Respiration is the process where organisms exchange respiratory gases with the external environment. This essential process involves the oxidation of food and the liberation of energy, which we need to maintain various life activities. Specialized organs, collectively called the respiratory system, carry out respiration.
Key Features of the Respiratory System
The respiratory system’s structure allows it to efficiently exchange respiratory gases. Therefore, it possesses several important features:
- Thin Membrane: A thin membrane facilitates efficient gas exchange.
- Increased Surface Area: The system provides an increased surface area to maximize gas exchange.
- Close Association with Blood Vessels: The respiratory epithelium and the endothelium of blood vessels are closely associated, allowing for effective gas exchange.
- Vascular Nature: The respiratory system is highly vascular, meaning it has a rich supply of blood vessels. Furthermore, its lining consists of involuted cells, which aids the respiratory process.
Division of the Respiratory System
We divide the respiratory system of mammals, including humans, into two distinct functional regions:
- The Respiratory Route: This involves structures that transport the air.
- The Respiratory Seat (or Lobe): This is where the actual gas exchange occurs.
The Respiratory Route
The respiratory route consists of several organs that transport respiratory gases.
1. External Nares (Nostrils)
These are the paired openings present at the tip of the nose.
2. Nasal Chambers
A vertical partition, called the nasal septum, divides the nasal cavity into two chambers. Internally, we can divide each nasal chamber into three regions:
- Vestibular Region: This is the outermost part of the nasal chambers. It is lined by stratified epithelium containing a few sebaceous glands and hair. This region acts as a sieve to filter incoming air.
- Respiratory Region: This is the middle region of the nasal chambers, lined by pseudostratified columnar epithelium. This region plays a significant role in warming and humidifying the air.
- Posterior Olfactory Region: The olfactory epithelium lines the posterior olfactory region, which functions as the organ of smell.
3. Internal Nares
The internal nares, an opening, allow the nasal chambers to open internally into the pharynx. For example, during swallowing, the soft palate closes the internal nares to prevent food from entering the nasal cavity.
4. Pharynx
The pharynx is a short muscular chamber located behind the soft palate and extending to the gullet and glottis. We divide it into three functional regions:
- Nasopharynx: The upper part connected to the nasal cavity.
- Oropharynx: The middle part behind the oral cavity.
- Laryngopharynx: The lower part leading to the larynx and esophagus.
5. Glottis and Epiglottis
The glottis is a slit-like opening on the floor of the laryngopharynx. However, a flap-like cartilage called the epiglottis covers it. The epiglottis closes the glottis during the passage of food to prevent the food from entering the respiratory tract.
6. Larynx (Voice Box)
The glottis leads into the larynx, a cartilaginous and ciliated chamber located in front of the laryngopharynx. In adult men, the larynx grows larger and becomes prominent on the front of the neck, forming the Adam’s apple. The larynx continues downward and opens into the trachea. Ultimately, in mammals, the larynx functions as the sound-producing organ.
7. Trachea (Windpipe)
The trachea, or windpipe, is a long tube about 19 cm long and 2.5 cm wide in adult humans. It runs down beneath the esophagus in the neck region and enters the thoracic cavity. Subsequently, at the level of the 5th and 6th ribs, it divides into a pair of primary bronchi, with each one entering one lung. C-shaped rings of cartilage (16-20) reinforce the walls of the trachea, which provides flexibility and protection against collapsing.
Furthermore, pseudostratified ciliated columnar epithelium containing mucus-secreting goblet cells lines the trachea internally, which keeps the trachea moist.
8. Bronchi
The trachea divides into two primary bronchi, which further divide into secondary bronchi. Then, each secondary bronchus divides into tertiary bronchi. Ciliated mucosa lines the inner walls of all these bronchi, and incomplete cartilaginous rings support them.
Lungs and Alveoli
Lungs
The lungs act as the primary respiratory organs in mammals and other vertebrates. They are located in the thoracic cavity, supported dorsally by the vertebral column, ventrally by the sternum, and laterally by the ribs. In addition, the thoracic diaphragm supports the lungs from below. The space between the lungs is called the mediastinum.
An inner visceral pleura and an outer parietal pleura line the pleural cavity, in which each lung is lodged. The fluid in the pleural cavity acts as a lubricant, preventing wear and tear and providing protection against mechanical shock.
Each lung has a spongy and elastic structure with a pinkish color and a conical shape. We call the top part the apex, and the broader bottom part the base. The left lung has two lobes, while the right lung has three lobes.
Internal Structure of the Lung Lobes
Internally, each lobe of the lung is divided into lobules. A terminal bronchiole enters and serves each lobule. The terminal bronchiole is further divided into respiratory bronchioles. The structure of the respiratory bronchiole is intricate and folded. Each respiratory bronchiole is again divided into alveolar ducts.
The alveolar ducts expand to form the atria. The atria lead into alveolar sacs, each of which contains multiple small, flask-shaped pouches called alveoli. Indeed, approximately 750 million alveoli exist in total, covering a surface area of about 100 square meters, which is 100 times greater than the surface area of human skin.
The alveoli contain lipoprotein in their membranes, which prevents the walls from collapsing during expiration. Squamous epithelium forms the lining of the alveoli. This lining receives a network of blood capillaries on its outer surfaces, along with some reticular and elastic connective tissue.
Cuboidal cells within the squamous epithelium of the alveoli secrete a thin film of surfactant, which is a lipoprotein. This surfactant reduces surface tension and prevents the alveoli from collapsing.
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