The Respiratory System: Structure, Function, and Physiology

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The Respiratory System

Overview and Organization

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. It is divided into the upper respiratory system (nose, paranasal sinuses, pharynx) and the lower respiratory system (larynx to alveoli). The system is further organized into conducting and respiratory zones.

Conducting zone: Passageways for air to reach sites of gas exchange (nose to terminal bronchioles).
Respiratory zone: Site of gas exchange (respiratory bronchioles, alveolar ducts, alveoli).
Respiratory muscles: Diaphragm and other muscles that promote ventilation.

Anatomy of the respiratory system, showing nasal cavity, pharynx, larynx, trachea, bronchi, and lungs

Major Functions of the Respiratory System

Processes of Respiration

The respiratory system's primary function is to supply oxygen and remove carbon dioxide. Four key processes are involved:

Pulmonary ventilation: Movement of air into and out of the lungs.
External respiration: Gas exchange between lungs and blood.
Transport: Movement of oxygen and carbon dioxide between lungs and tissues.
Internal respiration: Gas exchange between systemic blood vessels and tissues.

Upper Respiratory System

Structure and Function of the Nose

The nose is the only externally visible part of the respiratory system. It provides an airway, moistens and warms air, filters and cleans inspired air, serves as a resonating chamber for speech, and houses olfactory receptors.

External nose: Includes root, bridge, dorsum nasi, apex, philtrum, and external nares (nostrils).
Internal nasal cavity: Divided by the nasal septum, opens into the nasopharynx, and is bounded by the ethmoid and sphenoid bones (roof) and hard/soft palates (floor).

Surface anatomy of the nose, showing root, bridge, dorsum nasi, apex, philtrum, and naris Anatomical structure of the nose, showing bones and cartilages

Nasal Cavity and Associated Structures

The nasal cavity is lined with vibrissae (hairs) to filter particles, olfactory mucosa for smell, and respiratory mucosa that secretes mucus with lysozyme and defensins. The conchae increase surface area and air turbulence, aiding in filtering, heating, and moistening air.

Paranasal sinuses: Lighten the skull and help warm/moisten air.

Sagittal section of the head showing nasal cavity, conchae, sinuses, and pharynx

Pharynx

The pharynx is a muscular tube connecting the nasal cavity and mouth to the larynx and esophagus. It is divided into three regions:

Nasopharynx: Air passageway, lined with pseudostratified columnar epithelium.
Oropharynx: Common passageway for food and air, lined with stratified squamous epithelium.
Laryngopharynx: Common passageway for food and air, posterior to the epiglottis.

Lower Respiratory System

Larynx (Voice Box)

The larynx attaches to the hyoid bone and opens into the laryngopharynx. It provides an airway, routes air and food, and functions in voice production. The framework consists of hyaline cartilages (thyroid, cricoid, arytenoid, cuneiform, corniculate) and the elastic cartilage epiglottis.

Laryngeal cartilages and ligaments, anterior and sagittal views

Vocal Folds

Vocal folds (true vocal cords) are elastic fibers that vibrate to produce sound. The glottis is the opening between them. Vestibular folds (false vocal cords) are superior and do not produce sound.

Superior view of the larynx showing true and false vocal cords, glottis, and epiglottis

Trachea

The trachea is a flexible tube extending from the larynx into the mediastinum. It consists of three layers: mucosa (goblet cells, ciliated epithelium), submucosa (connective tissue), and adventitia (outer connective tissue with C-shaped hyaline cartilage rings).

Cross-section of the trachea showing mucosa, submucosa, cartilage, and muscle

Bronchial Tree and Lungs

The bronchi branch from the trachea into primary, secondary, and tertiary bronchi, undergoing 23 orders of branching. Bronchioles lack cartilage and have more smooth muscle. The lungs are divided into lobes and bronchopulmonary segments.

