Lung Physiology

The purpose of our pulmonary system is to deliver oxygen (oxygenate) and clear (ventilate) carbon dioxide from the blood stream. These goals are accomplished via a vast network of arterial / alveolar interfaces where gas exchange occurs. The anatomic "machine" that supplies endless cycles of fresh gas to the alveolar networks, is driven by the ribs, intercostal muscles, diaphragm and accessory muscles. This anatomic 'bellows', which is controlled by the medulla oblongata, pulls air in from the atmosphere and then holds it in the alveolar space long enough for the gas to distribute across 'open' alveolar units.

Factors that affect oxygenation:

Amount of inspiratory time
Lung compliance
Number of alveolar units engaged during a breath (a.k.a. 'recruitment) and in functional residual capacity (FRC)
Matching of ventilation (i.e. gas delivered to alveolus) to perfusion (capillary blood flow to peri-alveolar capillary networks). This idea of V/Q matching is important during pathophysiologic states affecting alveolar units (e.g. ARDs, inhalation injury, pulmonary contusion, pneumonia, etc.)

Factors that affect Carbon Dioxide clearance:

Amount of expiratory time
Lung compliance
Number of alveolar units engaged during a breath (a.k.a. 'recruitment) and in functional residual capacity (FRC)
Minute Ventilation = Respiratory Rate x Tidal Volume
- Expressed in L/minute
- Usually directly proportional to the amount of CO2 clearance

The network of alveolar units is extensive and alveolar groups can be affected by forces applied to any given segment of the lung. For example, when a patient lies supine for several hours, posterior lung regions begin to become atelectatic from the gravitational weight of the anterior lung & chest wall on the segments of posterior alveolar units. Another example is when a mucous plug obstructs a tertiary bronchiole, the alveolar units distal to this obstruction collapse from lack of air flow.

When intensivists manage respiratory failure, they consider the macro / micro lung anatomy and physiology to help determine therapy. Understanding how the respiratory apparatus works guides choices of ventilator modes and settings.

Respiratory Mechanics

When humans breath naturally, they use negative pressure to generate breaths. Respiratory mechanics is how the body controls air flow into and out of the lungs. Muscle contractions and elastic recoil affect change in pressure, flow of air and volume of each breath. During inspiration, the diaphragm and intercostals contract, expanding the chest cavity. The difference in pressure gradient between the atmosphere and intrapleural space causes an inward flow of oxygenated air. Expiration is typically a passive process of muscle relaxation and elastic recoil, though it becomes active during exercise or coughing.

Respiratory Mechanics

Inspiration (breathing in): This is an active process.
- the diaphragm contracts and moves down
- external intercostal muscles and accessory muscles contract, pulling the ribs up and out
- These actions expand the thoracic cavity, increasing it's volume and decreasing pressure inside the lungs to below atmospheric pressure. Air flows from the higher atmospheric pressure into the lungs.

Expiration (breathing out): This is typically a passive process.
- The inspiratory muscles relax.
- The elastic recoil of the lungs and chest wall causes them to return to their resting position. This increases the pressure inside the lungs, forcing air out.
- When active expiration occurs during exertion, involving the contraction of abdominal and internal intercostal muscles to forcefully push air out.

Pleura
- The pleural membranes (visceral pleura on the lungs and parietal pleura on the chest wall) and the fluid-filled pleural space between them act like two surfaces stuck together, ensuring the lungs move with the chest wall.

Definitions:

Pressure and flow: Respiratory mechanics are fundamentally about the relationship between pressure, volume, and flow, governed by physical laws.
Compliance: Measures how easily the lungs and chest wall can be stretched. It is the ratio of a change in volume to the corresponding change in pressure.
Resistance: Measures the opposition to gas flow in the airways. It is calculated by comparing the change in pressure to the flow rate.
Work of breathing: The total energy or work the respiratory muscles use to move air.
Clinical applications: Measuring these parameters helps monitor lung function, assess disease severity (like in ARDS), and adjust settings on mechanical ventilators.