In the normal lung, ventilation and perfusion are not evenly matched (known as V/Q mismatch) and this worsens with lung disease and dorsal or lateral recumbencies. It does no good to ventilate an alveolus that is not being perfused (alveolar dead space) or to perfuse an alveolus that is not being ventilated because of atelectasis. The "matching" of ventilation and perfusion is important to proper lung function. The amount of oxygen bound to hemoglobin is much larger than the amount dissolved in plasma. In the blood, oxygen is transported in two forms: dissolved in plasma (which is measured by PO2) and bound to hemoglobin (measured by SpO2). Gas exchange occurs through this alveolar-capillary membrane as oxygen moves into and carbon dioxide moves out of the bloodstream (perfusion). These blood vessels also branch into smaller and smaller units ending with capillaries, which are in direct contact with each alveolus. Blood vessels from the pulmonary arterial system accompany the bronchi and bronchioles. The oxygen transferred through the system is finally transferred to the bloodstream at the alveoli. At the end of each alveolar duct are clusters of alveoli. After many subdivisions, the bronchioles end at the alveolar ducts. The bronchi subdivide several times into smaller bronchi, which then divide into smaller and smaller branches called bronchioles. One bronchus serves the right lung and the other serves the left lung. The trachea divides into two main bronchi upon reaching the lungs. From there it passes through the pharynx on its way to the trachea. The smooth functioning of the entire system is directed by the brain and the autonomic nervous system.Īir containing oxygen enters the body through the nose and mouth. The heart creates the force to move the blood at the right speed and pressure throughout the body. The bloodstream takes oxygen from the lungs to the rest of the body and returns carbon dioxide to them to be removed. The bloodstream, heart and brain are also involved. It includes the diaphragm and chest muscles, the nose and mouth, the pharynx and trachea, the bronchial tree and the lungs. The respiratory system is the body's link to its supply of oxygen. Every cell in the body needs a constant supply of oxygen to produce energy to grow, repair or replace itself and to maintain normal vital functions.
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