Glomerular filtration rate Flashcards
Define glomerular filtration?
Process by which water and some solutes pass from the glomerular capillary to the bowman capsule
Give example of molecules which can’t fit through the podocyte slits?
- Albumin
- WBCs
- RBCs
What can freely flow through the the podocyte epithelium?
- Plasma protein (smaller ones)
- H2O
- Glucose
- electrolytes
- Amino Acids
- Fatty-acids
- Nitrogenous waste
When may Proteinuria or hematuria occur?
- caused by glomerular damage
- e.g. in Diabetes or hypertension or infection
When might you see leukocytes in the urine?
In UTIs
What forces will push fluid out of the capillaries?
- Blood flow
- Blood pressure
What forces will push fluid into the capillaries?
- Protein and salt sitting in the blood vessels which create concentration gradient (colloid osmotic pressure)
- fluid sucked back in
Where in the capillary will pressure be forcing fluid in and out?
Arterial forcing fluid out by hydrostatic pressure
Venous forcing fluid in due to decrease in hydrostatic pressure but no drop in colloid osmotic pressure
How do you calculate Net filtration pressure?
Hydrostatic pressure = HP Colloid osmotic pressure = Pi
Capillary = c
Interstital fluid = if
(HPc-HPif) - (Pi c - Pi if)
What affects hydrostatic pressure in the glomerulus?
- Arterial pressure
- Higher arteriolar resistance in efferent than afferent
What affects hydrostatic pressure in bowmans capsule?
- pressure of filtrate in the capsule, higher in bowman than glomerulus
How do you calculate net filtration pressure in the glomerulus?
(HPgc - HPbc) - (Pi gc - Pi bc)
Overall fluid forced out into bowman capsule
What determines the colloid osmotic pressure in bowmans and glomerulus?
Proteins in plasma and filtrate
more proteins in plasma than filtrate
What is GFR determined by?
- Net filtration pressure
- Ease of fluid to move through slits (Kf - constant of hydraulic conductivity and surface area of capillaries)
What unit is GFR expressed in?
mL/min or mL/min/1.73 m(squared)
Define glomerular filtration rate?
Clearance rate of a substance from the blood into the urine when any substance is freely filtered and is neither reabsorbed nor secreted by the kidneys
How can GFR be calculated?
(Urine concentration x Urine flow) / Plasma concentration
What substances can be used to work out GFR?
1) Inulin - given as continuous IV infusion, multiple blood and urine samples over 24h (invasive and can be inaccurate if incomplete bladder voiding)
2) Creatinine - clearance based on serum creatinine level used to estimate GFR (tends to overestimate due to peritubular secretion of creatinine)
How is creatinine naturally produced in the body?
Breakdown of creatinine phosphate in the muscles
How can renal function be estimated without blood tests?
Test urine for creatinine clearance then estimate plasma creatinine from this and so renal function
What lifestyle factors are also accounted for when estimating GFR?
Age, sex, weight and body SA
What factors may affect GFR?
- Kidney disease (reduced GFR)
- Hypertension (damage to glomeruli reduces GFR)
- Sleep (due to hormone and oncotic variation)
- Exercise (BF diverted to muscles)
- Renal BF - in renal stenosis less BF to kidney so GFR lower
What substance is used to measure Renal Blood Flow?
- Para-aminohippuric acid
- almost all extracted in one circuit and doesn’t affect kidney function
- PAH only in plasma so doesn’t directly measure renal BF
Why is it essential renal BF is maintained?
Need constant BF to keep constant GFR
