance equation

Answer 22

Clearance rate
of a substance
(ml/min)
=
Urine concentration of
substance (mg/ml urine)
Urine flow rate
X (ml/min)/ 
Plasma concentration of substance (mg/ml plasma)

Question 23

If a substance is freely filtered but not reabsorbed, what is it?

Answer 23

A substance that is freely filtered, but not reabsorbed or secreted is equal
to the glomerular filtration rate (GFR), which can be ‘measured’ using
exogenous inulin or ‘estimated’ using endogenous creatinine

Question 24

What do blood tests do?

Answer 24

serum creatinine (calculate eGFR), blood urea
nitrogen (BUN)

Question 25

What do urine tests do?

Answer 25

protein, red blood cells, colour, transparency, &

odour of urine

Question 26

What are the limitations of biomarkers?

Answer 26

– Modified by dietary intake (protein consumption), muscle mass, age,
gender, drugs interacting with plasma clearance such as steroids
– Only become abnormal when kidney function is reduced by >50%
– Cannot distinguish between types of kidney disease
– Great interest in identification of better biomarkers!

Question 27

What are the consequences of an impaired kidney?

Answer 27

Blood is not filtered - build up of toxic waste substances (e.g.
urea, creatinine)
• Fluid & solute balance affected & can lead to numerous
issues, including hypertension - cause & consequence of
kidney failure
• Acid-base balance compromised
• Anaemia - reduction in red blood cells
• Renal bone disease - calcium & phosphate imbalance

Question 28

Describe acute kidney injury? (AKI)

Answer 28

– Rapid ↓ in function
– Common in hospital & critically-ill patients
– Diagnosis: Reduced GFR, insufficient urine, elevated BUN and/or serum
creatinine, other abnormal blood results, biopsy (cell death, loss of tubular
cell adhesion)

Question 29

Describe Chronic kidney disease (CKD)

Answer 29

Progressive ↓ in function
– Substantial burden of illness & premature mortality. Can lead to kidney
failure, requiring dialysis or kidney transplant
– Diagnosis: loss of kidney function (eGFR) &/or kidney damage for over 3mo

Question 30

How to clinically differentiate?

Answer 30

To clinically differentiate:
Risk factors (hypertension & diabetes), rate of progression (slow
or sudden & identifiable cause), imaging studies (obstruction)