Functional Anatomy of the Respiratory System

The Upper Respiratory System: Warming, Humidifying, and Filtering Air

The upper respiratory system is responsible for conditioning inhaled air by warming, humidifying, and filtering it before it reaches the lower respiratory tract. It includes the nose, paranasal sinuses, and pharynx.

• Nose and Paranasal Sinuses: The nose provides an airway for respiration, moistens and warms air, filters and cleans incoming air, and houses olfactory receptors for smell.

• Pharynx: A muscular tube that connects the nasal cavity to the larynx and esophagus, serving as a passageway for air and food.

• Protective Mechanisms: Nasal hairs, mucus, and cilia trap and remove particles and pathogens from inhaled air.

Example: Sneezing and coughing are reflexes that help expel irritants from the upper respiratory tract.

Conducting and Respiratory Zone Structures

Distinction Between Conducting and Respiratory Zones

The conducting zone consists of all respiratory passageways that provide a route for air to reach the gas exchange sites, while the respiratory zone is where gas exchange occurs.

• Larynx: Attaches to the hyoid bone, opens into the laryngopharynx, and attaches inferiorly to the trachea. It contains vocal cords and is involved in sound production and airway protection.

• Trachea: A windpipe that extends from the larynx to the mediastinum, supported by C-shaped cartilages and lined with ciliated pseudostratified epithelium.

• Bronchi and Subdivisions: The trachea divides into right and left primary bronchi, which further branch into secondary and tertiary bronchi, bronchioles, and terminal bronchioles.

• Respiratory Zone: Includes respiratory bronchioles, alveolar ducts, and alveoli, which are the sites of gas exchange.

Additional info: The transition from conducting to respiratory zone is marked by the presence of alveoli.

Lung Structure and Pleural Cavities

Gross Structure of the Lungs and Pleura

The lungs occupy the thoracic cavity, each within its own pleural cavity. The right lung is larger and divided into three lobes, while the left lung has two lobes and a cardiac notch.

• Pleura: A double-layered serous membrane consisting of the parietal pleura (lining the thoracic wall) and visceral pleura (covering the lungs).

• Pleural Fluid: Fills the pleural cavity, reducing friction and allowing smooth movement during breathing.

Blood Supply: Pulmonary arteries carry deoxygenated blood to the lungs, while pulmonary veins return oxygenated blood to the heart.

Respiratory Pressures and Pulmonary Ventilation

Volume and Pressure Changes in Breathing

Breathing is driven by changes in thoracic volume, which create pressure gradients that move air in and out of the lungs.

• Intrapulmonary Pressure (Ppul): Pressure within the alveoli; rises and falls during breathing but always equalizes with atmospheric pressure.

• Intrapleural Pressure (Pip): Pressure within the pleural cavity; always negative relative to intrapulmonary pressure.

• Transpulmonary Pressure: The difference between intrapulmonary and intrapleural pressures; determines lung size.

Boyle's Law: Describes the inverse relationship between pressure and volume in a closed system:

Pulmonary Ventilation: Inspiration is an active process involving diaphragm and intercostal muscle contraction, while expiration is usually passive.

Measuring Respiratory Volumes, Capacities, and Flow Rates

Respiratory Volumes and Capacities

Respiratory volumes are measured to assess lung function and include:

• Tidal Volume (TV): Amount of air moved in and out of the lungs during normal breathing (about 500 ml).

• Inspiratory Reserve Volume (IRV): Amount of air that can be forcibly inhaled after a normal inspiration (2100–3200 ml).

• Expiratory Reserve Volume (ERV): Amount of air that can be forcibly exhaled after a normal expiration (1000–1200 ml).

• Residual Volume (RV): Air remaining in the lungs after forced expiration (about 1200 ml).

• Inspiratory Capacity (IC):

• Functional Residual Capacity (FRC):

• Vital Capacity (VC):

• Total Lung Capacity (TLC):

Example: Restrictive pulmonary disorders decrease VC, TLC, FRC, and RV, while obstructive disorders affect FEV (forced expiratory volume).

Gas Exchange by Diffusion Between Blood, Lungs, and Tissues

Dalton's and Henry's Laws

Gas exchange is governed by physical laws describing the behavior of gases in mixtures and solutions.

• Dalton's Law: The total pressure exerted by a mixture of gases is the sum of the pressures exerted by each gas.

• Henry's Law: The amount of gas that dissolves in a liquid is proportional to its partial pressure and solubility.

External Respiration: Involves O2 uptake and CO2 unloading between alveoli and blood, driven by partial pressure gradients and diffusion.

Example: At the alveoli, O2 diffuses into blood while CO2 diffuses out, maintaining efficient gas exchange.

Physical Factors Influencing Pulmonary Ventilation

Airway Resistance, Alveolar Surface Tension, and Lung Compliance

Several physical factors affect the ease of pulmonary ventilation:

• Airway Resistance: Friction encountered by air as it moves through the respiratory passages; increased resistance makes breathing more difficult.

• Alveolar Surface Tension: Surfactant produced by type II alveolar cells reduces surface tension, preventing alveolar collapse.

• Lung Compliance: The ease with which lungs can expand; decreased compliance is seen in fibrosis and other restrictive diseases.

Additional info: Surfactant is essential for normal lung function, especially in newborns.

Summary Table: Key Respiratory Volumes and Capacities