During normal quiet breathing, known as eupnea, inspiration is an active process that involves contraction of inspiratory muscles to expand the thoracic cavity. This expansion leads to a decrease in intraalveolar, intrapleural, and intracheastic pressure, allowing air to flow into the lungs down its pressure gradient. There are several key events that occur during a normal inspiration at rest:
Contraction of the diaphragm
The diaphragm is the primary muscle of inspiration during quiet breathing. It is a large, dome-shaped muscle that separates the thoracic and abdominal cavities. Upon contraction, the diaphragm flattens out, increasing the vertical dimension of the thoracic cavity. This causes the pleural pressure to fall, creating a pressure gradient that allows air to passively flow into the lungs.
Contraction of external intercostal muscles
The external intercostal muscles are located between the ribs. During inspiration, these muscles contract, causing the rib cage to expand and move upward and outward. This further expands the thoracic cavity, supplementing the action of the diaphragm. Contraction of the external intercostals also stabilize the expanded rib cage.
Accessory inspiratory muscle contraction
During deeper inspirations, additional accessory muscles are recruited to further expand the rib cage. These include the scalene and sternocleidomastoid muscles of the neck which raise the ribs when contracted. The pectoralis minor muscles can also assist in elevating the ribs.
Increased lung volume
As the thoracic cavity expands, the lungs follow due to their attachment to the thoracic wall. This causes the alveoli within the lungs to expand, creating a pressure gradient that allows air to flow into the alveoli down a pressure gradient. Lung volume increases during inspiration as more air fills the alveoli.
Opening of upper airway
For air to successfully travel into the lungs, the upper airway must first be opened. During inspiration, upper airway muscles such as the genioglossus and geniohyoid muscles of the tongue contract to maintain an open passageway.
Decreased intrathoracic pressure
As the volume of the thoracic cavity increases during inspiration, the intrathoracic pressure decreases. This creates a pressure gradient between the atmosphere (~760 mmHg) and the alveoli (~748 mmHg at rest) that allows air to rush into the lungs down its pressure gradient.
Increased lung compliance
As the lungs expand, lung compliance increases. Compliance refers to the ease of expansion of the lung tissue and is greatest at mid-lung volumes. The increased compliance allows more inspiration to occur for a given inspiratory muscular effort.
Pulmonary ventilation
Pulmonary ventilation is the rate at which gas enters the alveoli during breathing. It is determined by tidal volume (amount of air inhaled/exhaled with each breath) and respiratory rate. At rest, the average adult respiratory rate is 12-20 breaths/min and tidal volume is ~500 mL/breath, equaling a pulmonary ventilation of about 6-10 L/min.
Laminar airflow
Airflow within the conducting airways of the lungs is laminar or smooth during normal resting breathing. The air flows in straight lines along the bronchioles and bronchi without disruption until reaching the alveoli.
Airway Resistance
There is always some resistance to airflow within the respiratory tract due to friction between air and the airway walls. At rest, airway resistance is low, only accounting for ~20% of the total work of breathing. Airway resistance increases during exercise and obstructive lung diseases.
Surfactant function
Pulmonary surfactant is a substance composed of phospholipids and proteins that lines the alveoli. It serves to reduce surface tension within the alveoli, preventing their collapse during resting breathing. Normal surfactant function maintains alveolar inflation during inspiration.
Olfactory epithelium stimulation
As air enters the nostrils during inspiration, it flows over the olfactory epithelium. This stimulates olfactory receptors that transmit smell sensations to the brain. However, this olfactory stimulation is minimal during relaxed breathing.
Maintains partial pressures of gases
Inspiration helps maintain the partial pressures of gases within the alveoli and bloodstream. Oxygen (O2) diffuses into the blood as carbon dioxide (CO2) diffuses out, allowing for proper gas exchange. These partial pressures are balanced during normal inspiration at rest.
