The normal intrapleural pressure at the start/beginning of inspiration is ………………………………. cm of H20
The intrapleural pressure becomes more negative at the end of inspiration and less negative at expiration. 
It ranges between -3 and -6 mmHg. At the beginning of inspiration, the pressure is -3.8mmHg (-5 cm of water), and at the end of inspiration (beginning of expiration), it is -6mmHg ( -8 cm of water). 
The elasticity of lung and rib cage causes it to recoil and thus creating the negative pressure. 
The lymphatic drainage of the pleural fluid also creates a negative pressure.
–7.5
–5.0
–2.0
–0.5