Respiratory System

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• Provide oxygen to body tissues for cellular respiration.
• Remove waste products such as carbon dioxide.
• Help to maintain acid-base balance.
• Senses odors
• Speech production
• Straining
• Coughing
• Anatomy: nasal cavity, nostril, oral cavity, pharynx, larynx, trachea, left/right bronchus, left/right lung, diaphragm

Nose

• Major entrance and exit for the respiratory system.
• 2 major sections: external nose and internal nose
• Nares open into the nasal cavity, separated into left and right by the nasal septum.
• Conchae increase the surface area of the nasal cavity, and disrupt airflow to clean and warm it along the epithelium.
• Conchae and meatuses trap water during exhalation to prevent dehydration.
• The floor of the nasal cavity is composed of the hard palate and soft palate.
• Air exits the nasal cavities via the internal nares and moves into the pharynx.
• Paranasal sinuses warm and humidity incoming air and are lined with mucosa to produce mucus. The sinuses are named after the bones.
  • Frontal sinus
  • Maxillary sinus
  • Sphenoidal sinus
  • Ethmoidal sinus
• Nasal cavities are lined with mucous membranes, sebaceous glands, and hair follicles to protect the passage from large debris.
• Olfactory epithelium detects odors.
• Epithelium contains epithelial cells that produce mucus to trap debris; warms and humidify incoming air.
• Cilia help to remove mucus and debris by sweeping them towards the throat to be swallowed.
• Serous and mucous-producing cells secrete defensins, immune cells that patrol connective tissue and provide additional protection.

Pharynx

• Divided into 3 regions: nasopharynx, oropharynx, and laryngopharynx.
• Pharyngeal tonsil contains lymphocytes and is covered with ciliated epithelium to trap and destroy invading pathogens during inhalation.
• Uvula and soft palate move like a pendulum during the swallowing phase to close off the nasopharynx during ingestion.
• Auditory (Eustachian) tubes connect to each middle ear cavity.
• Oropharynx contains two sets of tonsils that trap and destroy pathogens:
  • Palatine tonsils: lateral in the oropharynx
  • Lingual tonsils: located at the base of the tongue

Larynx

• Formed by 3 large pieces of cartilage:
  • Thyroid cartilage: consists of the laryngeal prominence (Adam’s apple).
  • Epiglottis: helps move the vocal cords to produce speech.
  • Cricoid cartilage: forms a ring.
• When the epiglottis is in the “closed” position, the unattached end of the epiglottis rests on the glottis. A vestibular fold, or false vocal cord, is one of a pair of folded sections of mucous membrane. A true vocal cord is one of the white, membranous folds attached by muscle to the thyroid and arytenoid cartilages of the larynx on their outer edges. The inner edges of the true vocal cords are free, allowing oscillation to produce sound.
• The act of swallowing causes the pharynx and larynx to lift upward, allowing the pharynx to expand and the epiglottis of the larynx to swing downward, closing the opening to the trachea. These movements produce a larger area for food to pass through while preventing food and beverages from entering the trachea.

Trachea

• Formed by 16-20 stacked, C-shaped pieces of hyaline cartilage that are connected by dense connective tissue.
• The trachealis muscle and elastic connective tissue together form the fibroelastic membrane.
• The fibroelastic membrane allows the trachea to stretch and expand slightly during inhalation and exhalation, whereas the rings of cartilage provide structural support and prevent the trachea from collapsing.
• The trachealis muscle can be contracted to force air through the trachea during exhalation. The trachea is lined with pseudostratified ciliated columnar epithelium, which is continuous with the larynx.

Bronchial Tree

• The trachea branches into the right and left primary bronchi at the carina. These bronchi are also lined by pseudostratified ciliated columnar epithelium containing mucus-producing goblet cells.
• The carina is a raised structure that contains specialized nervous tissue that induces violent coughing if a foreign body, such as food, is present.
• Rings of cartilage, similar to those of the trachea, support the structure of the bronchi and prevent their collapse.
• The primary bronchi enter the lungs at the hilum. The bronchi continue to branch into bronchial a tree. A bronchial tree (or respiratory tree) is the collective term used for these multiple-branched bronchi. The main function of the bronchi, like other conducting zone structures, is to provide a passageway for air to move into and out of each lung.
• The mucous membrane traps debris and pathogens.
• A bronchiole branches from the tertiary bronchi. Bronchioles, which are about 1 mm in diameter, further branch until they become the tiny terminal bronchioles, which lead to the structures of gas exchange.

Respiratory Zone

• In contrast to the conducting zone, the respiratory zone includes structures that are directly involved in gas exchange.
• The respiratory zone begins where the terminal bronchioles join a respiratory bronchiole, the smallest type of bronchiole, which then leads to an alveolar duct, opening into a cluster of alveoli.

Alveoli

• An alveolar sac is a cluster of many individual alveoli that are responsible for gas exchange. An alveolus is approximately 200 μm in diameter with elastic walls that allow the alveolus to stretch during air intake, which greatly increases the surface area available for gas exchange.
• Alveoli are connected to their neighbors by alveolar pores, which help maintain equal air pressure throughout the alveoli and lung.

