Filtration

Question 1

What are the 3 basic renal processes?

Answer 1

Filtration
Reabsorption
Secretion

Question 2

What is filtration?

Answer 2

Formation at glomerular capillaries of an essentially protein free filtrate of plasma
Glomerular filtration rate is very high - 180l/day so kidneys regulate ECF volume and composition

Question 3

Explain kidney blood flow

Answer 3

Receive 1200mls/min so 20-25% of total CO
Receive such high blood flow so can be vulnerable to damage by vascular disease

Question 4

Where is plasma filtered?

Answer 4

No red blood cells and only a fraction of plasma is filtered through Bowman’s capsule
Remainder passes via efferent arterioles into peritubular capillaries then to renal vein

Question 5

What is the renal plasma flow?

Answer 5

660mls/min
As 55% of total BV (1200mls/min)

Question 6

What is the normal GFR?

Answer 6

120mls/min

Question 7

What is the filtration fraction?

Answer 7

125/660mls/min x 100 = 19%
So 19% of renal plasma becomes glomerular filtrate

Question 8

What is glomerular filtration dependant on the balance between?

Answer 8

Hydrostatic forces favouring filtration and oncotic pressure forces favouring reabsorption (Starling’s forces)

Question 9

What contributes to the perm-selectivity of the glomerular barrier?

Answer 9

Molecular size, electrical charge and shape determine filterability

Question 10

What are the 3 layers (filters) of the Bowman’s capsule?

Answer 10

1 - fenestration of glomerular endothelium (prevents blood cells)
2 - basal lamina of glomerulus (prevents large molecules)
3 - slit membrane between pedicles (prevents medium sized molecules)

Question 11

Why is the Glomerular capillary pressure (Pgc) higher than in most capillaries in the body?

Answer 11

Because the afferent arteriole is short and wide so offers little resistance to flow
So blood arriving to glomerulus still has a high hydrostatic pressure

Question 12

What is the unique arrangement of the efferent arteriole?

Answer 12

Long and narrow so offers a high post-capillary resistance

Question 13

What is the golden rule of the circulation?

Answer 13

If you have high resistance then hydrostatic pressure upstream is increased and pressure downstream is decreased - so both contribute to high Pgc

Question 14

What is pressure is higher in the glomerular capillaries?

Answer 14

Hydrostatic pressure favours filtration which exceed oncotic pressure

Question 15

What only occurs in glomerulus capillaries?

Answer 15

Filtration

Question 16

Explain the net filtration pressure

Answer 16

Ph - 55mmHg
Oncotic - 30 mmHg
P fluid - 15mmHg
55 - 30 -15 = 10mmHg which drives filtration

Question 17

What is the primary factor affecting GFR in normal physiology?

Answer 17

Afferent and efferent arteriolar diameters and therefore the balance of resistance between them

Question 18

What extrinsic control affects GFR?

Answer 18

Sympathetic VC nerves
Circulating catecholamines
Angiotensin II

Question 19

How does sympathetic VC nerves affect GFR?

Answer 19

Afferent and efferent constriction and greater sensitivity if afferent arteriole

Question 20

How does circulating catecholamines affect GFR?

Answer 20

Constriction primarily in afferent arterioles

Question 21

How does angiotensin II affect GFR?

Answer 21

Constriction of efferent at low levels and both afferent and efferent at high levels

Question 22

What happens if there is increased resistance in afferent arteriole?

Answer 22

Increased blood flow to other organs
Decreased GFR as decreased capillary BP

Question 23

What happens if there is increased resistance in efferent arteriole?

Answer 23

Increased GFR as increased Ph
Decreased RBF

Question 24

What is the intrinsic affect on GFR?

Answer 24

Adjusts resistance in response to changes in arterial BP
Keeps BF and GFR essentially constant - autoregulation

Question 25

What is the effective range of MBP?

Answer 25

60-130mmHg

Question 26

What happens if MBP falls below or increases?

Answer 26

Falls - filtration falls and ceases all together. Dilation occurs
Increase - increase in afferent arteriolar constriction so prevents rise in Pgc

Question 27

What is autoregulation independent of?

Answer 27

Nerves or hormones
Occurs in denerved and in isolated perfused kidneys

Question 28

What happens if blood volume/BP face serious compromise?

Answer 28

Activation of sympathetic VC nerves and AII
Can overrise autoregulation liberating blood for more important organs
800mls of blood can be given to perfuse other organs in expense of kidney

Question 29

What does prolonged reduction in renal BF result in?

Answer 29

Irreparable damage which may lead to death because kidney’s role in homeostasis is disrupted

Question 30

Explain marathons and kidneys

Answer 30

Endurance training events means adaption of skeletal muscle so requirement of blood flow is more efficient so less need to take from kidneys

Question 31

How much volume filters at Bowman’s capsule?

Answer 31

20% and then more than 19% is reabsorbed
So only less than 1% of volume is excreted to external environment

Question 32

What are responsible for reabsorption?

Answer 32

Peritubular capillaries as 180l/day is filtered but only 1-2l/day is excreted as urine
Efferent arterioles have important effects on Starling’s forces

Question 33

Describe the pressure in efferent arterioles

Answer 33

Offers resistance along entire length - large drop in pressure so that hydrostatic pressure is very low - Ppc is 15mmHg
Oncotic pressure higher so favours reabsorption

Question 34

How is the plasma protein conc. higher in peritubular capillaries?

Answer 34

As 20% of plasma filtered into Bowman’s capsule
Blood remaining in efferent and peritubular capillaries has higher conc. so higher oncotic pressure

Question 35

What is reabsorbed in the proximal convoluted tubule?

Answer 35

99% H2O
100% glucose
99.5% Na
50% urea