Pulmonary ventilation is the process of breathing, which is driven by pressure differences between the lungs and the atmosphere. Atmospheric pressure is the force exerted by gases present in the atmosphere. The force exerted by gases within the alveoli is called intra-alveolar (intrapulmonary) pressure, whereas the force exerted by gases in the pleural cavity is called intrapleural pressure. Typically, intrapleural pressure is lower, or negative to, intra-alveolar pressure. The difference in pressure between intrapleural and intra-alveolar pressures is called transpulmonary pressure. In addition, intra-alveolar pressure will equalize with the atmospheric pressure. Pressure is determined by the volume of the space occupied by a gas and is influenced by resistance. Air flows when a pressure gradient is created, from a space of higher pressure to a space of lower pressure. Boyle’s law describes the relationship between volume and pressure. A gas is at lower pressure in a larger volume because the gas molecules have more space to in which to move. The same quantity of gas in a smaller volume results in gas molecules crowding together, producing increased pressure.
Resistance is created by inelastic surfaces, as well as the diameter of the airways. Resistance reduces the flow of gases. The surface tension of the alveoli also influences pressure, as it opposes the expansion of the alveoli. However, pulmonary surfactant helps to reduce the surface tension so that the alveoli do not collapse during expiration. The ability of the lungs to stretch, called lung compliance, also plays a role in gas flow. The more the lungs can stretch, the greater the potential volume of the lungs. The greater the volume of the lungs, the lower the air pressure within the lungs.
Pulmonary ventilation consists of the process of inspiration (or inhalation), where air enters the lungs, and expiration (or exhalation), where air leaves the lungs. During inspiration, the diaphragm and external intercostal muscles contract, causing the rib cage to expand and move outward, and expanding the thoracic cavity and lung volume. This creates a lower pressure within the lung than that of the atmosphere, causing air to be drawn into the lungs. During expiration, the diaphragm and intercostals relax, causing the thorax and lungs to recoil. The air pressure within the lungs increases to above the pressure of the atmosphere, causing air to be forced out of the lungs. However, during forced exhalation, the internal intercostals and abdominal muscles may be involved in forcing air out of the lungs.
Respiratory volume describes the amount of air in a given space within the lungs, or which can be moved by the lung, and is dependent on a variety of factors. Tidal volume refers to the amount of air that enters the lungs during quiet breathing, whereas inspiratory reserve volume is the amount of air that enters the lungs when a person inhales past the tidal volume. Expiratory reserve volume is the extra amount of air that can leave with forceful expiration, following tidal expiration. Residual volume is the amount of air that is left in the lungs after expelling the expiratory reserve volume. Respiratory capacity is the combination of two or more volumes. Anatomical dead space refers to the air within the respiratory structures that never participates in gas exchange, because it does not reach functional alveoli. Respiratory rate is the number of breaths taken per minute, which may change during certain diseases or conditions.
Both respiratory rate and depth are controlled by the respiratory centers of the brain, which are stimulated by factors such as chemical and pH changes in the blood. These changes are sensed by central chemoreceptors, which are located in the brain, and peripheral chemoreceptors, which are located in the aortic arch and carotid arteries. A rise in carbon dioxide or a decline in oxygen levels in the blood stimulates an increase in respiratory rate and depth.
alveolar dead space
air space within alveoli that are unable to participate in gas exchange
anatomical dead space
air space present in the airway that never reaches the alveoli and therefore never participates in gas exchange
apneustic center
network of neurons within the pons that stimulate the neurons in the dorsal respiratory group; controls the depth of inspiration
atmospheric pressure
amount of force that is exerted by gases in the air surrounding any given surface
Boyle’s law
relationship between volume and pressure as described by the formula: P1V1 = P2V2
central chemoreceptor
one of the specialized receptors that are located in the brain that sense changes in hydrogen ion, oxygen, or carbon dioxide concentrations in the brain
dorsal respiratory group (DRG)
region of the medulla oblongata that stimulates the contraction of the diaphragm and intercostal muscles to induce inspiration
expiration
(also, exhalation) process that causes the air to leave the lungs
expiratory reserve volume (ERV)
amount of air that can be forcefully exhaled after a normal tidal exhalation
forced breathing
(also, hyperpnea) mode of breathing that occurs during exercise or by active thought that requires muscle contraction for both inspiration and expiration
functional residual capacity (FRC)
sum of ERV and RV, which is the amount of air that remains in the lungs after a tidal expiration
inspiration
(also, inhalation) process that causes air to enter the lungs
inspiratory capacity (IC)
sum of the TV and IRV, which is the amount of air that can maximally be inhaled past a tidal expiration
