Physiology of the Kidney Flashcards
How much blood enters each kidney every minute?
What % of total blood pumped by the heart is it?
1200ml
20%
How is accurate measurement of glomerular filtration rate (GFR) taken?
Requires injection of radioactive tracer
- Technetium (Tc99)
- Chromium-51 (51Cr-EDTA)
What is creatinine clearance used as?
Estimation of GFR
Where is creatinine released from?
Muscle at a relatively constant rate
Relative to muscle mass
How is creatinine then filtered?
Filtered by kidneys
Some secretion into the filtrate by the proximal tubule kidneys
Not that accurate for GFR (as some is secreted as well as filtered)
As your serum creatinine levels increase, what does this indicate?
Loss of kidney function
Be aware of muscly people –> will have high creatinine but good kidneys
What are problems with using serum creatinine for measuring kidney disease?
- Not specific for site of injury
- Delay in rise following acute kidney injury
What tends to be used instead of creatinine clearance?
Estimated Glomerular Filtration Rate (eGFR)
How is eGFR calculated?
- Age
- Sex
- Ethnicity
- Serum creatinine
What does eGFR correlate with?
% of kidney function
e.g. eGFR = 50 = 50% kidney function
Useful when discussing with patients
When will patients need to commence dialysis?
eGFR < 10mls/min/1.73m2
What is the best treatment for kidney failure?
Kidney transplant
What is dialysis?
Dialysis is a procedure to remove waste products and excess fluid from the blood when the kidneys stop working properly. It often involves diverting blood to a machine to be cleaned.
What % of kidney function is lost by the time creatinine levels rise above 104μmol/L (male) i.e. normal levels?
50% of kidney function lost
Where do 80% of the glomeruli lie?
Cortex of kidney
What is job of proximal convoluted tubule?
Recovers 70% of glomerular filtrate
- Water
- Electrolytes (Na+, Cl+, K+)
- Glucose
- Amino acids
- Phosphate
Recovery and regeneration of bicarbonate
What is the regeneration of bicarbonate dependent on?
Carbonic anhydrase (enzyme)
What can a failure to recover and regenerate bicarb lead to?
Acidaemia (disrupts acid-base balance)
What is taken up in the ascending limb of Henle?
- Na+
- K+
- 2Cl-
- H20
Sodium/Potassium/Chloride cotransporter (from lumen to inside cell)
What drug acts at the ascending limb of Henle?
Loop diuretics
How do loop diuretics work?
Block uptake of Na+/K+/2Cl+ cotransporter
So water is not reabsorbed and is lost in urine
When are loop diuretics used?
- Cardiac failure
- Oedema
- Chronic kidney disease
What are the side effects of diuretics?
- Low sodium (hyponatremia)
- Low potassium (hypokalaemia)
What can hyponatremia lead to?
Serious neurological problems (coma, death)
What can hypokalaemia lead to?
Cardiac arrhythmias
What is reabsorbed at the cortical collecting duct? What is this controlled by?
Na+ (with H20, Cl) in exchange for K+
- Na+, Cl-, H20 out into blood
- K+ into tubule
Controlled by aldosterone
What happens at the medullary collecting duct?
Site of urinary concentration
Water, Na+ Cl-, HCO3- reabsorption
Where does antidiuretic hormone (ADH) act?
Medullary collecting duct
Where is ADH stored?
Posterior pituitary
What is effect of ADH?
Stimulates water reabsorption –> concentrated urine
What happens when water needs to be preserved (e.g. dehydration)?
ADH levels increase
More water reabsorbed
More concentrated urine
What is osmolality?
“Osmolality” refers to the concentration of dissolved particles of chemicals and minerals – such as sodium and other electrolytes – in your serum. Higher osmolality means more particles in your serum. Lower osmolality means they’re more diluted
What is optimum plasma osmolality?
285 mOsm/L
What is the normal daily urine output?
1.5-2 L of urine
What is the normal urine osmolality?
50-1400 mOsm/L
50 –> very diluted
1400 –> very concentrated
What is urine osmolality regulated to maintain?
Plasma osmolality
What is it important to regulate urine osmolality?
To maintain plasma osmolality
Why do the kidneys concentrate urine during periods of decreased fluid intake?
To conserve salt and water
Maintain circulating volume and blood pressure
What is the mechanism responsible for urine concentration called?
Counter current mechanism
How does the counter current mechanism work?
High concentration gradient in the medulla
Enables reabsorption of water from the filtrate
- In proximal tubule
- In collecting duct
What is insensible loss?
Loss of fluid not through urine
Sweating, faeces, respiration
During states of fluid loss, what happens?
- Increased ADH
- Increased renin
What is effect of increased renin?
Renin converts angiotensinogen to angiotensin I
ACE converts angiotensin I to angiotensin II (vasoconstriction)
Angiotensin II stimulates release of aldosterone which increases Na+ reabsorption
What is glomerular filtration?
The movement of substances from the blood within the glomerulus into the capsular space
What is tubular reabsorption?
The movement of substances from the tubular fluid back into the blood
What is function of glomerulus?
Filters small solutes from blood
What is function of Bowman’s capsulse?
Collects what is filtered through the glomerulus
What is the function of the proximal convoluted tubule?
1) Reabsorbs 65% of filtrate volume: nutrients(glucose, amino acids)/ ions(Na/bicarb)/ water
2) Secretes toxins (ammonia, creatinine, organic acids, some drugs)
3) Adjusts filtrate pH
What is function of descending limb of loop of Henle?
Water reabsorption (aquaporins) = increased osmolality
What is function of ascending limb of loop of Henle?
Reabsorbs Na/Cl =r educes osmolalityUrea secreted (urea recycling)
What is function of distal convoluted tubule?
Hormone controlled
Aldosterone: reabsorb Na (Cl follows), secrete K
PTH: reabsorb Ca
Reabsorb bicarb and water, synthesizes bicarb
What is function of collecting duct?
Concentrates urine using osmotic gradient
ADH: reabsorb water
Reabsorb/secrete various ions (sodium, potassium, hydrogen ions, bicarb) to maintain blood pH
What is GFR?
Refers to the ultrafiltrate of plasma which crosses the glomerular barrier into the urinary space
Equal to the total filtration rates of all the functioning nephrons
What is used instead of GFR?
eGFR as bette predictor of risk
What does eGFR take into account?
Serum creatinine, sex, age, race
How does ethnicity affect GFR?
Higher average muscle mass in African patients –> higher creatinine generation rate = higher GFR at same level of sCr
How does sex affect GFR?
Men have more muscle mass
How does age affect GFR?
Younger people have greater muscle mass
