Glomerular Filtration

Question 1

what are the names of the basic renal processes?

Answer 1

filtration

reabsorption

secretion

Question 2

what is filtration?

Answer 2

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

Question 3

What is glomerular filtration rate? is it high or low? and what does this result in?

Answer 3

Glomerular filtration rate describes the flow rate of filtered fluid through the kidney

The glomerular filtration rate (GFR) is very high = 180 l/day. 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

Reabsorption: 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

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

Question 6

1. Process of Filtration:

Question 7

How much blood flow do the kidneys recieve and what is you BF/g tissue ratio like?

Answer 7

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

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

Question 8

The fact the kidneys recieve such high BF is a risk factor for what?

Answer 8

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

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

Question 9

Are red cells and plasma filtered through into the 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 percentage of blood does plasma make up?

Answer 10

Plasma constitutes around 55% of total BV

Question 11

how do you calculate renal plasma flow?

Answer 11

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

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

Question 12

GFR is normally 125mls/min

therefore what is the filtration fraction?

Answer 12

Plasma constitutes » 55% of total BV

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

Therefore the filtration fraction = 125/660 x 100 = 19% ie 19% of the renal plasma becomes glomerular filtrate

Question 13

Does Glomerular filtration occurs in the same way as what other process in the body?

Answer 13

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

Question 14

Glomerular filtration is dependent on a balance between what?

Answer 14

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

Question 15

what factors are improtant in regards to determaning permselectivity of glomerular barrier?

Answer 15

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

Question 16

describe the structure of the filtration membrane

Answer 16

Question 17

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

Answer 17

Glomerular capillary pressure PGC 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. 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

Question 18

Golden rule of the circulation:

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

Answer 18

Golden rule of the circulation:

If you have a high resistance, hydrostatic pressure upstream is _________, while the pressure downstream is ________

Question 19

What contributes to the high glomerular capillary pressure?

Answer 19

Therefore both the afferent and efferent arterioles contribute to the very high P_GC

Question 20

At glomerular capillaries, how does hydrostatic pressure compare to oncotic pressure?

Answer 20

Therefore at the glomerular capillaries the hydrostatic P favouring filtration always exceeds the oncotic P.

PGC >> oncotic p

Question 21

What is the only thing that happens at glomerular capillaries?

Answer 21

ONLY filtration occurs at the glomerular capillaries

Question 22

how is net filtration pressure calculated?

Answer 22

Question 23

what is the primary factor affecting GFR?

Answer 23

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

Question 24

afferent and efferent arteriolar diameter and therefore the balance of resistance between them is subject to extrinsic control via what?

Answer 24

a) Sympathetic VC nerves = afferent and efferent constriction, greater sensitivity of afferent arteriole

Sympathetic constriction means we can change the pressure in the glomerulus and therefore glomerulus filtration rate

b) Circulating catecholamines = constriction 1°ily afferent
c) Angiotensin II = constriction, of efferent at [low], both afferent and efferent at [high].

Question 25

diagram showing how relative diameter of afferent and efferent arterioles is crucial in determining GFR

Answer 25

In d) GFR would increase­ as the P_GC would increase­ and GFR would ­increase

Question 26

how are BF and GFR kept constant?

Answer 26

Renal vasculature also exhibits a well developed intrinsic ability to adjust its resistance in response to changes in arterial BP

This keeps BF and GFR essentially constant = autoregulation

Question 27

Over what range does autoregulation act?

Answer 27

In man, effective over a range of MBP from 60-130mmHg

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

Question 28

what happen if MAP increases? and what if it decreases?

Answer 28

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 29

Autoregulation is independent of ______ or ________, occurs in denervated and in isolated perfused kidneys

Answer 29

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

Question 30

what else also plays a cruical role in glomerular filtration?

Answer 30

blood volume

Question 31

There is interaction between intrinsic and extrinsic controls

In situations where blood volume/BP face serious compromise, eg in haemorrhage, what happens?

Answer 31

activation of sympathetic VC nerves and AII, can override autoregulation liberating blood for more immediately important organs.

Question 32

how much blood can be provided to other organs at the expense of the kidneys?

Answer 32

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

Question 33

what can prolonged reducion in renal BF lead to?

Answer 33

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 34

how does endurance training affect renal blood flow?

Answer 34

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 35

what is responsible for reabsorption?

Answer 35

Peritubular capillaries are responsible for reabsorption

Question 36

how much fluid each day is reabsorbed back into the peritubular capillaries?

Answer 36

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

Question 37

how does the efferent arteriole affect peritubular capillaries?

Answer 37

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 P_PC around 15mmHg

Question 38

How does reabsorption occur in the peritubular capillaries?

Answer 38

In peritubular capillaries:

P_PC very low because hydrostatic P overcoming frictional resistance in efferent arteriols

osmotic pressure high compared to normal, loss of 20% plasma concentrates plasma protein

osmotic pressure >> PPC only reabsorption

Question 39

what is the only thing to happen in the glomerular capillaries?

Answer 39

P_GC >> Osmotic Pressure

Only filtration occurs at glomerular capillaries

Question 40

filtration is driven by……………

reabsorption is driven by……………

Answer 40

Filtration - hydrostatic pressure

Reabsorption – driven by osmotic forces (starlings forces)

Question 41

Since aorund 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 what?

Answer 41

a higher concentration of plasma proteins and therefore increase osmotic pressure

Question 42

As a consequence, the net result of the low P_PC and the high oscmotic pressure causes what?

Answer 42

the net result of the low PPC and the high ismotic pressure is that the balance of Starling’s forces in the peritubular capillaries is entirely in favour of reabsorption

Question 43

what substances are reabsorbed and where about are they mainly reabsormbed?

Answer 43

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