Module 4 Section 3

Question 1

glomerulus

Answer 1

• a netowork of capillaries located at the beginning of a nephron
• blood is filtered across the walls of this capillary network through the glomerular membrane, which yields its filtrate into Bowmans capsule
• the filtrate then enters the renal tubule of the nephron
• receives its blood supply from an afferent arteriole and the glomerular capillaries exit into efferent arterioles
• 3 main functions are responsible for enabling glomerular filtration

Question 2

glomerular filtration rate (GFR)

Answer 2

the rate at which blood is filtered through all of the glomeruli, the measure of the overall renal function

Question 3

glomerular filtration - layer 1

Answer 3

glomerular capillary wall
- it consists of a single layer of endothelial cells
- it contains many large pores that make it 100 times more permeable to fluids and solutes than regular capillaries
- the pores are of such size that large plasma proteins cannot pass through, but smaller ones, such as albumin, can

Question 4

glomerular filtration - layer 2

Answer 4

basement membrane
- this layer contains no cells and is composed of collagen to provide structural strength, and glycoproteins to discourage the filtration of small plasma proteins
- because the glycoproteins are negatively charged, they help to repel any proteins that do get through the capillary walls
- only about 1% of filtered albumin will pass into bowmans capsule

Question 5

glomerular filtration - layer 3

Answer 5

inner layer of Bowmans capsule
- this layer is composed of podocytes that form narrow filtration slits between them that allow fluid to pass into Bowmans capsule

Question 6

forces that regulate glomerular filtration

Answer 6

• glomerular capillary blood pressure
• plasma-colloid oncotic pressure
• bowmans capsule hydrostatic pressure

Question 7

glomerular capillary blood pressure

Answer 7

• this is the pressure exerted by the blood in the glomerular capillaries
• while regular capillaries have a blood pressure of about 18mmHg, glomerular capillary pressure is average 55mmHg
• this is due mainly to the afferent arteriole diameter being larger than the diameter of efferent arterioles, increasing resistance to blood leaving glomerular capillaries
• also prevents glomerular capillary pressure from decreasing along their length, further favouring filtration

Question 8

plasma-colloid oncotic pressure

Answer 8

• the presence of large proteins in the plasma that cannot be filtered produces a oncotic force that resists the movement of water into bowmans capsule
• the plasma-colloid oncotic pressure i about 30mmHg

Question 9

bowmans capsule hydrostatic pressure

Answer 9

• this is the pressure of the fluid in bowmans capsule and it also resists the movement of water out of the glomerular capillaries
• bowmans capsule hydrostatic pressure is around 15mmHg

Question 10

what is glomerular filtration rate dependent on

Answer 10

• not only dependent on filtration pressure, but also the glomerular surface area available and how permeable the membrane is
• these properties are called filtration coefficient (Kf)

Question 11

pathological conditions that leads to changes in GFR

Answer 11

someone with a kidney stone that obstructs the urterer will have an increased bowmans capsule hydrostatic pressure, again decreasing GFR

Question 12

how GFR can be decreased

Answer 12

• someone with severe diarrhea will be dehydrated, as they are losing more fluid than they are taking in
• this results in decreased blood pressure due to the decreased plasma volume
• there is also an increase in plasma-colloid osmotic pressure that results in a decreased GFR

Question 13

equation used to determine net filtration pressure

Answer 13

even though plasma-colloid osmotic pressure and bowmans capsule hydrostatic pressure generally do not change, glomerular capillary blood pressure can be regulated

Question 14

equation to determine glomerular filtration rate

Answer 14

Question 15

autoregulation of GFR

Answer 15

• because changes in GFR are directly proportional to glomerular capillary blood pressure, autoregulatory (intrinsic) mechanisms are in place to prevent sudden swings in GFR
• primarily done by regulating the diameter of the afferent arterioles, such that constricting the afferent arterioles will decrease glomerular capillary blood pressure and dilating the afferent arterioles will increase it

Question 16

two intrarenal mechanisms allowing autoregulation

Answer 16

1. myogenic activity
2. tubuloglomerular feedback (TGF)

Question 17

myogenic activity

Answer 17

• when increased pressure stretches the afferent arteriole walls, they automatically constrict to reduce blood flow to the glomerular capillaries and thus prevent an increase in GFR
• the opposite is true that if blood pressure decreases, the afferent arterioles will dilate to increase blood flow and prevent a decrease in GFR

Question 18

tubuloglomerular feedback (TFG)

Answer 18

• specialized tubular cells in this area are collecitvely called the macula densa, which can sense changes in the salt elvel of the tubular fluid
• if there is an increased arterial pressure that increases the GFR, more fluid than normal will flow through the distal tubule
• this also means there is an increased salt delivery
• in response, the macula densa releases ATP, which is degraded to adenosine
• this adenosine acts of the afferent arterioles to cause constriction and reduce GFR (opposite is also true)

Question 19

sympathetic control of GFR

Answer 19

• GFR is also under extrinsic control, independent of fluctuations in arterial blood pressure
• this is controlled by the sympathetic nervous system, which innervates the afferent arterioles
• e.g. a sudden loss of blood volume followed by a drop in arterial pressure would be sensed by the baroreceptors which would initiate responses to normalize blood pressure

Question 20

sympathetic control of GFR at the level of the kidney

Answer 20

• this icnrease sympathetic activity would constrict the afferent arterioles, which would decrease glomerular capillary pressure, decreasing GFR, and reducing urine production
• this is a mechanism by whihc depleted plasma volumes can be corrected

Question 21

kidneys and cardiac output

Answer 21

• in a healthy person, about 20% of the plasma enters the kidneys and becomes the glomerular filtrate
• this means that if GFR=125ml/minute, the total blood flow to the kidneys must be 5 x 125 of 625ml/minute
• considering that only 55% of whole blood in filterable plasma, we can adjust renal blood flow to 1140ml/minute
• since total cardiac output equals 5000ml/minute at rest, we can calculate kidneys receive around 22% of total cardiac output
• exceeds what would be expected based on tissue size since kidney weight is only about 1% of total body weight

Question 22

significance of kidneys receiving a high proportion of total cardiac output

Answer 22

• its primary purpose is not to deliver oxygen and nutrients, but to deliver blood for “cleaning”
• delivery of this large volume of blood also allows the kidneys to maintain tight control of volume and electrolyte concentrations of the bodys water pools and to eliminate wastes efficiently