Lungs

• The lungs are pyramid-shaped, paired organs that are connected to the trachea by the right and left bronchi; on the inferior surface, the lungs are bordered by the diaphragm.
• The lungs are enclosed by the pleurae, which are attached to the mediastinum.
• The right lung is shorter and wider than the left lung, and the left lung occupies a smaller volume than the right.
• The cardiac notch allows space for the heart.
• Each lung is composed of smaller units called lobes. Fissures separate these lobes from each other.
• The right lung consists of three lobes: the superior, middle, and inferior lobes.
• The left lung consists of two lobes: the superior and inferior lobes.
• The parasympathetic system causes bronchoconstriction.
• The sympathetic nervous system stimulates bronchodilation.
• Reflexes such as coughing, and the ability of the lungs to regulate oxygen and carbon dioxide levels, also result from autonomic nervous system control.

Pleura of the Lungs

• Each lung is enclosed within a cavity that is surrounded by the pleura. The pleura (plural = pleurae) is a serous membrane that surrounds the lung. The right and left pleurae, which enclose the right and left lungs, respectively, are separated by the mediastinum.
  • The visceral pleura is the layer that is superficial to the lungs and extends into and lines the lung fissures.
  • The parietal pleura is the outer layer that connects to the thoracic wall, the mediastinum, and the diaphragm.
• The pleurae perform two major functions:
  • Produce pleural fluid that lubricates surfaces, reduces friction to prevent trauma during breathing, and creates surface tension that helps maintain the position of the lungs against the thoracic wall. This adhesive characteristic of the pleural fluid causes the lungs to enlarge when the thoracic wall expands during ventilation, allowing the lungs to fill with air.
  • The pleurae also create a division between major organs that prevents interference due to the movement of the organs, while preventing the spread of infection.
• The difference in pressures drives pulmonary ventilation because air flows down a pressure gradient, that is, air flows from an area of higher pressure to an area of lower pressure.
  • Air flows into the lungs largely due to a difference in pressure; atmospheric pressure is greater than intra-alveolar pressure, and intra-alveolar pressure is greater than intrapleural pressure.
  • Air flows out of the lungs during expiration based on the same principle; pressure within the lungs becomes greater than atmospheric pressure.
• Due to the adhesive force of the pleural fluid, the expansion of the thoracic cavity forces the lungs to stretch and expand as well. This increase in volume leads to a decrease in intra-alveolar pressure, creating a pressure lower than atmospheric pressure. As a result, a pressure gradient is created that drives air into the lungs.
• Inspiration: thoracic cavity expands, external intercostal muscles contract, diaphragm contracts.
• Expiration: thoracic cavity reduces, external intercostal muscles relax, the diaphragm relaxes.

Types of Breathing

• Quiet breathing, also known as eupnea, is a mode of breathing that occurs at rest and does not require the cognitive thought of the individual. During quiet breathing, the diaphragm and external intercostals must contract.
• Diaphragmatic breathing, also known as deep breathing, requires the diaphragm to contract. As the diaphragm relaxes, air passively leaves the lungs.
• Costal breathing, also known as a shallow breath, requires contraction of the intercostal muscles. As the intercostal muscles relax, air passively leaves the lungs.
• Forced breathing, also known as hyperpnea, is a mode of breathing that can occur during exercise or actions that require the active manipulation of breathing, such as singing.
  • During forced breathing, inspiration and expiration both occur due to muscle contractions. In addition to the contraction of the diaphragm and intercostal muscles, other accessory muscles must also contract.
  • During forced inspiration, muscles of the neck contract and lift the thoracic wall, increasing lung volume.
  • During forced expiration, accessory muscles of the abdomen contract, forcing abdominal organs upward against the diaphragm. This helps to push the diaphragm further into the thorax, pushing more air out. In addition, accessory muscles help to compress the rib cage, which also reduces the volume of the thoracic cavity.

Spirometry Testing

Spirometry testing is used to find out how well lungs are working by measuring air volume.

• Respiratory volume, describes the amount of air in a given space within the lungs, or which can be moved by the lung, and is dependent on a variety of factors.
• Tidal volume, refers to the amount of air that enters the lungs during quiet breathing, whereas inspiratory reserve volume is the amount of air that enters the lungs when a person inhales past the tidal volume.
• Expiratory reserve volume, is the extra amount of air that can leave with forceful expiration, following tidal expiration.
• Residual volume, is the amount of air that is left in the lungs after expelling the expiratory reserve volume.
• Respiratory capacity, is the combination of two or more volumes.
• Anatomical dead space, refers to the air within the respiratory structures that never participates in gas exchange, because it does not reach functional alveoli.
• Respiratory rate, is the number of breaths taken per minute, which may change during certain diseases or conditions.
• Both respiratory rate and depth are controlled by the respiratory centers of the brain, which are stimulated by factors such as chemical and pH changes in the blood. These changes are sensed by central chemoreceptors, which are located in the brain, and peripheral chemoreceptors, which are located in the aortic arch and carotid arteries. A rise in carbon dioxide or a decline in oxygen levels in the blood stimulates an increase in respiratory rate and depth

1. Betts, J.G., Young, K.A., Wise, J.A., Johson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J.E., Womble, M. & DeSaix, P. (2013). Anatomy and Physiology. OpenStax. http://cnx.org/content/col11496/latest/[↩]