Anatomy Of The Respiratory System Review Sheet 36

The respiratory system, a vital network responsible for the exchange of oxygen and carbon dioxide, is a fascinating and intricate part of human anatomy. Understanding its components and their functions is crucial for anyone in the medical field, from aspiring nurses to seasoned physicians. This review sheet delves into the anatomy of the respiratory system, providing a comprehensive overview of its structures, functions, and clinical relevance.

I. Introduction to the Respiratory System

The primary function of the respiratory system is to facilitate gas exchange. This process involves taking in oxygen from the air, which is essential for cellular respiration, and expelling carbon dioxide, a waste product of metabolism. Beyond gas exchange, the respiratory system also plays a role in:

• Voice Production: Air passing over the vocal cords in the larynx enables speech.
• Olfaction: The nasal cavity contains olfactory receptors for the sense of smell.
• Acid-Base Balance: The respiratory system helps regulate blood pH by controlling carbon dioxide levels.
• Protection: The respiratory system filters, warms, and humidifies inspired air, protecting the delicate tissues of the lungs from harmful substances.

II. The Upper Respiratory Tract

The upper respiratory tract consists of the structures located outside the chest cavity, responsible for conducting air into the lower respiratory tract.

A. Nose and Nasal Cavity

The nose is the primary entry point for air into the respiratory system. Its external structure is supported by bone and cartilage, shaped to optimize airflow and filtration. The nasal cavity, located behind the nose, is a hollow space lined with mucous membrane.

• Functions of the Nose and Nasal Cavity:

  • Airway: Provides a passage for air to enter the respiratory system.
  • Filtration: Nasal hairs (vibrissae) and mucus trap large particles, preventing them from entering the lungs.
  • Humidification: The nasal mucosa moistens the air, preventing the delicate tissues of the lower respiratory tract from drying out.
  • Warming: Blood vessels in the nasal mucosa warm the air, bringing it closer to body temperature.
  • Olfaction: Olfactory receptors in the superior nasal cavity detect odors.
  • Resonance: The nasal cavity contributes to the resonance of speech sounds.

• Key Structures:

  • Nares (Nostrils): External openings of the nose.
  • Nasal Septum: Divides the nasal cavity into right and left halves.
  • Conchae (Turbinates): Three bony projections (superior, middle, and inferior) that increase the surface area of the nasal cavity, enhancing filtration, humidification, and warming.
  • Mucosa: Epithelial lining of the nasal cavity, containing goblet cells that produce mucus and ciliated cells that move mucus and trapped particles towards the pharynx.
  • Paranasal Sinuses: Air-filled cavities within the skull bones (frontal, ethmoid, sphenoid, and maxillary) that connect to the nasal cavity; they lighten the skull, contribute to voice resonance, and produce mucus.

B. Pharynx (Throat)

The pharynx, commonly known as the throat, is a muscular tube that connects the nasal cavity and oral cavity to the larynx and esophagus. It serves as a common passageway for both air and food. The pharynx is divided into three regions:

• Nasopharynx:

  • The uppermost region, located behind the nasal cavity.
  • Contains the pharyngeal tonsil (adenoid), which plays a role in immunity.
  • The Eustachian tubes, which connect the middle ear to the nasopharynx, equalize pressure in the middle ear.
  • Passageway for air only.

• Oropharynx:

  • The middle region, located behind the oral cavity.
  • Contains the palatine tonsils and lingual tonsils, which are involved in immune responses.
  • Passageway for both air and food.

• Laryngopharynx:

  • The lowermost region, located behind the larynx.
  • Connects to the esophagus (for swallowing) and the larynx (for breathing).
  • Passageway for both air and food.

C. Larynx (Voice Box)

The larynx, or voice box, is a cartilaginous structure located in the anterior neck, superior to the trachea. It plays a critical role in voice production and protects the lower respiratory tract by preventing food and liquids from entering.

• Functions of the Larynx:

  • Voice Production: Contains the vocal cords, which vibrate to produce sound when air passes over them.
  • Airway Protection: The epiglottis, a flap of cartilage, covers the opening of the larynx during swallowing, preventing food and liquids from entering the trachea.
  • Airway Patency: The larynx maintains an open airway for breathing.

• Key Structures:

  • Epiglottis: A flap of cartilage that covers the opening of the larynx during swallowing.
  • Thyroid Cartilage: The largest cartilage of the larynx, forming the Adam's apple.
  • Cricoid Cartilage: A ring-shaped cartilage located inferior to the thyroid cartilage.
  • Arytenoid Cartilages: Paired cartilages that attach to the vocal cords and control their tension and position.
  • Vocal Cords: Folds of mucous membrane that vibrate to produce sound; composed of the vocal ligaments and the vocalis muscle.
  • Glottis: The opening between the vocal cords.

III. The Lower Respiratory Tract

The lower respiratory tract consists of the structures located within the chest cavity, responsible for conducting air to the lungs and facilitating gas exchange.

