The Kidneys Flashcards
What are the 3 major functions of the kidneys?
- Endocrine function (secreting hormones)
- Maintain balance of water, Salt and pH
- Excrete waste products
How much cardiac output does each kidney receive?
20% of cardiac output
5L/min
What is total renal blood flow?
1L/min
What is total urine flow?
1L/min
What is the pathway of renal blood supply from the renal artery?
Renal artery Segmental artery Interlobar artery Arcuate artery Interlobular artery Afferent arteriole (Nephron) glomerular capillary Efferent arteriole (Nephron) peritubular capillary
Each nephron has how many capillary beds and where?
2 capillary beds
One at the glomerulus and one of the peritubular area
Within each nephron, how many sets of capillaries are there and which ones?
2 sets of capillaries
Glomerular capillaries (glomeruli)
Peritubular capillaries
How are the 2 capillaries connected together?
By an efferent arteriole, the vessel that allows blood to leave the glomerulus
What is unusual about renal circulation?
2 sets of arterioles and capillaries
Where do peritubular capillaries supply blood to and why?
Supply the tubules with blood
Many of the tubular processes of secretion and reabsorption so are active (thus require oxygen and energy) so blood supply is crucial
Where do the peritubular capillaries go after?
Form the veins and blood leaves the kidneys
What is the entire capillary covered by?
Podocytes
What happens in glomerular filtration?
Where does fluid go from and to?
Passage of fluid from the blood into Bowman’s space to form filtrate
What is the distal part of the nephron (tubule) responsible for?
Secretion and reabsorption
What are the layers of the glomerular filtration barrier?
Capillary Capillary endothelium Basement membrane (basal lamina) Podocytes Bowman’s capsule
What factors determine what crosses the filtration barrier?
Pressure
Size and charge of molecule
Rate of blood flow
Binding to plasma proteins eg. Calcium, hormones such as thyroxine
What can pass the filtration barrier?
Small molecules and ions up to 10 kDa can pass freely eg. Glucose, Uric acid, potassium and creatine
What molecules are restricted?
Larger molecules are increasingly restricted
What charges are able to pass?
Fixed negative charge in the glomerular basement membrane repels negatively charges anions eg, albumin
Can albumin pass into the tubule and why?
Albumin has a molecular weight around 66kDa and is negatively charged so it cannot pass into the tubule easily
Is protein found in the filtered fluid in the tubules?
It is protein free
Only protein found is Tamm horsfall protein in urine which is produced by tubule
What can pressure determine in the kidneys?
Glomerular filtration rate
What are the 2 types of pressure found in the glomerular filtration?
Hydrostatic pressure (of glomerular capillary and bowman’s space - GC and BS) P(gc) bigger & P(bs) smaller
Osmotic pressure
ℼ(gc) - increases as you go along glomerular capillary ad proteins become more concentrated
ℼ(bs) - no osmotic pressure as there are no proteins = 0
What is the glomerular filtration rate?
It is the filtration volume from glomeruli into Bowman’s space per unit time (minutes)
What is the equation for GFR?
GFR = Kf [P(gc) - P(bs) - ℼ(gc)]
What does Kf stand for?
Filtration coefficient
The product of permeability of the filtration barrier and the SA available for filtration
In a 70kg person, the average GFR =
125ml/min
How can GFR be increased and decreased?
By constricting and increasing blood flow to afferent and efferent arterioles
What happens when afferent arterioles are constricted?
Decreased hydrostatic pressure in the glomerular capillaries P(gc)
Decreasing GFR
What happens when efferent arterioles are constricted?
Increases hydrostatic pressure in glomerular capillaries P(gc)
Increasing GFR
What happens when afferent arterioles are dilated?
Increases the P(gc) and thus GFR
What happens when efferent arterioles are dilated?
Decreases the P(gc) and thus GFR
What is GFR measured by?
Calculated by measuring the excretion of a marker substance (M)
What must the marker be? (3)
Freely filtered (same conc in blood and tubular fluid)
Not secreted or absorbed in the tubules
Not metabolised
Amount excreted per min =
Amount filtered per min
Equation for amount of M in fluid?
Conc in fluid x vol of fluid
Equation for GFR including conc of M in urine and plasma?
GFR = Um x urine flow rate / Pm
What is used to estimate GFR clinically and why?
Creatine Muscle metabolite, constant production Serum creatinine conc will vary with muscle mass Freely filtered by glomerulus Some additional secretion by the tubules
What is the filtration fraction? And equation?
It’s the Proportion of renal blood flow that gets filtered
GFR / renal plasma flow
Renal blood flow=1000ml/min
40% of blood is cells and rest is plasma
Renal plasma flow = ?
If GFR is 120ml/min then what is the filtration fraction?
Renal plasma flow=600ml/min
Fraction= 120/600 = 0.2 = 20%
What is renal clearance?
The vol of plasma from which a substance is completely removed from the kidney per unit time (usually a minute)
Why is substance M used?
It is freely filtered at the glomerulus and is neither reabsorbed nor secreted in the tubule
All the M that is filtered will end up in the urine
What is the clearance of substance M!
125ml/min
Equal to GFR
What is the equation for for clearance?
Urine conc x urine vol / plasma conc
