Healthy human lungs look pink and spongy from the outside. The left lung is a little smaller than the right lung to make room for the heart. The lungs and heart are both protected by the rib cage. In the lungs, each main bronchus branches off into many smaller bronchi. The bronchi become smaller and smaller, and the smallest bronchi are called bronchioles. There are about 30,000 bronchioles in each lung, each about the same thickness as a strand of hair. The inhaled air eventually ends up in tiny air sacs called alveoli at the end of each bronchiole. There are about 300 million alveoli in each lung. They are covered with capillaries, tiny blood vessels that are only one cell thick. Oxygen travels from the alveoli through the capillaries to the bloodstream, heart and other cells. Which of these best represents the progression of bronchi, bronchioles and alveoli in the lungs?

A
tree trunks that become smaller branches and twigs

B
a tight flower bud that has opened into a full bloom

C
whole apples that are sliced into separate wedges

D
a bridge that crosses two rivers