Filtration

Question 1

What is filtration?

Answer 1

1.Filtration: the formation at the glomerular capillaries of an essentially protein-free filtrate of plasma, (~20%)

Question 2

What is average GFR?

Answer 2

180 litres per day

• Allows regulation of ECF volume and control of blood composition (eliminating nasty substances)

Question 3

What is reabsorption?

Answer 3

Reabsorption: Substances that the body wants are reabsorbed, those it doesn’t want stay in the tubule and are excreted.

Question 4

What is secretion?

Answer 4

Secretion: Substances may be specifically removed from the body in this way.

Some drugs are eliminated this way

Proton excretion is important in the regulation of acid base balance

Question 5

How much blood flow does the kidney receive?

Answer 5

Kidneys receive a blood flow of » 1200mls/min, ie 20-25% of total cardiac output

Remarkable since the kidneys weigh less than 1% of the body weight

Question 6

How long would it take for the entire blood volume to pass through the kidney?

Answer 6

less than 5 mins

Question 7

Are red blood cells and blood plasma filtered into the bowmans capsule?

Answer 7

None of the red cells and only a fraction of the plasma is filtered through into Bowman’s capsule.

The remainder passes via the efferent arterioles into the peritubular capillaries and then to the renal vein.

Question 8

How much of renal plasma becomes glomerular filtrate?

Answer 8

Plasma constitutes » 55% of total BV.

55% of 1200mls/min = 660mls/min = renal plasma flow

GFR is normally 125mls/min

Therefore the filtration fraction = 125/660 x100 = 19% ie 19% of the renal plasma becomes glomerular filtrate.

Question 9

What forces govern filtration?

Answer 9

Glomerular Filtration occurs in exactly the same way as fluid filters out of any capillary in the body.

It is dependent on the balance between the hydrostatic forces favouring filtration and the oncotic pressure forces favouring reabsorption. (Starling’s forces).

Question 10

What determines the filterability of solutes across te glomerular filtration barrier?

Answer 10

Molecular size

Electrical charge

Shape

Question 11

Describe the structure of the glomerular endothelial cell

Answer 11

Fenestrated - prevents filtration of blood cells but allows all components of blood plasma to pass through

Question 12

Describe the function of the basal lamina of the glomerulus

Answer 12

Prevents the filtration of larger proteins

Question 13

Descrbe the function of the slit membrane between the pedicels

Answer 13

Prevents filtration of medium sized proteins

Question 14

Why is the glomerular capillary pressure high?

Answer 14

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

In addition, the unique arrangement of the efferent arteriole which is long and narrow offers a high post-capillary resistance.

Golden rule of the circulation:

If you have a high resistance, hydrostatic pressure upstream is increased, while the pressure downstream is decreased.

Question 15

Describe the difference in pressure between hydrostatic pressure and the oncotic pressure in the glomerular capillaries

Answer 15

At the glomerular capillaries the hydrostatic P favouring filtration always exceeds the oncotic P.

Question 16

How do we calculate net filtration pressure?

Answer 16

Hydrostatic pressure minus the colloid osmotic pressure plus the fluid pressure creasted by fluid in the bowmans capsule

Question 17

What is the number 1 factor affecting GFR?

Answer 17

In normal physiology, 1° factor is PGC and this is dependent on the afferent and efferent arteriolar diameter and therefore the balance of resistance between them.

Question 18

What extrinsically controles the GFR?

Answer 18

a) Sympathetic VC nerves - afferent and efferent constriction, greater sensitivity of afferent arteriole.
b) Circulating catecholamines - constriction 1°ily afferent
c) Angiotensin II - constriction, of efferent at [low], both afferent and efferent at [high].

Question 19

What is the affect of increasing the resistance of the afferent and efferent arteriole?

Answer 19

Question 20

What does the renal vasculature adjust its resistance in relation to?

Answer 20

Blood pressure

This keeps BF and GFR essentially constant = autoregulation.

Question 21

What is the effect on GFR of an increasing blood pressure and a decreasing blood pressure outwith the normal ranges?

Answer 21

Below 60mmHg, filtration falls and ceases altogether when MBP = 50mmHg.

If mean arterial P increases ­, there is an automatic ­ increase in afferent arteriolar constriction, preventing a rise in glomerular PGC . Dilatation occurs if P falls.

Question 22

What is autoregulation dependant on?

Answer 22

Autoregulation is independent of nerves or hormones, occurs in denervated and in isolated perfused kidneys.

Question 23

What is the autoregulatory range?

Answer 23

In man autoregulatory range is 60-130mmHg,

Question 24

What is the response in the kidneys if there is a haemorrhage?

Answer 24

In situations where blood volume/BP face serious compromise, eg in haemorrhage, activation of sympathetic VC nerves and AII, can override autoregulation liberating blood for more immediately important organs.

As much as 800mls of blood per minute can thus be provided to perfuse these other organs at the expense of the kidney.

Question 25

What is the effect of prolonged reduction in renal blood flow?

Answer 25

Can lead to irreparable damage which may then lead to death because of disruption of the kidney’s role in homeostasis.

Question 26

What percentage of blood volume is filtered, reabsorbed and excreted into the external environment?

Answer 26

Question 27

Why is there low blood pressure in the peritubular capillaries?

Answer 27

Hydrostatic pressure overcoming frictional resistance in efferent arterioles

Question 28

What is the oncotic pressure like in the peritubular capilaries?

Answer 28

High compared to normal because there is a loss of 20% of plasma

Question 29

Why do the peritubular capillaries favour reabsorption?

Answer 29

As a consequence, the net result of the low P_PC and the high Oncotic p is that the balance of Starling’s forces in the peritubular capillaries is entirely in favour of reabsorption.

Question 30

What things are reabsorbed and where are they reabsorbed?

Answer 30

99% H2O, 100% glucose, 99.5% Na+, 50% urea filtered at the glomerulus are reabsorbed within the tubule, mainly at the proximal convoluted tubule.