Airway Mechanics

Front 1

Describe how surface tension and sufactant are a major force contributing to lung compliance and hysteresis.

Back 1

Lungs consist of millions of alveoli (almost spheres) covered by a thin film of H20. Thus, surface tension has a large influence on the P-V behavior of the lung. This effect can be observed by comparing the compliance of saline-filled lungs and air-filled lungs. Saline filling has removed the air-liquid interface. The saline-filled lung has a compliance 3x greater than the air-filled lung.

The elastic properties of the tissue aren't changed and these dominate the P-V curve.

A normal compliance relationship occurs with hysteresis. This is also lost in saline filled lungs and is thought to result from the effects of surfactant.

At low lung volumes, alveoli collapse can occur. Thus the initial pressure goes to the opening of the alveoli. When the lung is relaxing, the alveoli are already open.

The hysteresis may also reflect the area-dependent action of surfactant.

Front 2

What is surface tension? How is it generated?

Back 2

Molecules in a sol'n have attractive forces acting between them. In the body of a fluid, these forces are equal in all directions. At the surface, a molecule is influenced by neighbors in the plane of the surface and by molecules deeper in the layer. Because of unequal forces, tension is developed at the surface.

Front 3

How does surface tension affect the lung?

Back 3

The lung consists of millions of little spheres (Alveoli) with connecting tubes. Each sphere is lined with fluid to dissolve gas.

Surface tension generates a force that favors the collapse of the alveolus and opposes lung distension. The extent of the force or pressure developed by ST is calculated according to LaPlace's law:

Pressure (bubble)= 4T/radius (T=tension).

An alveolus only has one surface affected, therefore:
P(alveoli)=2T/r

Thus, in a simple tube and bubble system, air at high pressure will move to sites of low pressure (alveoli with larger radii). The consequence of this, in a lung without sufactant, is that small alveoli would collapse as larger alveoli expand. This effect is referred to as alveolar instability and can be largely prevented by surfactant. A simple detergent cannot achieve the same effect.

Front 4

What type of alveolar cells secrete surfactant?

Back 4

Type II alveolar cells.

Front 5

What would happen to the lung if it was lined with water? What is nature's sol'n to this problem?

Back 5

Alveolar instability would result: all the small alveoli would try to collapse into the larger alveoli, which would reduce the surface area. Nature's sol'n to the problem: Surfactant: a complex phospholipid secreted by type II alveolar cells. Surfactant can intercalate btwn surface molecules and release the ST in the alveoli.

Front 6

Describe the physiological advantages of surfactant.

Back 6

Surface tension forces can be reduced by the addition of detergent. The interaction or alignment of detergent with the fluid molecules decreases their attractive forces. The lung has a unique form of detergent called surfactant, consisting of negatively charged phospholipid molecules that interact with the water molecules.

ST of water= 70 dynes/cm
ST of H2O + detergent= 30 dynes/cm
ST of H2O + surfactant= 5-45 dynes/cm.

Surfactant is secreted by alveoli type II cells that reside in the corners of alveoli (made of type I cells). It mainly consists of phospholipids, proteins, and calcium- (dipalmitoyl phosphatidyl choline) and is stored in a folded form in vesicles called lamella bodies, named because of their structure in the EM. The laminar structures unwind as surfactant is secreted and becomes tubular myelin.

Because surfactant is hydrophobic, it has a different action than detergent. When it's released, it spreads across the fluid surface. The hydrophobic region faces the airway. This is an important p

Front 7

Describe the relationship between area and surfactant, and the surface area/surface tension curve.

Back 7

Surfactant reduces the ST to a greater extent as the SA is decreased than when the SA is increased. This hysteresis in surfactant efficiency partly contributes to the hysteresis seen in the intact lung during inhalation and exhalation.

As alveoli get smaller, the pressure/ST forces developed are reduced. By constrast, as alveoli get bigger, the ST forces increase. These forces stabilize the alveoli.

