8. Renal structure and function 1 Flashcards
Describe the general structure of the kidney
Around the outside is the cortex
The inner part is the medulla - made up of pyramids of the medulla
Hilum is central - just inside the kidney
Ureter comes from hilum and carries urine from kidney to bladder - then carried out of bladder via urethra
Renal arteries pass through hilum and then branch
At the end of the arteries are glomeruli - whole vast of terminal arterioles end in this structure in the kidney cortex
Describe the structure of the glomeruli
Each glomerulus is enclosed in a Bowman’s capsule
Urine is filtered out of the glomerulus into the Bowman’s capsule and then into the proximal convoluted tubule
Blood enters the glomerulus of each nephron via the AFFERENT ARTERIOLE and leaves via the EFFERENT ARTERIOLE
About 20% of blood plasma is filtered through here and enters into the proximal convoluted tubule
Compare afferent arterioles to efferent arterioles and the implications of this
Blood enters the glomerulus via the afferent arteriole and leaves via the efferent arteriole
Afferent arterioles have larger diameters than efferent arterioles and so there is a considerable drop in pressure between the two - THIS IS BECAUSE the smaller diameter means there is an increased resistance and so a decreased flow and hence a reduced pressure through the efferent)
The increased resistance in the efferent results in an increased pressure in the glomerular cavity and this is known as the FILTRATION PRESSURE - this forces fluid through the endothelium of the capillaries into the capsular space
Define filtration pressure in the kidney
The pressure exerted by fluid on the capillary walls of the glomerulus or the walls of the Bowman’s capsule
How is filtration pressure regulated?
The pressure difference between the afferent and efferent arterioles
The pressure inside the glomerulus
The nature of the cells that do the filtering
Describe the capillaries of the glomerulus and their basic structure
These are called PODOCYTES and cover the outside of the capillaries i.e. the capsule side
They have slits between them which form the filtration mechanism
These allow the passing of water and small molecules e.g. glucose, NA+, K+, HCO3- but prevent the loss of proteins
If these become inflamed (or enlarged in renal disease) then this enables more solutes i.e. proteins through into the urine - sign of renal infection
Briefly describe the proximal convoluted tubule and it’s structure
Leads on immediately from the glomerulus/bowman’s capsule
Have many microvilli
From here, substances can either be reabsorbed or can go out into the urethra and be excreted
Briefly describe the structure of the loop of Henle
Thin descending limb - low permeability to ions and urea but highly permeable to water
Thin and thick regions of ascending limb - thin is impermeable to water but permeable to ions and thick is where reabsorption of ions occurs
Very briefly, give the movement of substances through the nephron
Fluid leaves the Bowman’s capsule and enters the proximal convoluted tubule
Then enters the loop of Henle
Goes into the distal convoluted tubule
Enters the collecting duct
From the collecting duct, fluid is drained down into the ureter and out to the bladder
What is the ‘juxtoglomerular apparatus’?
Where the distal convoluted tubule folds back and meets the vessels entering the glomerulus i.e. the afferent and efferent arterioles
Describe the flow of blood through the kidneys
Just under a quarter of systemic cardiac output flows through the kidney per minute
Cardiac output at rest is 5L/minute so about 1.2L/minute through the kidney - SO very high blood flow through the kidney
Explain what is meant by ‘renal plasma flow’
The volume of blood plasma delivered to the kidneys per unit time
e.g. 1.25L/minute is the GFR (to both kidneys)
Haematocrit is 45% so blood plasma is only 55%
SO the RPF is 0.55 x 1.25 = 680ml/minute
What is the ‘glomerular filtration rate’ (GFR)?
This is the total amount of general fluid through BOTH kidneys
It is about 120-125ml/minute
This is the best way to assess the health of the kidneys
Describe the glomerular filtration rate and how it is maintained
This is auto-regulated via the constriction of afferent and efferent arterioles
The GFR does not change with changes in blood pressure so the dilation of the arterioles is generally adjusted to maintain pressure of 55mm/Hg in the glomerular capillaries
How can you measure the glomerular filtration rate?
Measure by the ‘clearance’ of a selected material - clearance is measure in units of volume/time (litres/minute)
What is ‘clearance’?
