Chapter 15: The Respiratory System – Structure, Function, and Physiology

The Respiratory System: Overview

Introduction to the Respiratory System

The respiratory system is responsible for the movement of air into and out of the lungs and for gas exchange between the air and the bloodstream. Gas exchange occurs at specialized structures called alveoli. The system also plays roles in sound production and olfaction (sense of smell).

• Gas exchange between air and blood

• Movement of air along passageways to and from gas-exchange surfaces

• Protection of respiratory surfaces from dehydration, temperature changes, and pathogens

• Production of sounds for communication

• Assistance in olfaction

Organization of the Respiratory System

Anatomical Divisions

The respiratory system is divided into the upper respiratory system and the lower respiratory system:

• Upper respiratory system: Nose, nasal cavity, paranasal sinuses, pharynx. Functions to filter, warm, and humidify incoming air.

• Lower respiratory system: Larynx, trachea, bronchi, lungs (including bronchioles and alveoli).

Functional Zones

• Conducting portion: Nasal cavity to larger bronchioles; filters, warms, and humidifies air.

• Respiratory portion: Smallest bronchioles and alveoli; site of gas exchange.

Respiratory Mucosa and Defense Mechanisms

Respiratory Mucosa

The conducting portion is lined with respiratory mucosa, a ciliated columnar epithelium with mucous cells. The mucociliary escalator sweeps mucus and trapped debris toward the pharynx, where it is swallowed and destroyed in the stomach.

Structures of the Upper Respiratory System

The Nose and Nasal Cavity

Air enters through the nares (nostrils) into the nasal cavity, which is divided by the nasal septum. The hard palate forms the floor, and the soft palate lies posteriorly. Nasal conchae increase turbulence, warming and humidifying air. The nasal cavity is flushed by mucus and tears, which help trap debris and pathogens.

The Pharynx

The pharynx (throat) is a shared chamber for the respiratory and digestive systems. It is divided into three regions:

• Nasopharynx: Posterior to nasal cavity; contains pharyngeal tonsil and auditory tube openings.

• Oropharynx: Posterior to oral cavity; contains palatine tonsils.

• Laryngopharynx: Posterior to larynx; leads to esophagus.

Structures of the Lower Respiratory System

The Larynx

The larynx (voice box) surrounds and protects the glottis. It is composed of nine cartilages, including the epiglottis (prevents food entry during swallowing), thyroid cartilage (Adam's apple), and cricoid cartilage. The larynx contains the vocal cords for sound production.

Vocal Cords and Sound Production

• False vocal cords: Upper, inelastic; protect true vocal cords.

• True vocal cords: Lower, elastic; produce sound when air passes through.

• Pitch is determined by the length, thickness, and tension of the vocal cords.

The Trachea and Bronchi

The trachea (windpipe) is a flexible tube supported by C-shaped cartilages, which keep the airway open and allow food to pass through the esophagus. The trachea branches into the right and left main bronchi, which enter the lungs.

The Bronchial Tree and Bronchioles

The bronchial tree consists of branching airways: main bronchi, lobar bronchi (2 left, 3 right), segmental bronchi, and bronchioles. As airways branch, their diameter decreases and cartilage is replaced by smooth muscle. Bronchioles control airflow via muscle contraction (bronchodilation and bronchoconstriction).

Gas Exchange Structures

Terminal and Respiratory Bronchioles

Terminal bronchioles are the smallest conducting airways, each supplying a pulmonary lobule. They branch into respiratory bronchioles, which may participate in gas exchange and lead to alveolar ducts.

Alveolar Ducts and Alveoli

Alveolar ducts end in alveolar sacs composed of many alveoli. Each lung contains about 150 million alveoli, providing a large surface area (~140 m2) for rapid gas diffusion.

Alveolar Cells and Surfactant

• Pneumocytes type I: Thin squamous cells for gas exchange.

• Pneumocytes type II: Produce surfactant, reducing surface tension and preventing alveolar collapse.

• Alveolar macrophages: Phagocytize debris and pathogens.

Lung Anatomy and Pleural Cavities

Lobes and Surfaces of the Lungs

• Right lung: Three lobes (superior, middle, inferior)

• Left lung: Two lobes (superior, inferior), with a cardiac notch for the heart

• Apex: Top of the lung; base: rests on diaphragm

Pleural Cavities and Membranes

Each lung is surrounded by a pleural cavity lined by serous membranes:

• Visceral pleura: Covers lung surface

• Parietal pleura: Lines thoracic wall and diaphragm

• Pleural fluid: Reduces friction between layers

Mechanics of Breathing (Pulmonary Ventilation)

Pressure and Volume Changes

Air moves into and out of the lungs due to pressure gradients created by changes in thoracic volume. When lung volume increases, pressure decreases and air flows in (inhalation). When volume decreases, pressure increases and air flows out (exhalation).

Muscles of Respiration

• Diaphragm: Main muscle of inspiration; contraction increases thoracic volume.

• External intercostals: Assist in elevating the rib cage.

• Accessory muscles: Used during forced breathing.

Lung Compliance

Compliance is the ease with which the lungs expand. High compliance means lungs expand easily; low compliance requires more effort. Factors affecting compliance include alveolar damage, surfactant levels, and thoracic mobility.

Lung Volumes and Capacities

Definitions and Values

• Tidal volume (TV): Air moved in or out during quiet breathing (~500 mL)

• Expiratory reserve volume (ERV): Air expelled after normal exhalation

• Inspiratory reserve volume (IRV): Air inhaled above tidal volume

• Vital capacity: $TV + IRV + ERV$

• Residual volume: Air remaining after maximal exhalation

• Anatomic dead space: Air in conducting passages not involved in gas exchange (~150 mL)

Summary Table: Lung Volumes and Capacities

Volume/Capacity	Definition	Typical Value (Male)	Typical Value (Female)
Tidal Volume (TV)	Air moved per breath at rest	500 mL	500 mL
Expiratory Reserve Volume (ERV)	Max air exhaled after normal exhalation	1000 mL	700 mL
Inspiratory Reserve Volume (IRV)	Max air inhaled after normal inhalation	3300 mL	1900 mL
Residual Volume	Air remaining after maximal exhalation	1200 mL	1100 mL
Vital Capacity	TV + IRV + ERV	4800 mL	3100 mL

Key Terms and Concepts

• External respiration: Exchange of gases between lungs and blood

• Internal respiration: Exchange of gases between blood and tissues

• Hypoxia: Low tissue oxygen

• Anoxia: Absence of oxygen, leading to cell death

Clinical Notes

• Pneumothorax: Air in pleural cavity causing lung collapse

• Pulmonary embolism: Blockage of pulmonary artery branch

• Respiratory distress syndrome: Lack of surfactant, leading to alveolar collapse