What happens if intrapleural pressure equal atmospheric pressure?

Answer 1

The intrapleual pressure is always below atmospheric pressure. Because of the connection between the two plurae which is similar to two wet pieces of paper adhered to each other, the negative intrapleural pressure helps to expand the lungs during ventilation. If intrapleural pressure was equal to atmospheric pressure, the lungs would collapse. Such a case is seen in a penetration of the thoracic cavity (pneumothorax), where a puncture in the thoracic cavity, and subsequently the plurae, will result in a collapsed lung.