Glomerular filtration Flashcards
What is a nephron?
A functional unit of the kidney which regulates the concentration of water and solutes through filtration, reabsorption and secretion.
What is a nephron composed of?
The glomerulus surrounded by Bowman’s capsule and its tubular system to the renal pelvis.
What is the RPF received by a kidney?
600ml.min of the 1.1L.min of blood passing through
How much plasma is filtered by both kidneys in total?
120ml.min
180L.day
How does the plasma become filtrate in the nephron and what forces are contributing?
The filtrate forms within Bowman’s space, separated from plasma by the endothelial cells of the capillaries and their BM. Ions move between the plasma and filtrate at equilibrium so only oncotic has an effect but as hydrostatic pressure is higher in the capillaries than oncotic, fluid moves into Bowman’s space.
What is the osmolarity of the filtrate as it enters the tubule?
285osm.L as equal to plasma
Which two forces drive filtration?
Hydrostatic - Higher in capillaries so fluid moves out
Oncotic - Higher in capillaries as the proteins are impermeable. As more filtrate moves out along the length of the capillaries the flow of filtrate reduces as the oncotic force increases.
How can the volume of filtrate entering be altered?
Increasing glomerular hydrostatic pressure will increase the filtrate. This can be achieved by dilating afferent capillary or constricting efferent.
Where does the efferent capillary travel and what forces play a role?
Leaves the glomerulus to enter the portal vein and form a second capillary bed that surrounds the loop of Henle. As the capillary has a low hydrostatic but a high oncotic pressure it can recieve the reabsorbed products from the tubules.
Define net perfusion pressure.
Glomerulus hydrostatic pressure - (Bowman’s space hydrostatic pressure + colloid)
Define net hydrostatic pressure.
Glom. hydrostatic - Bowman’s space hydrostatic
What forms the diffusion barrier of the glomerulus?
Endothelial cells
BM (collagen = -ve charge)
Podocytes
What are podocytes?
Cells of the Bowman’s capsule that have small processes that surround a capillary loop to produce filtration slits. They have a -ve charge on glycocalyx to prevent entry of proteins.
Which molecules can pass through the filtration barrier and which cannot?
Cationic (+) and neutral molecules pass through and are at EQUILIBRIUM to the plasma. All small molecules will filter e.g. Na, K, Cl, Hco3, glucose, urea
Anions (-) do not pass due to the -vely charged membrane e.g. albumin
What is the glomerular filtration rate?
The amount of blood passing through the glomeruli for filtartion per minute. = 120ml.min (total of both)
What does GFR show?
Kidney function
What is the snGFR?
Single nephron GFR: 1 million nephrons per kidney = 30-50nl.min
What is the filtration fraction?
FF = GFR/RPF
The proportion of fluid entering the kidney which passes into the tubules = 20%
How can GFR be estimated? Why is the substance chosen?
GFR = ConcCr.U x V / ConcCr.P
Uses Creatinine as it is freely filtered with no absorption or secretion so rate of excretion is equal to rate of filtration.
What is required to calculate GFR?
24hr urine collection and a blood sample
What is renal clearance?
The volume of plasma completely cleared of a substance in one minute.
How can renal clearance be used to determine the nature of a substance?
If clearance < GFR = reabsorbed e.g. glucose
If clearance > GFR = secreted by tubules
If clearance = GFR it is freely filtered with no absorption or secretion
What is the relationship between GFR and ConcCr.P?
Inversely proportional as the production is equal to loss.
Why can creatinine be used to calculate eGFR?
Cr is produced at a constant rate correlated to sex, mass and age.
How is eGFR calculated?
eGFR = (140-age) x mass x c / Ccr.p c = 1.23 men and 1.04 women
What is a normal eGFR?
90-120ml.min
<60ml.min shows kidney damage
Why is life limited in the absence of other causes of death?
Both renal function and Cr production fall with age
What is proteinuria and why does it occur?
Excess protein in urine due to glomerular dysfunction.
What renal condition is proteinuria a sign for?
Glomerulonephritis - A group a diseases that cause renal damage and dysfunction due to inflammation of the glomeruli or vessels.
What other signs indicates Glomerulonephritis?
Nephrotic syndrome - Inflammation affecting the podocytes increases protein permeability of nephron causing oedema and protein loss through urine. Leads to renal failure and necrosis.
Nephritic syndrome - Blood in urine and decreased volume of urine as the inflammation damages the filtration barrier.
