Renal blood flow and glomerular filtration Flashcards
Why do the kidneys receive 20% of the cardiac output if they only contribute to 0.5% of the body weight?
Reflects major role that the kidney plays in the regulation of extracellular fluid compartment (ECF); blood volume regulation and rapid waste disposal
Why is the renal system able to function if a person only has 1 kidney?
Each kidney has a million nephrons which is enough for the kidney to still carry out all of its functions (produce urine).
Why is almost all of the fluid volume that’s filtered into the bowman’s capsule reabsorbed back into the peritubular capillaries?
To prevent volume depletion/dehydration as if non of the fluid was reabsorbed back into the circulation it would result in a loss of 7.2 L of fluid per hour.
Briefly describe the process of glomerular filtration
Hydrostatic pressure within the capillaries of the glomerulus forces fluids and solutes through the glomerular capillary membrane into the bowman’s capsule. This process is called passive ultrafiltration and produces an ultrafiltrate.
What are the 2 things that regulate glomerular filtration rate?
Autoregulation
Renal sympathetic vasomotor nerve activity
Why is sampling urine for plasma proteins (proteinurea) used as a test of renal function?
Normally plasma protein concentration in glomerular fluid should be almost 0 as plasma proteins aren’t able to move across glomerular capillary membrane. This means if plasma proteins are found in urine it’s a sign of renal/urinary tract disease as
Describe the experiment by Richards and Wearns used to sample glomerular fluid?
They blocked the tubule of the glomerulus using a small piece of mercuray which prevented any of the glomerular fluid draining away.
They then punctured the glomerulus using a very fine pipette and took a sample of the glomerulur fluid.
They then measured the contents of the glomerular fluid and compared it to teh contents of plasma.
How did the experiment by Richards and Wearns prove that the glomerular membrane filters out solutes from plasma by molecular size?
When they took they ratio of certain substances, e.g. Glucose, in the filtrate compared to the plasma they found it was one - same conc. in plasma and in filtrate but others they found the ratio to be less.
When they sorted the contents by molecular size they saw a clear pattern - as molecular size increases the ratio of that substance in the filatrate compared to plasma decreased.
Explain how an imbalance in starling’s forces in the glomerular capillaries allows for glomerular filtration
Glomeruluar capillary blood pressure produces an outward force which drives fluid movemnt out of the capillary into the bowman’s capsule.
However, the plasma colloid osmotic pressure (exerted by the plasma proteins) and the pressure in the bowman’s capsule produce an inward force that drives fluid movement into the capillary.
The inward force genrated is less than the outward force so filtration occurs - fluid moves out of capillary and into bowman’s capsule (glomerular filtration)
Describe how these starlings forces change as fluid is filtered into bowman’s capsule
Do the forces still favour filtration?
As the blood flows through the capillary there’s a slight drop in pressure from the afferent end to the efferent end.
Plasma also gets more concentrated as the blood flows along due to fluid loss so there’s slight increase in plasma colloid osmotic pressure.
Also slight increase in pressure of bowman’s capsule as more fluid is filtered into it.
These chnages aren’t enough to make inward force generted greater than outward force so filtration is still favoured.
Explain how the imbalance of starling’s forces in the peritubular capillaries allows for tubular reabsorption
As you go from the glomerular capillaries to the peritubular capillaries almost all of the plasma has been filtered into the bowman’s capsule.
This means that the concentration of plasma proteins, and therefore the plasma colloid osmotic pressure, within the peritubular capillaries is a lot higher than in the glomerular capillaries.
Also, due to the high amount of fluid loss the capillary blood pressure in the peritubular capillaries is a lot lower than in the glomerular capillaries.
Both of these changes to the starling’s forces mean the inward force generated by plasma proteins is now highter than the outward force genereted by capillary blood pressure. This favours reabsorption allowing tubular resorbption to occur.
What is the name given to the specialised cells formed from invaginations of the epithelium of the bowman’s capsule that wrap around the capillaries of the glomerulus?
Podocytes
What are the pedicels and what is their function?
Pedicels are projections (processes) of the podocytes that are also wrapped around the glomerular capillaries.
Their arrangement around the glomerular capillaries leaves gaps (filtration slits) that allow soultes to filter from the capillaries to the urinary space of the bowman’s capsule.
Why are there fenestrations (gaps) within the endothelium of the glomerular capillaries?
It allows small molecules to move through them across the basmemnt lamina, through the filtration slits created by the pedicels and into the urinary space of the bowman’s capsule.
What is the width of the fenestrations of the endothelium of the glomerular capillary?
What is the width of the filtration slits?
Fenestrations 50-100 nm
Filtration slits: 30 nm
Some plasma proteins such as albumin are small enough to filter through the fenestrations and the flitration slits so why is it that no plasma proteins are seen in the ultrafiltrate?
Because the filtration slits are further subdiveded into 4 nm wide pores by a cytoskeletal arrangement of the proteins nephrin and podocin.
This cytoskeletal arrangement is quite ladder-like in structure.
Describe an experiment that could be used to test which layer of the glomerular membrane acts a molecular sieve
Inject Ferritin into the blood plasma of the test animal.
You would leave the animal for a certian length of time to allow the ferritin to circulate within the blood before sacrificing the animal and disecting its kidneys.
You would then prepare the kidneys for microscopy and then view a section of the kidney under the microsccope.
Under microscope you would see that Ferritin molecules aren’t able to pass through basal lamina basal lamina acts as a molecular sieve.
