Learning Objectives
By the end of this section, you will be able to:
- Describe the functions of respiratory anatomy and physiology
- Define the concepts of oxygenation and ventilation
- Discuss the relationship of respiratory physiology on oxygenation and ventilation
- Define the concept of perfusion
- Discuss the relationship of respiratory physiology on perfusion
An adequately functioning respiratory system is vital to survival. Each breath provides an opportunity for the body to receive oxygen, remove carbon dioxide, and help maintain acid-base balance. A functioning respiratory system has adequate airflow in and out of the lungs, provides oxygen to fuel the body’s tissues, and allows sufficient clearance of carbon dioxide during exhalation. Nonessential tasks of the respiratory system include sensing odors and speech production. Because of the vital functions of the respiratory system, protection is necessary. The ribs, as well as protective membranes, safeguard the lungs from injury. The body also has multiple defenses to prevent pathogens from entering the respiratory tract, such as mucus and nasal mucosa.
Respiratory Anatomy and Physiology Review
The respiratory system allows the body to breathe in air, eliminate carbon dioxide, and oxygenate blood. It is made up of two zones: the conducting zone and the respiratory zone. The conducting zone includes all parts that help deliver air to the lower airway for ventilation and perfusion. These areas serve as passageways for air to flow into and out of the lungs. The respiratory zone describes the parts that perform gas exchange. These are the areas where oxygen from the outside air enters the bloodstream and is swapped for carbon dioxide.
The major structures of the respiratory system are shown in Figure 11.2.
Link to Learning
Watch this Crash Course on the respiratory system that reviews respiratory anatomy and physiology.
Components of the Conducting Zone
The conducting zone starts at the nose and mouth, where air enters and exits the body. The primary purposes of this zone are to humidify and warm air and to minimize the likelihood of pathogens and debris entering the body. The nasal passages and paranasal sinuses are lined with respiratory epithelium. Respiratory epithelium generates mucus, which helps to catch and trap debris. The cilia of respiratory epithelium are short, hairlike structures that move continuously, sweeping mucus and debris toward the throat so it can be swallowed instead of advancing into the lower respiratory tract. Because the epithelium is moist, it adds humidification to air as it enters the body. Capillaries under the epithelium are a source of warmth. When inhaled air is warmed and humidified, it is less likely to damage delicate tissues in the lungs. Some cells in this area provide protection by secreting antibacterial enzymes and proteins. Protective immune cells exist in the connective tissue underneath the respiratory epithelium.
The pharynx is a muscular tube with an inner mucous membrane lining; it connects the nasal passages and oral cavity to the trachea and esophagus (Figure 11.3). Its three regions are the nasopharynx, oropharynx, and laryngopharynx. The nasopharynx allows air to pass from the nasal cavity toward the trachea. The oropharynx allows passage of both air and food. The laryngopharynx is the most inferior portion of the pharynx. It is located superior to both the trachea and esophagus. Like the oropharynx, it allows passage of both air and food. The upper portion of the laryngopharynx is lined with epithelial cells capable of secreting mucus, which helps to trap pathogens and debris. Additionally, cilia beat continuously, sweeping mucus and debris upward so it can be swallowed instead of remaining in the respiratory tract.
The larynx is made of cartilage and joins the pharynx to the trachea (Figure 11.4). It regulates how much air goes into and out of the lungs. The thyroid cartilage, epiglottis, and cricoid cartilage are the key structures in the larynx. The laryngeal structures protect the opening of the trachea during swallowing and allow vibrations in the vocal cords to produce sound.
The trachea is anterior to the esophagus and extends downward from the larynx to the lungs (Figure 11.5). This tube is made up of sixteen to twenty C-shaped pieces of hyaline cartilage, joined by connective tissue. The cartilage rings provide rigid support and keep the trachea open. A fibroelastic membrane encases the posterior trachea. The fibroelastic membrane provides flexibility to permit the trachea to shift and stretch slightly during breathing. The trachea is lined with epithelial cells that secrete mucus and beat consistently to sweep debris up and out of the airway.
The carina is the point where the trachea splits into the right and left primary bronchi. Specialized nervous tissue in the carina can detect foreign bodies (such as food) and will cause forceful coughing to expel it. The bronchi are lined with epithelial cells capable of producing mucus. Each primary bronchi enters the lung and continues to split into a bronchial tree. The bronchi serve as a passage for air to enter and exit the lung. The bronchi continue to branch into smaller and smaller airways called bronchioles. Bronchioles have muscular walls that can flex to increase or decrease airflow. With a diameter of approximately one millimeter, these tiny airways continue branching off until they end in small terminal bronchioles. Terminal bronchioles lead to the site where gas exchange occurs.
The lungs connect to the trachea via the right and left bronchi (Figure 11.6). The diaphragm is adjacent and inferior to the lungs. To accommodate space for the heart, the left lung is smaller than the right lung. The left lung has two lobes, and the right lung has three lobes. The ribs and sternum provide protection for the lungs and heart. The lungs are encased in protective membranes called pleurae. The parietal pleura is the outer layer, and the visceral pleura is the inner layer. The pleural space is the small area between the parietal and visceral pleura.
A small volume of pleural fluid provides lubrication and creates surface tension to keep the lungs expanded. Most of the pressure changes that drive inspiration and expiration are caused by movement in the intercostal muscles and diaphragm.
