Filtration

Question 1

define filtration?

Answer 1

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

Question 2

what is the glomerular filtration rate?

Answer 2

very high = 180 l/day

Question 3

what is the purpose of a high glomerular filtration rate?

Answer 3

This means that the kidney has ample opportunity to precisely regulate ECF volume and composition and eliminate “nasty” substances.

Question 4

what is reabsorption?

Answer 4

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

Question 5

what is secretion?

Answer 5

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

Question 6

how much blood flow do the kidneys usually receive?

Answer 6

a blood flow of ≈ 1200mls/min, ie 20-25% of total cardiac output.

In < 5mins, a volume of blood equal to the total BV passes through the renal circulation.

Question 7

what do kidneys weigh in relation to blood flow received?

Answer 7

Kidneys weigh <1% of BW and therefore have almost the highest BF/g tissue of any tissue in the body.

Question 8

what does a high blood flow to kidneys make them susceptible to?

Answer 8

vulnerability to damage by vascular disease.

Question 9

what of blood constituents are filtered through into Bowmans capsule?

Answer 9

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 10

what does plasma constitute of total blood volume, how can you use this to calculate renal plasma volume?

Answer 10

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

Question 11

what is the filtration fraction?

Answer 11

Therefore the filtration fraction = 125/660 x100 = 19%

ie 19% of the renal plasma becomes glomerular filtrate.

Question 12

what is glomerular filtration dependent on?

Answer 12

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

Question 13

how is the glomerular barrier selective?

Answer 13

Molecular size, electrical charge and shape determine the filterability of solutes across the glomerular filtration barrier

Question 14

what are the three layers of the filtration membrane?

Answer 14

Question 15

Glomerular capillary pressure PGC is higher than in most of the capillaries in the body why is this?

Answer 15

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

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

Question 16

what is the golden rule of circulation?

Answer 16

high resistance, hydrostatic pressure upstream is increased, while the pressure downstream is decreased

Question 17

do both the afferent and efferent arterioles contribute to the very high PGC?

Answer 17

yes

Question 18

at the glomerular capillaries the hydrostatic P favouring filtration always _______ the oncotic P?

Answer 18

exceeds

Question 19

what is the only process that occurs in the glomerular capillaries?

Answer 19

filtration

Question 20

what factors affect the GFR?

Answer 20

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

Question 21

what effects the resistance of afferent and efferent arterioles?

Answer 21

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 22

Renal vasculature also exhibits a well developed intrinsic ability to adjust its resistance in response to changes in arterial BP, what does this do?

Answer 22

This keeps BF and GFR essentially constant = autoregulation

Question 23

what is the auto regulatory range for a man?

Answer 23

In man, effective over a range of MBP from 60-130mmHg.
Below 60mmHg, filtration falls and ceases altogether when MBP = 50mmHg.

Question 24

is auto regulation dependent on anything?

Answer 24

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

Question 24

what is an example of interaction between intrinsic and extrinsic controls?

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.

This is particularly important in haemorrhage, but, prolonged reduction in renal BF can lead to irreparable damage which may then lead to death because of disruption of the kidney’s role in homeostasis.

Question 25

what effect does marathons have on kidneys?

Answer 25

Endurance training for very long events involves adaptation of skeletal muscle so that its requirement for blood flow becomes more efficient and therefore there is less need to take it from the kidneys.

Question 26

what is responsible for reabsorption?

Answer 26

Peritubular capillaries

Question 27

describe the role of peritubular capillaries?

Answer 27

Since 180 l/day are filtered through the glomerulus into the renal tubule, but only 1-2 l/day are excreted as urine an enormous amount of fluid must be reabsorbed back into peritubular capillaries.

The unique efferent arteriole has important effects on Starling’s forces in the peritubular capillaries.

Because it offers resistance along its entire length, there is a large P drop so that hydrostatic pressure is very low, ie PPC ≈ 15mmHg.

Question 28

why does the peritubular capillaries and blood remaining in efferent arterioles have a higher concentration of plasma proteins?

Answer 28

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 ∴ ↑Πp.

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

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