Review Sheet 36 Anatomy Of The Respiratory System

The respiratory system, a complex network of organs and tissues, enables us to breathe, delivering life-sustaining oxygen to every cell in our body and removing carbon dioxide, a waste product of cellular metabolism. Understanding its intricate anatomy is crucial for anyone in the medical field, as well as for those simply seeking to comprehend the miracle of human life. This review sheet will delve into the key structures of the respiratory system, their functions, and their interrelationships.

Overview of the Respiratory System

The respiratory system can be divided into two main parts: the upper respiratory tract and the lower respiratory tract. The upper respiratory tract consists of the nose, nasal cavity, pharynx (throat), and larynx (voice box). Its primary functions include filtering, warming, and humidifying incoming air, as well as facilitating the sense of smell and vocalization. The lower respiratory tract, on the other hand, comprises the trachea (windpipe), bronchi, bronchioles, and alveoli (air sacs) within the lungs. This portion is primarily responsible for gas exchange – the critical process of transferring oxygen from the air into the blood and carbon dioxide from the blood into the air to be exhaled.

Understanding the mechanics of breathing, which involves the coordinated action of the diaphragm and intercostal muscles, is also fundamental to grasping the respiratory system's function.

The Upper Respiratory Tract: Pathway to the Lungs

1. The Nose and Nasal Cavity: First Line of Defense

The nose, the visible structure projecting from the face, serves as the entry point for air into the respiratory system. The external nose is supported by bone and cartilage. Internally, the nasal cavity is a large, air-filled space behind the nose.

• Functions: The nose and nasal cavity perform several vital functions:

  • Filtering: Nasal hairs (vibrissae) and mucus trap large particles, preventing them from entering the lungs.
  • Warming: Blood vessels in the nasal mucosa warm the incoming air to body temperature.
  • Humidifying: Mucus secreted by goblet cells moistens the air, preventing the delicate lung tissues from drying out.
  • Olfaction: Olfactory receptors in the superior nasal cavity detect odors.
  • Resonance: The nasal cavity contributes to the resonance of speech.

• Key Structures:

  • Nasal Septum: Divides the nasal cavity into right and left halves.
  • Nasal Conchae (Turbinates): Three bony projections (superior, middle, and inferior) that increase the surface area of the nasal cavity, enhancing filtering, warming, and humidifying.
  • Nasal Mucosa: A ciliated pseudostratified columnar epithelium lining the nasal cavity, containing goblet cells that secrete mucus.
  • Paranasal Sinuses: Air-filled cavities within the bones of the skull (frontal, ethmoid, sphenoid, and maxillary bones) that connect to the nasal cavity. They lighten the skull, contribute to resonance, and produce mucus.

2. The Pharynx: The Crossroads

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

• Nasopharynx: Located posterior to the nasal cavity, it serves as an airway. It contains the pharyngeal tonsil (adenoids) and the opening of the Eustachian tube (auditory tube), which connects to the middle ear.
• Oropharynx: Located posterior to the oral cavity, it serves as a passageway for both air and food. It contains the palatine tonsils and the lingual tonsils.
• Laryngopharynx: Located posterior to the larynx, it also serves as a passageway for both air and food. It connects to the esophagus (for food) and the larynx (for air).

3. The Larynx: Voice Box and Airway Protection

The larynx, or voice box, is a short passageway that connects the pharynx to the trachea. It's primarily responsible for voice production but also plays a crucial role in protecting the lower respiratory tract.

• Functions:

  • 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 to prevent food and liquids from entering the trachea.
  • Airflow Regulation: The larynx can regulate airflow into the lungs.

• Key Structures:

  • Epiglottis: A flap of elastic 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: Two small cartilages that articulate with the cricoid cartilage and are involved in vocal cord movement.
  • Vocal Cords: Two folds of mucous membrane that vibrate to produce sound. The space between them is called the glottis.

