1 The primary job of the respiratory system is respiration or breathing In humans and other animals breathing is how oxygen moves into the lungs The human body can store many nutrients but it can t store oxygen Oxygen is essential for all cells to function properly That s why humans have to breathe continually to stay alive Respiration also moves carbon dioxide a waste product out of the body Which of these best describes the relationship between respiration and oxygen A Oxygen is the process by which respiration happens C Respiration is how the body takes in oxygen B Respiration and oxygen are both types of gases D Oxygen comes before respiration 2 Respiration begins when air enters the mouth and nose As air goes down the trachea or windpipe tiny hairs called cilia gently wave to keep dirt and mucus out At the bottom of the trachea are two tubes called bronchi One bronchus goes into the left lung and the other goes into the right lung When you inhale the diaphragm muscle between the lungs and abdomen contracts making room for the lungs to expand Which of these best describes how the lungs and diaphragm affect each other A As the lungs expand the diaphragm expands C As the lungs contract the diaphragm contracts B As the lungs expand the diaphragm contracts D The lungs and diaphragm have no relationship