Physiology 1 - glomerular Filtration

Question 1

What is the normal GFR?

Answer 1

~180l/day

100-120ml/min/1.73m^2

of which only ~2l ends up in urine so almost all of it is reabsorped too

Question 2

What is the plasma filtration fraction at the glomerulus?>

Answer 2

1200ml/min of blood passes through
Of which 55% = 660ml/min is plasma
GFR of plasma is 125ml/min

Therefore 125/660 = ~19%

Question 3

Explain the forces behind filtration?

Answer 3

Hydrostatic pressure favours filtration
Oncotic pressure favours reabsorption back into glomerular capillaries

For filtration to work capillary hydrostatic pressure (PGC- Glomerular Capillary Pressure) must > fluid oncotic pressure

Question 4

What factors influence filtration rate of a specific particle?

Answer 4

Size
Charge
Shape

Small particles like Na+/K+/Water are almost completely filtered out.

Question 5

Describe the physical structure of the capillary walls that allows filtration?

Answer 5

1) Fenestrated endothelial cells allow plasma to pass but not RBC
2) Then basal lamina prevents large proteins passing through
3) Then slit membrane between pedicals under the basal lamina prevent medium proteins passing

Question 6

How is a high capillary Hydrostatic pressure maintained?

Answer 6

Afferent arterioles are wide and short to put up little resistance
Efferent arterioles are long and narrow to put up high resistance

Question 7

Describe extrinsic control of Glomerular Capillary Pressure

Answer 7

Sympathetic vasoconstrictive effect (afferent arteriole)
Circulating catecholamines (afferent arteriole)
Angiotensin II (efferent arteriole)

Question 8

How do circulating catecholamines affect glomerular Capillary Pressure (Pgc)

Answer 8

Cause vasoconstriction primarily in the afferent arteriole. Thus lowering renal blood flow and Pgc to provide more blood to other tissues e.g. heart and muscle

Question 9

How does angiotensin II affect Pgc?

Answer 9

Causes constriction of the efferent at low conc and both efferent/afferent at high concs

Question 10

Renal vessels also have an intrinsic ability to control Pgc. whats the purpose of this?

Answer 10

To allow the vessels to react to changes in MAP and so maintain Pgc
Aka autoregulation of the GFR

Question 11

How do renal vessels autoregulate GFR?

Answer 11

The afferent arterioles constrict and dilate in response to changes in blood volume in order to main Pgc.

Question 12

When would autoregulation of the GFR be overridden?

Answer 12

When blood volume faces serious problems such as a large haemorrhage in order to liberate blood for more immediately essential tissues

(Symp nerves and angiotensin II)

Question 13

What controls autoregulation of the GFR?

Answer 13

The renal vessels themselves.

Its independant of nerves and hormones, occuring even in denervated and isolated kidneys

Question 14

How are the peritubular capillaries capable of resorping almost 180l a day?

Answer 14

1) Extremely low hydrostatic pressure thanks to the high resistance efferent arteriole upstream
2) High oncotic pressure in peritubular capillaries thanks to losing 20% of the plasma but still containing 100% of normal blood protein

Net starling forces are much more in favour of reabsorption leading to reabsorption of 50% of urea, 99% H2O, 100% Glc and 99.5% of Na.