The respiratory system is composed of the nasal cavity, the pharynx, the larynx, the trachea, the bronchi, and the bronchioles which transmit oxygen from air to the alveoli. This is a process which is called ventilation.
The bronchi divide distally and become progressively smaller to form the bronchioles. Bronchioles do not have cartilage in contrast to the bronchi which do have cartilage. They have smooth muscle walls and terminate in the alveoli.
The epithelium of the bronchi is columnar, while in the bronchioles it is cuboidal. The bronchi contain goblet cells which specialize in the secretion of mucus.
The lung is composed of two main structures: the lobules and the acini. The pleura is a sac that encloses the lung. It is a layer of mesothelial cells which is called the visceral pleura. This layer is continuous with the internal lining of the chest wall. The pleura contains fluid that help lubricating it. This process facilitates the movement of the lungs with respect to the chest wall.
The respiratory system has two sources of blood supply. These are the bronchial arteriolar branches which have mainly a role in supplying the lung with nutrients. The other source of blood supply to the lungs comes from the pulmonary artery. This artery divides into the capillaries network. It functions by doing gas exchange with the alveoli. The function of the lungs is to exchange oxygen with carbon dioxide.
Under normal conditions the arterial partial pressure of oxygen is 95 mmHg while that of carbon dioxide is 40 mmHg.
The lungs have three main functions. These are ventilation which involves the passage of air through the lungs to the acini. The ventilation pace is controlled by the respiratory center in the medulla of the brain stem. The respiratory center in the medulla is under the control of arterial carbon dioxide pressure and the pressure of oxygen. The respiratory center of the medulla controls the respiratory muscles and the diaphragm.
The other function of the lungs is done by perfusion, which is a process in which the blood flows from the pulmonary arterial blood to the alveolar capillaries. These alveolar capillaries then leads to gas exchange.
The third function of the lungs is manifested by a process that is called diffusion. The process of diffusion of gases happens through the alveolar membrane. The alveolar membrane is composed of capillaries, endothelial cells, and alveolar epithelial cells in addition to a basement membrane.
When the diffusion of gases is abnormal, hypoxemia occurs. CO2 is much more diffusible than oxygen. Therefore its diffusion is not affected. When diffusion is abnormal the hypoxemia stimulates the respiratory center causing hyperventilation and hypocapnia.
Pulmonary dysfunction has several manifestations. These include dyspnea which is a sensation of obstruction to the flow of air with active awareness of the breathing process.
Cyanosis is another manifestation of lung dysfunction. It is manifested as bluish discoloration of the skin and mucus membrane that is caused by increased level of reduced hemoglobin in the blood.
The other manifestation of lung dysfunction is chest pain. Most pulmonary diseases do not have pain associated with them. The pain if it occurs is associated with lung pleura. An example of such a pain occurs in bacterial pneumonia.
Cough is a usual symptom of lung dysfunction. It occurs due to stimulation of the cough reflex or due to mucus accumulation in the respiratory tract. This type of coughing is called productive coughing.
Hemoptysis or coughing blood is a symptom of a serious lung disorder. It can occur for example in lung carcinoma.
