Respiratory System: Structure and Function of the Nose, Lungs, and Airways
Respiratory System: Internal Structures of the Nose
The internal structures of the nose perform three essential functions:
- Filter, warm, and humidify incoming air
- Detect olfactory stimuli (odors)
- Modify sound vibrations during speech
Nasopharynx
The uppermost part of the pharynx connects to the two internal nostrils and has openings leading to the Eustachian tubes, which connect to the middle ear. The back wall houses the pharyngeal tonsil. The nasopharynx exchanges air with the nasal cavity and traps dust and mucus, which are then moved towards the mouth by cilia in its pseudostratified epithelium. Small amounts of exchanged air via the Eustachian tubes equalize air pressure between the pharynx and middle ear.
Structure of Alveoli
Alveoli are tiny, rounded air sacs clustered within the lungs. If spread out, their total surface area would be approximately 130 square meters. Key features of alveoli include:
- Surrounded by specialized hair-like structures
- Rounded shape with a clustered arrangement
- Presence of endothelial tissue and pneumocytes
- Production of surfactant to keep alveoli open
- Type II epithelial cells and phagocytic cells (macrophages) for defense
Pulmonary Ventilation
Airflow between the atmosphere and the lungs is driven by differences in air pressure. Inspiration occurs when lung pressure is lower than atmospheric pressure, while expiration occurs when lung pressure is higher.
Inspiration
Inspiration is the process of air entering the lungs. Before inspiration, lung pressure is equal to atmospheric pressure (760 mmHg). To draw air in, lung pressure must decrease. This is achieved by increasing the volume of the thoracic cavity through the contraction of the diaphragm and intercostal muscles. The diaphragm, a dome-shaped muscle innervated by the phrenic nerve, contracts and flattens during inhalation, increasing the volume of the thoracic cavity. Simultaneously, the intercostal muscles contract, lifting the ribs and sternum outward and upward, further increasing thoracic volume. This expansion lowers the pressure within the lungs, allowing air to flow in until intrapulmonary pressure equalizes with atmospheric pressure.
There are two main types of inspiration:
- Costal breathing: More superficial, involving the contraction of external intercostal muscles and expansion of the chest.
- Diaphragmatic breathing: Deeper breathing, primarily using the diaphragm and expanding the abdomen.
Forced Inspiration: An active process involving the contraction of numerous skeletal muscles, including the diaphragm, intercostal muscles, sternocleidomastoid, scalenes, pectoralis major, and others.
Deglutition and Epiglottis
The epiglottis is a flap of elastic cartilage that acts as a lid over the larynx. During swallowing, the pharynx and larynx elevate. The pharynx widens to receive food or drink, while the larynx elevates, causing the epiglottis to fold down and cover the laryngeal opening. This prevents food or liquid from entering the lower airways, directing it instead into the esophagus.
External Respiration
External respiration, also known as pulmonary gas exchange, involves the diffusion of oxygen from the air in the alveoli into the blood within the pulmonary capillaries and the diffusion of carbon dioxide in the opposite direction. This process oxygenates deoxygenated blood from the right side of the heart before it returns to the left side of the heart.
Upper Airway
The upper airway includes the nose, pharynx, and associated structures.
Thorax and Abdomen During Breathing
During inspiration, the volume of the thoracic cavity increases while pressure decreases. Conversely, during expiration, thoracic volume decreases while pressure increases. The opposite occurs in the abdomen. These pressure changes facilitate blood flow between the chest and abdomen, aided by valve systems.
Lung Volume and Pressure
As lung volume increases during inspiration, the air molecules within the lungs spread out, leading to a decrease in air pressure. This pressure difference allows air to flow into the lungs from the atmosphere. Conversely, during expiration, lung volume decreases, causing air pressure to increase and forcing air out of the lungs.
Bicarbonate Ion Reaction
Approximately 70% of carbon dioxide is transported in the blood plasma as bicarbonate ions (HCO3-). When CO2 diffuses from tissues into red blood cells, it combines with water to form carbonic acid (H2CO3) with the help of the enzyme carbonic anhydrase. Carbonic acid then dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-), which are transported in the plasma.