Glomerular Filtration

Question 1

What three process occur in the nephron?

Answer 1

1. Filtration (protein free filtrate of plasma)
2. Reabsorption (i.e. NaCl and water)
3. Secretion

Question 2

What is filtered at the renal corpuscle?

Answer 2

No red ells and only a fraction of the plasma, the remainder passes via the efferent arterioles –> peritubular capillaries –> renal vein

Question 3

What is the renal plasma flow?

Answer 3

• Kidney receives blood flow of 1200ml/min
• Plasma is 55% of total BV

55% of 1200 = 660ml/min

Question 4

What is the normal GFR?

Answer 4

125ml/min

Question 5

What is the filtration fraction?

Answer 5

• GFR = 125ml/min
• Renal plasma flow = 660ml/min

125/660 x100 = 19%

Question 6

What is glomerular filtration dependent on?

Answer 6

Balance between the hydrostatic forces favouring filtration and the oncotic pressure forces favouring reabsorption - starling’s forces

Primary factor is P(GC) which is dependent on aff/efferent arteriolar diameter

Question 7

What three characteristics of a substance determine its filterability across glomerular filtration barrier?

Answer 7

Molecular size, electrical charge and shape

- Na highest and serum albumin lowest

Question 8

Describe the renal arterioles

Answer 8

• Fenestrated capillaries
• Basal lamina of glomerulus
• Slit membrane between pedicels

Question 9

Why does blood arriving at the glomerulus have a high hydrostatic pressure?

Answer 9

Glomerular capillary pressure P(GC) is higher than in most of the capillaries in the body because the afferent arteriole is short and wide and offers little resistance to flow.

Question 10

Describe the efferent arteriole

Answer 10

Long and narrow which offers a high post-capillary resistance

Question 11

What is the relationship between peripheral resistance and hydrostatic pressure?

Answer 11

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

• Compression of an arteriole would increase pressure in front and decreases pressure after

Question 12

What is the balance between hydrostatic pressure and oncotic pressure which favours filtration?

Answer 12

P(GC) > Oncotic P

Only filtration occurs at the glomerular capillaries

Question 13

How do extrinsic controls affect afferent and efferent arteriolar diameter (and therefore P(GC))?

Answer 13

• Sympathetic vasoconstrictor nerves -> afferent and efferent constriction
• Circulating catecholamines -> constriction primarily afferent
• Angiotensin II -> constriction, of efferent at [low], both afferent and efferent at [high].

Question 14

What is the physiological response to increased resistance in afferent arteriole?

Answer 14

Constriction of afferents arterioles; pressure after this decrease and producing resistance -> this causes intra G pressure to decrease

Question 15

What is the physiological response to increased resistance in efferent arteriole?

Answer 15

Constriction of efferent arterioles; increased pressure in glomerulus as it is before the constriction –> so increases GFR

Question 16

What is the physiological response to decreased resistance in afferent arteriole?

Answer 16

GFR would increase as the P(GC) would increase (efferent needs to be consitricted more to compensate for decreased resistance in afferent)

Question 17

What is the autoregulation of renal vasculature?

Answer 17

Intrinsic ability to adjust its resistance in response to changes in arterial BP - dependent on nerves and hormones.

If mean arterial P increases, there is an automatic increase in afferent arteriolar constriction, preventing a rise in glomerular P(GC) (prevents too more blood being pumped in due to high pressure and pressure decreases after constriction).
Dilatation occurs if P falls (to allow more blood in).

Question 18

What are the the percentages of filtration, reabsorption and secretion?

Answer 18

1. PV entering afferent arteriole = 100%
2. 20% of volume filters
3. > 19% of fluid is reabsorbed
4. > 99% of plasma entering kidney returning to systemic circulation
5. < 1% of volume is excreted to external environment

Question 19

What vessel is responsible for reabsorption?

Answer 19

Peritubular capillaries

Question 20

Why is a large pressure drop occur in the peritubular capillaries?

Answer 20

Because it is long and thin so offers resistance along its entire length, there is a large pressure drop so that hydrostatic pressure is very low, ie P(PC) ~ 15mmHg.

Question 21

What is the balance between P(GC) and P(PC)?

Answer 21

P(GC)&raquo_space; P(PC)

• P(PC) very low because hydrostatic P overcoming frictional resistance in efferent arterioles
• P(Plasma) high compared to normal, loss of 20% plasma concentrates plasma protein
• P(P)&raquo_space; P(PC) - only reabsorption

Since ~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 therefore increase in oncotic pressure.

Question 22

What is the balancer of starlings forces which favours reabsorption?

Answer 22

Low P(PC) and high oncotic pressure (due to higher concentration of plasma proteins)

Question 23

How much water, Glc Na and urea that is filtered out is reabsorbed?

Answer 23

99% H2O, 100% glucose, 99.5% Na+ and 50% urea