When the air pressure within the alveolar spaces falls below atmospheric pressure, air enters the lungs (inspiration), provided the larynx is open; when the air pressure within the alveoli exceeds atmospheric pressure, air is blown from the lungs (expiration).
 
        
             
        
        
        
Answer:
Atom, smallest unit into which matter can be divided without the release of electrically charged particles. It also is the smallest unit of matter that has the characteristic properties of a chemical element. As such, the atom is the basic building block of chemistry.
Atom: has equal number of +ve charge (protons) and -ve charge electrons and hence atom has charge.
Neutrons (having no charge) and protons are present in nucleus. Electrons keep revolving around the nucleus in different shells.