Physiology of the Kidney Flashcards
How much blood enters the kidney every minute?
1200 ml blood enters both kidneys each minute
This is 20% of total cardiac output
How much blood is purified by the kidneys in one day?
How much urine does this produce?
1700 litres of blood is purified by the kidneys in 1 day
This produces 1.5 - 2 litres of urine
What is glomerular filtration rate used for?
It is a measurement of kidney function
What is normal glomerular filtration rate?
100-120 ml/min/1.73m2
What does accurate measurement of GFR require?
The injection of a radioactive tracer
e.g. technetium
What is creatinine clearance used to measure?
Where does creatinine come from?
It is an estimation of glomerular filtration rate
Creatinine is released from muscle at a relatively constant rate
What happens to creatinine in the kidneys?
Why is creatinine clearance not entirely accurate
It is filtered by the kidneys
But there is some secretion into the filtrate by the proximal tubule
As creatinine clearance is rarely used in clinical practice, what is used instead?
- blood tests
2. 24-hour urine collection
What is serum creatinine used to measure?
Why?
It measures kidney function
Creatinine will accumulate in kidney disease
What are the drawbacks of using serum creatinine to measure kidney function?
- it is not specific for the site of injury
2. there is a delay in serum creatinine rise following AKI
Where does creatinine come from?
What does serum creatinine involve?
It is released by the muscles and removed by kidneys
Serum creatinine is a simple blood test
What is normal serum creatinine for males and females?
Male - 64-104 micromol/L
Female 60 - 93 micromol/L
What is eGFR?
What is it used instead of?
estimated glomerular filtration rate
It is used instead of creatinine clearance
It is used alongside serum creatinine
What factors are taken into account when calculating eGFR?
- age of patient
- sex of patient
- ethnicity
- serum creatinine
What does eGFR correlate with?
It correlates with the % of kidney function
e.g. eGFR = 50
This means 50% kidney function
At what eGFR will patients need to commence dialysis?
eGFR < 10 ml/min/1.73m2
What happens to kidney function by the time creatinine rises above 104 micromol/L in males?
50% of kidney function has been lost by this stage
What is the main function of the proximal convoluted tubule?
It recovers 70% of glomerular filtrate
- water via aquaporins
- electrolytes
- glucose
- amino acids
What is the other funcrtion of the proximal convoluted tubule?
It is involved in the recovery and the generation of bicarbonate
What moves into the nephron at the level of the proximal convoluted tubule?
H+
What moves out of the nephron at the level of the proximal convoluted tubule?
- glucose and amino acids
- phosphate
- Na+ and Cl-
- K+
- water
- bicarbonate
What enzyme is needed for the reabsorption of bicarbonate in the PCT?
Carbonic anhydrase
In acute or chronic kidney disease, how is the PCT affected?
There is a failure to reabsorb bicarbonate
This leads to metabolic acidaemia
What is the passage of bicarbonate from the lumen of the PCT into the interstitial fluid/blood?
- H+ ions excreted from the PCT are combined with bicarb to make H2CO3
- carbonic acid is converted into CO2 and H2O, which enters the PCT cell
- Carbonic anhydrase converts it back to carbonic acid
- This dissociates to give bicarbonate that can enter the blood
What is there an uptake of in the ascending limb of the loop of Henle?
Na+
K+
2Cl-
H2O
How do K+, Mg2+, Ca2+ and NH4+ move from the lumen into the blood?
They move paracellularly through paracellin-1
How do K+, 2Cl- and Na+ move from the lumen into the cell of the nephron?
They enter the cell via NKCC2
This is the Na-K-Cl cotransporter
What happens to K+, Cl- and Na+ once they have entered the cell of the ascending loop of Henle?
- Na+ leaves the cell via the Na/K+ ATPase
- Each Cl- ion leaves via barttin
- K+ reenters the lumen of the loop of Henle via ROMK
What controls reabsorption in the cortical collecting duct?
What is reabsorbed here?
Na+ is reabsorbed (along with Cl- and H2O)
in exchange for K+
Controlled by aldosterone
What is the function of the medullary collecting duct?
What hormone acts here?
It is the site of urinary concentration
Antidiuretic hormone (ADH) acts to increase water reabsorption
What value is plasma osmolality maintained at?
Why?
285 mOsm/L
This is optimal for cellular function
What is normal daily urine output and urine osmolality?
daily urine output of 1.5 - 2 L
urine osmolality of 50-1400 mOsm/L
What is meant by osmolality?
The number of osmoles (Osm) of solute per kg of solvent
Why is urine osmolality regulated?
regulating urine osmolality allows maintenance of plasma osmolality
What is urine osmolality dependent upon?
The volume of water ingested
What is the osmolality and colour of urine when volume of water intake is increased?
The urine is dilute and colourless
It is around 50 mOsm/L
What is the osmolality and colour of urine in decreased water intake or increased fluid loss?
The urine is concentrated and dark yellow
Osmolality is 1400 mOsm/L
Why do the kidneys concentrate urine during periods of decreased fluid intake?
- to conserve salt and water
2. to maintain circulating volume and blood pressure
What is the name of the mechanism responsible for concentrating the urine?
What does this involve?
Counter current mechanism
It establishes a high conc. gradient in the medulla
This enables water reabsorption from the filtrate in the PCT and CD
What is healthy fluid balance?
Fluid intake = fluid loss
How is fluid lost from the body?
- urine
- insensible loss (around 500 ml/24hrs)
this includes sweating, faeces, respiration
What conditions can lead to fluid loss?
- vomiting and diarrhoea
- burns
- haemorrhage
- fever
What does fluid loss lead to increased secretion of and why?
- ADH as it inserts channels into the CD to allow water reabsorption
- RENIN as it increases Na+ absorption and causes vasoconstriction
