Pulmonary Ventilation

Page 1.  Introduction

Pulmonary ventilation, or breathing, is the exchange of air between the atmosphere and the lungs.

As air moves into and out of the lungs, it travels from regions of high air pressure to regions of low air pressure

Page 2.  Goals

To relate Boyle's law to ventilation.

To identify the muscles used during ventilation.

To understand how volume changes in the thoracic cavity cause pressure changes that lead to breathing.

To identify factors which influence airway resistance and lung compliance.

Page 3.  Boyle's Law:  Relationship Between Pressure and Volume

In order to understand ventilation, we must first look at the relationship between pressure and volume. 

Pressure is caused by gas molecules striking the walls of a container. 

The pressure exerted by the gas molecules is related to the volume of the container.

This large sphere contains the same number of gas molecules as the original sphere. Notice that in this larger volume, the gas molecules strike the wall less frequently, thus exerting less pressure.

In this small sphere, the gas molecules strike the wall more frequently, thus exerting more pressure. Notice that the number of gas molecules has not changed.

These demonstrations illustrate Boyle's Law, which states that the pressure of a gas is inversely proportional to the volume of its container. Thus, if you increase the volume of a container, the pressure will decrease, and if you decrease the volume of a container, the pressure will increase.

Page 4.  Quiet Inspiration: Muscle Contraction

The volume of the thoracic cavity is changed by muscle contraction and relaxation.

During quiet inspiration, the diaphragm and the external intercostal muscles contract, slightly enlarging the thoracic cavity. 

As we learned from Boyle's Law, increasing the volume decreases the pressure within the thoracic cavity and the lungs.

Notice how the diaphragm flattens and moves inferiorly while the external intercostal muscles elevate the rib cage and move the sternum anteriorly. These actions enlarge the thoracic cavity in all dimensions.

As we learned from Boyle's Law, increasing the volume decreases the pressure within the thoracic cavity and the lungs.

Page 5.  Quiet Expiration: Muscle Relaxation

Quiet expiration is a passive process, in which the diaphragm and the external intercostal muscles relax, and the elastic lungs and thoracic wall recoil inward. 

This decreases the volume and therefore increases the pressure in the thoracic cavity.

As the diaphragm relaxes, it moves superiorly. As the external intercostal muscles relax, the rib cage and sternum return to their resting positions. These actions decrease the size of the thoracic cavity in all dimensions, and therefore increase the pressure in the thoracic cavity.

Page 6.  Muscles of Deep Inspiration and Expiration

Deep breathing uses forceful contractions of the inspiratory muscles and additional accessory muscles to produce larger changes in the volume of the thoracic cavity during both inspiration and expiration.

Label the muscles in this diagram.  Indicate which muscles are used during each of the following processes:  a. quiet inspiration b. quiet expiration c. deep inspiration  d. forceful expiration

  

During deep inspiration, the diaphragm and the external intercostal muscles contract more forcefully than during quiet breathing. Additionally, the sternocleidomastoid and scalenes contract, lifting the rib cage higher. These actions further increase the volume. As we learned from Boyle's Law, this decreases the pressure within the thoracic cavity. 

 

Deep or forceful expiration is an active process. The internal intercostal muscles  depress the rib cage, and the external oblique, internal oblique, transversus abdominis and rectus abdominis muscles compress the abdominal organs, forcing them superiorly against the diaphragm. These actions can dramatically decrease the volume, and further increase the pressure within the thoracic cavity, producing forceful expiration.

 

 

Page 7.  Intrapulmonary Pressure Changes

Now let's look at the specific pressure changes that occur in the lungs during breathing. For reasons described later, the lungs closely follow the movements of the thoracic wall.

The pressure within the lungs is called the intrapulmonary, or intra-alveolar, pressure.

Between breaths, it equals atmospheric pressure, which has a value of 760 millimeters of mercury at sea level.  When discussing respiratory pressures, this is generally referred to as zero.

During inspiration, the volume of the thoracic cavity increases, causing intrapulmonary pressure to fall below atmospheric pressure. This is also known as a negative pressure. Since air moves from areas of high to low air pressure, air flows into the lungs. Notice that at the end of inspiration, when the intrapulmonary pressure again equals atmospheric pressure, airflow stops. 

During expiration, the volume of the thoracic cavity decreases, causing the intrapulmonary pressure to rise above atmospheric pressure. Following its pressure gradient, air flows out of the lungs, until, at the end of expiration, the intrapulmonary pressure again equals atmospheric pressure.

Draw the pressure changes that occur during inspiration and expiration on this graph:

 

 

Page 8.  Intrapleural Pressure

Label this diagram:

 

Intrapleural pressure is the pressure within the pleural cavity. Intrapleural pressure is always negative, which acts like a suction to keep the lungs inflated.

The negative intrapleural pressure is due to three main factors:

1. The surface tension of the alveolar fluid.

The surface tension of the alveolar fluid tends to pull each of the alveoli inward and therefore pulls the entire lung inward.  Surfactant reduces this force.

2. The elasticity of the lungs. 

The abundant elastic tissue in the lungs tends to recoil and pull the lung inward. As the lung moves away from the thoracic wall, the cavity becomes slightly larger.  The negative pressure this creates acts like a suction to keep the lungs inflated.

3. The elasticity of the thoracic wall.

The elastic thoracic wall tends to pull away from the lung, further enlarging the pleural cavity and creating this negative pressure.  The surface tension of pleural fluid resists the actual separation of the lung and thoracic wall.

