Lecture 16: Micturition And GFR

Question 1

Describe the steps that occur during the micturition reflex

Answer 1

• Contractions begin to appear more as bladder fills more.
• Sensory signals: sacral region- pelvic nerves, reflexively back to bladder-parasympathetic nerves
• Reflex contractions relax when only partly filled.
• Micturition is self-regenerative…it fatigues after a few seconds and the bladder relaxes.
• When powerful enough, it causes 2nd reflex.
• pudendal nerves inhibit external sphincter.
• Pons keeps partly inhibited when needing to micturate.
• cortical centers can facilitate sacral micturition centers to help initiate a micturition reflex and at the same time inhibit external urinary sphincter.

Question 2

Be able to list the various functions of the nephrons

Answer 2

Rid of wastes

Regulate water and electrolytes, body fluid osmolarity, arterial pressure (excrete electrolytes (long term) and hormones- renin(short term))

Regulate acid-base balance

Secretion, metabolism, and excretion of hormones

Question 3

List and describe the 3 processes carried out by nephrons that determine the composition of the urine.

Answer 3

Filtration
Reabsorption
Secretion

Question 4

Express the above filtration, reabsorption, and secretion mathematically:

Answer 4

Urinary excretion rate=

Filtration rate-Reabsorption rate+ secretion rate

Question 5

List the major components of the glomerular filtrate

Answer 5

Water
Ions
Glucose
Urea

Question 6

Define filtration fraction and express mathematically:

Answer 6

Fraction of renal plasma flow that is filtered.

GFR / Renal plasma flow

20% of plasma flowing through kidney is filtered.

Question 7

List the major characteristics that determine whether or not a substance is filtered by the nephron.

Answer 7

Neutral and positive charges are filtered by the nephron better because the layers of the filtration barrier are negative.

Radius is a factor in filterability as well.

Question 8

Describe the 3 components of the filtration barrier.

Answer 8

Endothelium: w/ fenestrae and NEGATIVE charges

Basement membrane: w/ collagen and proteoglycan fibers and strong NEGATIVE charges.

Podocytes: w/ NEGATIVE charges

Question 9

List the factors that determine glomerular filtration rate

Answer 9

Balance of hydrostatic and Colloid osmotic force acting across capillary membrane
—the same as the CV system starling forces.

Capillary filtration coefficient

Question 10

Define capillary filtration coefficient.

Answer 10

Product of permeability and filtering surface area of capillaries (K1)

Question 11

Know that the GFR is:

Answer 11

125ml/min = 180L /day

Question 12

Recognize and define: minimal change nephropathy and hydronephrosis

Answer 12

Minimal change neuropathy: loss of negative charges on the basement membrane

Hydronephrosis: dissension and dilation of renal pelvis and calyces

Question 13

Be able to mathematically express the glomerular filtration rate in terms of starling forces:

Answer 13

GFR= K1 (glomerular hydrostatic pressure - bowman’s capsule hydrostatic pressure - glomerular capillary colloid osmotic pressure + colloid osmotic pressure -bowmans capsule)

Question 14

Describe how changes in the capillary filtration coefficient affects GFR

Answer 14

Increased K1 = increased GFR

Decreased K1 = decreased GFR

Question 15

List and describe factors that affect GFR:

Arterial pressure
Afferent arteriolar resistance
Efferent arteriolar resistance

Answer 15

Misc. factors: arterial plasma colloid osmotic pressure, filtration fraction, increasing filtration fraction

Increased A pressure= increased glomerular hydrostatic pressure= INCREASES GFR

Increased afferent arteriolar resistance= decreases glomerular hydrostatic pressure= DECREASES GFR

Increase in efferent arteriolar resistance= increase in glomerular hydrostatic pressure = slightly INCREASES GFR

Question 16

Explains the effect of the sympathetic system on GFR.

Answer 16

Sympathetic activation of kidney leads to strong activation of:

• constriction of renal arterioles
• decrease renal blood flow and GFR

Moderate activation has little effect.

Question 17

List and describe hormones that control GFR and how they effect GFR:

Answer 17

Norepinephrine and Epinephrine: sympathetic-(less GFR)

Angiotensin II:constrictor-more GFR d/t efferent constriction
Endothelin: constrictor-less GFR

NO:dilator
Prostaglandins and Bradykinin: dilator

Question 18

List source of the previous hormones:

Answer 18

Norepi and epi: adrenal medulla

Endothelin: damaged vascular endothelial cells of the kidneys and other tissue

Angiotensin II: from liver

NO: endothelial cells

Prostaglandins and bradykinins: EFAs

Question 19

Kidney autoregulation overview

Answer 19

Autoregulation assists in decreasing flow rate in loop of henle leading to:

• decreased GFR
• increased reabsorption of Na+ and Cl- in the ascending limb
• decreased NaCl at macula densa

[this decrease in NaCl results in signal from macula densa that:

• decreases resistance to blood in afferent arterioles
• increases renin release from JG cell (major storage of renin)
• increases angiotensin II
• increased efferent arteriolar resistance—-increasing GFR! Balance!]

Question 20

Define autoregulation as it r/t kidneys:

Answer 20

Trying to maintain appropriate blood flow at all times.

Precise control of renal excretion of water and solutes

Prevents large changes in GFR and renal excretion that would otherwise occur w/ changes in blood pressure

Question 21

Describe the functions of autoregulation and explain why it is important

Answer 21

SLight increases in BP w/o autoregulation could increase GFR to 225L/ day from 180L/day. Urine flow would increase from 1.5L/day to 46.5L/day.

2 components to the autoregulation:

• tubuloglomerular feedback mechanism (afferent/efferent feedback)
• juxtaglomerular complex

Question 22

List and describe the 2 compartments of the autoregulation feedback mechanism:

Answer 22

tubuloglomerular feedback mechanism 2 components:

afferent/efferent feedback mechanism

Question 23

Relate the macula densa to the JG complex

Answer 23

Macula densa is in distal tubule which is adjacent to JG cells in afferent and efferent arterioles.

Question 24

Describe in detail the role of the JG complex in autoregulation:

Answer 24

JG cells release Renin-angiotensin II cascade when the [NaCl] decreases at the macula densa.

Question 25

Understand the concept of GFR, renal clearance and determine their values

Answer 25

GFR: estimates how much blood passes through glomeruli / min

Renal clearance: The volume of plasma that is completely cleared of the substance by the kidneys per unit time

Values: GFR >60, Clearance=625 ml/min