<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> <P> Cartilage is present until the small bronchi . In the trachea they are C - shaped rings of hyaline cartilage, whereas in the bronchi the cartilage takes the form of interspersed plates . Smooth muscle starts in the trachea, where it joins the C - shaped rings of cartilage . It continues down the bronchi and bronchioles, which it completely encircles . Instead of hard cartilage, the bronchi and bronchioles are composed of elastic tissue . </P>

Where does the upper respiratory tract begin and end