The Thoracic cage
The thoracic cage is formed by the vertebral column behind, the ribs and intercostal spaces on either side laterally and the sternum and costal cartilages anteriorly in front. Above, it communicates with the root of the neck through the ‘thoracic inlet’; and below, it is separated from the abdominal cavity by the diaphragm.
Its a dagger-shaped bone, and forms the anterior part of the thoracic cage. It consists of three parts.
- The manubrium - Almost triangular in shape. The clavicles and the first costal cartilage and the upper part of the 2nd costal cartilages are attached to the manubrium.
- The body of the sternum – is attached to the manubrium at the manubriosternal joint (the angle of Louis). Its lateral border is notched to join with the lower part of the 2nd costal cartilage and 3rd to the 7th costal cartilages.
- The xiphoid process is the smallest part of the sternum and usually remains cartilaginous well into adult life.
Respiratory Zone Structures
The respiratory zone begins as the terminal bronchioles give rise to respiratory bronchioles within the lung. Protruding from the smallest bronchioles are scattered alveoli (thin-walled air sacs).The respiratory bronchioles lead into winding alveolar ducts.The alveolar ducts lead into terminal clusters of alveoli called alveolar sacs.The alveolar sac is similar to a bunch of grapes, and the alveoli are the individual grapes. About 300 million gas-filled alveoli in the lungs take up most of the lung volume and provide a large surface area for gas exchange similar in surface area to a tennis court ( of about 70 m2)
The alveoli are surrounded by pulmonary capillaries . In most areas, air and blood are separated only by the alveolar epithelium and the capillary endothelium (known as the alveolar capillary membrane or respiratory membrane), and they are about 0.5 µm apart .
The Alveolar Capillary Membrane (Respiratory Membrane): The walls of the alveoli are composed of a single layer of squamous epithelial cells, surrounded by a basement membrane. The external surfaces of the alveoli are densely covered with a “network” of pulmonary capillaries . The alveolar and capillary walls and their fused basement membranes together form the respiratory membrane, a 5-µm-thick air-blood barrier that has alveolar gas on one side and pulmonary capillary blood flowing past on the other side . Gas exchange occurs readily by simple diffusion across the respiratory membrane— O2 passes from the alveolus into the blood and CO2 leaves the blood to enter the gas-filled alveolus.
The thoracic cage is mainly formed by twelve pairs of ribs. The first seven of these are connected anteriorly through their costal cartilages to the sternum. The cartilages of the 8th, 9th and 10th ribs attach to the cartilage of the rib above it . The 7th rib and therefore they are known as ‘false ribs’ and the last two ribs are free anteriorly and therefore known as ‘floating ribs’.
The lungs fill the thoracic cavity apart from a small area called the mediastinum (where the heart and the great vessels are located). Both lungs are well protected within the bony rib cage. Each lung is a cone shaped structure with an apex and a base. Each lung is surrounded by its own pleural cavity and covered by the pleura. It is connected to the central mediastinum by vascular and bronchial attachments, called the hilum or the lung root. The anterior, lateral, and posterior lung surfaces lie in close contact with the ribs (separated by the pleura and form the costal surface). Just deep to the clavicle, the narrow superior tip of the lung is located and is known as the apex. The concave, inferior surface of the lung rests on the diaphragm and is known as the base of the lung.
The lungs of both sides are not symmetrical. The liver bulges upwards on the right side elevating the diaphragm and the heart takes up space on the left chest. The left lung is thinner and longer than the right lung.
The right lung has three main lobes (superior, middle and inferior) and the left lung has only two lobes (superior and inferior). Each lobe is further subdivided into bronchopulmonary segments. Each broncho-pulmonary segment is served by individual bronchi and bronchioles and functions independantly. In diseases of the lung such as cancer a bronchopulmonary segment can be removed surgically without causing much damage to the rest of the lung. Bronchopulmonary segments contain smaller subdivisions known as lobules which are surrounded by elastic connective tissue. Each lobule is an independent unit with hundreds of alveoli, and has its own bronchiole, a lymphatic vessel, a small pulmonary venule and a small pulmonary arteriole.
The lungs are covered by pleural membranes that are continuous with each other. The layer of pleura directly in contact with the surface of the lung is known as the visceral pleura and the layer of pleura lining the inner part of the thoracic cavity is the parietal pleura. The space between the visceral and parietal pleura is the pleural cavity or pleural space, and it is filled with a fluid, known as pleural fluid.
Functions of the pleura:
1. The visceral pleura slides on the parietal pleura with each breath and the pleural fluid provides the necessary lubrication for this movement.
2. The air pressure in the pleural cavity is negative to the atmospheric air pressure and therefore it aids in breathing
On the mediastinal surface of each lung is the hilum, through which pulmonary and systemic blood vessels enter and leave the lungs. Each main bronchus enters the lung at the hilum on its own side and begins to branch almost immediately. All conducting and respiratory passageways distal to the main bronchi are located with in the lungs.
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