The Lower Respiratory Tract: Gas Exchange Hub

1. The Trachea: Windpipe to the Lungs

The trachea, or windpipe, is a cylindrical tube that extends from the larynx to the bronchi. It's composed of C-shaped rings of hyaline cartilage that provide support and prevent the trachea from collapsing. The posterior wall of the trachea is made of smooth muscle, allowing for some flexibility.

• Function: Conducts air from the larynx to the bronchi.

• Key Structures:

  • C-shaped Cartilage Rings: Provide support and prevent collapse.
  • Trachealis Muscle: Smooth muscle on the posterior wall that allows for flexibility and constriction.
  • Mucosa: Lined with ciliated pseudostratified columnar epithelium, which traps debris and propels it upward towards the pharynx (the mucociliary escalator).

2. The Bronchial Tree: Branching Airways

The bronchi are the two main branches of the trachea that enter the lungs. The right bronchus is shorter, wider, and more vertical than the left bronchus, making it more likely for inhaled objects to lodge there.

• Primary Bronchi: The right and left bronchi that branch from the trachea.
• Secondary (Lobar) Bronchi: The primary bronchi divide into secondary bronchi, each of which supplies a lobe of the lung (three lobes in the right lung, two lobes in the left lung).
• Tertiary (Segmental) Bronchi: The secondary bronchi divide into tertiary bronchi, each of which supplies a bronchopulmonary segment (a functionally independent unit of the lung).
• Bronchioles: The tertiary bronchi branch into smaller and smaller tubes called bronchioles. Bronchioles lack cartilage and are primarily composed of smooth muscle.
• Terminal Bronchioles: The smallest bronchioles, which lead into the respiratory bronchioles.

3. The Lungs: Organs of Gas Exchange

The lungs are the primary organs of respiration, located in the thoracic cavity. They are cone-shaped organs, with the base resting on the diaphragm and the apex extending above the clavicle.

• Lobes: The right lung has three lobes (superior, middle, and inferior), while the left lung has two lobes (superior and inferior).
• Fissures: Deep grooves that separate the lobes of the lungs. The right lung has two fissures (horizontal and oblique), while the left lung has one fissure (oblique).
• Pleura: A double-layered membrane that surrounds each lung. The visceral pleura covers the surface of the lung, while the parietal pleura lines the thoracic cavity. The space between the two layers is called the pleural cavity, which contains a small amount of pleural fluid that reduces friction during breathing.

4. Alveoli: The Site of Gas Exchange

Alveoli are tiny air sacs clustered at the ends of the respiratory bronchioles. They are the primary site of gas exchange in the lungs.

• Structure: Alveoli are surrounded by a dense network of capillaries. The walls of the alveoli and the capillaries are very thin, allowing for efficient diffusion of oxygen and carbon dioxide.

• Cell Types:

  • Type I Alveolar Cells: Simple squamous epithelial cells that form the majority of the alveolar wall.
  • Type II Alveolar Cells: Secrete surfactant, a substance that reduces surface tension in the alveoli, preventing them from collapsing.
  • Alveolar Macrophages: Phagocytic cells that remove debris and pathogens from the alveoli.

• Gas Exchange: Oxygen diffuses from the alveoli into the capillaries, where it binds to hemoglobin in red blood cells. Carbon dioxide diffuses from the capillaries into the alveoli to be exhaled.

Muscles of Respiration: Powering the Breath

Breathing is an active process that requires the coordinated action of several muscles.

• Diaphragm: The primary muscle of respiration. It is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. When the diaphragm contracts, it flattens, increasing the volume of the thoracic cavity and drawing air into the lungs.
• External Intercostal Muscles: Located between the ribs, they contract to elevate the rib cage, further increasing the volume of the thoracic cavity.
• Internal Intercostal Muscles: Located between the ribs, they contract to depress the rib cage, decreasing the volume of the thoracic cavity (used primarily during forced exhalation).
• Accessory Muscles: Muscles that assist in breathing during strenuous activity or respiratory distress, such as the sternocleidomastoid and scalene muscles in the neck, and the pectoralis minor in the chest.

