Glomerular Filtration And Tubular Function

Front 1

Describe basic renal processes (6)

Back 1

1. Glom. filtration in Bowman's capsule
2. 70% filtrate reabsorbed in proximal tubule
3. High salt conc by active transport in Loop of Henlé
4. Secretion/reabsorption in distal tubule
5. control osmolarity of urine in collecting duct
6. Ureter

Front 2

describe glomerular filtration (4)

Back 2

1. each afferent arteriole divides to multiple capillary branches (tuft of vessels: large SA) forms 1 glomerulus

2. in Bowman's capsule (blind end/origin of nephron). Blood in glomerular capillaries and filtrate in Bowman's separated by 2 layers of epi cells, sharing a BM. BM thus is main barrier to filtration

3. Ultrafiltration: capillary pressure gradient to tubule forces water to filter from blood through membrane into lumen of Bowman's. Soluble substances also enter tubule depending on size and charge.

4. Glycoproteins on surface of glom. capillary cells negatively charged.

Front 3

to what extent may substances enter tubule with the water in glomerular filtration? (2)

Back 3

1. SIZE (ø)
< 4 nm (enter freely)
4-8 nm (partial entry)
> 8 nm (excluded)

2. CHARGE
~ -ve charged filter < half rate of neutral substances
~ +ve charged filter slightly > rate than neutral substances

∴ albumin (-ve, ø 7nm) doesn't filter through 8 nm pore

Front 4

what is the net effect of glomerular filtration

Back 4

1. water filtered, 125 mL/min or 180 L
2. cells completely retained
3. plasma proteins completely retained
4. small molecules and ions enter freely
~ Na+ radius 0.1 nm
~ Cl- radius 0.2 nm
~ glucose radius 0.3 nm

5. charge has no effect on very small ions

Front 5

why may there be increased glomerular permeability? (2)

Back 5

1. physical enlargement of pore

2. loss of -ve charged glycoprotein from surface of capillary cells e.g. nephritis [hence albuminuria]

Front 6

how to control glomerular filtration? (3)

Back 6

balance between
1. forces pushing fluid out of capillary
2. forces drawing fluid into capillary
3. filtration constant

too high: excessive loss of water and solute
too low: inadequate excretion of waste products

Front 7

how to calculate filtration (hence GFR?)

Back 7

Filtration = K [(Pgc - Pbc) - (πgc - πbc)]

1. K: filtration coefficient (permeability x SA)
2. Pgc: glomerular capillary blood pressure (constant along the whole capillary)
3. Pbc: Bowman's capsule fluid pressure
4. πgc: glomerular capillary oncotic pressure (rises along the capillary)
5. πbc: Bowman's capsule oncotic pressure

net filtration P = 15 mmHg at arterial end, may reach 0 mmHg at venous end

Front 8

what forces favour filtration? (2)

Back 8

1. Glomerular capillary blood pressure
2. Bowman's capsule Oncotic pressure

Front 9

what forces oppose filtration?

Back 9

1. glomerular oncotic pressure
2. Bowman's capsule fluid pressure

Front 10

How to change GFR?
1. a, b, c
2. a, b
3. a, b

Back 10

1. Change Pgc
a. constrict/dilate afferent or efferent arteriole
b. constrict afferent arteriole ↑Raff, ↓Pgc, ↓GFR
c. constrict efferent arteriole ↑Reff, ↑Pgc, ↑ GFR
[if both aff & eff arteriole constrict, capillary pressure remains unchanged but flow decreases

2. Change plasma oncotic pressure
a. dehydration leads to ↑πc and ↓GFR
b. starvation leads to ↓πc and ↓GFR

3. Change in filtration coefficient
a. change SA for filtration (no. of perfused capillaries)
b. contract mesangial cells (reduce perfusion of cap.)