A. Trachea (Windpipe)

The trachea, or windpipe, is a cartilaginous tube that extends from the larynx to the bronchi. It provides a clear and unobstructed pathway for air to reach the lungs.

• Structure of the Trachea:

  • C-Shaped Cartilage Rings: Provide support to the trachea, preventing it from collapsing during breathing. The open part of the "C" faces posteriorly, allowing the esophagus to expand during swallowing.
  • Trachealis Muscle: A smooth muscle located on the posterior aspect of the trachea, connecting the ends of the cartilage rings. It contracts during coughing to reduce the diameter of the trachea, increasing the velocity of airflow and helping to expel mucus and foreign objects.
  • Mucosa: The inner lining of the trachea, consisting of ciliated pseudostratified columnar epithelium with goblet cells. The cilia beat upwards, moving mucus and trapped particles towards the larynx for swallowing or expectoration.

B. Bronchi

The bronchi are the two main branches of the trachea that enter the lungs. The trachea bifurcates into the right and left primary bronchi at the level of the sternal angle.

• Primary Bronchi (Main Bronchi):

  • The right primary bronchus is shorter, wider, and more vertical than the left primary bronchus, making it more likely for inhaled foreign objects to enter the right lung.
  • Each primary bronchus enters the lung at the hilum, an indentation on the medial surface of the lung.

• Secondary Bronchi (Lobar Bronchi):

  • The primary bronchi divide into secondary bronchi, also known as lobar bronchi.
  • The right lung has three lobes (superior, middle, and inferior), and the right primary bronchus divides into three secondary bronchi, one for each lobe.
  • The left lung has two lobes (superior and inferior), and the left primary bronchus divides into two secondary bronchi, one for each lobe.

• Tertiary Bronchi (Segmental Bronchi):

  • The secondary bronchi divide into tertiary bronchi, also known as segmental bronchi.
  • Each tertiary bronchus supplies a specific bronchopulmonary segment of the lung.
  • There are typically 10 bronchopulmonary segments in each lung, although variations can occur.

• Bronchioles:

  • The tertiary bronchi divide into smaller and smaller tubes called bronchioles.
  • Bronchioles are less than 1 mm in diameter and lack cartilage support in their walls.
  • The walls of bronchioles contain smooth muscle, which allows them to constrict or dilate, regulating airflow to the alveoli.

• Terminal Bronchioles:

  • The smallest bronchioles are called terminal bronchioles.
  • They are the last part of the conducting zone of the respiratory system, which conducts air to the gas exchange surfaces of the lungs.

C. Lungs

The lungs are the primary organs of respiration, responsible for gas exchange between the air and the blood. They are located within the thoracic cavity, separated by the mediastinum, which contains the heart, great vessels, trachea, and esophagus.

• Structure of the Lungs:

  • Lobes: The right lung has three lobes (superior, middle, and inferior), while the left lung has two lobes (superior and inferior). The lobes are separated by fissures.
  • Fissures: The right lung has two fissures: the horizontal fissure, which separates the superior and middle lobes, and the oblique fissure, which separates the middle and inferior lobes. The left lung has one fissure: the oblique fissure, which separates the superior and inferior lobes.
  • Pleura: Each lung is surrounded by a double-layered membrane called the pleura. The visceral pleura covers the surface of the lung, while the parietal pleura lines the thoracic cavity. The space between the visceral and parietal pleura, called the pleural cavity, contains a thin layer of pleural fluid, which reduces friction during breathing.
  • Hilum: The hilum is an indentation on the medial surface of each lung where the bronchi, blood vessels, and nerves enter and exit the lung.

• Alveoli:

  • The terminal bronchioles lead into respiratory bronchioles, which have alveoli budding from their walls.
  • Alveoli are tiny air sacs where gas exchange takes place.
  • The walls of alveoli are very thin, consisting of a single layer of squamous epithelial cells called type I alveolar cells.
  • Type II alveolar cells secrete surfactant, a substance that reduces surface tension in the alveoli, preventing them from collapsing.
  • Alveolar macrophages, also known as dust cells, are phagocytic cells that engulf and remove debris and pathogens from the alveoli.
  • The respiratory membrane, where gas exchange occurs, consists of the alveolar wall, the capillary wall, and their fused basement membranes.

D. Alveolar-Capillary Gas Exchange

The alveoli are surrounded by a dense network of capillaries. The close proximity of the alveoli and capillaries facilitates the diffusion of oxygen from the alveoli into the blood and the diffusion of carbon dioxide from the blood into the alveoli.

• Factors Affecting Gas Exchange:

  • Partial Pressure Gradients: Gases move from areas of high partial pressure to areas of low partial pressure. The partial pressure of oxygen is higher in the alveoli than in the blood, while the partial pressure of carbon dioxide is higher in the blood than in the alveoli.
  • Surface Area: The large surface area of the alveoli (approximately 70 square meters) provides ample space for gas exchange.
  • Thickness of the Respiratory Membrane: The thinness of the respiratory membrane facilitates rapid diffusion of gases.
  • Ventilation-Perfusion Matching: The amount of air reaching the alveoli (ventilation) should match the amount of blood flowing through the pulmonary capillaries (perfusion) for efficient gas exchange.

