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The human respiratory system plays a critical role in gas exchange, facilitating the uptake of oxygen and the elimination of carbon dioxide, which are essential for cellular respiration and metabolic processes. The human respiratory system has been well-adapted for gas exchange with atmospheric oxygen. The parts of the human respiratory system are,
The path from the nose to the alveolar ducts is known as the respiratory tract.
The nasal cavity is a large air-filled space located above and behind the nose in the middle of the face. It opens to the nostril from the front, and it extends posteriorly to the nasopharynx. The nasal cavity is divided into two chambers by the nasal septum.
The anterior part of the nasal septum is made of cartilage, and the posterior part is made of bone. The nasal cavity is lined with a pseudostratified mucociliary epithelium. It traps dust, pathogens, and other airborne particles in the inhaled air. In the upper part of the nasal cavity, there is the “Olfactory epithelium” that contains olfactory receptor cells for the sense of smell.
The pharynx is a muscular, funnel-shaped tube that extends from the base of the skull to the level of the sixth cervical vertebra. The pharynx is a shared pathway for both the respiratory system and the digestive system. The pharynx is also lined with a mucous membrane. There are three anatomical regions in the pharynx.
The nasopharynx is the anterior part of the pharynx. It is located posterior to the nasal cavity and above the soft palate. It contains pharyngeal tonsils and an opening to the Eustachian tube.
The oropharynx is located behind the oral cavity. It extends from the soft palate to the epiglottis. This part of the pharynx serves both the respiratory and the digestive systems.
The laryngopharynx is located behind the larynx. It extends from the epiglottis to the cricoid cartilage.
The larynx is a cartilaginous chamber located in the anterior portion of the neck, spanning the third to sixth cervical vertebrae (C3–C6). Interior, it opens to the trachea, and posteriorly it connects to the esophagus. The larynx is lined with a pseudostratified mucociliary epithelium.
The larynx is composed of several cartilages.
There are vocal cords in the larynx. They are mucous membranes. The epiglottis prevents food from entering the trachea. Here, the epiglottis depends and the larynx rises, closing the airway during swallowing.
The vocal cords generate sound by vibrating. The vocal cords are brought closer by muscle contraction and cartilage movements. When the air passes through the vocal cords, they vibrate. Thus, the sound is generated.
The trachea is a tube-like structure that starts from the posterior part of the larynx. It extends through the thoracic chamber and divides into the left bronchus and the right bronchus at the level of the 5th cervical vertebrae. The trachea gets its rigidity from C-shaped cartilaginous rings.
They are stacked on top of each other and covered with connective tissues. The tracheal wall contains smooth muscles. The tracheal tube is lined with a pseudostratified mucociliary epithelium.
The left and right bronchi consist of complete cartilaginous rings. The left trachea is located slightly off the midline, making it slightly longer and narrower. The left trachea is shorter and wider. Both left and right trachea enter to left and right lungs, respectively. Then they will be divided into bronchi. The left trachea is divided into two bronchi, and the right trachea is divided into three bronchi.
In the walls of the bronchi, there are irregular-shaped cartilaginous rings for their rigidity. The bronchi will be further divided into bronchioles, terminal bronchioles, respiratory bronchioles, and alveolar ducts. The diameter of these tubes is decreasing gradually.
It reduces the cartilaginous support when it comes to the terminal bronchioles. The ciliated nature of the epithelium gradually decreases. Also, it decreases the smooth tissues and the epithelial tissues on the walls. When it comes to the alveolar ducts, the wall has become a simple squamous epithelium.
The lungs are located in the thoracic cavity on either side of the midline, filling the entire thoracic cavity. The left lung is located slightly off the midline because the heart is located slightly left in the thoracic cavity.
The lung is roughly a cone-shaped organ with a narrow apex, a wide base, a broad costal surface, and a smaller mediastinal surface. The costal surface lies against the rib cage, while the mediastinal surface faces the mediastinum.
The lung root (hilum), which contains the bronchi, pulmonary vessels, lymphatic vessels, and nerves, is located on the mediastinal surface. The pulmonary artery enters, the pulmonary vein exits, and the bronchi enter the lung from the lung root. Additionally, lymphatic vessels and nerves also enter the lung from the lung root.
Each lung is covered with two pleural membranes, which are the visceral pleura and parietal pleura. The visceral pleura is in contact with the lung wall. The parietal pleura is the outer layer, and it attaches to the chest wall. Between the two pleural layers, the pleural cavity contains a fluid called pleural fluid.
A lung is further divided into lobes. The left lung is divided into two lobes, as inferior lobe and the superior lobe. The right lung is divided into three lobes as the superior lobe, the middle lobe, and the inferior lobe. Each lobe is subdivided into bronchopulmonary segments, which are smaller segments of the lobe.
A bronchus that enters a lung will be divided into bronchioles. The bronchioles will enter each lobe and divide into alveolar ducts and alveoli. Inside a lung, there are parts from the bronchioles to the alveoli that are covered with blood vessels, epithelial tissues, and smooth tissues.
The functional unit of the human respiratory system is the alveoli. An alveolus is a bulb-shaped structure. There are about 70 million alveoli in the lungs. And they have about 70 m2 of surface area combined. Many alveoli come together to form an alveolar sac. An alveolar sac is surrounded by a network of blood capillaries.
An alveolus is lined with a simple squamous epithelium. There are surfactant cells in this epithelium. Also, there are macrophage cells in the alveoli.
The cover image was created using an image by OpenClipart-Vectors from Pixabay
Figure 01: Contains an image by BruceBlaus, licensed under CC BY-SA 4.0, via Wikimedia Commons
Figure 02: Contains an image by LadyOfHats, licensed under CC0, via Wikimedia Commons