inspiratory reserve volume (IRV)
amount of air that enters the lungs due to deep inhalation past the tidal volume
intra-alveolar pressure
(intrapulmonary pressure) pressure of the air within the alveoli
intrapleural pressure
pressure of the air within the pleural cavity
peripheral chemoreceptor
one of the specialized receptors located in the aortic arch and carotid arteries that sense changes in pH, carbon dioxide, or oxygen blood levels
pneumotaxic center
network of neurons within the pons that inhibit the activity of the neurons in the dorsal respiratory group; controls rate of breathing
pulmonary ventilation
exchange of gases between the lungs and the atmosphere; breathing
quiet breathing
(also, eupnea) mode of breathing that occurs at rest and does not require the cognitive thought of the individual
residual volume (RV)
amount of air that remains in the lungs after maximum exhalation
respiratory cycle
one sequence of inspiration and expiration
respiratory rate
total number of breaths taken each minute
respiratory volume
varying amounts of air within the lung at a given time
thoracic wall compliance
ability of the thoracic wall to stretch while under pressure
tidal volume (TV)
amount of air that normally enters the lungs during quiet breathing
total dead space
sum of the anatomical dead space and alveolar dead space
total lung capacity (TLC)
total amount of air that can be held in the lungs; sum of TV, ERV, IRV, and RV
transpulmonary pressure
pressure difference between the intrapleural and intra-alveolar pressures
ventral respiratory group (VRG)
region of the medulla oblongata that stimulates the contraction of the accessory muscles involved in respiration to induce forced inspiration and expiration
vital capacity (VC)
sum of TV, ERV, and IRV, which is all the volumes that participate in gas exchange
Watch this video to learn more about lung volumes and spirometers. Explain how spirometry test results can be used to diagnose respiratory diseases or determine the effectiveness of disease treatment.
Patients with respiratory ailments (such as asthma, emphysema, COPD, etc.) have issues with airway resistance and/or lung compliance. Both of these factors can interfere with the patient’s ability to move air effectively. A spirometry test can determine how much air the patient can move into and out of the lungs. If the air volumes are low, this can indicate that the patient has a respiratory disease or that the treatment regimen may need to be adjusted. If the numbers are normal, the patient does not have a significant respiratory disease or the treatment regimen is working as expected.
1. Which of the following processes does atmospheric pressure play a role in?
A) pulmonary ventilation
B) production of pulmonary surfactant
C) resistance
D) surface tension
A
2. A decrease in volume leads to a(n) ________ pressure.
A) decrease in
B) equalization of
C) increase in
D) zero
C
3. The pressure difference between the intra-alveolar and intrapleural pressures is called ________.
A) atmospheric pressure
B) pulmonary pressure
C) negative pressure
D) transpulmonary pressure
D
4. Gas flow decreases as ________ increases.
A) resistance
B) pressure
C) airway diameter
D) friction
A
5. Contraction of the external intercostal muscles causes which of the following to occur?
A) The diaphragm moves downward.
B) The rib cage is compressed.
C) The thoracic cavity volume decreases.
D) The ribs and sternum move upward.
D
6. Which of the following prevents the alveoli from collapsing?
A) residual volume
B) tidal volume
C) expiratory reserve volume
D) inspiratory reserve volume
A
1. Describe what is meant by the term “lung compliance.”
Lung compliance refers to the ability of lung tissue to stretch under pressure, which is determined in part by the surface tension of the alveoli and the ability of the connective tissue to stretch. Lung compliance plays a role in determining how much the lungs can change in volume, which in turn helps to determine pressure and air movement.
2. Outline the steps involved in quiet breathing.
Quiet breathing occurs at rest and without active thought. During quiet breathing, the diaphragm and external intercostal muscles work at different extents, depending on the situation. For inspiration, the diaphragm contracts, causing the diaphragm to flatten and drop towards the abdominal cavity, helping to expand the thoracic cavity. The external intercostal muscles contract as well, causing the rib cage to expand, and the rib cage and sternum to move outward, also expanding the thoracic cavity. Expansion of the thoracic cavity also causes the lungs to expand, due to the adhesiveness of the pleural fluid. As a result, the pressure within the lungs drops below that of the atmosphere, causing air to rush into the lungs. In contrast, expiration is a passive process. As the diaphragm and intercostal muscles relax, the lungs and thoracic tissues recoil, and the volume of the lungs decreases. This causes the pressure within the lungs to increase above that of the atmosphere, causing air to leave the lungs.
3. What is respiratory rate and how is it controlled?
3. What is respiratory rate and how is it controlled?
Respiratory rate is defined as the number of breaths taken per minute. Respiratory rate is controlled by the respiratory center, located in the medulla oblongata. Conscious thought can alter the normal respiratory rate through control by skeletal muscle, although one cannot consciously stop the rate altogether. A typical resting respiratory rate is about 14 breaths per minute.