Respiratory System Structure Overview

The Structure of the Respiratory System

The respiratory system is responsible for maintaining the body's oxygen levels and removing carbon dioxide. It is a complex system that includes the upper and lower respiratory tract and is divided into conducting and respiratory zones.

Upper Respiratory Tract

The upper respiratory tract, which includes the nose, pharynx, and larynx, is located outside the thorax. The structures of the upper airways are crucial for protecting the respiratory system against environmental agents such as dust, pollen, and pathogens. They are lined with mucus, which traps particles and is moved out of the lungs by cilia, a specialized type of cell. These upper airways also contain smooth muscle and cartilage, which help maintain the airway structure and prevent collapse during exhalation.

Lower Respiratory Tract

The lower respiratory tract, which includes the trachea, bronchi, bronchioles, alveolar ducts, and alveoli, is located within the thorax. The structure of these conducting airways differs from that of the respiratory region, as they have relatively thick walls of mucosa, smooth muscle, and cartilage. This thick wall helps to support the airway structure and prevent collapse during exhalation. The mucosa is ciliated, forming an 'escalator' that carries dust out of the lungs to the mouth.

The respiratory airways, such as the alveolar ducts, have a thin, moist, and vascular structure. They form a typical respiratory surface, which is where gas exchange with the blood occurs. Blood vessels, nerves, and lymph vessels run parallel with the airways, which is essential for the function of the respiratory system.

Trachea and Main Bronchi

The trachea is a hollow conduit for gases and bronchial secretions, extending from the level of C6 (cricoid cartilage) to the carina, approximately located at the level of T4-T5. It has 16 to 22 horseshoe bands (c-shaped) of cartilages, with the posterior tracheal wall lacking cartilage and supported by the trachealis muscle. The posterior wall of the trachea can flatten or bow slightly forward during inspiration, depending on the level of inspiration.

The trachea divides into the right and left main bronchi at the carina, which is close to the aortic arch and the division of the pulmonary artery into its left and right branches. The main bronchi, pulmonary arteries, and veins penetrate each lung at the hila, which is the root of the lung.

Lungs and Alveoli

Each lung is anatomically divided into lobes, made up of segments, which are subdivided into lobules. The lungs lie on both sides of the mediastinum, which contains the trachea, heart, major blood vessels, nerves, and oesophagus. The lungs are covered by a thin layer of tissue called the visceral pleura, and the mediastinum and chest wall are lined by the parietal pleura.

The lungs are filled with alveoli, which are small air sacs where gas exchange occurs. These air sacs are surrounded by a thin layer of alveolar tissue, which provides support and facilitates gas exchange. The alveoli are organized into clusters called acini, which are the functional tissues of the lung.

In summary, the structure of the respiratory system is intricate and designed to ensure efficient gas exchange while also providing protection against environmental agents. The upper and lower respiratory tracts work together to bring air in and out of the body, and the alveoli are the site of gas exchange between the air and the blood.