Week 3 Filtration

Question 1

How much blood is filtered within the glomerolus and where does the rest go?

Answer 1

20% of blood from renal artery filtered at any one time, the other 80% flows through efferent arteriole

Question 2

In the glomerolus what is filtered and what remains?

Answer 2

Water, salts and small molecules pass through, cells and large proteins don’t get filtered- filtrate is identical to plasma but without large proteins and cells

• 100mg/100ml glucose in plasma and ultra filtrate
• 140 Na+ mmol/L in both
• 15mg/dl urea in both
• 60-120umol/L creatinine in both

Question 3

What 3 layers make up the filtration barrier?

Answer 3

1. Capillary endothelium permeable- filtrate a move between cells
2. Basement membrane- permeable to small proteins, glycoproteins (-charge) repel protein movement
3. Podocytes layer- filtration slits- define size of particle that can pass

Question 3

How does disease often affect the filtration barrier?

Answer 3

Removed negative charge allowing proteins to pass through the barrier more readily- proteinurea

Question 4

What are the 3 forces involved in the filtering of plasma to ultra filtrate and how is the net filtration pressure calculated?

Answer 4

Hydrostatic pressure in capillary (A 50mmHg)- regulated
Hydrostatic pressure in bowmans capsule (B 15mmHg)
Oncotic pressure difference between capillary and tubular lumen (C 25mmHg)

Net filtration pressure = A -(B+C) = 10mmHg

Question 5

How does the diameter of the afferent and efferent arterioles affect filtration?

Answer 5

As afferent arteriole bigger than efferent it means as the blood arrives in the tuft there is a build of pressure which drives things through into the ultra filtrate

Question 6

How does the oncotic pressure difference arise?

Answer 6

As proteins cannot pass through barrier there are many left in the afferent arteriole which tries to draw back water

Question 7

What autoregulation mechanisms are in place to keep GFR Within normal limits when blood pressure changes?

Answer 7

Increase in BP- afferent arteriole constricts- GFR unchanged
Decrease in BP- afferent arteriole dilate- GFR unchanged
- only when BP within physiological limits 80-180mmHg

Question 8

Describe myogenic response involved in autoregulation?

Answer 8

Smooth muscle within arterioles don’t like to be stretched
-if you try and force mor fluid through them they will constrict
-if you don’t stretch them they will dilate in response to fall in pressure
Mainly affects afferent arteriole, if affects efferent will affect GFR

Question 9

Describe TG feedback involved in autoregulation?

Answer 9

Tubular glomerulus feedback between the afferent and efferent arteriole
- macula densa cells detect changes in Na and Cl in the distal tubule and alter afferent arteriole tone by stimulating juxtamedulllary apparatus to release chemical

If arterial pressure increases= increases glomerular capillary pressure= increased GFR= increased Na and Cl in DT- sensed by MD cells and adenosine released= vasoconstriction of AA
In pressure drops and Na drops= prostaglandins releases= vasodilation of AA

Question 10

Describe reabsorption in the PCT

Answer 10

The bulk transport occurs here
reabsorption is isosmotic
driven by Na uptake

Question 11

What are some differences between cortical and juxtamedulllary nephrons?

Answer 11

Cortical 90%;

• small glomerolus in cortex,
• loose arrangement of peritubular capillaries,
• shorter loop of henle
• afferent arteriole bigger than efferent

Juxtamedulllary 10%;

• big glomerolus close to medulla,
• capillaries parallel to loop (vasa recta),
• longer loop of henle deeper into medulla
• afferent and efferent arteriole same size

Question 12

describe Na reabsorption and how this drives the absorption of other things

Answer 12

• Na pumped out across baso-lateral membrane (into blood) by the 3Na/2K ATPase
• Na then moves across apical membrane from the tubule lumen down its conc grad
• energy generated by ATPase drives secondary transport of glucose
• H2O follows Na down osmotic gradient

Question 13

what is the difference between antiporters and symporters?

Answer 13

antiporters- transport involved in secondary AT of 2 different ions across the plasma membrane in OPPOSITE directions
symporters-transport involved in secondary AT of 2 different ions across plasma membrane in SAME direction

Question 14

What is tubular reabsorption?

Answer 14

Reabsorption of Solutes and water into the blood from the tubule

Question 15

why is the process of secretion necessary and what main substances are secreted?

Answer 15

as only 20% of plasma filtered each time passes through kidneys allows second route of entry for solutes that need to be secreted

• protons
• potassium
• organic anions (-ve charged ion) and cations (+ve charged ion)

Question 16

how are cations and anions secreted in PCT?

Answer 16

enter by passive carrier mediated diffusion (uniporter) across basolateral membrane down favourable conc and electrical gradient set up by 2Na-3K ATPase pump
- Na moved into tubular cell down conc grad in exchange for H
- H moved back in in exchange for OC+ (organic cation) (antiporter)
SAME for anion but with anion channel
- DRUGS are cations and need to be aware kidneys are removing them

Question 17

name some different Na transporters present within different segments of the tubule?

Answer 17

PCT- NA-H transporter, Na-Glucose symporter
Loop of H- Na-K-2Cl symporter
early DCT- Na-Cl symporter
late DCT and CD- EnaC

Question 18

What if GFR and how is it calculated?

Answer 18

GFR represents how well the kidneys are functioning specifically how much blood passes through the glomerulus each minute
- need to be able to measure a substance as it is filtered to assess function of kidneys- substance must not be altered as passes

Question 19

what are normal GFRs?

Answer 19

male- 112-125ml/min

female- 90-100ml/min

Question 20

how is renal plasma flow (RPF) calculated?

Answer 20

renal blood flow= 1.1L/min
haematocrit (vol of RBCs in blood) usually 0.45
RPF= 1.1 x 0.55 = 605ml/min of plasma

Question 21

how do you calculate the filtration fraction?

Answer 21

as 605ml of plasma enters glomerulus and 20% is filtered 125ml is filtered through bowmans capsule (GFR)
FF = GFR/RPF = 20%

Question 22

What is renal clearance and how is it calculated?

Answer 22

renal clearance of any substance is the volume of plasma that is completely cleaned of the substance by the kidneys per unit tine (min)- indicated how well kidneys working

conc. of it in urine (mg/ml) x urine volume (ml/min) / conc of it in plasma
- used to determine GFR and detects glomerulus damage

Question 23

what characteristics must a substance have to be used to calculate GFR and give an example of one?

Answer 23

must be freely filtered, not reabsorbed and not secreted so its renal clearance = GFR
creatinine and inulin

Question 24

what is filtered load?

Answer 24

normal glucose plasma conc is 100mg/100ml or 1mg/ml
freely filtered and ultra filtrate has same conc
-GFR - 125ml/mins so filtered load will be 1mg/ml x 125ml/min = 125mg/min
- if plasma conc increases to 200mg/100ml or 2mg/ml filtered load would be 250mg/ml

Question 25

what is transport max (Tm)?

Answer 25

maximum plasma conc of a substance before it starts spilling into urine