Respiratory System Flashcards

Learning Outcomes

• By the end of this session, you will be able to:

  • Relate Boyle’s Law to ventilation in greater detail.

  • Describe the events that cause inhalation and exhalation, including the muscles involved and pressure changes.

Breathing

• Inhalation (Inspiration): The process of taking air into the lungs.

• Exhalation (Expiration): The process of expelling air from the lungs.

• Breathing is a mechanical process that relies on the contraction and relaxation of respiratory muscles, leading to changes in the volume of the thoracic cavity. These volume changes result in pressure gradients that drive airflow into and out of the lungs.

Boyle’s Law

• Boyle's Law explains the relationship between the volume and pressure of a gas: at a constant temperature, the volume of a gas varies inversely with its pressure.

• Mathematically represented as: P1V1 = P2V2 where:

  • P1 and V1 are the initial pressure and volume, respectively.

  • P2 and V2 are the final pressure and volume, respectively.

• This law is crucial in understanding how changes in the volume of the thoracic cavity during breathing cause air to flow into or out of the lungs.

Inhalation/Exhalation

• Inhalation:

  • Contraction of respiratory muscles, such as the diaphragm and external intercostals, increases the volume of the thoracic cavity. This increase in volume leads to a decrease in intra-alveolar pressure, causing air to flow into the lungs from an area of higher pressure (the atmosphere).

• Exhalation:

  • Relaxation of the respiratory muscles decreases the volume of the thoracic cavity. This decrease in volume leads to an increase in intra-alveolar pressure, causing air to flow out of the lungs to an area of lower pressure (the atmosphere).

Normal (Quiet) Inspiration

• During normal inspiration:

  • The diaphragm and external intercostal muscles contract.

  • This contraction increases the volume of the thoracic cavity.

  • As volume increases, pressure decreases inside the lungs (intra-alveolar pressure becomes lower than atmospheric pressure).

  • Air flows into the lungs, typically around 500 mL (tidal volume).

Normal (Quiet) Expiration

• Normal expiration is a passive process:

  • It depends on the natural elasticity of the lungs and the relaxation of the inspiratory muscles.

  • The inspiratory muscles relax, and the chest and lungs recoil.

  • This recoil decreases the volume of the thoracic cavity.

  • As volume decreases, pressure increases inside the lungs (intra-alveolar pressure becomes higher than atmospheric pressure).

  • Air flows out of the lungs.

Forced Inspiration

• During forced inspiration:

  • The sternocleidomastoid muscles raise the sternum, further increasing thoracic volume.

  • Scalene and pectoralis minor muscles raise the 1st to 5th ribs, contributing to a greater expansion of the thoracic cavity.

• These actions result in a larger decrease in intra-alveolar pressure, allowing more air to flow into the lungs than during normal inspiration.

Forced Expiration

• During forced expiration:

  • The internal intercostal and abdominal muscles contract.

  • This pulls the ribs down and forces the (relaxed) diaphragm upwards, further decreasing the volume of the thoracic cavity.

• This leads to a greater increase in intra-alveolar pressure, forcing more air out of the lungs than during normal expiration.

Muscles of Breathing - Summary

• Inspiration:

  • Normal: Diaphragm, External intercostal muscles

  • Forced: Diaphragm, External intercostal muscles, Sternocleidomastoid muscles, Scalene muscles, Pectoralis minor muscles

• Expiration:

  • Normal: None (passive)

  • Forced: Internal intercostal muscles, Abdominal muscles

Factors Affecting Pulmonary Ventilation

• Airway resistance

• Surface tension of alveolar fluid

• Lung compliance

Airway Resistance

• Airway resistance is the resistance to airflow in the respiratory passageways.

• Obstructing the airways or reducing their diameter (e.g., during bronchoconstriction) increases resistance.

• Increased resistance requires more pressure (and work) to force air through the airways, making breathing more difficult.

Surface Tension of Alveolar Fluid

• Surface tension is the attraction of liquid molecules to one another at a liquid-gas interface.

• Surfactant, produced by type II alveolar cells, decreases surface tension within the alveoli.

• It creates a thin elastic film, preventing the alveoli from collapsing and making it easier to inflate the lungs.

Lung Compliance

• Compliance refers to the effort required to stretch the lungs and chest wall.

• Lungs normally have high compliance, meaning they expand easily in response to pressure changes.

• This is due to elastic fibers in the lung tissue and surfactant reducing alveolar surface tension.

• Decreased lung compliance is common in lung disorders, such as:

  • Tuberculosis (scar tissue)

  • Fluid in lung tissue (pulmonary edema)

  • Surfactant deficiency

• Conditions that decrease lung compliance increase the work required to breathe.