Check us out at http://www.tutorvista.com//videos •The human lungs are the organs of respiration in humans. Humans have two lungs, with the left being divided into two lobes and the right into three lobes. Together, the lungs contain approximately the same length as 1500 miles (2,400 km) of airways and 300 to 500 million alveoli, having a total surface area of about 70 m2 in adults — roughly the same area as one side of a tennis court. Furthermore, if all of the capillaries that surround the alveoli were unwound and laid end to end, they would extend for about 620 miles. The conducting zone contains the trachea, the bronchi, the bronchioles, and the terminal bronchioles •The respiratory zone contains the respiratory bronchioles, the alveolar ducts, and the alveoli. The conducting zone and the respiratory stuffers (but not the alveoli) are made up of airways. The conducting zone has no gas exchange with the blood, and is reinforced with cartilage in order to hold open the airways. The conducting zone warms the air to 37 degrees Celsius and humidifies the air. It also cleanses the air by removing particles via cilia located on the walls of all the passageways. The lungs are surrounded by the rib cage. The respiratory zone is the site of gas exchange with blood. •the sympathetic nervous system via noradrenaline acting on the beta receptors and causes bronchodilation •the parasympathetic nervous system via acetylcholine which acts on the M-1 muscarinic receptors and maintains the resting tone of the bronchiolar smooth muscle. This action is related, although considered distinct from bronchoconstriction •many other non-autonomic nervous and biochemical stimuli including, for example, carbon dioxide. The pleural cavity is the potential space between the parietal pleura lining the inner wall of the thoracic cage and the visceral pleura lining the lungs. The lung parenchyma is strictly used to refer solely to alveolar tissue with respiratory bronchioles, alveolar ducts and terminal bronchioles. However, it often includes any form of lung tissue, also including bronchioles, bronchi, blood vessels and lung interstitium
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