Ans 1) The opening between the vocal cords at the upper part of the voice box is called glottis . At the top of the trachea there is a small flap known as the epiglottis. When you swallow food. it closes off the trachea to prevent food going into the lungs. 
Ans 2) The lung alveoli are covered with blood capillaries, so that maximum exchange of gases takes place.Ans 3) The walls of the trachea have cartilage rings so that the trachea does not collapse when there is less air in it.