3. Renal Blood Flow and Glomerular Filtration Flashcards
What are 5 of the basic functions of the kidney
- Excretion of metabolic products e.g. urea
- Excretion of foreign substances e.g. durgs
- Homeostasis of cell volume - regulation of body fluids
- Regulation of blood pressure
- Secretion of hormone e.g. erythropoietin
What is the filtration barrier impermeable to?
Cells, proteins and drugs etc. which are bound to plasma proteins
What drives glomerular filtration?
- Hydrostatic pressure in glomerular capillaries
* Caused by high blood pressure in afferent renal arteriole
Which 2 forces determine the net ultrafiltration pressure?
- Hydrostatic pressure
* Oncotic pressure
What is the normal range for net ultrafiltration pressure?
10-20mmHg
Apart from pressure, what else influences the glomerular filtration rate?
- Permeability of tubule
* Surface area of membrane available for filtration
What is the definition of the glomerular filtration rate?
Amount of fluid that moves from the glomeruli capillaries into the Bowman’s capsule per unit time (ml/min)
• ultrafiltration pressure x ultrafiltration coefficient
• renal plasma flow x friction fraction
How much blood do the kidneys received?
- 20% of cardiac output
- 1L/min
- Renal plasma flow - 0.6L/min
What is the friction fraction (FF)?
- Ratio between renal plasma flow and amount of filtrated filtered by the glomerulus
- Normally 20%
What is the normal value for the glomerular filtration rate?
120ml/min (kept within a narrow range)
How is the GFR regulated (+ example during exercise)?
- Neural or hormonal output to the afferent/efferent arteriole
- e.g. during exercise - afferent arteriole constricts, efferent arteriole dilates (avoid fluid loss when BP increases)
What are the mechanisms of (GFR) autoregulation?
• Myogenic mechanism
- arterial pressure rises => afferent arteriole stretches
- arteriole contracts => vessel resistance increases => blood flow reduces and GFR remains constant
• Tubuloglomerular feedback
- macula densa response to NaCl increase
- release of ATP as signalling molecule
- afferent arteriole vasoconstricts => reduced filtration (negative feedback loop association)
How do the following change the GFR: • Severe haemorrhage • Obstruction in nephron tubule • Reduced plasma protein concentration • Small increase in blood pressure
- Severe haemorrhage - decrease
- Obstruction in nephron tubule - decrease
- Reduced plasma protein concentration - increase
- Small increase in blood pressure - constant
What is clearance?
- The extent to which substances are removed from the blood
- Number of litres of plasma completely cleared of the substances per unit time
- concentration of substance in urine x (rate of urine production)/(concentration of substance in plasma)
What affects clearance?
- Processes following ultrafiltration
- If molecule is freely filtered (neither reabsorbed nor secreted following ultrafiltration), clearance = GFR e.g. inulin (plant polysaccharide) has a clearance value of 120ml/min = GFR
