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Respiratory System Anatomy and
Physiology
Respiratory System
Functions of the Respiratory System
The respiratory system serves several important functions:
Extensive surface area for gas exchange : The lungs provide a large surface area for the exchange of oxygen and carbon dioxide between the air and the blood.
Air movement : The respiratory system moves air to and from the gas exchange surfaces of the lungs.
Defense : The respiratory system contains mucosa-associated lymphoid tissue (MALT) that helps defend against pathogens.
Sound production : The respiratory system facilitates the production of sounds, including speech.
Olfaction : The respiratory system enables the detection of olfactory stimuli by the olfactory receptors.
Processes of Respiration
The respiratory system works in conjunction with the cardiovascular system to facilitate the processes of respiration:
Pulmonary ventilation (breathing) : The movement of air into and out of the lungs.
External respiration : The exchange of oxygen and carbon dioxide between the lungs and the blood.
Transport : The transport of oxygen and carbon dioxide in the blood.
Internal respiration : The exchange of oxygen and carbon dioxide between the systemic blood vessels and the tissues.
Gross Anatomy
The respiratory system is divided into two main parts:
Upper Respiratory Tract : Structures located in the head and neck, including the nose, nasal cavity, and pharynx.
Lower Respiratory Tract : Structures located in the thoracic cavity, including the larynx, trachea, bronchi, and lungs.
The respiratory muscles, including the diaphragm and other respiratory muscles, promote ventilation.
Functional Anatomy
The respiratory system can be further divided into two functional zones:
Conducting Zone : Structures that serve as conduits to the gas exchange sites, including the nose, pharynx, larynx, trachea, bronchi, and bronchioles. These structures also clean, warm, and humidify the air.
Respiratory Zone : The site of gas exchange, including the respiratory bronchioles, alveolar ducts, and alveoli.
Upper Airway
Nose and Nasal Cavity
The nose and nasal cavity contain two types of mucosa:
Olfactory mucosa : Contains the olfactory epithelium, which is responsible for the sense of smell.
Respiratory mucosa : Produces mucous and serous secretions that contain lysozyme and defensins. The mucosa also has a ciliary escalator that helps remove particles and pathogens.
The nasal cavity also has sensory nerve endings that can trigger sneezing, and the inspired air is warmed by the plexuses of capillaries and veins.
The nasal cavity is lined with ciliated columnar respiratory epithelium.
Homeostatic Imbalance: Rhinitis
Rhinitis is an inflammation of the nasal mucosa. Since the nasal mucosa is continuous with the mucosa of the respiratory tract, the inflammation can spread to the tear ducts and paranasal sinuses. This can lead to blocked sinus passageways, air absorption, and a vacuum that causes sinus headaches.
Pharynx
The pharynx is divided into three parts: the nasopharynx, oropharynx, and laryngopharynx. This marks the beginning of the lower respiratory tract.
Gross Anatomy of the Lungs
The lungs occupy the thoracic cavity, except for the mediastinum. They have a costal surface (anterior, lateral, and posterior) and are composed primarily of alveoli, with a balance of stroma (elastic connective tissue) that provides elasticity.
The lungs have an apex (superior tip), a base (inferior surface resting on the diaphragm), and a hilum (on the mediastinal surface where blood vessels, bronchi, lymphatic vessels, and nerves enter and exit).
The left lung is smaller than the right, with a cardiac notch for the heart. The right lung is divided into superior, middle, and inferior lobes, while the left lung is divided into superior and inferior lobes.
The lungs also contain bronchopulmonary segments, which are the smallest subdivisions that can be individually removed if diseased, and lobules, which are the smallest subdivisions visible to the naked eye.
Blood Supply
The pulmonary circulation is a low-pressure, high-volume system. Pulmonary arteries deliver systemic venous blood to the lungs for oxygenation, and the pulmonary veins carry the oxygenated blood from the respiratory zones to the heart.
Pleurae of the Lungs
The lungs are surrounded by two layers of pleura: the visceral pleura, which covers the surface of the lungs, and the parietal pleura, which lines the inner surface of the thoracic cavity. The space between the two layers is the pleural cavity, which contains a small amount of pleural fluid that reduces friction during breathing.
Pulmonary Respiration: Pressures
The key pressures involved in pulmonary respiration are:
Atmospheric pressure (Patm) : The pressure of the air surrounding the body. Alveolar pressure (Ppul) : The pressure within the alveoli and airways. Intrapleural pressure (Pip) : The pressure within the pleural cavity.
The negative intrapleural pressure is caused by opposing forces: the elastic recoil of the lungs, which decreases lung size, and the elasticity of the chest wall, which pulls the thorax outward. This negative pressure keeps the airways open (transpulmonary pressure).
Conditions like atelectasis (lung collapse) and pneumothorax (air in the pleural cavity) can disrupt the normal pressure relationships and lead to lung collapse.
Pulmonary Ventilation
Pulmonary ventilation, or breathing, is the mechanical process of moving air into and out of the lungs. It depends on changes in the volume of the thoracic cavity, which in turn cause changes in pressure.
Inspiration is an active process where the inspiratory muscles (diaphragm and external intercostals) contract, increasing the thoracic volume and decreasing the intrapulmonary pressure, causing air to flow into the lungs.
Expiration is normally a passive process where the inspiratory muscles relax, the thoracic cavity volume decreases, and the elastic lungs recoil, increasing the intrapulmonary pressure and causing air to flow out of the lungs. Forced expiration is an active process that uses the abdominal and internal intercostal muscles.
Respiratory Volumes and Capacities
Respiratory volumes are used to assess respiratory status:
Tidal volume (TV) Inspiratory reserve volume (IRV) Expiratory reserve volume (ERV) Residual volume (RV)
Respiratory capacities are combinations of these volumes:
Inspiratory capacity (IC) Functional residual capacity (FRC) Vital capacity (VC) Total lung capacity (TLC)
Dead Space
Dead space is the volume of air that does not participate in gas exchange:
Anatomical dead space : The air remaining in the passageways, approximately 150 mL. Alveolar dead space : Non-functional alveoli due to collapse or obstruction. Total dead space : The sum of anatomical and alveolar dead space.
Carbon Dioxide Transport
Carbon dioxide is transported in the blood through the following process:
CO2 + H2O ⇌ H2CO3 ⇌ HCO3- + H+ This reaction is catalyzed by the enzyme carbonic anhydrase.
