Glomerular Filtration

Question 1

Name the 3 basic renal processes.

Answer 1

1. Filtration
2. Reabsorption
3. Secretion

Question 2

What is the glomerular filtration rate (GFR) in L/day? and ml/min?

Answer 2

180L/day

125ml/min

Question 3

The normal GFR is very high; what does this allow the kidneys to do?

Answer 3

To precisely regulate ECF volume and composition, as well as eliminate “bad” substances.

Question 4

What substances does the proximal convoluted tubule absorb?

Answer 4

• NaCl
• Water
• Amino Acids
• Glucose

Question 5

What substances does the distal convoluted tubule absorb?

Answer 5

• NaCl

- Water

Question 6

What substances does the proximal convoluted tubule secrete?

Answer 6

• Organic ions
• Drugs
  (Tubules can actively secrete certain types of substances.)

Question 7

What substances does the distal convoluted tubule secrete?

Answer 7

K+ and H+ ions.

Question 8

Describe the blood flow that the kidneys receive and why this is importantly clinically.

Answer 8

Kidneys receive a blood flow of 1200ml/min, i.e. 20-25% of total cardiac output.

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

Question 9

Describe the composition of the fluid filtered through the Bowman’s capsule, as well as where the rest is filtered.

Answer 9

No RBCs and only a fraction of the plasma (essentially protein-free) 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 10

What does glomerular filtration depend upon?

Answer 10

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

Question 11

What dictates the filterability of solutes across the glomerular filtration barrier?

Answer 11

• Molecular size
• Electrical charge
• Shape

Question 12

Detail the filtration membrane of the Bowman’s capsule.

Answer 12

1. Fenestration of glomerular epithelium: prevents filtration of RBCs but allows all components of blood plasma to pass through.
2. Basal lamina of glomerulus: prevents filtration of larger proteins.
3. Slit membrane between pedicels: prevents filtration of medium-sized proteins.

Question 13

Why is glomerular capillary pressure (Pgc) higher than in most capillaries?

Answer 13

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

Question 14

Describe the relationship between Pgc and oncotic pressure in the glomerular capillaries.

Answer 14

Pgc&raquo_space; oncotic pressure.

The hydrostatic Pic always exceeds oncotic pressure. Thus, ONLY filtration occurs in the glomerular capillaries.

Question 15

Name 3 extrinsic factors that affect GFR.

Answer 15

1. Sympathetic nerves –> afferent and efferent vasoconstriction, greater sensitivity of afferent.
2. Circulating catecholamines –> afferent vasoconstriction.
3. Angiotensin II –> efferent constriction at [low], afferent and efferent at [high].

Question 16

What effect does afferent vasoconstriction have on GFR?

Answer 16

Decreased RBF (and increased BF to other organs) –> increased resistance in afferent arteriole –> decreased Pgc –> decreased GFR.

Question 17

What effect does efferent vasoconstriction have on GFR?

Answer 17

Decreased RBF –> increased Pgc –> increased GFR, (increased resistance in efferent arteriole).

Question 18

What effect does afferent vasodilation have on GFR?

Answer 18

Decreased resistance in afferent arteriole –> increased Pgc –> increased GFR.

Question 19

Describe the auto regulation of RBF and GFR.

Answer 19

Renal vasculature can adjust its resistance in response to changes in arterial BP –> keeps BF and GFR constant.

e.g. MAP increases –> afferent vasoconstriction occurs –> prevents rise in Pgc.

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

Question 20

Describe the interaction between intrinsic and extrinsic controls of RBF.

Answer 20

In situations where blood volume/pressure face serious compromise, extrinsic controls can override autoregulation. This allows more blood to flow to more important organs.

Max. 800ml/min can be spared at the expense of the kidney. However, prolonged reduction leads to irreparable kidney damage.

Question 21

Glomerular pressure (Pgc) vs. peritubular pressure (Ppc) in mmHg.

Answer 21

Pgc = 55mmHg
Ppc = 15mmHg

Question 22

Describe the relationship between Ppc and oncotic pressure in the peritubular capillaries.

Answer 22

Oncotic pressure&raquo_space; Ppc

Ppc is very low because it has to overcome frictional resistance in efferent arterioles. Oncotic pressure is higher than normal due to the peritubular capillaries having a higher [plasma proteins] (no protein in glomerulus).

Thus, ONLY reabsorption occurs in the peritubular capillaries.

Question 23

What % of water, glucose, sodium and urea is reabsorbed?

Answer 23

Water = 99%
Glucose = 100%
Sodium = 99.5%
Urea = 50%