Chapter 7: Mechanics of Breathing Flashcards
What spinal segments does the phrenic nerve origniate from in dogs and cats?
C5, C6, C7 (some also C4)
What does a paradoxical movement of the diaphragm indicate?
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
Compare the direction of the two layers of intercostal muscles and how they affect breathing
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
List accessory muscles of inspiration
- scalene muscles
- sternomastoids (in people sternocleidomastoids, but dogs don’t have clavicles)
- alae nasi
- small neck and head muscles
What are the muscles of expiration?
muscles of the abdominal wall - rectus abdominis, internal and external oblique muscles, transversus abdominis
Internal intercostal muscles
What happens to the lung volume if the intrathoracic pressure rises above atmospheric pressure
airway closure
little further air is lost because the small airways will close, trapping gas in the alveoli
What is transpulmonary pressure?
difference in pressure between the inside and outside of the lung - alveolar pressure and intrathoracic pressure
How does the pressure - volume curve of the lung change in spontaneous versus mechanically ventilated patients?
spontaneous - pressure around lungs (negative value) on X-axis
ventilated - airway pressure on X-axis (generated by machine)
What is the equation for compliance and what is its unit?
Complicance = change in volume/change in pressure
mL/cm H2O
Name conditions that can lower pulmonary compliance
- pulmonary fibrosis
- pulmonary edema
- atalectasis/collapse
- increased surface tension
- increased pulmonary venous pressure
Name conditions that can increase pulmonary compliance
- pulmonary emphysema
- aging lung
what is lung elastance?
the lungs tendency to return to its resting volume after distension
fibers of elastin and collagen
Name the Laplace law for alveolar surface tension
P (collapsing pressure) = 2 T(surface tension)/radius
the smaller the radius the higher the collapsing pressure
What is the main constituent of surfactant?
phospholipid –> DPPC
dipalmitoyl phosphatidylcholine
How does pulmonary blood flow affect surfactant production?
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
Explain briefly how surfactant decreases surface tension
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
What are the 3 main pathophysiologic consequences of lack of surfactant?
- decreased compliance –> harder to expand collapsing lungs
- atalactasis
- transudate filling alveolir –> surface tension sucks fluid out of capillaries into alveoli
What is pulmonary interdependence?
most alveoli are arranged with many surrounding alveoli –> create stabilizing traction forces/expanding forces –> keeps alveoli open
How does the distribution of ventilation between the apex and base of the lung change from normal lung volumes to very low lung volumes?
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
What part of the airways collapses first during airway closure?
respiratory bronchioles –> leads to air trapping in the distal alveoli
Describe Poiseuille’s law for Flow
P - pressure difference/driving pressure
n - viscosity
l - lenght
Describe Poiseuille’s law for resistance
n - viscosity
l - lenght
If the radius is halved, by how much does the resistance increase?
16-fold
If the length is doubled, by how much does the resistance change?
doubled as well
In laminar flow, how much faster does the center of the tube move?
twice as fast as the average velocity
“velocity profile”
How do you calculate the likelihood of flow to be laminar or turbulent
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
Explain how intrapleural and alveolar pressures change during inspiration and expiration
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
Where is the major site of airway resistance?
medium-sized bronchi (i.e., segmental bronchi)
Why does airway resistance increase at low lung volumes?
bronchi are supported by radial traction (similar to extraalveolar vessles) –> caliber increased when lungs expand
reduced lung volume –> bronchi collapse –> resistance increases
How does parasympathetic acitvity affect airway resistance?
causes bronchoconstriction –> increases resistance
Explain why forced expiration fails to increase the speed at which a subject reaches the residual volume
forced expiration –> more chest wall compression –> increased intrapleural pressure –> lead to dynamic airway compression, increasing resistance, making flow effort-independent
Explain these graphs, explain the disease processes in B and C
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
What is pulmonary resistance?
different from airway resistance!
pressure required to overcome the viscous forces within tissues as they slide over each other
Explain the work of breathing with this graph
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
What percentage of the resting O2 consumption does the work of breathing require?
5%
(voluntary hyperventilation can increase it as much as 30%)
