Which of the following describes tight junctions? a. junctions among epithelial cells lining the digestive tract b. junctions within electrically excitable tissues, such as the heart and smooth muscle cell c. junctions in areas that are subjected to great mechanical stress, such as the skin and heart muscles d. junctions where cells are held together by cadherins

a) Junctions among epithelial cells lining the digestive tract

Explanation: Tight junctions provide an intercellular barrier between epithelial cells within the gastrointestinal tract and liver and, by reducing the movement of water and solutes through the intercellular space, thereby maintaining separation of tissue compartments.

air enters through the nose, moves through the nasal cavity, the pharynx, the larynx, enters the trachea, moves through the bronchi andbronchioles till the alveoli.