Why don't we run out of the important gases that we need to stay alive?

The reason why we don't run out of the important gases that we need to stay alive is due to existence of the natural recycling processes such as the carbon cycle and oxygen cycle makes use of gases produced by living organisms, which are carbon dioxide and oxygen to reproduce the gases used

The important gases we need to stay alive are;

The aerobic respiration and photosynthesis processes along with other natural occurring processes through which oxygen and carbon dioxide are produced  creates a cycle that sustains the gases that we need to stay alive so that they do not run out

The cycles are known as the carbon and oxygen cycles

Learn more about the carbon and oxygen cycles here:

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mass and weight are closely related because mass affects the weight of an object experiencing the effects of gravity. weight is the measure of the force of gravity on an objects's mass, while mass is the measure of how much matter there is in an object.