Lock and Key Model. Each type of enzyme has a unique 3-D shape, including a surface groove called an active site, which fits its target substrate much like a key fits in a lock. Other substances that donât fit can't enter the active site and no reaction occurs.
Some of the many causes of hypoxia are the following: 1  Chemical or gas poisoning (for example cyanide, carbon monoxide) 2  Low or absent concentration of oxygen (for example, high altitudes reached without supplemental oxygen as seen in mountain climbing, and aviation, drowning or fires)  Lung problems, for example:
Hypoxemia caused by decreased alveolar oxygen tension or underventilation improves with oxygen therapy. Hypoxemia resulting from shunting of blood from the right side of the heart to the left side without exchange of gases in the lungs is treated with bronchial hygiene and positive end-expiratory pressure. Also spelled hypoxaemia. Compare hypoxia.