The most common examples of respiration occur in two places: in human lungs as part of breathing and in cells through a process called "cellular respiration." Humans breathe to oxygenate their blood, and cells breathe to acquire oxygen to be able to break down glucose for the energy that they need.
Air moves into the lungs when certain muscles in the body contract. Then oxygen transfers into the blood through the alveoli, and carbon dioxide is transferred from the blood into the lungs.
Each cell in the human body needs energy to perform its necessary tasks, and each cell uses oxygen in breaking down sugar to obtain it. Carbon dioxide and water are produced as a by-product of this process called "aerobic respiration."
The mitochondria are often referred to as the power plant of the cell, and it is within this structure of the cell where respiration occurs to obtain the power needed to complete other processes.
Respiration begins with hydrogen being released from the sugar, releasing energy and giving off carbon dioxide and water as by-products.
Respiration usually occurs in the presence of oxygen, but there are cases where respiration takes place without oxygen (anaerobic respiration). Anaerobic respiration produces less energy than aerobic respiration but is used by some bacteria.