January 12, 2016 - Respiratory Mechanics

Question 1

Muscles of Respiration

Answer 1

Inspiration is an active process. The diaphragm is the most important muscle, but the external intercostals are also involved, along with the accessory muscles, the scalenes and sternocleidomastoids. These are involved during respiratory distress.

Expiration is a passive process during rest. Expiration becomes active during exercise, or in respiratory distress in which the abdominal muscles and internal intercostals become involved.

Question 2

Intercostal Muscles

Answer 2

Muscles that run between the ribs.

External intercostals are involved in inhalation (particularly during respiratory distress) as they pull the ribs out and expand the chest cavity.

Internal intercostals are involved during forced exhalation or during respiratory distress as they pull the chest cavity in to expulge the air. (Abdominal muscles are also used during respiratory distress)

Question 3

Pressure Change in Lungs

Answer 3

The pressure in the lungs will drop with inspiration; this negative pressured causes air to flow from the mouth to the alveoli in the lungs down a pressure gradient.

Question 4

Balance of Pressure in Lungs

Answer 4

The intra-pleural space has a more negative pressure than the middle of the lungs.

This causes the chest wall to want to spring out, and the lung to want to recoil in.

Question 5

Lung Compliance

Answer 5

The ability of the lungs to expand under pressure.

Change in lung volume per change in transpulmonary pressure.

Question 6

Surface Tension

Answer 6

Because the alveoli are spherical and are coated with water, the water’s polar bonds want to come into contact with each other. This would mean collapsing the alveoli.

There needs to be a sufficient gas pressure to balance the collapsing force of the surface tension. Because the alveoli are so small, this results in a large balancing pressure needed to prevent collapse.

Type II cells in the alveolus secrete surfactant which works to reduce surface tension within the alveoli and prevent the collapse of alveoli and they remain inflated.

Question 7

Surfactant

Answer 7

Secreted by Type II cells in the alveoli.

Prevents the collapse of the alveoli due to the surface tension of the water molecules.

Clinically useful for intubation in premature babies. When the babies come out, their lungs wont open up unless you intubate their lungs with surfactant. This is saved many lives.

Question 8

Clinical Use of Surfactant

Answer 8

Clinically useful for intubation in premature babies. When the babies come out, their lungs wont open up unless you intubate their lungs with surfactant. This is saved many lives.

Question 9

Ventilation and Perfusion are Not Uniform

Answer 9

The lung has weight.

Alveoli at the top are stretched open while alveoli at the base are smaller and more crowded because the base weighs more.

Ventilation and perfusion are higher at the bases than the apices.

Question 10

Relaxation Pressure-Volume Curve

Answer 10

When you inhale, you are using ATP as chemical energy to contract the muscle and breath in, but because of the elastin that surrounds the alveolar sacs, when you exhale at rest, you are using “free” elastic potential energy.

At the FRC (functional residual capacity) in the graph, this is the volume of air present in the lungs at the end of passive expiration. At FRC, the opposing elastic recoil forces of the lungs and the chest wall are in equilibrium and there is no exertion by the diaphragm or any other respiratory muscles.

Question 11

Elastin

Answer 11

Acts as a “rubber band” around the alveoli.

When they are stretched open during inspiration, they will want to recoil back.

Question 12

Airway Resistance

Answer 12

Dependent on…

1. Gas viscosity
2. Length of the tube
3. Radius of the tube

If the radius is halved, the resistance increases 16-fold.

Question 13

Why is Flow Decreased in COPD?

Answer 13

1. Increased resistance (mucus/remodelling)
2. Lung destruction –> Loss of small airways
3. Lower recoil due to loss of elastic tissue
4. Decreased radial traction on airways by the lung

Question 14

Dynamic Hyperinflation

Answer 14

Dynamic hyperinflation is a phenomenon that occurs when a new breath begins before the lung has reached the static equilibrium volume.

When a patient is exercising with a disease, they don’t exhale all the airs out of their lungs after each new breath, and breaths “stack” on top of each other. Functional residual capicity increases over time and the patient becomes very uncomfortable.

Question 15

The Significance of FRC

Answer 15

1. It is the equilibrium volume where the elastic recoil of the lung is balanced by the tendency for the chest wall to spring out
2. It represents the residual volume of the lung
3. It is the volume where the small airways collapse, trapping air in the chest