Gas Exchange in Mammals
The mammalian respiratory system consists of the nasal cavity, pharynx, glottis, larynx, trachea, bronchi, bronchioles and air sacs (alveoli).
The respiratory passages are lined by the presence of cilia and mucus-secreting cells, which keep them moist and trap dust, dirt and bacteria. The lungs are encased a rib cage. The rib cage is composed of the breastbone or sternum and ribs. At the bottom of the rib cage is a muscular diaphragm. Breathing in mammals. During inhalation, the rib cage is pulled outwards and upwards by the contraction of the intercostal muscles, while the diaphragm is lowered or flattened. The volume of the thoracic cavity is increased, the air enters the lungs. During exhalation, the intercostal relax, the rib cage is lowered and pulled inwards while the diaphragm contracts and is raised. This reduces the volume of the thoracic cavity, and the air is forced out of the lungs. During inhalation, the intercostal muscles of the ribs contract, pulling the rib cage upwards and outwards. This stretches the diaphragm outwards, creating a large empty space in the thoracic cavity. The air is then drawn into the lungs. During inhalation the intercostal muscles relax, the rib cage falls back to its original position and the diaphragm arches upwards. The result is that the volume of the thoracic cavity is reduced and the air is expelled from the lungs. The amount of air inhaled at each breath is called the tidal volume and is about 500 ml for an adult human being. Only about 350 ml of the tidal volume reaches the lung. The remaining 150-ml of air, known as the dead space volume remains within the respiratory passages. Under a strong physical exercise, the lungs can hold up to between 4500 ml and 5000 ml of air. This is called the vital capacity of the lungs. It is impossible to empty the lungs completely. Some air always remains in the lungs, which is called the residual volume. Control of Breathing
Breathing is under the...
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