Blood/Gas test
This test measures the ratio between a liquid and a gas. Since blood is a liquid and oxygen is a gas, the oxygen and carbon dioxide levels in the blood is an indication of how efficient the lungs work. Any imbalance between the three (oxygen, carbon dioxide, blood) is a sign of a problem.
This test is also used to determine the PH of the blood and how acidic it is. PH (power of hydrogen) is a measure of acidity (bitterness). The scale runs from 0 to 14 with 7 being neutral. Blood is slightly alkaline between 7 and 8. The urine litmus test should be near neutral to indicate the correct PH of the kidneys.