Respiratory System - Chapter 22

• Major organs function to:

  • Provide oxygen to body tissues for cellular respiration, a vital process that produces energy for cellular functions.

  • Remove waste products like carbon dioxide, which is produced as a byproduct of metabolism and can be toxic if accumulated.

  • Sense odors (non-vital function) through specialized olfactory receptors located in the nasal cavity.

  • Speech production (non-vital function) facilitated by the larynx, which houses the vocal cords.

  • Protect from dust and microbes through mucous secretions and cilia that trap and eliminate particulates and pathogens.

Functional Divisions

• Conducting Zone:

  • Organs and structures not directly involved in gas exchange, including the nose, pharynx, larynx, trachea, bronchi, and bronchioles.

  • Function:

    • Provide a passageway for incoming and outgoing air, essential for breathing and ventilation.

    • Remove debris and pathogens from incoming air via mucous membranes and ciliary action.

    • Warm and humidify incoming air to protect delicate respiratory tissues and enhance gas exchange efficiency.

• Respiratory Zone:

  • The site where gas exchange occurs, primarily in the alveoli, tiny air sacs that maximize the surface area for exchange.

Upper Airway

• Major entrance and exit for airflow: the nose, equipped with external nares (nostrils) that open into the nasal cavity, beginning with the nasal vestibule.

• Nasal cavity:

  • Separated into left and right sections by the nasal septum, contributing to airflow regulation and olfaction.

• Nares (anterior portion of the nasal cavity):

  • Lined with mucous membranes containing sebaceous glands and hair follicles, which act as a natural filtration system to prevent large debris from entering the respiratory system.

• Olfactory epithelium:

  • Located deeper in the nasal cavity, specialized for detecting odors, essential for taste and environmental sensing.

• Nasal conchae (superior, middle, and inferior):

  • Bony projections that increase surface area within the nasal cavity, allowing air to be cleaned, warmed, and humidified efficiently as it bounces along the epithelium.

• Conchae, meatuses, and paranasal sinuses:

  • Lined by respiratory epithelium (pseudostratified ciliated columnar epithelium), cilia actively transport mucus and debris up toward the throat for swallowing, enhancing respiratory health.

• Air exits the nasal cavity via the internal nares into the pharynx, continuing the airflow pathway.

Pharynx (Throat)

• A muscular tube lined with mucous membrane, continuous with the nasal cavity and connecting to the larynx and esophagus.

• Three major regions:

  • Nasopharynx:

    • Located posterior to the nasal cavity and ending at the soft palate, serves only as an airway and contains the auditory tubes (Eustachian tubes) that connect to the middle ear, helping in equalizing pressure.

  • Oropharynx:

    • Passageway for both air and food, bordered superiorly by nasopharynx and anteriorly by the oral cavity, with a change in epithelial tissue reflecting its dual function.

  • Laryngopharynx:

    • Inferior to the oropharynx, it directs air into the larynx and food into the esophagus, ensuring the proper separation of respiratory and digestive pathways.

Larynx (Voice Box)

• A cartilaginous structure inferior to the laryngopharynx connecting the pharynx to the trachea, crucial for respiration and phonation.

• Regulates the volume of air entering and leaving the lungs.

• Composed of several pieces of cartilage, including:

  • Thyroid cartilage:

    • The largest component, more prominent in males due to the laryngeal prominence (Adam's apple).

  • Epiglottis:

    • A flexible flap of cartilage that prevents food from entering the trachea during swallowing.

  • Cricoid cartilage:

    • A complete ring of cartilage providing structural support.

• Glottis:

  • The part of the larynx containing the vocal cords and the opening between them, critical for sound production.

• Various smaller paired cartilages (arytenoids, corniculates, cuneiforms) provide support and movement, essential for adjusting tension on the vocal cords and sound modulation.

• The vestibular fold (false vocal cords) serves as additional support for the true vocal cords, which vibrate to produce sound.

• The superior portion of the larynx contains stratified squamous epithelium transitioning to pseudostratified ciliated columnar epithelium, reflecting changing functions along the airway.

Trachea (Windpipe)

• A flexible tube extending from the larynx, supported by C-shaped rings of hyaline cartilage that prevent collapse.

• Composed of 16 to 20 rings connected by dense connective tissue, allowing for flexibility and stability.

• The trachealis muscle at the posterior side aids in regulating airflow and pressure changes during breathing, under autonomic nervous system control.

• The trachea branches into the right and left primary bronchi, providing passageways to each lung.

Bronchial Tree

• The bronchi branch extensively into a bronchial tree, providing a passageway for air and conducting it to the alveoli.

• The main function is to trap debris and pathogens via mucous membranes, playing a critical role in immune defense.

• Secondary bronchi:

  • The first branches off the primary bronchi, one for each lung lobe (two on the left, three on the right).

• Tertiary bronchi:

  • Form around 9-10 branches supplying distinct bronchopulmonary segments within the lungs.

• As the bronchi continue to branch, the cartilages diminish, and the diameter decreases until reaching the bronchioles, where cartilage disappears completely.

• Bronchioles:

  • Smaller airways approximately one millimeter in diameter, leading to terminal bronchioles for gas exchange preparation.

Respiratory Zone

• Terminal bronchioles transition into respiratory bronchioles with smooth muscle allowing diameter adjustments, critical for airflow regulation.

• Respiratory bronchiole leads into an alveolar duct, which eventually opens into an alveolar sac.

• Alveolar sacs contain clusters of individual alveoli, resembling grapes, responsible for gas exchange.