Mechanics of Breathing: Inspiration and Expiration

Breathing, or pulmonary ventilation, consists of two phases: inspiration (inhalation) and expiration (exhalation).

• Inspiration:

  • The diaphragm and external intercostal muscles contract.
  • The volume of the thoracic cavity increases.
  • The pressure inside the lungs (intrapulmonary pressure) decreases below atmospheric pressure.
  • Air flows into the lungs until intrapulmonary pressure equals atmospheric pressure.

• Expiration:

  • The diaphragm and external intercostal muscles relax.
  • The volume of the thoracic cavity decreases.
  • The pressure inside the lungs (intrapulmonary pressure) increases above atmospheric pressure.
  • Air flows out of the lungs until intrapulmonary pressure equals atmospheric pressure.
  • Forced expiration involves contraction of the internal intercostal muscles and abdominal muscles.

Control of Respiration: The Respiratory Centers

Breathing is controlled by the respiratory centers located in the medulla oblongata and pons of the brainstem.

• Medulla Oblongata:

  • Ventral Respiratory Group (VRG): Contains neurons that control both inspiration and expiration.
  • Dorsal Respiratory Group (DRG): Receives input from chemoreceptors and stretch receptors and relays information to the VRG.

• Pons:

  • Pneumotaxic Center: Inhibits inspiration, regulating the rate and depth of breathing.
  • Apneustic Center: Stimulates inspiration, prolonging inhalation.

• Factors Influencing Respiration:

  • Chemoreceptors: Detect changes in blood levels of carbon dioxide, oxygen, and pH. Increased carbon dioxide or decreased oxygen or pH stimulates increased ventilation.
  • Stretch Receptors: Located in the lungs and airways, they prevent overinflation of the lungs (Hering-Breuer reflex).
  • Higher Brain Centers: The cerebral cortex can voluntarily control breathing to some extent (e.g., holding your breath).

Common Respiratory Diseases: Disrupting the Breath

Understanding the anatomy of the respiratory system is crucial for understanding respiratory diseases. Here are a few examples:

• Asthma: Chronic inflammatory disease of the airways, characterized by bronchospasm (constriction of the bronchioles), inflammation, and mucus production.
• Chronic Obstructive Pulmonary Disease (COPD): A group of lung diseases that block airflow and make it difficult to breathe. Includes emphysema and chronic bronchitis.
• Pneumonia: Infection of the lungs that causes inflammation of the alveoli and filling of the alveoli with fluid.
• Lung Cancer: Uncontrolled growth of abnormal cells in the lungs.
• Cystic Fibrosis: A genetic disorder that causes the production of thick, sticky mucus that can clog the airways and lead to lung infections.

Review Questions: Test Your Knowledge

1. What are the two main divisions of the respiratory system?
2. What are the functions of the nasal cavity?
3. What are the three regions of the pharynx?
4. What is the function of the epiglottis?
5. What is the structure and function of the trachea?
6. Describe the branching pattern of the bronchial tree.
7. What are the lobes and fissures of the lungs?
8. What are the different types of cells found in the alveoli?
9. What are the muscles involved in breathing?
10. How is respiration controlled by the brain?

Conclusion: The Breath of Life

The respiratory system is a marvel of biological engineering, enabling the vital exchange of gases that sustains life. From the filtering mechanisms of the nose to the intricate network of alveoli in the lungs, each component plays a crucial role in ensuring that our cells receive the oxygen they need and that waste products are efficiently removed. A thorough understanding of the anatomy and physiology of the respiratory system is essential for healthcare professionals and anyone seeking to appreciate the complexity and beauty of the human body. By studying the structures, functions, and regulatory mechanisms of this vital system, we gain a deeper appreciation for the breath of life.