IV. Muscles of Respiration

Breathing, or ventilation, is the process of moving air into and out of the lungs. It involves the coordinated action of several muscles.

A. Diaphragm

The diaphragm is the primary muscle of respiration. It is a dome-shaped muscle that separates the thoracic cavity from the abdominal cavity.

• Inspiration: During inspiration, the diaphragm contracts, pulling downward and flattening. This increases the volume of the thoracic cavity, decreasing the pressure within the lungs and causing air to flow into the lungs.
• Expiration: During expiration, the diaphragm relaxes, returning to its dome shape. This decreases the volume of the thoracic cavity, increasing the pressure within the lungs and causing air to flow out of the lungs.

B. External Intercostal Muscles

The external intercostal muscles are located between the ribs.

• Inspiration: During inspiration, the external intercostal muscles contract, lifting the rib cage upward and outward. This increases the volume of the thoracic cavity, decreasing the pressure within the lungs and causing air to flow into the lungs.

C. Internal Intercostal Muscles

The internal intercostal muscles are located between the ribs, deep to the external intercostal muscles.

• Forced Expiration: During forced expiration, such as during coughing or exercise, the internal intercostal muscles contract, pulling the rib cage downward and inward. This decreases the volume of the thoracic cavity, increasing the pressure within the lungs and causing air to flow out of the lungs.

D. Accessory Muscles of Respiration

Several other muscles can assist in breathing, especially during strenuous exercise or respiratory distress. These include:

• Sternocleidomastoid: Elevates the sternum, increasing thoracic volume.
• Scalenes: Elevate the upper ribs, increasing thoracic volume.
• Pectoralis Minor: Elevates the ribs, increasing thoracic volume.
• Abdominal Muscles (Rectus Abdominis, External Oblique, Internal Oblique, Transversus Abdominis): Contract during forced expiration, increasing abdominal pressure and pushing the diaphragm upward.

V. Control of Respiration

Breathing is controlled by the respiratory centers in the brainstem, specifically the medulla oblongata and the pons.

A. Medulla Oblongata

The medulla oblongata contains the primary respiratory centers:

• Ventral Respiratory Group (VRG): Contains both inspiratory and expiratory neurons. It is primarily responsible for forced breathing.
• Dorsal Respiratory Group (DRG): Primarily contains inspiratory neurons. It receives input from chemoreceptors and stretch receptors and relays information to the VRG.

B. Pons

The pons contains the pontine respiratory group (PRG), which modifies the activity of the VRG and DRG, smoothing out the transitions between inspiration and expiration.

C. Factors Influencing Respiration

Several factors can influence the rate and depth of breathing, including:

• Chemical Factors: Chemoreceptors in the medulla oblongata and carotid and aortic bodies detect changes in blood pH, carbon dioxide levels, and oxygen levels. Increased carbon dioxide levels or decreased pH stimulate respiration, while decreased oxygen levels stimulate respiration to a lesser extent.
• Lung Stretch Receptors: Stretch receptors in the lungs detect the degree of lung inflation and send signals to the respiratory centers, preventing overinflation of the lungs (Hering-Breuer reflex).
• Irritant Receptors: Irritant receptors in the airways respond to irritants such as dust, smoke, and fumes, triggering reflexes such as coughing and sneezing.
• Higher Brain Centers: The cerebral cortex can voluntarily control breathing, such as during speech or singing. The hypothalamus and limbic system can also influence respiration through emotional responses.

VI. Clinical Significance

Understanding the anatomy of the respiratory system is essential for diagnosing and treating respiratory diseases. Some common respiratory conditions include:

• Asthma: Chronic inflammatory disease of the airways, characterized by bronchospasm, mucus production, and airway obstruction.
• Chronic Obstructive Pulmonary Disease (COPD): Progressive lung disease characterized by airflow limitation, often caused by smoking.
• Pneumonia: Infection of the lungs, causing inflammation and fluid accumulation in the alveoli.
• Lung Cancer: Malignant tumor of the lung tissue.
• Cystic Fibrosis: Genetic disorder that causes the production of thick mucus, leading to airway obstruction and infection.
• Pneumothorax: Presence of air in the pleural cavity, causing lung collapse.

VII. Conclusion

The respiratory system is a complex and vital organ system responsible for gas exchange, voice production, olfaction, acid-base balance, and protection. A thorough understanding of its anatomy, including the structures of the upper and lower respiratory tracts, the muscles of respiration, and the control of respiration, is crucial for healthcare professionals. By mastering this knowledge, one can better diagnose and treat respiratory diseases, ultimately improving patient outcomes. This review sheet serves as a valuable tool for reinforcing your understanding of the anatomy of the respiratory system. Remember to supplement your study with visual aids, such as diagrams and models, to enhance your learning experience. Good luck!