Chapter 7: Mechanics of Breathing

Question 1

What spinal segments does the phrenic nerve origniate from in dogs and cats?

Answer 1

C5, C6, C7 (some also C4)

Question 2

What does a paradoxical movement of the diaphragm indicate?

Answer 2

when one side of the diaphragm is paralized and cannot contract, it will move into the opposite direction –> towards the thorax, because it is pulled by the falling intrathoracic pressure

Question 3

Compare the direction of the two layers of intercostal muscles and how they affect breathing

Answer 3

External intercostal muscles –> run from caudoventral to craniodorsal –> raise ribs up and front and widen the chest cavity - INSPIRATION

Internal intercostal muscles –> run from caudodorsal to cranioventral –> pull ribs down and together and narrow chest cavity - EXPIRATION

intercostal muscles are supplied from the same spinal segments where their ribs originate

Question 4

List accessory muscles of inspiration

Answer 4

• scalene muscles
• sternomastoids (in people sternocleidomastoids, but dogs don’t have clavicles)
• alae nasi
• small neck and head muscles

Question 5

What are the muscles of expiration?

Answer 5

muscles of the abdominal wall - rectus abdominis, internal and external oblique muscles, transversus abdominis

Internal intercostal muscles

Question 6

What happens to the lung volume if the intrathoracic pressure rises above atmospheric pressure

Answer 6

airway closure

little further air is lost because the small airways will close, trapping gas in the alveoli

Question 7

What is transpulmonary pressure?

Answer 7

difference in pressure between the inside and outside of the lung - alveolar pressure and intrathoracic pressure

Question 8

How does the pressure - volume curve of the lung change in spontaneous versus mechanically ventilated patients?

Answer 8

spontaneous - pressure around lungs (negative value) on X-axis
ventilated - airway pressure on X-axis (generated by machine)

Question 9

What is the equation for compliance and what is its unit?

Answer 9

Complicance = change in volume/change in pressure

mL/cm H2O

Question 10

Name conditions that can lower pulmonary compliance

Answer 10

• pulmonary fibrosis
• pulmonary edema
• atalectasis/collapse
• increased surface tension
• increased pulmonary venous pressure

Question 11

Name conditions that can increase pulmonary compliance

Answer 11

• pulmonary emphysema
• aging lung

Question 12

what is lung elastance?

Answer 12

the lungs tendency to return to its resting volume after distension

fibers of elastin and collagen

Question 13

Name the Laplace law for alveolar surface tension

Answer 13

P (collapsing pressure) = 2 T(surface tension)/radius

the smaller the radius the higher the collapsing pressure

Question 14

What is the main constituent of surfactant?

Answer 14

phospholipid –> DPPC
dipalmitoyl phosphatidylcholine

Question 15

How does pulmonary blood flow affect surfactant production?

Answer 15

surfactant turn-over is rapid and is synthesized from fatty acids that are mostly supplied through the blood –> if blood flow drops (e.g., lung emblolus) –> surfactant can be rapidly depleted

Question 16

Explain briefly how surfactant decreases surface tension

Answer 16

surfactant has a hydrophobic and hydrophilic end –> have intermolecular repulsive forces –> oppose the normal attracting forces of the liquid surface molecules on the alveolar surface

Question 17

What are the 3 main pathophysiologic consequences of lack of surfactant?

Answer 17

• decreased compliance –> harder to expand collapsing lungs
• atalactasis
• transudate filling alveolir –> surface tension sucks fluid out of capillaries into alveoli

Question 18

What is pulmonary interdependence?

Answer 18

most alveoli are arranged with many surrounding alveoli –> create stabilizing traction forces/expanding forces –> keeps alveoli open

Question 19

How does the distribution of ventilation between the apex and base of the lung change from normal lung volumes to very low lung volumes?

Answer 19

the dependent/lower base of the lung has the most ventilation during normal lung volumes because of a low resting volume and high change in volume

at very low volumes the intrapleural pressure can rise above atmospheric pressure at the base leading to airway closure less ventilation at the base

Question 20

What part of the airways collapses first during airway closure?

Answer 20

respiratory bronchioles –> leads to air trapping in the distal alveoli

Question 21

Describe Poiseuille’s law for Flow

Answer 21

P - pressure difference/driving pressure
n - viscosity
l - lenght

Question 22

Describe Poiseuille’s law for resistance

Answer 22

n - viscosity
l - lenght

Question 23

If the radius is halved, by how much does the resistance increase?

Answer 23

16-fold

Question 24

If the length is doubled, by how much does the resistance change?

Answer 24

doubled as well

Question 25

In laminar flow, how much faster does the center of the tube move?

Answer 25

twice as fast as the average velocity

“velocity profile”

Question 26

How do you calculate the likelihood of flow to be laminar or turbulent

Answer 26

With the Reynolds number, Re

Re = 2rvd/n

r - radius
v - velocity
d - density
n - viscosity

in a straight, smooth tube –> turbulence likely if Re >2,000

Question 27

Explain how intrapleural and alveolar pressures change during inspiration and expiration

Answer 27

before inspiration
* intrapleural pressure is about -5 in the normal person
* alveolar pressure is 0 (i.e., same as athmospheric) becasue no airflow present and no pressure gradient

inspiration
* intrapleural pressure drops while chest expands due to elastic recoil forces
* alveolar pressure drops - only minimal drop needed to establish airflow (usually drops by -1)

expiration
* intrapleural pressure increases
* alveolar pressure increases

Question 28

Where is the major site of airway resistance?

Answer 28

medium-sized bronchi (i.e., segmental bronchi)

Question 29

Why does airway resistance increase at low lung volumes?

Answer 29

bronchi are supported by radial traction (similar to extraalveolar vessles) –> caliber increased when lungs expand
reduced lung volume –> bronchi collapse –> resistance increases

Question 30

How does parasympathetic acitvity affect airway resistance?

Answer 30

causes bronchoconstriction –> increases resistance

Question 31

Explain why forced expiration fails to increase the speed at which a subject reaches the residual volume

Answer 31

forced expiration –> more chest wall compression –> increased intrapleural pressure –> lead to dynamic airway compression, increasing resistance, making flow effort-independent

Question 33

Explain these graphs, explain the disease processes in B and C

Answer 33

FEV - forced expiratory volume
FVC - forced vital capacity, i.e., total volume exhaled - includes the effort-independent expiration

A - normal FEV is about 80% of FVC
B - obstructive disease - forced expiratory volume is reduced due to dynamic dynamic compression of airways
C - restrictive - primary problem is expandingon inhalation

Question 34

What is pulmonary resistance?

Answer 34

different from airway resistance!

pressure required to overcome the viscous forces within tissues as they slide over each other

Question 35

Explain the work of breathing with this graph

Answer 35

pressure-volume curve showing the intrapleural pressure needing to be generated to achieve the required volume

• 0-A-B-C-D-0 = work required
• 0-A-E-C-D-0 = work required to overcome elastic forces
• A-B-C-E-A = work required to overcome viscous (airway and tissue) resistance

reduced compliance –> more elastic work required and will take small rapid breaths

airway obstruciton –> slow breathing

Question 36

What percentage of the resting O2 consumption does the work of breathing require?

Answer 36

5%

(voluntary hyperventilation can increase it as much as 30%)