<P> The lungs are the largest organs in the lower respiratory tract . The lungs are suspended within the pleural cavity of the thorax . The pleurae are two thin membranes, one cell layer thick, which surround the lungs . The inner (visceral pleura) covers the lungs and the outer (parietal pleura) lines the inner surface of the chest wall . This membrane secretes a small amount of fluid, allowing the lungs to move freely within the pleural cavity while expanding and contracting during breathing . The lungs are divided into different lobes . The right lung is larger in size than the left, because of the heart's being situated to the left of the midline . The right lung has three lobes--upper, middle, and lower (or superior, middle and inferior), and the left lung has two--upper and lower (or superior and inferior), plus a small tongue - shaped portion of the upper lobe known as the lingula . Each lobe is further divided up into segments called bronchopulmonary segments . Each lung has a costal surface, which is adjacent to the ribcage; a diaphragmatic surface, which faces downward toward the diaphragm; and a mediastinal surface, which faces toward the center of the chest, and lies against the heart, great vessels, and the carina where the two mainstem bronchi branch off from the base of the trachea . </P> <P> The alveoli are tiny air sacs in the lungs where gas exchange takes place . The mean number of alveoli in a human lung is 480 million . When the diaphragm contracts, a negative pressure is generated in the thorax and air rushes in to fill the cavity . When that happens, these sacs fill with air, making the lung expand . The alveoli are rich with capillaries, called alveolor capillaries . Here the red blood cells absorb oxygen from the air and then carry it back in the form of oxyhaemaglobin, to nourish the cells . The red blood cells also carry carbon dioxide (CO) away from the cells in the form of carboxyhemoglobin and releases it into the alveoli through the alveolor capillaries . When the diaphragm relaxes, a positive pressure is generated in the thorax and air rushes out of the alveoli expelling the carbon dioxide . </P> <P> The respiratory tract is covered in epithelium, which varies down the tract . There are glands and mucus produced by goblet cells in parts, as well as smooth muscle, elastin or cartilage . Most of the epithelium (from the nose to the bronchi) is covered in ciliated pseudostratified columnar epithelium, commonly called respiratory epithelium . The cilia beat in one direction, moving mucus towards the throat where it is swallowed . Moving down the bronchioles, the cells get more cuboidal in shape but are still ciliated . </P> <P> Glands are abundant in the upper respiratory tract, but there are fewer lower down and they are absent starting at the bronchioles . The same goes for goblet cells, although there are scattered ones in the first bronchioles . </P>

Where are the cilia found in the respiratory tract