One common sign of aging is when a person cannot maintain as high a level of physical activity as in the past.  One of the major causes of this decease in activity is the deceased ability of the respiratory organs to acquire and deliver oxygen to the arterial blood.  This is due to a number of structural changes to the respiratory system.

The cartilage in the walls of the trachea and bronchi undergoes a progressive calcification causing them to become increasingly rigid with aging.  These changes cause a gradual decrease in maximum breathing capacity.  Additionally the walls of the alveoli deteriorate.  This reduces the surface area of the alveoli where gas exchange takes place.  Finally with age the lungs lose some of their elastic recoil and thus offer less resistance to expansion.