Front 11

How to regulate GFR?
(2 mechanisms)
1. (6)
2. (7)

Back 11

tubuloglomerular feedback: change in GFR sensed and Pgc adjusted to return GFR to previous value.
If renal blood flow changes a lot, GFR can't be kept constant
2 mechanisms

1. ↓ NaCl delivered to macula densa (NaCl arrives at macula densa in DCT at proportional rate to GFR)
2. ↓ NaCl uptake and ATP breakdown
3. ↓ adenosine
4. ↓ afferent arteriole vasoconstriction
5. ↑ Pgc
6. ↑ GFR

a. ↓ Na+ & Cl- at macula densa cells
b. renin secreted from JG cells
c. angiotensinogen → angiotensin I
d. angiotensin I → angiotensin II (with ACE)
e. ↑ efferent arteriole vasoconstriction
f. ↑ glomerular capillary pressure
g. ↑ increase GFR

Front 12

function of tubular cell (4)

Back 12

They may
1. reabsorb some or all of substance back to blood
2. secrete more of substance to filtrate
3. do both: reabsorb in one part of tubule, secrete in a different part.
4. active transport of charged substances by epithelial cells creates potential difference between tubule lumen and ECF

Front 13

what is reabsorbed in tubule?

Back 13

~179/180 L of filtered water reabsorbed
~concentration of substances in filtrate ↑ as water removed (provides conc. gradient for passive reabsorption processes)

Front 14

what are tubule walls made of?

Back 14

single layer of tight epithelial cells,
~ providing minimal barrier to transport
~ controlling transport processes.

Front 15

transport mechanisms of renal tubules

Back 15

1. simple diffusion
~ driving force is concentration gradient
~ no carriers required

2. facilitated diffusion
~ driving force is concentration gradient
~ carriers required

3. primary active transport
~ uses ATP to transport substance
~ carrier pump required

4. secondary active transport
~ carrier transports a substance, plus another substance down conc. gradient
~ energy sets up conc. gradient for 1 substance

Front 16

PCT transport mechanisms

Back 16

iso-osmotic reabsorption of 70% (65-85%) filtered fluid
~70% filtered water
~70% Na+, Cl-, HCO3(-)
~all filtered K+, HPO4(2-)

Front 17

for reabsorption processes, state how it is transported at the basolateral border and luminal border:
1. Na+ reabsorption
2. K+ reabsorption
3. Cl-, glucose, amino acid reabsorption
4. Water reabsorption

Back 17

1. (B) active transport, Na+/K+ pump
(L) facilitated diffusion: subject to tubular maximum (transport maximum: if all available carriers saturated no further in ↑ in reabsorption occurs)
affects susbstances w. relatively few transporters: Na/glucose cotransporter

2. (B) passive diffusion through K+ channels
(L) active transport

3. (B) diffusion, Na+/K+ pump for Na
(L) co-transport with Na+ (sec. active transport)

4. (B) osmosis
(L) osmosis

Front 18

what happens in the loop of henle (2)

Back 18

1. salt transported out of loop to interstitial space

2. fluid leaving loop and entering DCT is hypotonic to plasma

Front 19

what are the DCT transport mechanisms? (3)

Back 19

1) 5% water reabsorbed
2) further Na+ reabsorption, K+ secretion
3) Aldosterone modifies extent of salt secretion/reabsorption

Front 20

for reabsorption processes, state how it is transported at the basolateral border and luminal border:
1. Na+ reabsorption
2. K+ secretion
3. K+ reabsorption
4. Cl- reabsorption

Back 20

1. (b) Na+/K+ pump if aldosterone present
(l) facilitated diffusion, pump pumps out Na+ which then enters at luminar border

2. (b) Na+/K+ pump if aldosterone present
(l) diffusion~passes through channels

3. (b) diffusion through basolateral channels to be reabsorbed.
(l) active transport if no aldosterone

4. (b) diffusion
(l) co-transport with Na+

Front 21

what happens in the collecting duct (3)

Back 21

1. little more Na reabsorption in uppermost part of collecting duct

2. Na crosses apical membrane using mainly epithelial Na channel (contrast to few of these channels in PCT/DCT: those were predominantly NaCl cotransporter)

3. water reabsoprtion under hormonal control

Front 22

what is the distribution for Na reabsorption in the kidney?

Back 22

1. 67% from proximal tubule
2. 25% from loop of Henle
3. 5% from distal tubule
4. 3% from cortical collecting duct

proportions depending on hormones present