 Page 9.  Intrapleural Pressure Changes

Intrapleural pressure changes during breathing:

As the thoracic wall moves outward during inspiration, the volume of the pleural cavity increases slightly, decreasing intrapleural pressure.

As the thoracic wall recoils during expiration, the volume of the pleural cavity decreases, returning the pressure to minus 4, or 756 millimeters of mercury.

Draw the changes in intrapleural pressure on this graph:

 

 

Page 10.  Effect of Pneumothorax

If you cut through the thoracic wall into its pleural cavity, air enters the pleural cavity as it moves from high pressure to low pressure. This is called a pneumothorax.

Normally, there is a difference between the intrapleural and intrapulmonary pressures, which is called transpulmonary pressure. The transpulmonary pressure creates the suction to keep the lungs inflated. In this case, when there is no pressure difference there is no suction and the lung collapses.

The lungs are completely separate from one another, each surrounded by its own pleural cavity and pleural membranes. Therefore, changes in the intrapleural pressure of one lung do not affect the other lung.

Page 11.  Events During Inspiration

Label this graph as you work through this page.

Let's review all the events that occur during inspiration.  The upper graph shows the intrapulmonary pressure, that is, the pressure within the lungs.  The middle graph shows the intrapleural pressure, the pressure within the pleural cavity. The region between the two graphs is the transpulmonary pressure, the pressure difference between the intrapulmonary and intrapleural pressures.  The lower graph shows the volume of air which enters and leaves the lungs during quiet breathing. This is called the tidal volume.

During inspiration, the diaphragm and external intercostal muscles contract, increasing the volume of the thoracic cavity. This causes the intrapleural pressure to become more negative, which increases the transpulmonary pressure, causing the lungs to expand.  The expansion of the lungs lowers the intrapulmonary pressure below atmospheric pressure.  Air, following its pressure gradient, now flows into the lungs.

 

Page 12.  Events During Expiration

During expiration, the diaphragm and external intercostal muscles relax, decreasing the volume of the thoracic cavity. The intrapleural pressure becomes less negative, the transpulmonary pressure decreases, and the lungs passively recoil. This increases the intrapulmonary pressure so that it rises above atmospheric pressure. Air, following its pressure gradient, moves out of the lungs. Watch how the three pressures change together on the graph.

Page 13.  Events During Inspiration and Expiration

Let's correlate the graphs with the movements of the thoracic cavity during inspiration and expiration.

Page 14.  Other Factors Affecting Ventilation

 Two other important factors play roles in ventilation:

1. The resistance within the airways.

2. Lung compliance.

Page 15.  Resistance Within Airways

As air flows into the lungs, the gas molecules encounter resistance when they strike the walls of the airway. Therefore the diameter of the airway affects resistance.

When the bronchiole constricts, the diameter decreases, and the resistance increases.  This is because more gas molecules encounter the airway wall. Airflow is inversely related to resistance.

This relationship is shown by the equation:

Airflow equals the pressure difference between atmosphere and intrapulmonary pressure, divided by the resistance.

As the resistance increases, the airflow decreases.

As the resistance decreases, the airflow increases. 

In healthy lungs, the airways typically offer little resistance, so air flows easily into and out of the lungs.

Page 16.  Factors Affecting Airway Resistance

Several factors change airway resistance by affecting the diameter of the airways. They do this by contracting or relaxing the smooth muscle in the airway walls, especially the bronchioles.

Parasympathetic neurons release the neurotransmitter acetylcholine, which constricts bronchioles. As you can see in the equation, increased airway resistance decreases airflow.

Histamine, released during allergic reactions, constricts bronchioles. This increases airway resistance and decreases airflow, making it harder to breathe.

Epinephrine, released by the adrenal medulla during exercise or stress, dilates bronchioles, thereby decreasing airway resistance. This greatly increases airflow, ensuring adequate gas exchange.

Page 17.  Lung Compliance: Elastic Fibers

Another important factor affecting ventilation is the ease with which the lungs expand, also known as lung compliance. It is primarily determined by two factors: 

1. The stretchability of the elastic fibers within the lungs.

2. The surface tension within the alveoli. 

Healthy lungs have high compliance because of their abundant elastic connective tissue.

Low lung compliance occurs in some pathological conditions, such as fibrosis, in which increasing amounts of less flexible connective tissue develop.

Page 18.  Lung Compliance: Surface Tension

The second factor affecting lung compliance is surface tension within the alveoli.

Some premature infants do not produce surfactant. Is their lung compliance high or low? 

Without surfactant, alveoli have high surface tension, and they tend to collapse. Collapsed alveoli resist expansion, so lung compliance is low. This condition is known as respiratory distress syndrome of the newborn. Natural or synthetic surfactant may be sprayed into the infant's respiratory passageways. Surfactant lowers surface tension and increases lung compliance.

Page 19.  Summary

Muscle activity causes changes in the volume of the thoracic cavity during breathing.

Changing the thoracic cavity volume causes intrapulmonary and intrapleural pressure changes, which allow air to move from high pressure to low pressure regions.

Airway resistance is normally low, but nervous stimulation and chemical factors can change the diameter of bronchioles, thereby altering resistance and airflow.

Lung compliance is normally high due to the lung's abundant elastic tissue and surfactant's ability to lower the surface tension of the alveolar fluid.

 

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