Lecture 3

Question 1

During Glomerular Filtration, Fluid filtered from the glomerulus into Bowman’s capsule must pass through the three layers that make up the glomerular membrane:

Answer 1

1. The glomerular capillary wall
2. The basement membrane
3. The inner layer of Bowman’s capsule

• These layers function as a fine molecular sieve

Question 2

Question 3

Question 4

The glomerular capillary wall is perforated by many large pores

Answer 4

• 100 times more permeable to H2O and solutes

Question 5

The endothelial cells of the glomerular membrane themselves are also

Answer 5

perforated by large holes or fenestrations

Question 6

The basement membrane of the glomerular membrane is an

Answer 6

acellular (lacking cells) gelatinous layer composed of collagen and glycoproteins that is sandwiched between the glomerulus and Bowman’s capsule

Question 7

Plasma proteins are almost completely excluded from the filtrate

Answer 7

Urine is normally protein free

Question 8

The inner layer of Bowman’s capsule consists of

Answer 8

podocytes
- octopus like epithelial cells that encircle the glomerulus.

Question 9

The glomerular membrane also has

Answer 9

Filtration slits

Question 10

To accomplish glomerular filtration, a force must drive a part of the plasma in the glomerulus through the openings in the glomerular membrane

Answer 10

Passive physical forces similar to those acting across capillaries elsewhere accomplish glomerular filtration.
[Glomerular capillary blood pressure]

Question 11

Glomerular capillary blood pressure
The same principles of fluid dynamics apply here that cause ultrafiltration across other capillaries, except for two important differences:

Answer 11

1. The glomerular capillaries are more permeable than capillaries elsewhere, so more fluid is filtered for a given filtration pressure
2. The balance of forces across the glomerular membrane is such that filtration occurs the entire length of the capillaries.

Question 12

Three physical forces are involved in glomerular filtration

Answer 12

1. Glomerular capillary blood pressure
2. Plasma-colloid osmotic pressure
3. Bowman’s capsule hydrostatic pressure.

Question 13

Question 14

Glomerular Filtration Rate

Answer 14

The net filtration pressure of 10 mm Hg forces large volumes of fluid from the blood through the highly permeable glomerular membrane.

Question 15

The actual rate of filtration

Answer 15

= the glomerular filtration rate (GFR)

Question 16

the glomerular filtration rate (GFR) depends on

Answer 16

The net filtration pressure
How much glomerular surface area is available for penetration
How permeable the glomerular membrane is

[These properties of the glomerular membrane are collectively referred to as the filtration coefficient (Kf)]

Question 17

GFR = Kf x net filtration pressure

Answer 17

Average GFR of 125 mL/min in males

Question 18

Unregulated Influences on the GFR

Answer 18

Plasma-colloid osmotic pressure and Bowman’s capsule hydrostatic pressure normally do not vary much and cannot be regulated.

Question 19

Controlled Adjustments in the GFR

Answer 19

• Glomerular capillary blood pressure can be controlled to adjust the GFR to suit the body’s needs

Question 20

Two major control mechanisms regulate the GFR:

Answer 20

1. Autoregulation
2. Extrinsic sympathetic control

Question 21

Mechanisms Responsible for Autoregulation of the GFR

Answer 21

GFR would increase in direct proportion to an increase in arterial pressure
A fall in arterial blood pressure would cause a decline in GFR
Autoregulation by the kidneys
The net filtration pressure and GFR can be reduced to normal by constriction of the afferent arteriole
Glomerular pressure can be increased to normal by vasodilation of the afferent arteriole

Question 22

Two mechanisms contribute to autoregulation of the GFR:

Answer 22

1. A myogenic mechanism
2. A tubuloglomerular feedback mechanism

Question 23

A myogenic mechanism

Answer 23

• which responds to changes in pressure within the nephron’s vascular component

Question 24

A tubuloglomerular feedback mechanism

Answer 24

• which senses changes in salt level in the fluid flowing through the nephron’s tubular component.
  > Granular cells
  > Macula densa

Question 25

Macula densa - function in the Autoregulation of the GFR

Answer 25

chief cells within the kidney, playing key sensory and regulatory functions in the maintenance of body fluid, electrolyte homeostasis, and blood pressure.
- responds to the change in the Na concentration in the distal tubules, signals the juxtaglomerular cells to release renin.
- this triggers contraction of the afferent arteriole, reducing flow of blood to the glomerulus and the glomerular filtration rate.

> Release ATP and adenosine, both of which act locally as a paracrine

• Nitric oxide in macula densa can dilate afferent arterioles.

Question 26

Question 27

Importance of Autoregulation of the GFR

Answer 27

• The myogenic and tubuloglomerular feedback mechanisms work in unison to autoregulate the GFR within the mean arterial blood pressure range of 80 to 180 mm Hg
• Autoregulation is important because unintentional shifts in GFR could lead to dangerous imbalances of fluid, electrolytes, and wastes.
• When changes in mean arterial pressure fall outside the autoregulatory range, these mechanisms cannot compensate.

Question 28

Importance of Autoregulation of the GFR

Answer 28

• The myogenic and tubuloglomerular feedback mechanisms work in unison to autoregulate the GFR within the mean arterial blood pressure range of 80 to 180 mm Hg
• Autoregulation is important because unintentional shifts in GFR could lead to dangerous imbalances of fluid, electrolytes, and wastes.
• When changes in mean arterial pressure fall outside the autoregulatory range, these mechanisms cannot compensate.

Question 29

Importance of Extrinsic Sympathetic Control of the GFR

Answer 29

Extrinsic control mechanisms override the autoregulatory responses
Mediated by sympathetic nervous system input to the afferent arterioles

Question 30

The parasympathetic nervous system

Answer 30

does not exert any influence on the kidneys

Question 31

Baroreceptor Reflex in Extrinsic Control of the GFR

Answer 31

• The afferent arterioles are innervated with sympathetic vasoconstrictor fibers to a far greater extent than the efferent arterioles are
• Increase/decrease in GFR
• Increased/decreased tubular reabsorption of H2O and salt regulate plasma volume

Question 32

GFR depends on

Answer 32

the filtration coefficient (Kf) as well as on the net filtration pressure

Question 33

Kf is subject to change under physiologic control

Question 34

The surface area and the permeability of the glomerular membrane can be modified by

Answer 34

contractile activity within the membrane.

Question 35

The surface area available for filtration - The inner surface of the glomerular capillaries that comes into contact with blood

Answer 35

mesangial cells
Podocytes

Question 36

Answer 36

Question 37

The kidneys and the cardiac output

Answer 37

20% of the plasma that enters the kidneys is converted into glomerular filtrate
An average GFR of 125 mL/min - the total renal plasma flow must average about 625 mL/min.
55% of whole blood consists of plasma - the total flow of blood through the kidneys averages 1140 mL/min.
This quantity is about 22% of the total cardiac output of 5 L/min
Most of the blood goes to the kidneys not to supply the renal tissue but to be adjusted and purified by the kidneys