Renal blood flow and Glomerular filtration Flashcards
(35 cards)
List the main functions of the kidney.
- To control volume and composition of body fluids
- To get rid of waste material from the body
- Acid-base balance
- As an endocrine organ – EPO, renin, vitaminD
Name and describe the functional unit of the kidney.
Nephron (approx. 4cm long), nephron has two elements to it, the glomerulus and a tubule. The glomerulus (a tuft of blood vessels) is contained within the Bowmans capsule. The glomerulus is unusual in that is has an artery entering (afferent arteriole) and the vessel leaving is also an artery (efferent arteriole) before becoming the peritubular capillaries the surround/are adjacent to the tubule. Importantly, the blood and tubule meet again through the peritubular capillaries.
How many nephrons are there in each kidney?
1 Million nephrons in each kidney.
**Nephrons cannot be regenerated.
How do the Glomeruli contribute to urine formation?
Glomeruli produce the liquid. Blood flowing through the renal artery enters the afferent arteriole, goes into the ball shaped glomerular capillaries. Here fluid is filtered out (by ultrafiltration) into the tubule at a relatively fast rate of normally about 120ml/min. Blood leaves via the efferent arteriole and goes to the peritubular capillaries, which is where a lot of reabsorption occurs.
How do the tubules contribute to urine formation?
Tubule modifies the volume and composition of the liquid. 120ml is filtered through and enters the tubule, most of it is reabsorbed into blood, around 119ml/min. So, what you’re left with in the tubule is about 1ml/min formation of urine, this equates to about 1.5L of urine a day. If you fall below 5ml/day then you’re considered to be in renal failure. We also reabsorb any useful things, such as glucose and AA, anything that isn’t useful we leave in the urine and it is excreted.
How is the Glomerular Filtration Rate (GFR) determined?
- Autoregulation
2. Renal sympathetic vasomotor nerve activity
What encloses the Glomerulus?
Glomerulus is completely enclosed by epithelium of the Bowman’s capsule, though they are specialised to form podocytes (epithelium is ‘invaginated’ to coat the capillaries).
What type of ultrafiltrate is Glomerular fluid?
Passive ultrafiltrate of plasma i.e. plasma from which the proteins have been filtered out (so has very similar composition, just without as many proteins).
What key features must the Glomerular fluid have for it to be an ultrafiltrate?
- For small solutes, such as NaCl, glucose and urea the concentration of glomerular fluid should equal the concentration in the plasma.
- For plasma proteins, concentration in the glomerular fluid is almost zero. Hence urine is routinely tested on wards for protein (proteinuria). Proteinuria is a sign of renal/urinary tract disease.
- It is a net pressure drop across the glomerular membrane drives the ultrafiltration process.
How do we know that we the glomerular membrane sieves out solutes by molecular size?
It was found the ratio of glomerular filtrate/plasma was 1 for urea and glucose (because they are equal). But albumin was below 0.01. So, it was concluded that the glomerular membrane sieves out solutes from plasma by molecular size.
What is the main force trying to push fluid out of the Glomerular capillary? (Give a value if possible)
Capillary blood pressure (Pc), which is around 50mmHg.
*An imbalance of Starling Forces drives glomerular fluid formation (filtration).
What are the forces trying to push fluid back into the capillary? (Give values if possible)
Plasma oncotic pressure (Pi-p) around 25mmHg due to plasma proteins in the blood and there is also pressure in the Bowman’s space trying to push fluid in (Pu), around 10mmHg.
What is the equation for the net filtration force?
Capillary BP – (Plasma oncotic pressure (or COP) + Pressure in Bowmans space)
How do we know we get fluid filtration throughout the whole Glomerular capillary?
Along the whole capillary, despite the oncotic pressure rising, it is always less than Pc.
What is the ‘filtration fraction’?
Tells us the proportion of fluid that is being filtered from the volume of blood that is flowing through. The glomerulus has a filtration fraction of 20% (in most capillary beds it is only 1%). i.e. 20% of blood which flows through is filtered.
What happens to the plasma oncotic pressure (AKA Colloid osmotic pressure - COP) in the efferent arteriole?
Pressure starts to drop and COP rises because fluid has been lost from capillaries and protein is getting more concentrated. Therefore, it is exerting a greater pulling force. Hence in the peritubular capillaries, the COP rises above the hydrostatic BP and so we get reabsorption of fluid back into the blood.
Describe the barriers in place, which can be seen in a cross section of the Glomerular membrane.
In the capillary we have a fenestrated endothelium, sitting on a basement membrane.
There are then the pedicels of the podocytes attaching to the capillary, in between them being the gaps called filtration slits.
What is the fenestrae in the glomerulus?
The fenestrae are areas where there are no membrane present, these cover about 30% of the surface and are about 50-100nm wide. The RBCs are too large to pass through, but any substance that is less than 50nm wide can pass through the fenestrae, through the basal lamina
How wide is the filtration slit?
The filtration slit, through which the glomerular fluid emerges from, is only about 30nm wide.
How come albumin doesn’t pass through? Even though an albumin molecule is approx. 4nm across
Because the filtration slits are further sub-divided into even smaller slits, in the formation of a central spine with lateral rungs sticking out.
This sub-division results in slits only 4nm wide. The sub-division is made of the proteins nephrin and podocin.
What molecules are freely able to pass in glomerular filtration?
*What does it mean when protein is found in urine?
Substances like water, glucose, NaCl, urea, creatinine can freely move through.
*Proteins will leak out (proteinuria) if the filtration slits of basement membrane are damaged. Which is why it is important to test urine for protein.
What happens to GFR when blood pressure increases?
Would increase GFR (normally 120ml/min), if this happened and the rate of reabsorption remained the same (as body only has certain capacity to reabsorb) then urine volume would increase. So, you would end up losing blood volume.
Name the internal mechanism which keeps GFR constant.
The mechanism holding GFR constant is an internal one called ‘autoregulation’. This means
changes in urine production (diuresis, antidiuresis) are not usually due to changes in GFR but due to changes in tubular reabsorption. Auto regulation is an intrinsic property of the kidney.
What would happen to the kidney if there was no autoregulation?
Then a relatively small increase in bp (from 100-125mmHg) would cause a similar 25% increase in GFR i.e. increasing from 180-225L/day. If tubular reabsorption remained constant then urine flow would increase by 30-fold (difference between GFR and tubular reabsorption), thereby depleting blood volume very quickly.
