Respiratory System Structure and Function Study Guide

Structural Plan and Overview of the Respiratory System

The basic structural plan of the respiratory system is often compared to an inverted tree. In this metaphor, if the tree were hollow, the leaves would be comparable to the alveoli. These alveoli are microscopic sacs enclosed by extensive networks of capillaries. The primary mechanism responsible for the exchange of gases during respiration is the passive transport process known as diffusion. Structurally, the respiratory system is divided into the upper respiratory tract and the lower respiratory tract. The upper respiratory tract consists of the nose, the pharynx, and the larynx. The lower respiratory tract is composed of the trachea, the bronchial tree, and the lungs. A specialized membrane called the respiratory mucosa lines the air distribution tubes throughout the respiratory tree. This membrane is prolific, producing more than $125\,mL$ of mucus each day to form what is referred to as a "mucous blanket." This mucus serves a critical air purification function by trapping inspired irritants such as dust and pollen. Furthermore, specialized mucosal cells contain cilia that beat in only one direction, effectively moving the mucus upward toward the pharynx for eventual removal.

Anatomy and Physiology of the Upper Respiratory Tract

The nose is the first structure of the upper respiratory tract, featuring a nasal septum that separates the interior into two distinct cavities. These cavities are lined with a mucous membrane. Several paranasal sinuses drain into the nose, including the frontal, maxillary, sphenoidal, and ethmoidal sinuses. Other associated structures include the lacrimal sac and the superior, middle, and inferior conchae. Functionally, the nose warms and moistens inhaled air and contains the specialized sense organs for smell. The pharynx, commonly known as the throat, is approximately $12.5\,cm$ ($5\,\text{inches}$) in length. It is divided into three sections: the nasopharynx, the oropharynx, and the laryngopharynx. It serves as a common passageway for food, liquids, and air. Multiple openings lead into the pharynx, including two nasal cavities, the mouth, the esophagus, the larynx, and the auditory (eustachian) tubes. The nasopharynx contains the pharyngeal tonsils (adenoids) and the openings of the auditory tubes, while the oropharynx contains the palatine and lingual tonsils. The larynx, or voice box, is formed by a framework of several pieces of cartilage, the largest of which is the thyroid cartilage, also known as the "Adam's apple." The epiglottis is a specific cartilage that partially covers the opening into the larynx. The interior of the larynx contains vocal cords and is lined with mucosa. The larynx functions both as an air distribution passageway to and from the lungs and as the primary organ for voice production.

Anatomy and Physiology of the Lower Respiratory Tract

The trachea, or windpipe, is a tube approximately $11\,cm$ ($4.5\,\text{inches}$) long that extends from the larynx into the thoracic cavity. It is held open by a series of C-shaped rings of hyaline cartilage and is lined with a mucous membrane. The trachea functions as an essential passageway for air; however, obstruction of this tube can be fatal. A complete blockage occludes the airway and can cause death within minutes. In the United States, tracheal obstructions are responsible for more than $4000$ deaths annually. The trachea branches into the right and left primary bronchi, which then branch into secondary bronchi and eventually into smaller tubes called bronchioles. These bronchioles terminate in clusters of microscopic alveolar sacs. The walls of these sacs are composed of alveoli, which are the sites of gas exchange between the air and the blood. The respiratory membrane consists of the alveolar epithelium, the capillary endothelium, and their respective basement membranes. Red blood cells (RBCs) pass through these capillaries for gas exchange. Specialized type II cells within the alveoli produce surfactant to prevent collapse. The lungs themselves are large organs that fill most of the chest cavity, with the exception of the mediastinum, which contains the heart and large blood vessels. The narrow upper part of each lung, located under the collarbone, is called the apex, while the broad lower part resting on the diaphragm is the base. The lungs are covered by the pleura, a moist, smooth, and slippery membrane. The visceral pleura covers the surface of the lungs, while the parietal pleura lines the chest cavity. The intrapleural space between them contains fluid to reduce friction during breathing.

Mechanics and Regulation of Respiration

Respiration involves the exchange of gases between a living organism and its environment. External respiration includes pulmonary ventilation (breathing) and pulmonary gas exchange. Internal respiration encompasses systemic gas exchange and cellular respiration. Pulmonary ventilation is driven by volume and pressure changes in the thorax. Inspiration is an active process where the diaphragm flattens and the external intercostal muscles contract to elevate the ribs, increasing the size of the thoracic cavity and drawing air into the lungs. Expiration is normally a passive process involving the elastic recoil of lung tissues. However, forceful expiration involves the internal intercostals, which depress the rib cage, and the abdominal muscles, which elevate the diaphragm to decrease thoracic volume. Pulmonary volumes are measured using a spirometer. The tidal volume ($TV$) is the amount of air taken in or expelled during a normal breath, typically $500\,mL$. The vital capacity ($VC$) is the maximum amount of air that can be inhaled and exhaled. The expiratory reserve volume ($ERV$) is the air that can be forcibly exhaled after a normal tidal expiration, and the inspiratory reserve volume ($IRV$) is the air that can be forcibly inspired above a normal inhalation. Breathing is regulated by respiratory control centers in the medulla (inspiratory and expiratory centers) and the pons (PRG and apneustic center). Under resting conditions, these centers maintain a rate of $12\text{ to }18$ breaths per minute. While the cerebral cortex allows for limited voluntary control, respiration is largely reflexive. Chemoreceptors in the carotid and aortic bodies respond to levels of $\text{CO}_2$, $\text{O}_2$, and blood pH ($H^+$ ions). Additionally, pulmonary stretch receptors respond to the inflation of the lungs.

Gas Exchange, Transport, and breathing Patterns

During pulmonary gas exchange, $\text{CO}_2$ moves out of the lung capillary blood into the alveolar air to be expired, while $\text{O}_2$ moves from the alveoli into the blood. In the blood, oxygen combines with hemoglobin ($Hb$) to form oxyhemoglobin ($HbO_2$). In systemic gas exchange, oxyhemoglobin breaks down to release $\text{O}_2$ into tissue cells. Simultaneously, $\text{CO}_2$ moves from the tissues into the blood. Some $\text{CO}_2$ binds to hemoglobin to form carbaminohemoglobin ($HbCO_2$). Most $\text{CO}_2$ combines with water ($H_2O$) to form carbonic acid ($H_2CO_3$), which then dissociates into bicarbonate ions ($HCO_3^-$) and hydrogen ions ($H^+$). This process is reversed in the lungs to release $\text{CO}_2$. Various breathing patterns are recognized clinically: eupnea is normal breathing; hyperventilation describes rapid and deep respirations; hypoventilation describes slow and shallow respirations; dyspnea refers to labored or difficult breathing; apnea is the temporary cessation of breathing; and respiratory arrest is the failure to resume breathing following a period of apnea.