Module 2 Lecture 5 Flashcards by Vivienne Young

What concentration are body fluids maintained at?

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

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Why is keeping a constant solute concentration important?

This prevents cells from swelling (hypotonic) or shrinking

(hypertonic) due to osmotic movement of water

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How do the kidneys adjust to variations of fluid intake?

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

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What do nephrons do?

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

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How does fluid flow? LABEL DIAGRAM

Fluid flows in opposite directions of

adjacent segments of the same tube

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What is counter current multiplier

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

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What is counter current exchanger?

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

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Why is countercurrent mechanisms so important?

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

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LABEL THE CYCLE for long nephron loops

memorize them Vivienne

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

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)

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

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

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

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)

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What happens with maximal ADH?

With maximal ADH, 99% water in filtrate is

reabsorbed & returned to blood

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How is variable water re-absorption is regulated by ADH

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

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What happens when dehydrated?

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

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What happens when hydrated?

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

What do nephrons do again?

nephrons create the vertical osmotic gradient - countercurrent multipliers

What do vasa recta do?

Vasa recta preserve the vertical osmotic gradient

What do the distal and collecting tubules do>

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

What is urine excreation?

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

What is plasma clearance?

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

What is the clearance rate of substance equation

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)

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

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

What do blood tests do?

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

What do urine tests do?

protein, red blood cells, colour, transparency, & | odour of urine

What are the limitations of biomarkers?

– 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!

What are the consequences of an impaired kidney?

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

Describe acute kidney injury? (AKI)

– 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)

Describe Chronic kidney disease (CKD)

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

How to clinically differentiate?

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