The word hypoxia means "low oxygen". To put in other words, hypoxia is the reduced supply or availability of oxygen to the body tissues. Hypoxia can be generalized, affecting the whole body or it can be localized, confined to one part or region of the body. What makes hypoxia so dangerous is its ability to permanently damage body organs, like the brain. That’s because cells require an uninterrupted supply of oxygen to thrive. Once they lose that, they start to wither.
To understand hypoxia, you must know a little about how oxygen is supplied to different body parts. Our lungs are the main sites for gaseous exchange in the body. Each lung is composed of a huge number of tiny air pockets or sacs called "alveoli", which are covered with extremely small blood vessels called the capillaries. When air is inhaled into the lungs, the oxygen present in it moves through the walls of alveoli into the blood present in the capillaries and from there into the blood circulation. From here onwards, oxygen is transported to the tissues of body through the hemoglobin (a protein) in red blood cells. Once hemoglobin takes oxygen to the target tissues, oxygen detaches itself from hemoglobin and is utilized by body tissues.