Trachea are generally highly elastic but they may calcify with age. The trachealis muscle connects the ends of the incomplete rings and contracts during coughing, reducing the size of the lumen of the trachea to increase the rate of air flow. The esophagus lies posteriorly to the trachea, adjoining along the tracheoesophageal stripe. Circular horizontal bands of fibrous tissue called the annular ligaments of trachea join the tracheal rings together. The cartilaginous rings are incomplete to allow the trachea to collapse slightly so that food can pass down the esophagus. A flap-like epiglottis closes the opening to the larynx during swallowing
Human body and about 23 litres of fluid inside cells. The content, acidity and composition of the water inside and outside cells is carefully maintained. The main electrolytes in body water outside cells are sodium and chloride, whereas within cells it is potassium and other phosphates. The body contains trillions of cells, the fundamental unit of life. At maturity, there are roughly 30–37 trillion cells in the body, an estimate arrived at by totalling the cell numbers of all the organs of the body and cell types. The body is also host to about the same number of non-human cells as well