Lecture 15: Micturition and Glomerular Filtration

Question 1

Micturition Reflex

Answer 1

Contraction begins to appear as bladder fills; relax spontaneously when bladder is partially filled; self-regenerative - causes a 2nd reflex when powerful enough

Question 2

Where do sensory signals from bladder stretch receptors conduct?

Answer 2

Sacral region of spinal cord via pelvic nerves and back to bladder via parasympathetic nerves

Question 3

How do adults keep micturition inhibited (unless desired)?

Answer 3

Via higher brain centers in the pons

Question 4

Describe the reflex process that occurs during urination

Answer 4

Cortical centers facilitate sacral micturition centers to initiate micturition reflex; simultaneously, the external urinary sphincter is inhibited so that urination can occur

Question 5

What are the functions of nephrons?

Answer 5

Get rid of waste materials, regulate H2O/electrolyte balance, regulate body fluid osmolarity, regulate arterial pressure, regulate acid-base balance, secretion/metabolism/excretion of hormones, gluconeogenesis

Question 6

What are the three processes that determine the rates at which different substances are excreted in the urine?

Answer 6

Filtration, Reabsorption, Secretion

Question 7

Urinary excretion rate

Answer 7

Filtration rate - Reabsorption rate + Secretion rate

Question 8

What is the first step in urine formation?

Answer 8

Filtration

Question 9

Filtration fraction

Answer 9

GFR/Renal plasma flow

Question 10

How much of the plasma flowing through the kidneys is filtered?

Answer 10

20%; Filtration fraction = 0.2

Question 11

What two factors influence how substances are filtered?

Answer 11

Molecular size and charge; larger negative molecules are repelled

Question 12

What are the three layers of the filtration barrier?

Answer 12

Endothelium (fenestrae, negative charges), Basement membrane (colagen, proteoglycan fibers, negative charges), Podocytes (filtration slits, negative charges)

Question 13

List the 2 major determinants of Glomerular Filtration Rate

Answer 13

Balance of hydrostatic and colloid osmotic forces acting across capillary membranes (Starling Forces), Capillary filtration coefficient (product of permeability and filtering surface area of capillaries)

Question 14

Average GFR

Answer 14

125 ml/min = 180 L/day

Question 15

Capillary filtration coefficient

Answer 15

K1 = permeability x filtering surface area of capillaries; depends on leakiness of pores

Question 16

Minimal change nephropathy

Answer 16

Loss of negative charges on the basement membrane

Question 17

Hydronephrosis

Answer 17

Distension and dilation of renal pelvis and calyces

Question 18

What diseases lower glomerular capillary filtration coefficient?

Answer 18

Chronic uncontrolled hypertension, diabetes mellitus

Question 19

Express GFR in terms of Starling Forces

Answer 19

GFR = K1 x Net Filtration Pressure

Question 20

How does K1 affect GFR?

Answer 20

Increasing K1 raises GFR, decreasing K1 lowers GFR

Question 21

What effect does increasing arterial pressure have on hydrostatic pressure and GFR?

Answer 21

Increases glomerular hydrostatic pressure, Increases GFR

Question 22

What effect does increasing afferent arteriolar resistance have on hydrostatic pressure and GFR?

Answer 22

Decreases glomerular hydrostatic pressure, Decreases GFR

Question 23

What effect does increasing efferent arteriolar resistance have on hydrostatic pressure and GFR?

Answer 23

Increases glomerular hydrostatic pressure, Increases GFR

Question 24

Factors that determine renal blood flow

Answer 24

O2 consumption related to high rate of sodium reabsorption, tubular sodium reabsorption (closely related to GFR)

Question 25

Renal Blood Flow

Answer 25

(Renal artery pressure - Renal vein pressure)/Total vascular resistance

Question 26

What effect does the sympathetic system have on GFR?

Answer 26

Strong activation: constricts renal arterioles, decreases renal blood flow and GFR; Moderate activation: little effect

Question 27

Norepinephrine/epinephrine

Answer 27

Parallel the sympathetic system (constrict renal arterioles, decrease GFR)

Question 28

Endothelin

Answer 28

Released by damaged vascular endothelial cells; may contribute to renal vasoconstriction and reduced GFR

Question 29

Angiotensin II

Answer 29

Formed in situations associated with decreased arterial pressure/volume depletion. Preferentially constricts efferent arterioles, increases GFR; Afferent arterioles protected against effects

Question 30

Nitric Oxide

Answer 30

Derived from endothelial cells; Maintains renal vasodilation

Question 31

Prostaglandins and bradykinin

Answer 31

Vasodilators that may offset effects of sympathetic system and angiotensin

Question 32

Autoregulation

Answer 32

Maintains relatively constant GFR, allows precise control of renal excretion of water/solutes, prevents large changes in GFR/excretion that would normally occur with BP changes

Question 33

Normal daily fluid excretion

Answer 33

1.5 L/day

Question 34

What are the two components of the tubuloglomerular feedback mechanism for autoregulation?

Answer 34

Afferent arteriolar feedback mechanism, efferent arteriolar feedback mechanism

Question 35

JG complex

Answer 35

Macula densa in distal tubule, JG cells in afferent and efferent arterials (close together)

Question 36

How does JG complex participate in autoregulation?

Answer 36

Lower GFR = slow flow rate in loop of Henle = more reabsorption of Na+ and Cl- and loweded [NaCl] at macula densa = dilated afferent arterioles = increased renin release from JG cells = increased angiotensin II = increased efferent arteriolar resistance = INCREASED GFR