Which one of the following statement(s) is/are correct?
1. Inspiration occurs only when there is negative pressure in the lungs with respect to the atmosphere.
2. Expiration takes place only when intrapulmonary pressure is lower than atmospheric pressure.
3. Pressure gradients are generated with the help of the diaphragm and intercostal muscles.
4. Inspiration is initiated by the relaxation of the diaphragm.
5. Contraction of the diaphragm reduces pulmonary volume and increases intrapulmonary pressure causing expiration.