Answer:
1. too little oxygen in the inhaled air
2. inadequate ventilation (=the chest muscles and diaphragm moving air in and out of the lungs)
3. ventilation/perfusion (=blood flow) mismatch
4. Shunting (venous, unoxygenated blood flows through completely unventilated areas of the lung and mixes with oxygenated blood from functional areas of the lung going into the body - a severe form of no. 3, which doesn't improve much by giving oxygen)
5. inadequent diffusion of oxygen and carbon dioxide across the membrane between the air-filled alveoli and the blood (as in severe pneumonia or pulmonary edema)
(6. "venous admixure effect" does not cause, but does worsen hypoxia - when the blood flowing into the lungs contains too much carbon dioxide (CO2) for the lungs to clear, as in acute heart failure or septic shock)