A test for arterial blood gases determines the amount of carbon dioxide, oxygen and carbon monoxide bound to hemoglobin in a volume of blood from an artery. It can also test for the acidity of blood. It is an important test of heart and lung function. The term "blood gases" is a bit of misnomer. There are always dissolved gases in the liquid part of blood that are generally inert, such as nitrogen. What a physician needs to know is whether or not oxygen and carbon dioxide are properly bound to blood in the bloodstream.
Nowadays, oxygen levels in arterial blood in the capillaries can be indirectly monitored on an ongoing basis without having to draw blood through the use of a pulse oximeter. Many new smart phones improvise a pulse oximeter from it's camera sensor. Ideally, oxygenation of hemoglobin in arteries should be well over 95%, and is often close to 100% in a normal patient. However, pollution and smoking can reduce this percentage nominally. Similarly, blood in veins should be almost totally saturated with carbon dioxide. The exceptions, of course, are the pulmonary vein and pulmonary artery, where the percentages should be precisely the reverse.
However, in patients with severe lung damage, blood in the arteries is less oxygenated. This forces the heart to pump faster, even at rest, to supply the body's cells with oxygen. As such, lower oxygen levels in the arteries in a seemingly normal patient is often an indication of underlying lung problems.
Similarly, if oxygen is not being properly processed by cells due to a metabolic problem, the oxygen level in veins will be too high. The patient will feel fatigued even when given oxygen, and giving the patient more oxygen will have no effect.
One blood gas that is not monitored is carbon monoxide, which should not be present in more than negligible amounts in a patient unless they are suffering from carbon monoxide poisoning. However, it can be tested for by drawing blood from the patient.