The respiratory organs of the human body are the lungs. The human respiratory system can be divided into respiratory tract and lungs. The main function of the respiratory tract is as a gas in and out of the channel, and the lungs for the exchange of internal and external gases, including the nose, pharynx, larynx, trachea, bronchi, etc.; the lungs are to carry out the exchange of gases in the body, including alveoli, bronchial tree and so on. Lungs are located in the thoracic cavity, on both sides of the mediastinum, divided into the right and left lungs, due to a part of the heart occupies, the two lungs are not identical in shape, the left lung is narrow and long, the right lung is wide and short, the left lung is divided into upper and lower lobes through the diagonal fissure, and the right lung is divided into upper, middle and lower lobes through the horizontal fissure and the diagonal fissure. Generally the normal lungs are light red in color, soft, elastic and spongy in the living state. If you smoke for a long time, it can lead to darkening of the lungs and so on. There is also a difference between fetal lungs and adult lungs. Fetal lungs do not contain air and have a larger specific gravity and can be sunk under water; lungs that have breathed contain air and have a smaller specific gravity and can float on water.