Urinary System 3 - GFR and liver failure Flashcards
Define glomerular filtration
- Formation of an ultrafiltrate of plasma in the glomerulus via a passive process
- Blood is driven through semi-permiable walls of glomerular capillaries into the bowmans capsule space by hydrostatic pressure of the heart
Define freely filtered
Same concentration of a solute in the filtrate and in the plasma
What is ultrafiltrate?
- A clear fluid free from blood and proteins, is produced containing electrolytes and small solutes
- ‘primary urine’
What is the amount excreted equal to?
Amount filtered + Amount secreted - Amount absorbed
How is net ultrafiltration pressure calculated?
Hydrostatic pressure in glomerular capillaries (Pgc) - Hydrostatic pressure of tubule (Pt) - Osmotic pressure of plasma proteins in glomerular capillaries (πgc)
What is the net ultrafiltration pressure?
10-20mmHg
How is glomerular filtration rate calculated?
Ultrafiltration rate x Kf (ultrafiltration coefficient)
List the factors affecting the ultrafiltration constant.
- Kidney disease reducing functioning nephrons and therefore reducting SA
- Dilation of glomerular arterioles by drugs/hormones will increase Kf
Define glomerular filtration rate
The amount of fluid filtered from the glomeruli into the Bowmans capsule per unit of time (ml/min).
What is renal blood flow?
Approx 1L
What is renal plasma flow?
Approx 0.6L
What is the filtration fraction?
- 0.2
- Ratio between renal plasma flow and the amount of filtrate filtered by the glomerulus
What is normal glomerular filtration rate?
Renal plasma flow x filtration fraction = 0.12
What does glomerular filtration rate depend upon?
- Glomerular capillary pressure
- Plasma oncotic pressure
- Tubular pressure
- Glomerular capillary surface area or permeability
- Altered by neural and hormonal input
Describe the mechanism of autoregulation in the kidney.
- Myogenic mechanism
- Vascular smooth muscle contracts when stretched, keeping GFR constant when blood pressure rises
- Tuboglomerular feedback (macula densa cell response to high sodium concentration)
Define renal clearance
- As substances in the blood pass through the kidney they are filtered to different degrees.
- The extent to which they are removed from the blood is called clearance.
- Clearance is the number of litres of plasma that are completely cleared of the substance per unit time.
What is the equation for clearance?
concentration of substance in urine x rate of urine production / concentration of substance in the plasma
How can GFR be measured using clearance? Name two substances that can be used
- Clearance can be measured if a molecule is freely filtered and neither reabsorbed nor secreted in the nephron
- Amount filtered = amount excreted
- Inulin, a plant polysaccharide, can be used
- So can creatinine
List the properties of inulin that make it an accurate measure of GFR, and why it is not desirable
- Plant polysaccharide - freely filtered, neither reabsorbed nor secreted
- Not toxic
- Measurable in the urine and plasma
- However, not made by humans so must be transfused with catheterisation
List the properties of creatinin that make it good and bad for GFR use
- Waste product made in muscle metabolism
- Constant amount released
- If there is stable renal function, amount of creatinine in the urine is stable
- Low values both low and high values indicate renal failure
- Not as accurate as inulin
What is the normal value of renal clearance of inulin and creatinine?
120ml/min
How is renal plasma flow measured?
- Measured by PAH (625l/min)
- This is because all RPF is removed from the blood by the renal system, first by filtration then actively secreted at the tubule
- Clearance of PAH matches renal plasma flow
Describe the pathway through the kidney of most solutes
- Clearance less than 120mL/min, with reabsorption as well as secretion and excretion
- Controlled excretion
What is the cardinal feature of renal disease?
- A fall in GFR
- Excretory products build up in plasma, so creatinine concentration is raised
Why is renal disease important to consider when administering drugs?
Doses need to be monitored in case of overdose
What is the equation you would use to calculate the rate PAH enters the kidneys per minute in renal arteries?
Renal plasma flow rate x plasma PAH concentration
What is the equation you would use to measure rate of excretion of PAH in urine?
Urine PAH concentration x urine flow rate
If the plasma arterial inulin concentration is 1mmol/L, what is the plasma inulin concentration in the efferent arteriole?
- 1mmol/L
- 20% filtered from blood, so 20% less water and 20% less inulin. Therefore, concentration remains the same
If the plasma arterial inulin concentration is 1mmol/L, what will the inulin concentration be in the renal vein? Why?
- 0.8mmol/L
- Though 20% water enters the filtrate, it is reabsorbed into the renal vein. Inulin is not reabsorbed, so the concentration of inulin decreases by 20%
- Say initial water in the blood is 1L and 200ml enters filtrate, that 200ml is reabsorbed into the renal vein.
- 0.8mmol of inulin remains in the blood from the efferent arteriole. 0.8/1=0.8mmol/L
