Lung

The lung is the essential organ of respiration in air-breathing vertebrates.

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 evolutionary 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.

Related Topics:
Energy - Organism - Life - Evolution - Adaptation

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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 musculo-skeletal system is used. In the mammal, a large muscle, the diaphragm, drives 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.

Related Topics:
Alveoli - Capillaries - Diaphragm

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