lecture 32 Flashcards
what is filtration?
- Type of ultrafiltration where blood is filtered across a membrane
- It creates a plasma-like filtrate of the blood
- not very selective
what determines glomerular filtration?
- renal blood flow
- filtration barrier
- driving forces
what is the renal blood flow?
- 20% of cardiac output
- 1100-1200mL blood/min
- has a high flow for filtration, rather than metabolism
what happens at the filtration barrier?
- blood enters to glomerulus via afferent arterioles and gets filtered through the membrane into the capsular space
- small substances are freely filtered
- large substances are not filtered
what are positive and negative pressure?
- positive pressure favours filtration
- negative pressure opposes filtration
what are hydrostatic and colloid osmotic pressure?
hydrostatic
- pressure due to the volume of fluid. more fluid = more hydrostatic pressure
colloid osmotic
- osmotic pressure due to proteins
what are the 4 driving forces?
- Glomerular Hydrostatic pressure
- Blood colloid osmotic pressure
- capsular hydrostatic pressure
- capsular colloid osmotic pressure
what is glomerular hydrostatic pressure?
blood pressure - +50mmHg
what is blood colloid osmotic pressure?
proteins pulling towards itself since they cant be filtered e.g. albumin (-25mmHg)
what is capsular hydrostatic pressure?
pressure of the filtrate already present (-15mmHg)
what is capsular colloid osmotic pressure?
no protein in capsular space (+0mmHg)
what do the 4 pressures determine?
Net filtration pressure
(+50)+(-25)+(-15)+(0) = 10mmHg
what is renal blood flow?
20% of cardiac output per minute
- 1100-1200 mL blood/min
what is renal plasma flow?
55% of blood is plasma
renal plasma flow = 625mL plasma/min
what is glomerular filtration rate?
20% of renal plasma flow is filtered
80% remains in the glomerular capillaries and exits via efferent arterioles and goes to peritubular capillaries
so GFR = 125ml plasma/min
how do you calculate filtration fraction?
how much of the kidneys blood (plasma) flow is filtered
- GFR/RPF
= 125/625
= 20%
define glomerular filtration rate?
how much plasma is filtered per minute
- normall 125ml/min or 180L/day
- only 1.5L of urine is produced though
what are the characteristics of glomerular filtration rate?
- tightly regulated
- variation from person to person
- declines slowly from age 30
define filtered load
how much of a substance is filtered per minute
what is renal clearance of creatinine and inulin?
125ml/min (same as GFR)
how do you calculate filtered load?
GFR x concentration of solute in the plasma
define renal clearance
the volume of plasma that is cleared of a substance by the kidneys per unit time
what can clearance be used for?
- quantify how a substance is handled by the kidneys
- estimate glomerular filtration rate
how do you calculate clearance?
Cx = Ux * Vx/ Px
clearance = concentration of X in urine * volume of urine produced per unit time / concentration of X in plasma
Ux = concentration of X in urine
V = volume of urine produced per unit time
Px = concentration of X is plasma
what does the equation for clearance describe?
the clearance for all substances that can be detected in plasma and urine
how can another substance be used to measure GFR?
to measure GFR, a substance must:
- be freely filtered
- not be reabsorbed from the tubule
- not be secreted into the tubule
which substances meet the criteria to be used to measure GFR?
- inulin (not insulin)
polysaccharide, not metabolised by body. not found in body so must be injected - creatinine
waste product produced by muscles
already in body so most commonly use clinically
how does creatinine meet the criteria for being used to measure GFR?
creatinine is filtered freely at the glomerulus but is not reabsorbed or secreted so measuring the renal clearance of creatinine is ideal for clinically estimated GFR
what is the calculation of renal clearance used to measure GFR?
C = [creatinine]U x V/ [creatinine]P = estimated GFR
how is plasma creatinine concentration involved in kidney function?
plasma creatinine is an indicator of kidney function
- if both kidneys are working (GFR=125ml/im) plasma creatinine is low
- if one kidney is working (GFR=60ml/min) plasma creatinine is fairly normal
what happens when GFR is less than 25ml/min?
plasma creatinine concentration increases as the kidneys ability to clear waste products from blood is reduced
what is renal clearance of sodium?
less than 1ml/min
what is renal clearance of glucose?
0ml/min
what is PAH renal clearance?
625ml/min (same as renal plasma flow)
