M23: Urinary System - Glomerular Filtration

Question 1

glomerular filtration rate (GFR)

Answer 1

• amount of filtrate formed in all renal corpuscles of both kidneys/minute
• average adult rates
  
  • female: 105 mL/min
  • male: 125 mL/min
• homeostasis requires constant GFR
• changes in net filtration pressure affects GFR

Question 2

3 main processes in urine production

Answer 2

1. Glomerular filtration
   - water and solutes from blood move into Bowman’s capsule
2. Tubular reabsorption
   - water and useful substances reabsorbed into blood
3. Tubular secretion
   - further wastes removed from blood and secreted into urine

Question 2

Glomerular filtration

Answer 2

• first step of glomerular filtration
• in glomerulus, blood plasma and solutes removed from blood and filtered into bowman’s capsule

Question 3

Tubular reabsorption

Answer 3

• water and useful substances reabsorbed into blood
• renal tubule and collecting duct
• tubular secretion happens simultaniously

Question 4

Tubular Secretion

Answer 4

• wastes removed from blood and secreted into urine
• renal tubule and collecting duct
• tubular reabsorption happens simultaneously

Question 5

Renal blood flow (amount of blood going to kidneys) equivalent to…

Answer 5

20-25% of cardiac output (~1.2L/min)

Question 6

renal blood flow vs renal plasma flow rate

Answer 6

Renal blood flow: amount of blood going to kidneys (1.2L/min)

Renal plasma flow rate: amount of plasma going to kidneys (650mL/min)
=55% of renal blood flow because plasma makes up ~55% of blood

Question 7

filtration fraction

Answer 7

amount of renal plasma that becomes filtrate (16-20%)
- 125mL/min or 150-180L/day

Question 8

How much glomerular filtrate is reabsorbed? How does this effect urine production?

Answer 8

99%
a lot of fluid is filtered, but only 1-2L urine produced per day because of reabsorption

Question 9

glomerular blood hydrostatic pressure (GBHP)

Answer 9

55mmHg
- pressure entering into glomerulus from afferent arteriole
- drives filtrate into glomerular capsule
- cause: efferent arteriole smaller than afferent, creating a driving pressure towards filtration membrane

Question 10

pressures that oppose filtration

Answer 10

1. CAPSULAR HYDROSTATIC PRESSURE (CHP)
   - 15mmHg
   - fluid in capsule pushes back on fluid in glomerulus
2. BLOOD COLLOID OSMOTIC PRESSURE
   - 30mmHg
   - proteins remaining in glomerular fluid attract water in glomerular capsule via osmosis

Question 11

net filtration pressure (NFP) in glomerulus

Answer 11

= GBHP–CHP–BCOP
= 55mmHg – 15mmHg – 30mmHg
= 10 mmHg

GBHP = glomerular blood hydrostatic pressure
CHP = capsular hydrostatic pressure
BCOP = blood colloid osmotic pressure

Question 12

At what pressure would glomerular filtration stop?

Answer 12

If GBHP drops below 45mmHg
filtration keeps going within a mean arterial pressure range 80-180

GBHP = glomerular blood hydrostatic pressure

Question 13

2 ways that glomerular filtration rate is regulated

Answer 13

1. ADJUSTING BLOOD VOLUME
2. ALTERING GLOMERULAR CAPILLARY SURFACE AREA

Under 3 mechanisms of control
1. Renal auto-regulation
2. Neural regulation
3. Hormonal regulation

Question 14

Renal auto-regulation of glomerular flow rate

Answer 14

MYOGENIC MECHANISM (fast)
- stretch the arteriole walls due to increased blood pressure
- smooth muscle contraction - reduces blood floow
- GFR returns to normal

TUBULOGLOMERULAR FEEDBACK (slower)
- if GFR high, not enough time to reabsorb solutes and water
- macula densa detects this: inhibits release of a vasodialator (nitric oxide) from the juxtraglomerular apparatus
- afferent arterioles constrict and reduce blood flow, reducing GFR

Question 15

neural regulation of glomerular flow rate

Answer 15

Controlled by SYMPATHETIC ANS innervation, causing vasoconstriction

AT REST
- renal blood vessels maximally dilated
- SNS activity minimal, most control from auto-regulation mechanisms

MODERATE SNS STIMULATION
- minimal impact on filtrate formation
- equal afferent and efferent contraction

HIGH SNS STIMULATION
- afferent arterioles most constricted
- GFR reduced
- lowers urine output and permits blood flow to other tissues (e.g. muscles for exercise)

Question 16

Hormonal control of glomerular filtration rate

Answer 16

ATRIAL NATRIURETIC PEPTIDE (ANP)
- increases GFR
- hormone released by heart
- triggered by stretching of the atria because of increase in blood volume
- relaxes glomerular mesengial cells , increasing capillary surface area and GFR

ANGIOTENSIN II
- reduces GFR
- potent vasoconstrictor
- narrows afferent and efferent arterioles, reducing blood flow and GFR

Question 17

how much urine produced per day?

Answer 17

1-2L

Question 18

Which glomerular arteriole has smaller diameter? why?

Answer 18

Efferent arteriole
- creates pressure in afferent arteriole to create glomerular blood hydrostatic pressure

Question 19

Atrial naturetic peptide (ANP) role in GFR regulation

Answer 19

- increases GFR
- hormone released by heart
- triggereda by stretching of the atria because of increase in blood volume
- relaxes glomerular mesengial cells , increasing capillary surface area and GFR

Question 20

Angiotensin II role in GFR regulation

Answer 20

- reduces GFR
- potent vasoconstrictor
- narrows afferent and efferent arterioles, reducing blood flow and GFR
- afferent arteriole constricted more