Chapter I - Respiratory System

The lung is the essential organ of respiration in air-breathing vertebrates. Its principal function is to transport oxygen from the atmosphere into the bloodstream, and excrete carbon dioxide from the bloodstream into the atmosphere. This it accomplishes with its mosaic of specialized cells that form millions of tiny, exceptionally thin-walled air sacs where gas exchange takes place. Lungs also have nonrespiratory functions.

Medical terms related to the lung often begin with pulmo-, from the Latin pulmonarius ("of the lungs"),

The respiratory function of the lung
Energy production in living organisms often uses oxygen and produces carbon dioxide. Hence, life necessitates an efficient means of oxygen delivery to cells and carbon dioxide excretion from cells. In smaller organisms, such as single-celled bacteria, this process of gas exchange can take place entirely by simple diffusion. In larger organisms this is not possible; only a small proportion of cells are situated close enough to the surface for oxygen from the atmosphere to enter them through diffusion. Two major adaptations made it possible for organisms to attain great multicellularity: an efficient circulatory system that conveyed gases to and from the deepest tissues in the body, and a large respiratory system that centralized the task of obtaining oxygen from the atmosphere and bringing it into the body, whence it could rapidly be distributed to all tissues via the circulatory system. In air-breathing vertebrates, respiration occurs in a series of steps. Air is brought into the animal via the airways - in reptiles, birds and mammals this often consists of the nose, the pharynx, the larynx, the trachea, the bronchi and bronchioles, and the terminal branches of the respiratory tree.

The lungs of these animals are a rich lattice of alveoli, which provide an enormous surface area for gas exchange. A network of fine capillaries transports blood over the surface of alveoli. Oxygen from the air inside the alveoli diffuses into the bloodstream across the exceptionally thin alveolar membranes, and carbon dioxide moves from the blood to the alveoli via the same process. The drawing and expulsion of air is driven by muscular action; in early tetrapods, air was driven into the lungs by the pharyngeal muscles, whereas in reptiles, birds and mammals a more complicated musculoskeletal system is used. In the mammal, a large muscle, the diaphragm (in addition to the internal intercostal muscles), drive ventilation by periodically altering the intra-thoracic volume and pressure; by increasing volume and decreasing pressure, air is sucked into the airways, and by reducing volume and increasing pressure, the reverse occurs. During normal breathing, expiration is passive and no muscles are contracted (the diaphragm relaxes).

Nonrespiratory functions of the lung
In addition to respiratory functions such as gas exchange and regulation of hydrogen ion concentration, the lungs also:

• Influence the concentration of biologically active substances and drugs used in medicine in arterial blood
• Filter out small blood clots formed in the systemic veins
• Serve as a physical layer of soft, shock-absorbent protection for the heart, which the lungs flank and nearly enclose.

The lungs of mammals have a spongy texture and are honeycombed with epithelium having a much larger surface area in total than the outer surface area of the lung itself. The lungs of humans are typical of this type of lung. The environment of the lung is very moist, which makes it a hospitable environment for bacteria. Many respiratory illnesses are the result of bacterial or viral infection of the lungs.

Breathing is largely driven by the diaphragm below, a muscle that by contracting expands the cavity in which the lung is enclosed. The rib cage itself is also able to expand and contract to some degree.

As a result, air is sucked into and pushed out of the lungs through the trachea and the bronchial tubes or bronchi; these branch out and end in alveoli which are tiny sacs surrounded by capillaries filled with blood. Here oxygen from the air diffuses into the blood, where it is carried by hemoglobin.

The deoxygenated blood from the heart reaches the lungs via the pulmonary artery and, after having been oxygenated, returns via the pulmonary veins.

ANATOMY

The lungs are located inside the thoracic cavity, protected by the bony structure of the rib cage and enclosed by a double-walled sac called pleura. The inner layer of the sac (visceral pleura) adheres tightly to the lungs and the outer layer (parietal pleura) is attached to the wall of the chest cavity. The two layers are separated by a thin space called the pleural cavity that is filled with pleural fluid; this allows the inner and outer layers to slide over each other, and prevents them from being separated easily. The left lung is smaller than the right one to give way for the heart.

The lungs attach to the heart and trachea through structures that are called the "roots of the lungs." The roots of the lungs are the bronchi, pulmonary vessels, bronchial vessels, lymphatic vessels, and nerves. These structures enter and leave at the hilus of the lung.

The lungs are divided into lobes by the horizontal and oblique fissures. The right lung has three lobes and the left lung has two. A unique feature of the left lung is the cardiac notch, which helps create the lingula (Latin for "tongue") of the left lung.

The lungs are connected to the upper airway by the trachea and bronchi. The trachea runs down the neck and divides into left and right bronchi behind the sternal angle. The right main bronchus is shorter and runs more vertically than the left. For this reason, it is more common to aspirate foreign objects into the right lung. The bronchi enter the lung and branch out to form the bronchial tree. The bronchi divide into smaller bronchioles, which terminate into alveoli. An alveolus is composed of respiratory tissue and is the site of gas exchange in the lung.

The blood supply to the lungs is from two sources: the pulmonary vessels and the bronchial vessels. The bronchial vessels support the nonrespiratory tissue and the pulmonary vessels provide support to the respiratory tissue.

The pulmonary arteries carry deoxygenated blood that has returned to the heart from the venous system to the lungs to be reoxygenated. The pulmonary veins carry oxygenated blood back to the heart to go to the arterial system. The right and left pulmonary arteries arise from the pulmonary trunk and carry "venous" blood to their respective lungs. The pulmonary veins, two on each side, carry "arterial" blood to the left atrium of the heart.

The bronchial arteries that supply the nonrespiratory tissue of the lung arise from different sources. The left bronchial arteries come off of the thoracic aorta, however, the right bronchial artery has a variable source.