Bronchial tree and lobes of the lungs

Respiratory Zone Structures

The respiratory zone includes respiratory bronchioles, alveolar ducts, and alveoli. Alveoli are the primary sites of gas exchange, with approximately 300 million in the lungs, providing a large surface area.

Diagram of respiratory bronchioles, alveolar ducts, and alveolar sacs Histology and diagram of alveoli and alveolar pores Alveolar structure showing capillaries, elastic fibers, and smooth muscle

Respiratory Membrane

The respiratory membrane consists of alveolar and capillary walls and their fused basal laminas. Type I cells allow gas exchange by diffusion; Type II cells secrete surfactant. Alveoli contain pores and macrophages for air pressure equalization and sterility.

Microscopic structure of the respiratory membrane and alveolar wall

Pleura and Thoracic Cavity

The pleura is a double-layered serosa: parietal pleura lines the thoracic wall, and visceral pleura covers the lungs. The pleural cavity contains fluid to reduce friction during breathing.

Transverse section of the thoracic cavity showing pleura, lungs, and mediastinum

Mechanics of Breathing

Pressure Relationships

Breathing depends on pressure differences between the atmosphere, alveoli, and pleural cavity. Intrapulmonary pressure equalizes with atmospheric pressure; intrapleural pressure is always less, preventing lung collapse. Transpulmonary pressure keeps airways open.

Diagram of pressure relationships in the thoracic cavity

Inspiration and Expiration

Inspiration occurs when inspiratory muscles contract, increasing thoracic volume and decreasing intrapulmonary pressure, causing air to flow in. Expiration is usually passive, resulting from muscle relaxation and lung recoil, increasing intrapulmonary pressure and expelling air.

Sequence of events during inspiration Sequence of events during expiration

Physical Factors Influencing Ventilation

Airway resistance, alveolar surface tension, and lung compliance affect ventilation. Surfactant reduces surface tension, preventing alveolar collapse. Lung compliance is determined by tissue distensibility and alveolar surface tension.

Graph of airway resistance in conducting and respiratory zones

Gas Exchange and Transport

External and Internal Respiration

Gas exchange occurs by diffusion across partial pressure gradients. Oxygen diffuses from alveoli to blood; carbon dioxide diffuses from blood to alveoli. Internal respiration is the exchange between systemic capillaries and tissues.

Diagram of oxygen and carbon dioxide exchange in the body Graph of oxygen partial pressure in pulmonary capillary

Oxygen Transport

Oxygen is transported bound to hemoglobin (Hb) and dissolved in plasma. The oxygen-hemoglobin dissociation curve shows the relationship between PO2 and hemoglobin saturation. Factors such as pH, temperature, and BPG affect oxygen release (Bohr effect).

Carbon Dioxide Transport

Carbon dioxide is transported dissolved in plasma, bound to hemoglobin, and as bicarbonate ions. The chloride shift maintains ionic balance during CO2 transport. The Haldane effect describes increased CO2 transport when PO2 is low.

Oxygen release and carbon dioxide pickup at the tissues Oxygen pickup and carbon dioxide release in the lungs

Control of Respiration

Neural Regulation

Respiratory centers in the medulla (DRG and VRG) and pons (apneustic and pneumotaxic centers) regulate breathing rhythm and depth. Reflexes and higher brain centers (hypothalamus, cortex) also influence respiration.

Chemical Regulation

Central and peripheral chemoreceptors respond to changes in CO2, O2, and pH. Increased CO2 (hypercapnia) stimulates increased ventilation. Substantial drops in O2 are required to stimulate breathing.

Clinical and Developmental Aspects

Respiratory Volumes and Capacities

Key volumes include tidal volume (TV), inspiratory reserve volume (IRV), expiratory reserve volume (ERV), and residual volume (RV). Capacities are combinations of these volumes. Pulmonary function tests (e.g., spirometry) assess respiratory health.

Respiratory Adjustments

Exercise and high altitude require adjustments in ventilation and oxygen transport. Acclimatization involves increased ventilation and erythropoietin release. Respiratory efficiency decreases with age.