The air we breathe in goes into our trachea and is transferred to small air passages called bronchioles. The air then reaches the air sacs in the lungs called alveoli, which allow oxygen to enter the bloodstream and get distributed to every cell in the body. The alveoli also absorb the carbon dioxide emitted by the cells and then expel it using the same route.
Although carbon dioxide is thought to be a harmful gas because it is released from the body, the respiratory system actually needs a certain level of carbon dioxide to function properly. Faster, deeper breathing may lead to build-up of too much oxygen and lack of carbon dioxide in our body, which can cause respiratory alkalosis. This occurs when the normal pH level of the blood is elevated due to oxygen-carbon dioxide imbalance.
When the cells do not have adequate carbon dioxide for gas exchange, oxygen absorption decreases and some undesirable conditions such as weakness, dizziness, nausea, and convulsions may occur. When you hold your breath, you give your cells more time to absorb oxygen and release carbon dioxide. Therefore, holding your breath can increase the amount of carbon dioxide in your cells and can be a lifesaver in case of hyperventilation, for instance during a panic attack.
In some breathing exercises, oxygen-carbon dioxide balance is intentionally disrupted and then restored via help of breath retention. Especially in breathing exercises which incorporate hyperventilation, breath holds help more oxygen to get into cells.
Holding your breath also has a calming effect on the nervous system. Additionally, breath retention helps focus intensely on one thing and one thing alone: The next breath. This trains the mind to grow more mindful and let go of other worries.
However, you should always listen to your body and make sure that you’re not holding your breath past the point where you feel comfortable.