Front 8

Aside from surfactant, what else increased alveoli stability?

Back 8

Alveolar stability is enhanced by the interdependency of the structures. All alveoli are interconnected. Therefore, it's difficult for one alveolus to collapse while surrounding adjacent alveoli enlarge. This is also know as tethering.

Front 9

How does surfactant reverse the flow of air between large and small alveoli?

Back 9

Due to surfactant, alveoli with small radii will have low pressure inside and alveoli with large radii will have greater pressure inside. Consequently, air will flow from large alveoli to small ones until the two volumes equilbrate. Without surfactant, air would flow from small alveoli to large ones, causing alveoli to collapse.

Front 10

What is the major reason for reduction in lung compliance? Give one example for how this can occur.

Back 10

Lack of surfactant is a major cause of reduced compliance. This can occur in respiratory distress syndrome.

Surfactant is synthesized late in fetal development: type II cells appear ~24 weeks of gestation and increase in # by 32 weeks. Thus, premature babies often lack sufficient amounts of lung surfactant and, as a result, the compliance of the premature lung is greatly decreased. Also, the alveoli are much smaller at birth, which increases the effects of ST. The consequence is that newborns suffer from respiratory difficulties known as respiratory distress syndrome.

You can have the same effects when something shuts down surfactant production in adults, leading to Acute Respiratory Distress Syndrome.

Front 11

Describe problems with airway collapse and forced ventilation in emphysema.

Back 11

The rate of exhalation is limited by airway compliance and is particularly relevant to airways with increased compliance (i.e., emphysema).

When you try to rapidly exhale, you quickly reach a maximum rate of air flow. A greater effort to exhale will not increase this flow rate. The reason for this is that your small airways (w/o cartilage) begin to collapse and prevent exhalation.

The pt at which collapse occurs is affected by lung volume. At large volumes, airways can be held open by keeping the lung well inflated--referred to as tethering. However, as the lungs begin to deflate, the effects of airway collapse become noticeable at mid-lung volumes.

At FRC, PA=0. Inhalation occurs by decreased Ppl (making it more negative). Air flows in until the steady-state returns where PA=0 (w/ mouth open).

At the onset of exhalation, the Ppl is rapidly increased (becomes more positive) by decreasing chest volume. This establishes a transairway pressure gradient that ranges from PA in the alveoli to zero at the mou

Front 12

Summary of Airway Collapse

Back 12

-Upper airways have cartilage support.
-Occurs mainly in the small airways: alveolar ducts, respiratory bronchioles, terminal bronchioles.
- Pressure gradient is enhanced by faster moving air (lower pressure).
- Easier collapse if compliance of the lung is increased and mechanical tethering is lost (i.e., with Emphysema).

Front 13

How does the Bernoulli effect impact Alveolar Pressure?

Back 13

When air moves faster, the pressure exerted by the air falls. In the lungs, air is still in the alveoli. But as it begins to move up the airways, the flow rate increases. Thus, the pressure in the small airways can fall during exhalation due to air flow as a result of the Bernoulli effect.

Front 14

How do emphysema patients compensate for airway collapse?

Back 14

They learn to overcome the problem of airway collapse by changing their breathing method.

By using pursed lips, the pts effectively shift the pressure drop between the alveoli and the mouth to higher up in the respiratory tract where the airways are stiffer. This also maintains the lung at a higher volume, using tethering to help open the airways, and maintains greater pressure inside than outside of the airway. This shift in the pressure gradient avoids the collapsing pressures in the airways without cartilage support.

Front 15

Describe how exhalation is limited by compliance and lung volume but can also be independent of effort.

Back 15

These effects can be seen in the flow-loops of a normal pt. Expirations are performed from TLC with increasing effort until maximum forced expiration occurs.

Although the initial flow rates increase with effort, the later flow rates at lower lung volumes converge to a single rate. This zone is referred to as being effort-independent. In other words, a greater effort cannot expel the air from the lung at low volumes.

