As the blood cells release the oxygen they pick up carbon dioxide , a waste product of metabolism. The carbon dioxide is then carried back to the lungs and released into the alveoli. With each exhalation , carbon dioxide is expelled from the bronchi out through the trachea. View a graphic image illustrating the anatomy of lung. The major function of the lungs is to perform gas exchange, which requires blood from the pulmonary circulation. This blood supply contains deoxygenated blood and travels to the lungs where erythrocytes, also known as red blood cells, pick up oxygen to be transported to tissues throughout the body.
The pulmonary artery is an artery that arises from the pulmonary trunk and carries deoxygenated, arterial blood to the alveoli.
The pulmonary artery branches multiple times as it follows the bronchi, and each branch becomes progressively smaller in diameter. One arteriole and an accompanying venule supply and drain one pulmonary lobule. As they near the alveoli, the pulmonary arteries become the pulmonary capillary network. The pulmonary capillary network consists of tiny vessels with very thin walls that lack smooth muscle fibers. The capillaries branch and follow the bronchioles and structure of the alveoli. It is at this point that the capillary wall meets the alveolar wall, creating the respiratory membrane.
Once the blood is oxygenated, it drains from the alveoli by way of multiple pulmonary veins, which exit the lungs through the hilum. Dilation and constriction of the airway are achieved through nervous control by the parasympathetic and sympathetic nervous systems.
The parasympathetic system causes bronchoconstriction , whereas the sympathetic nervous system stimulates bronchodilation. Reflexes such as coughing, and the ability of the lungs to regulate oxygen and carbon dioxide levels, also result from this autonomic nervous system control. Sensory nerve fibers arise from the vagus nerve, and from the second to fifth thoracic ganglia. The pulmonary plexus is a region on the lung root formed by the entrance of the nerves at the hilum.
The nerves then follow the bronchi in the lungs and branch to innervate muscle fibers, glands, and blood vessels. Each lung is enclosed within a cavity that is surrounded by the pleura. The right and left pleurae, which enclose the right and left lungs, respectively, are separated by the mediastinum.
The pleurae consist of two layers. The visceral pleura is the layer that is superficial to the lungs, and extends into and lines the lung fissures Figure 2. In contrast, the parietal pleura is the outer layer that connects to the thoracic wall, the mediastinum, and the diaphragm. The visceral and parietal pleurae connect to each other at the hilum. The pleural cavity is the space between the visceral and parietal layers. The pleurae perform two major functions: They produce pleural fluid and create cavities that separate the major organs.
Pleural fluid is secreted by mesothelial cells from both pleural layers and acts to lubricate their surfaces. This lubrication reduces friction between the two layers to prevent trauma during breathing, and creates surface tension that helps maintain the position of the lungs against the thoracic wall. This adhesive characteristic of the pleural fluid causes the lungs to enlarge when the thoracic wall expands during ventilation, allowing the lungs to fill with air. The pleurae also create a division between major organs that prevents interference due to the movement of the organs, while preventing the spread of infection.
The burning of a tobacco cigarette creates multiple chemical compounds that are released through mainstream smoke, which is inhaled by the smoker, and through sidestream smoke, which is the smoke that is given off by the burning cigarette.
Second-hand smoke, which is a combination of sidestream smoke and the mainstream smoke that is exhaled by the smoker, has been demonstrated by numerous scientific studies to cause disease.
Only a relatively small proportion of alveoli in the lungs are perfused with blood and actually take part in gas exchange. Too low perfusion and a higher ratio indicates alveolar dead space, while too low ventilation and a lower ratio indicates a shunt, which is a lack of air supply relative to perfusion.
The lungs are located on either side of the heart and are separated by fissures into lobes, three in the right and two lobes in the left. The lungs are located in two chambers of the thoracic cavity on either side of the heart.
Though similar in appearance, the two lungs are not identical, nor wholly symmetrical. Fissures are double folds of pleura that divide the lung into lobes. There are three lobes in the right lung and two in the left lung. The lobes are further divided into segments and then into lobules, which are hexagonal divisions of the lungs that are the smallest visible subdivision. The lobes are further divided into segments and then into lobules, hexagonal divisions of the lungs that are the smallest subdivision visible to the naked eye.
The lobes of the lungs : The right lung has three lobes and the left lung has two. The right lung is five centimeters shorter than the left lung to accommodate the diaphragm, which rises higher on the right side over the liver; it is also broader.
The volume, the total capacity, and the weight of the right lung is greater than that of the left. The right lung is divided into three lobes.
The upper lobe is the largest lobe of the right lung. It extends from the apex of the lung down to the horizontal and oblique fissures. It bears apical, anterior, and posterior bronchopulmonary segments.
The middle lobe is the smallest lobe of the right lung, located between the horizontal and oblique fissures. It bears medial and lateral bronchopulmonary segments. The lower lobe is the bottom lobe of the right lung. It lies beneath the oblique fissure. It bears medial, lateral, superior, anterior, and posterior bronchopulmonary segments.
The Left Lung : This has a concave depression that accommodates the shape of the heart, called the cardiac notch. The human left lung is smaller and narrower that the right lung, and is divided into two lobes, an upper and a lower, by the oblique fissure. The left lung has only two formal lobes because of the space taken up in the left side of the chest cavity by the heart, though it does have the lingula, which is similar to a lobe.
The left lung has a depression on the medial side of its surface called the cardiac notch, a concave impression molded to accommodate the shape of the heart. The upper lobe of the left lung contains anterior and apicoposterior bronchopulmonary segments.
It is above the oblique fissure. The lower lobe of the left lung contains superior, anterior, posterior, medial, and lateral bronchopulmonary segments. The lingula is not formally considered to be a lobe. It is a small, tongue-like projection of the left lung that is analogous to the middle lobe of the right lung.
It contains superior and inferior bronchopulmonary segments. Above and behind the cardiac impression is a triangular depression named the hilum.
The hilum is the root of the lung where that contains structures that supply the lungs with blood, lymph fluid, and innervation, such as the pulmonary vein, pulmonary artery, pulmonary nerves, and lymphatic vessels.
These structures are enclosed by pleura. There is a hilium for each of the lungs found in the mediastinum backside of the lungs. The hilium is thinner in the left lung compared to the right lung because it lies between the cardiac notch and the groove for the aorta. The hilium is important because it is the primary way in which the respiratory system links with the cardiovascular and nervous systems. Pulmonary circulation transports oxygen-depleted blood away from the heart to the lungs and returns oxygenated blood back to the heart.
Bronchial circulation by the bronchial arteries supplies blood to the tissues of the bronchi and the pleura, and is considered part of systemic circulation. The right side of the heart deals with pulmonary circulation. At the end of systemic circulation, the veins take blood back to the heart through the vena cava. The vena cava fills the right atrium with blood, which then ejects blood into the right ventricle by passing through the tricuspid valve.
After blood fills in the right ventricle, it contracts and pumps the blood through the pulmonary valve, and into the pulmonary arteries.
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