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

Because as times goes on the atmosphere and plants produce more of the gases we need to stay alive. It is because that new gas is produced as a part of cycle, because plants recycle our waste gasses and replace them with the oxygen we need.

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;

Oxygen: Needed for aerobic respiration in the production of energy for life in animals and other living organisms, in the process, carbon dioxide is producedCarbon dioxide: Needed by plant to manufacture plant food which contains energy and oxygen is also produced as a by product, in the process known as photosynthesis. Sunlight energy is used to enable the process

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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Biochemistry, it deals with the chemical constitution, the breakdown and the synthesis of plant and animal products.

Abay where the river empties into the sea