Minimal alteration in blood gases
Relaxed, resting inspiration does not significantly alter blood oxygen or carbon dioxide levels. Partial pressures in the bloodstream remain relatively constant during normal eupneic breathing.
Activation of stretch receptors
As the lungs expand during inspiration, stretch receptors located in the smooth muscle of the airways are activated. Known as pulmonary stretch receptors, these send nerve impulses to the respiratory centers of the brainstem to regulate breathing rate and depth.
Undetection by chemoreceptors
Chemoreceptors located in the brainstem and carotid arteries detect changes in blood oxygen, carbon dioxide, and pH. However, during normal resting breathing, no significant changes occur in these levels for the chemoreceptors to detect.
Summary of the key events of inspiration at rest:
- Contraction of the diaphragm and external intercostal muscles
- Enlargement of the thoracic cavity
- Expansion of lung volume
- Decrease in intrathoracic pressure
- Airflow into lungs down a pressure gradient
- Opening of upper airways
- Increased lung compliance
- Pulmonary ventilation of ~6-10 L/min
- Laminar airflow in airways
- Minimal airway resistance
- Surfactant function maintained
- Stretch receptor activation
- Minimal stimulatory effects on other receptors
- Stable blood gas levels
Neural control of inspiration
Inspiration during resting breathing is an involuntary process controlled by the brainstem respiratory centers including the:
- Dorsal respiratory group (DRG)
- Ventral respiratory group (VRG)
- Nucleus ambiguous
- Pontine respiratory group (PRG)
These areas stimulate the diaphragm and intercostal muscles to contract via the phrenic and external intercostal nerves. The VRG functions as the primary respiratory rhythm generator in the brainstem.
Table summarizing neural control of inspiration:
| Brainstem respiratory centers | Function |
|---|---|
| Dorsal respiratory group | Generates rhythmic inspiratory activity |
| Ventral respiratory group | Primary respiratory rhythm generator |
| Nucleus ambiguous | Innervates laryngeal muscles to open airway |
| Pontine respiratory group | Regulates rate and depth of breathing |
Biomechanics of inspiration
There are important biomechanical principles that allow normal inspiration to occur:
- Boyle’s Law – Volume and pressure within the thorax have an inverse relationship. As volume increases during inspiration, pressure decreases.
- Poiseuille’s Law – Rate of airflow is proportional to the pressure gradient and radius of the airway. A larger pressure gradient and wider airways allow greater inspiratory airflow.
- Compliance – The ease of expansion of the lungs increases during inspiration, permitting greater volume changes.
- Airway Resistance – Is low during resting breathing due to laminar airflow in the large conducting airways.
- Alveolar Law – The alveoli expand causing intrapulmonary pressure to drop, creating the pressure gradient for inspiratory airflow.
These physical laws govern movement of gases into the lungs during inspiration.
Metabolic effects
Inspiration has minimal metabolic effects during resting breathing. It does not significantly impact oxygen consumption, carbon dioxide production, cardiac output, or other metabolic processes in the body. This allows resting cellular metabolism to be maintained during eupneic breathing.
Comparison to forced inspiration
Forced inspirations, such as during exercise, involve much more robust contractions of the inspiratory muscles. More accessory muscles are recruited, the diaphragm descends further, and deeper expansions of the thorax occur. This leads to a much larger tidal volume and inspiratory flow rates. The respiratory rate also increases. In addition, forced inspirations can alter blood gas and pH levels, stimulating the peripheral and central chemoreceptors to provide feedback to the respiratory centers.
Conclusion
In summary, normal inspiration at rest involves coordinated contractions of the diaphragm, external intercostals, and accessory muscles to expand the thoracic cavity. This reduces intrathoracic pressure, creating airflow into the lungs down a pressure gradient. Lung volume increases as air fills the alveoli, while resistance remains low. Inspiration is regulated by brainstem respiratory centers and follows key biomechanical principles. It has minimal effects on metabolism or stimulation of chemoreceptors during resting eupneic breathing.
