lecture 32

Question 1

what is filtration?

Answer 1

• Type of ultrafiltration where blood is filtered across a membrane
• It creates a plasma-like filtrate of the blood
• not very selective

Question 2

what determines glomerular filtration?

Answer 2

• renal blood flow
• filtration barrier
• driving forces

Question 3

what is the renal blood flow?

Answer 3

• 20% of cardiac output
• 1100-1200mL blood/min
• has a high flow for filtration, rather than metabolism

Question 4

what happens at the filtration barrier?

Answer 4

• 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

Question 5

what are positive and negative pressure?

Answer 5

• positive pressure favours filtration
• negative pressure opposes filtration

Question 6

what are hydrostatic and colloid osmotic pressure?

Answer 6

hydrostatic
- pressure due to the volume of fluid. more fluid = more hydrostatic pressure

colloid osmotic
- osmotic pressure due to proteins

Question 7

what are the 4 driving forces?

Answer 7

• Glomerular Hydrostatic pressure
• Blood colloid osmotic pressure
• capsular hydrostatic pressure
• capsular colloid osmotic pressure

Question 8

what is glomerular hydrostatic pressure?

Answer 8

blood pressure - +50mmHg

Question 9

what is blood colloid osmotic pressure?

Answer 9

proteins pulling towards itself since they cant be filtered e.g. albumin (-25mmHg)

Question 10

what is capsular hydrostatic pressure?

Answer 10

pressure of the filtrate already present (-15mmHg)

Question 11

what is capsular colloid osmotic pressure?

Answer 11

no protein in capsular space (+0mmHg)

Question 12

what do the 4 pressures determine?

Answer 12

Net filtration pressure
(+50)+(-25)+(-15)+(0) = 10mmHg

Question 13

what is renal blood flow?

Answer 13

20% of cardiac output per minute
- 1100-1200 mL blood/min

Question 14

what is renal plasma flow?

Answer 14

55% of blood is plasma
renal plasma flow = 625mL plasma/min

Question 15

what is glomerular filtration rate?

Answer 15

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

Question 16

how do you calculate filtration fraction?

Answer 16

how much of the kidneys blood (plasma) flow is filtered

• GFR/RPF
  = 125/625
  = 20%

Question 17

define glomerular filtration rate?

Answer 17

how much plasma is filtered per minute

• normall 125ml/min or 180L/day
• only 1.5L of urine is produced though

Question 18

what are the characteristics of glomerular filtration rate?

Answer 18

• tightly regulated
• variation from person to person
• declines slowly from age 30

Question 19

define filtered load

Answer 19

how much of a substance is filtered per minute

Question 20

what is renal clearance of creatinine and inulin?

Answer 20

125ml/min (same as GFR)

Question 21

how do you calculate filtered load?

Answer 21

GFR x concentration of solute in the plasma

Question 22

define renal clearance

Answer 22

the volume of plasma that is cleared of a substance by the kidneys per unit time

Question 23

what can clearance be used for?

Answer 23

• quantify how a substance is handled by the kidneys
• estimate glomerular filtration rate

Question 24

how do you calculate clearance?

Answer 24

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

Question 25

what does the equation for clearance describe?

Answer 25

the clearance for all substances that can be detected in plasma and urine

Question 26

how can another substance be used to measure GFR?

Answer 26

to measure GFR, a substance must:
- be freely filtered
- not be reabsorbed from the tubule
- not be secreted into the tubule

Question 27

which substances meet the criteria to be used to measure GFR?

Answer 27

• 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

Question 28

how does creatinine meet the criteria for being used to measure GFR?

Answer 28

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

Question 29

what is the calculation of renal clearance used to measure GFR?

Answer 29

C = [creatinine]U x V/ [creatinine]P = estimated GFR

Question 30

how is plasma creatinine concentration involved in kidney function?

Answer 30

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

Question 31

what happens when GFR is less than 25ml/min?

Answer 31

plasma creatinine concentration increases as the kidneys ability to clear waste products from blood is reduced

Question 32

what is renal clearance of sodium?

Answer 32

less than 1ml/min

Question 33

what is renal clearance of glucose?

Answer 33

0ml/min

Question 34

what is PAH renal clearance?

Answer 34

625ml/min (same as renal plasma flow)