The medulla oblongata in the brain is the respiratory center and controls breathing by responding to shifts in carbon dioxide, oxygen, and blood pH (potential hydrogen). When changes in the pH of cerebrospinal fluid (CSF) are detected in the medulla oblongata, it can change the respiratory rate so that pH shifts back into the normal range. Autonomic functions control the rate and depth of respiration based on the chemical changes discussed preceedingly.
Components of the Respiratory Zone
The respiratory zone begins where terminal bronchioles meet a respiratory bronchiole (Figure 11.7). This connects to an alveolar duct and opens into a cluster of alveoli. An alveolus is an individual, grapelike sac in the lungs where gas exchange occurs, and an alveolar sac is a group of alveoli. Alveoli are elastic and stretch during inspiration, increasing the available surface area for gas exchange.
Link to Learning
Watch this Crash Course on the respiratory system that reviews diffusion, the mechanics of breathing, and gas exchange.
Oxygenation and Ventilation
The movement of air into and out of the lungs, which allows gas exchange to occur, is called ventilation. Air flows from an area of high pressure to an area of low pressure. During inhalation, air passes through the conducting zone and travels to the respiratory bronchiole and the alveolar sacs.
The process when oxygen from the air moves into the bloodstream is called oxygenation. When air enters the lungs and travels to the alveoli, oxygen molecules pass into the capillaries by diffusion, which is the movement of substances from an area of high concentration to an area of low concentration (Figure 11.8). The term partial pressure refers to how much of a gas is dissolved in the blood. Because there is a higher partial pressure of oxygen in the alveoli than the capillaries, the pressure gradient pulls oxygen into the bloodstream. Carbon dioxide in the blood diffuses across the capillaries into the alveoli, where it leaves the body during exhalation.
Because oxygen does not dissolve well in liquids, it uses red blood cells as a means of transportation. Oxygen molecules attach to hemoglobin in the red blood cells. When oxygen binds to hemoglobin, a chemical reaction occurs, and oxyhemoglobin is formed. Oxyhemoglobin is a bright red molecule and contributes to the vivid red color observed in arterial blood. Once oxygen is in the blood, it can travel through the body and be transferred to the tissues.
Link to Learning
View a real-time MRI of the thorax during breathing. Note how the diaphragm lowers and the ribs expand during inspiration. Note how the diaphragm raises and the ribs contract during expiration.
Factors That Affect Oxygenation and Ventilation
Many factors affect oxygenation and ventilation (Table 11.1). Any condition that creates a decreased respiratory rate or limitation of chest wall/diaphragmatic expansion can create a decrease in oxygenation either through hypoventilation or other factors. Narrowed airways, found in conditions such as asthma or angioedema (soft tissue swelling in the deep layers of the skin, most commonly in the mouth, eyelids, and genitals), can limit airflow into the body, decreasing the amount of oxygen the lungs can utilize. In conditions, such as anemia or hemorrhage, a decreased volume of red blood cells reduces hemoglobin levels and limits the body’s oxygen-carrying capability. Damaged alveoli and pulmonary capillaries can occur in lung diseases, such as chronic obstructive pulmonary disease (COPD), decreasing the areas that are available for ventilation and causing inadequate oxygenation. In atelectasis as well as pneumonia, some alveoli are unventilated, leaving fewer capillaries available for gas exchange. Alveolar filling disorders, such as hemorrhage, affect ventilation and oxygenation due to a decrease in available areas of gas exchange (Theodore, 2022).
| Condition | Effect |
|---|---|
| Asthma or angioedema | Narrowed airways can limit airflow into the body, decreasing the amount of oxygen the lungs can utilize. |
| COPD | Damage to alveoli and pulmonary capillaries can decrease the areas that are available for ventilation and cause inadequate oxygenation. |
| Atelectasis and pneumonia | Some alveoli are unventilated, leaving fewer capillaries available for gas exchange. |
| Alveolar filling disorders (e.g., hemorrhage) | Ventilation and oxygenation are affected due to a decrease in available areas of gas exchange. |
| Anemia or hemorrhage | A decreased volume of red blood cells reduces hemoglobin levels and limits the body’s oxygen-carrying capability. |
| Perfusion defects, such as pulmonary embolus or blood clot | The oxygen-carbon dioxide exchange can be impaired. |
Perfusion
While the term perfusion is used to describe blood flow throughout all parts of the body, certain concepts specifically relate to perfusion of the respiratory system. Perfusion occurs when blood flows through the pulmonary capillaries. This flow allows:
- deoxygenated blood to flow toward the alveoli to receive oxygen by way of diffusion
- oxygenated blood to flow into the pulmonary vein and enter the left side of the heart, to be pumped out through the circulatory system
Factors That Affect Perfusion
Factors that affect blood flow through the blood vessels of the lungs affect perfusion. When blood vessels are blocked or narrowed, it decreases the lung’s ability to oxygenate blood. When areas of the lung are inadequately ventilated, more problems occur. Capillaries become constricted and blood flow is diverted to alveoli that are sufficiently ventilated. Conversely, in alveoli that are adequately ventilated, the diameter of the pulmonary arterioles increases in response to a greater partial pressure of oxygen in the alveoli. The increased diameter allows greater blood flow through these functioning parts of the lung.