• Alveolar pores connect adjacent alveoli, helping maintain equal pressure throughout the lungs.

• Surrounding capillary beds play a vital role in gas exchange, supplying a massive surface area through approximately 50,000,000 alveoli in each lung.

Alveolar Wall

• Composed of three major cell types essential for function:

  • Type One Alveolar Cells:

    • Squamous epithelial cells comprising about 97% of the alveolar surface area, allowing for rapid gas diffusion due to their thin structure.

  • Type Two Alveolar Cells:

    • Secrete a surfactant that lowers alveolar surface tension and prevents collapse, ensuring efficient gas exchange.

  • Alveolar Macrophages:

    • Immune cells that protect the lungs by removing pathogens and debris through phagocytosis.

• The respiratory membrane, approximately 0.5 micrometers thick, consists of Type One alveolar cells, an elastic basement membrane, and the endothelial membrane of capillaries, facilitating gas diffusion.

Lungs

• Major organs of the respiratory system, pyramid-shaped and comprising two paired structures connected to the trachea.

• Divided into lobes separated by fissures; the right lung has three lobes (superior, middle, inferior) while the left lung has two lobes (superior, inferior).

• The left lung is smaller due to the cardiac notch, accommodating space for the heart.

• Each lung is enveloped in pleural membranes containing two layers:

  • Visceral pleura: Directly covering the surface of the lungs.

  • Parietal pleura: Lining the thoracic cavity wall.

• The pleural cavity, the space between these layers, contains pleural fluid that lubricates surfaces during breathing and maintains lung position against the thoracic wall.

Pulmonary Ventilation (Breathing)

• The process of air movement into and out of the lungs, essential for gas exchange.

• It depends on the pressure differentials created by changes in lung volumes.

• Boyle's Law governs this process, indicating that at a constant temperature, a decrease in volume leads to an increase in pressure, and vice versa:
  P1V1 = P2V2

• The lungs themselves do not actively create pressure changes; rather, they passively stretch and compress alongside the thoracic cavity during respiratory movements.

• Major muscular actions contributing to breathing include contraction and relaxation of the diaphragm and the intercostal muscles.

Inspiration

• When at rest, the diaphragm is dome-shaped.

• Upon contraction, it moves downward into the abdominal cavity, enlarging the thoracic cavity and decreasing pressure inside.

• Additionally, contraction of the external intercostals lifts the rib cage upward and outward, further enhancing capacity, resulting in a pressure gradient that draws air into the lungs.

Expiration

• Normal expiration is generally passive, driven by elastic lung recoil upon relaxation of the diaphragm and intercostals.

• This decrease in thoracic volume elevates intrapulmonary pressure, forcing air out of the lungs.

Quiet Breathing

• Characterized by normal, relaxed breathing patterns with little active muscular force required for expiration.

Forced Breathing

• Active mode of breathing required during intense exercise or singing, which involves muscle contractions for both inspiration and expiration.

• Accessory muscles (e.g., abdominal muscles and internal intercostals) assist in rapidly compressing the rib cage, facilitating quicker expiration.

Respiratory Volumes and Capacities

• Respiratory volume refers to the various amounts of air moved in the respiratory cycle, measured using a spirometer.

• Total lung capacity averages about 6,000 mL for males and approximately 4,200 mL for females.

• Four major types of respiratory volumes:

  • Tidal Volume:

    • The typical amount of air inhaled or exhaled during quiet breathing, around 500 mL.

  • Expiratory Reserve Volume (ERV):

    • The air that can be forcefully exhaled after a normal expiration, approximately 1,200 mL in males.

  • Inspiratory Reserve Volume:

    • The additional air that can be inhaled beyond normal tidal volume, roughly 3,000 mL.

  • Residual Volume:

    • The air remaining in the lungs post-expiration, preventing alveolar collapse.

• Respiratory capacities:

  • Combinations of volumes providing comprehensive metrics of lung function.

  • Total Lung Capacity:

    • All combined volumes totaling approximately 6 liters in males and 4.2 liters in females.

  • Vital Capacity:

    • The amount of air a person can move in and out of the lungs excluding residual volume.

  • Inspiratory Capacity:

    • Maximum air that can be inhaled post-tidal expiration (sum of tidal volume and inspiratory reserve volume).

  • Functional Residual Capacity:

    • The sum of expiratory reserve volume plus residual volume.

Control of Breathing

• Breathing rate is regulated by central respiratory control centers in the brain responding to fluctuations in carbon dioxide, oxygen, and pH levels in the blood.

• Medulla Oblongata houses two respiratory groups:

  • Dorsal Respiratory Group (DRG):

    • Maintains a constant rhythm of breathing, stimulating diaphragm and external intercostals for inspiration.

  • Ventral Respiratory Group (VRG):

    • Involved in active and forced breathing, integrating sensory information and adjusting the respiratory rate as needed.

• The Pons contains the Pontine Respiratory Group (PRG), modulating the rhythm generated by the DRG to facilitate deeper or more controlled breaths when necessary.

Gas Exchange

• Gas exchange occurs in two main locations in the body:

  • In the Lungs:

    • Involves external respiration, where oxygen enters the bloodstream, and carbon dioxide is expelled. This exchange occurs at the alveoli, where oxygen diffuses from alveoli into capillaries, and carbon dioxide diffuses from blood into the alveoli for exhalation.

  • In the Tissues:

    • Involves internal respiration, where oxygen is delivered to cells and carbon dioxide is removed. Oxygen diffuses out of capillaries into tissues, while carbon dioxide diffuses back into blood. This process relies on the principles of simple diffusion and concentration gradients.