Front 16

How are Flow-loops diagnostically helpful?

Back 16

The curve of the flow-loops will change for different lung diseases, allowing them to be used diagnostically.

Front 17

What is the influence of gravity on the shape of the lung?

Back 17

The lung consists of compressible tissue and has mass. In the absence of gravity, this would be evenly distributed.

But, under the influence of gravity, the shape of the lungs will be distorted. Just like a column of air or water, the tissue generates a gradient of pressure from top to bottom--slinky effect.

Tissue at the bottom is compressed, while tissue at the top is stretched out, forming a pear shape.

Front 18

How does gravity influence the pleural pressure and the position of the lung and chest?

Back 18

Under zero gravity, the negative pleural pressure evenly inflates the lung to fill the chest cavity. With gravity, the uniform effect of the pleural pressure is distorted by the pressures generated by the weight of the lung.

At the top of the chest, the lung tissue is trying to pull away from the chest wall. The result of this is that the effective Ppl in this region becomes more negative. So here the alveoli are expanded.

At the bottom of the chest, the lung is pressing up against the wall. The effect of this is that the Ppl becomes less negative. Here the alveoli are expanded less (compressed more).

Thus, the combination of uniform Ppl and a pressure gradient generated by the mass of the lung with gravity in an enclosed space can be expressed as a gradient of the Ppl. This gradient has significant consequences for the distribution of ventilation.

Front 19

Describe how gravity affects the distribution of ventilation.

Back 19

Air is not distributed evenly in the lungs. This is the result of: 1) the gradient in pleural pressure, and 2) the non-linear compliance curve of the lungs.

Lungs have a mass and this causes the bottom of the lungs to be compressed (Ppl is less negative) while at the top of the lungs, the tissue is expanded (Ppl is more negative). For example, if the Ppl is -7.5 cm H2O and the lung generates a pressure gradient from -2.5 to +2.5 cm H2O (the tissue pressure counteracts the Ppl), the effective Ppl pressure gradient would range from -10 to -5 cm H2O.

The effects of the differences in Ppl on alveolar size can be seen by examining the lung compliance curve.

Assume the lung is at the end of expiration and at FRC. In a lung with a uniform Ppl gradient, each alveolus would be inflated to the same size.

With gravity and the Ppl gradient, alveoli at different locations are inflated to different sizes. The relative amt of inflation can be determined from the compliance (PV) curve of the lung.

For alveoli near the

Front 20

How does gravity effect the distribution of ventilation during inhalation?

Back 20

From FRC, the chest is expanded to inhale. We'll assume that this changes the pleural pressure throughout the chest by -10 cm H20. The gradient due to gravity is still present. You're applying the same pressure change to each alveoli, but because of where they fall on the Ppl curve, the volume is going to expand more for alveoli at the bottom of the lungs than it will for alveoli at the top of the lungs. So, the air you're bringing in is going to the bottom of your lung preferentially and not as much is going to the top of your lung. This is due to gravity and comes back to the concept of V and Q matching.

For a given pressure change, alveolar volume changes occur in this order:
change in Vtop< change in Vmiddle< change in Vbottom.

--> The lower lung is ventilated more than the upper lung.

The same principle applies if the pt is sitting up or lying down, because the distribution of ventilation relates to gravity and not to a specific area of the lung. The gradient is less when lying down because the dime

Front 21

How does gravity affect V and Q matching?

Back 21

Due to gravity and its effects on the distribution of ventilation, the bottom of the lung is ventilated MUCH MORE than the top of the lung. Blood is also distributed in a similar way. Because of gravity, we don't have ideal matching at all parts of the lung. Overall, lungs seem to be well matched, but we have these local disparities, which affects blood gases, which affects the alveolar gradient.

The whole lung compliance curve is made for the whole lung, but depending on where that part of the lung is sitting at any one point due to gravity, the volume will differ depending on the Ppl.