Glomerular filtration

Question 1

Basic functions of the kidney (3)

Answer 1

Filtration
Re-absorption of substances the body wants
Secretion

Question 2

How much blood do the kidneys receive per min on average

Answer 2

1200mls/min

Question 3

What % of the total cardiac output do the kidneys receive? Why is this crucial in vascular disease?

Answer 3

20-25%

The fact that the kidneys normally receive such a high BF explains their greater vulnerability to damage by vascular disease.

Question 4

What is the renal plasma flow?

Answer 4

Plasma constitutes  55% of total BV.
55% of 1200mls/min = 660mls/min = renal plasma flow

Question 5

What is Glomerular filtration dependent on?

Answer 5

The balance between the hydrostatic forces favouring filtration and the oncotic pressure forces favouring reabsorption (Starling’s forces).

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

Question 6

Which things affect the filterability of solutes across the glomerular filtration barrier?

Answer 6

Molecular size

Electrical charge

Shape

Question 7

What prevents the filtration of larger proteins?

Answer 7

The Basal Lamina of the glomerulus

Question 8

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

Answer 8

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 (force it exerts on vessel wall)

Question 9

What is the golden rule of circulation?

Answer 9

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

Question 10

How does only filtration occur at the glomerular capillaries?

Answer 10

Both the afferent and efferent arterioles contribute to the very high glomerular capillary pressure.

Therefore at the glomerular capillaries the hydrostatic P favouring filtration always exceeds the oncotic P (brings water in).

So only filtration occurs at the glomerular capillaries

Question 11

What is the efferent arteriole like?

Answer 11

The efferent arteriole is long and narrow and offers a high post-capillary resistance.

Question 12

Factor affecting the GFR

Answer 12

Glomerular capillary pressure

Question 13

What is the Glomerular capillary pressure dependent on

Answer 13

The afferent and efferent arteriolar diameter and therefore the balance of resistance between them.

Question 14

3 methods of extrinsic control of afferent and efferent arteriolar diameter

Answer 14

Sympathetic VC nerves => afferent and efferent constriction, greater sensitivity of afferent arteriole.

Circulating catecholamines e.g epinephrine => constriction primarily afferent

Angiotensin II => constriction, of efferent at [low], both afferent and efferent at [high].

Question 15

If there is increased resistance in the afferent arteriole what happens?

Answer 15

Increased blood flow to other organs

Decreased Renal blood flow

Decreased capillary blood pressure

Decreased GFR

Question 16

If there is increased resistance in the efferent arteriole what happens?

Answer 16

Decreased Renal blood flow

Normal flow to other organs

Increased pressure in the capillaries

Increased GFR

Question 17

If there is decreased resistance in the afferent arteriole what happens?

Answer 17

GFR increases as the glomerular capillary pressure increases

Question 18

What is MBP?

Answer 18

MBP = mean arterial blood pressure

You can work it out by doubling the diastolic adding it to the systolic and then dividing the whole thing by 3

Question 19

At what MBP does filtration cease all together

Answer 19

50 mmHg

Question 20

If mean arterial pressure increases how do the renal arteries respond

Answer 20

There is an automatic increase in afferent arteriolar constriction, preventing a rise in glomerular capillary pressure.

Question 21

What is autoregulation of blood flow to the kidneys and GFR independent of?

Answer 21

Independent of nerves or hormones

Question 22

What is the effective autoregulatory range ie what range of BPs can the GFR be maintained?

Answer 22

60-130mmHg

Question 23

What can be overridden in situations where blood volume/BP face serious compromise, eg in haemorrhage?

Answer 23

Autoregulation of the kidneys therefore more blood can be given to more immediately important organs at the expense of the kidneys

Question 24

What % of plasma entering the kidney returns into the systemic circulation?

Answer 24

> 99%

Question 25

What % of volume is excreted to the external environment?

Answer 25

<1%

Question 26

What volume is filtered through the glomerulus into the renal tubule per day?

Answer 26

180 L a day

only 1-2 L are excreted as urine in a day though

Question 27

How is the efferent arteriole involved in reabsorption?

Answer 27

Because it offers resistance along its entire length, there is a large pressure drop so that the hydrostatic pressure is very low in the peritubular capillaries (extension of the efferent arteriole).

However, because 20% of the plasma has filtered into Bowman’s capsule in the glomerulus, (filtration fraction), the blood remaining in the efferent arteriole and then the peritubular capillaries has a higher concentration of plasma proteins and so IIp increases

As a consequence, the net result of the low Ppc (hydrostatic) and the high IIp (oncotic) is that the balance of Starling’s forces in the peritubular capillaries is entirely in favour of reabsorption.