It is the effective volume of plasma completely cleared of a substance per minute
It’s units is volume/time e.g. litres/minute
Explain the different possible values of clearance and what these mean
Clearance of glucose would be 0 because no blood would be cleared of glucose as it is all reabsorbed
If 100% of a substance is filtered through the glomerulus and NONE of it is reabsorbed then the clearance of this substance is the same as the GFR
NB. the portion of plasma which is not filtered and enters the efferent arteriole will still contain the normal concentration of this material - some will be present in the renal venous flow (as opposed to the urethra)
If 100% of the material is filtered out of the blood plasma into the efferent arteriole but then all of the material is SECRETED into the urine then the renal venous flow has NO material in it because all of the blood passing through the kidney will have been cleared of the material - in this case, the clearance will = renal plasma flow
SO clearance of a material can range from 0 to renal plasma flow
Summarise the different possible measurements of clearance
Not removed at all by the kidneys: clearance = 0
Removed at the same rate as water passing through the glomeruli: clearance = GFR
Completely removed from the blood passing through the kidney: clearance = RPF
What are the implications of the GFR and the RPF if the kidneys are damaged?
Damaged kidneys can result in a decreased GFR but the RPF may remain normal
What is the formula to measure clearance?
Clearance = (urine concentration/plasma concentration) x urine flow
C = (U/P) x V
NB. as long as the concentration units are the same, it does not matter what they are
What is required to be able to measure clearance (on paper/theoretically)?
Measure the concentration of the substance in the plasma
Collect urine for a fixed period of time to get the urine flow (ml/minute)
Measure the concentration of the substance in the collected urine
What can be used to practically measure clearance in theory?
What are the problems with using this method?
Inulin can be used - polysaccharide that is completely filtered from the plasma and not reabsorbed
Not secreted or reabsorbed so the clearance of this is the GFR
BUT inulin does not occur naturally occur in plasma SO to measure the insulin clearance, have to infused inulin IV over a couple of hours to reach a steady plasma concentration - hence, this method of inulin clearance becomes impractical
What is instead used to measure GFR in a clinical setting?
Creatinine is used
This is produced naturally in the body (breakdown product of creatine phosphate)
This is freely filtered by the glomerulus BUT IT IS ALSO actively secreted by the PERITUBULAR capillaries in small amounts - THIS MEANS that the CREATININE CLEARANCE OVERESTIMATES THE ACTUAL GFR BY ABOUT 10-20%
However, due to the ease of measuring creatinine clearance, this margin of error is accepted - creatinine is already at a stead-state concentration in the blood
If the measurement of urinary excretion of creatinine is not possible, how can you measure the clearance?
Can use a formula method (look on notes for the formula)
What is the regulatory response if there is a high filtration pressure and GFR?
The afferent arterioles relax and the efferent arterioles constrict
What is the regulatory response if there is a low filtration pressure and GFR?
Afferent arterioles constrict and efferent arterioles relax
How is the balance between the afferent and efferent arterioles controlled? Give the mechanism of the macula densa to maintain this
By the juxtaglomerular apparatus - structure where the distal tubule folds back and contacts with the glomerulus at the point where the afferent and efferent arterioles enter
Here, there are MACULA DENSA cells which line the distal tubule where it contacts the afferent and efferent arterioles
The macula densa sense the sodium concentration in the distal tubule:
- In the proximal convoluted tubule, Na+ is removed at a relatively constant rate
- If the flow in the proximal tubule is low (i.e. the pressure is low), then more Na+ is removed and the Na+ concentration goes down
- If the pressure is high, less Na+ is removed and the Na+ concentration increases
- SO if the Na+ at the macula densa is too low, this means that the GFR is too low
- When this occurs, chemical factors are released from the macula densa that selectively constricts the efferent arteriole (? NEED TO CHECK - SHOULD IT NOT RELAX TEH EFFERENT TO INCREASE THE PRESSURE?) to increase the GFR back to normal
- At the same time, Renin is released from the juxtaglomerular cells and this diffuses into the blood
Describe the role of Renin within the blood and describe the mechanism that occurs
This is released into the blood and travels in the veins to the liver
At the liver, this cleaves the protein ANGIOTENSINOGEN to release the peptide ANGIOTENSIN 1
This travels to the lungs where it is converted to ANGIOTENSIN 2 by ANGIOTENSIN CONVERTING ENZYME (ACE)
This acts on the adrenal cortex to release ALDOSTERONE
Aldosterone increases the Na+ retention in the distal tubule
This renin-angiotenin-aldosterone system (RAAS) is a negative feedback mechanism to maintain the plasma volume of Na+, volume of blood and hence the blood pressure
