Lung Ventilation and Pressure

Components of Breathing

• Ventilation refers to how air moves into and out of the lungs
• Inspiration is the process of air flowing into the lungs
• Expiration is the process of air leaving the lungs
• The rest period is the brief pause between inspiration and expiration

Pressures

• Air flow direction is determined by the difference between atmospheric and alveolar pressure
• Atmospheric pressure refers to the pressure of air in the environment
• Alveolar pressure refers to the pressure of the air inside the alveoli
• Intrapleural pressure (intrathoracic pressure) refers to the pressure of fluid inside the pleural cavity that surrounds the lungs
• Intrapleural pressure is usually negative compared to alveolar/atmospheric pressures
• Transmural pressure is the difference between alveolar pressure and intrapleural pressure
• Airways remain open when transmural pressure is positive
• Lungs tend to collapse during rest, while the chest wall tends to expand, acting as opposing forces

Tidal Volume

• Tidal volume is the volume of air that fills the alveoli with an inspiration plus the volume of air that fills the airways

Respiratory Center

• A group of neurons control ventilation

Medulla Oblongata

• The medulla oblongata contains the dorsal and ventral respiratory groups
• The dorsal respiratory group (DRG) initiates respiration and determines rhythm of breathing
• The ventral respiratory group (VRG) facilitates forced expiration
• The VRG sends inhibitory impulses to the apneustic center in the pons
• The VRG decreases duration of inspiration, allowing more time for a longer expiration

Pons

• The pons contains pontine respiratory group
• The pontine respiratory group contains the pneumotaxic and apneustic centers
• The pneumotaxic center allows for expiration to happen
• Activation of the pneumotaxic center limits activation by the DRG, decreasing action potentials of the phrenic nerve and stopping inspiration
• The apneustic center is activated when the body requires more oxygen than normal
• Stimulation of neurons in the apneustic center excites the dorsal respiratory group and prolongs action potentials in the phrenic nerve, prolonging contraction of the diaphragm and inspiration

Chemoreceptors

• Chemoreceptors monitor the concentration of CO2 and O2 in the blood
• Chemoreceptors send signals to the respiratory center neurons if concentration is abnormal
• Normal arterial partial pressure of oxygen (PaO2) is 100 mmHg
• Normal arterial partial pressure of carbon dioxide (PaCO2) is 40 mmHg
• Normal arterial pH is 7.4

Muscles

Quiet Breathing

• The diaphragm increases the length of the thoracic cavity
• External intercostal muscles pull ribs up and out, increasing the width of the thoracic cavity

Accessory Muscles

• Accessory muscles contract during vigorous inspiration to increase the volume of the thoracic cavity beyond tidal volume by moving the ribs up and out
• Examples of accessory muscles: sternocleidomastoid and scalene muscles

Nerves

• The phrenic nerve innervates the diaphragm
• Intercostal nerves innervate the external intercostal muscles

Breathing Cycle

• In the rest phase, the diaphragm is at its balanced position
• Alveolar pressure is equal to atmospheric pressure during the rest phase, so no air is moving in or out of the lungs
• Transmural pressure is +5cm during the rest phase, so airways are open

Inspiration

• Increased PaCO2 is detected by chemoreceptors
• The DRG sends a command via the phrenic nerve to the diaphragm and intercostal nerves to external intercostal muscles
• Muscles contract and the volume of the thorax increases
• Lung volume increases, causing alveolar pressure to decrease below atmospheric pressure, this creates a pressure gradient so air moves from higher pressure to lower pressure, into the alveoli
• Flow of air stops when pressure in the alveoli equals pressure in the atmosphere

Expiration

• The diaphragm relaxes, so air leaves the lungs and the lungs return to their initial size
• Alveolar pressure increases and exceeds atmospheric pressure, creating a pressure gradient so air moves from high pressure to lower pressure, out of the lungs
• All volumes and pressures return to their values at rest, and a new breathing cycle begins