For respiration to function properly the lungs need to inspire and expire successfully to do this the lungs need to create pressures inside of the organ which allows for the movement of air in and out. Negative pressure is need for inspiration this is achieved by the use of respiratory muscles which include the diaphragm, intercostal muscles and auxiliary respiratory muscles (Tortora & Derrickson 2011)which expand the thoracic cavity to creating  negative pressure. Also the lungs are protected by a double layer serous membrane called the pleural membrane, which consists of superficial layer the parietal pleura and the deep layer the visceral pleura. The space in-between is the pleural cavity which contains lubricating fluid secreted by membranes; this allows the pleura to slide over each other with minimal friction. Plural fluid also causes the two membranes to adhere to one another which create surface tension (Marieb & Hoehn 2014) which keeps the lungs patent and aids the respiratory muscles by keeping the lungs attached to the thoracic cavity. Inside the alveolus surfactant is produced to lower surface tension of fluid within the alveoli (Marieb & Hoehn 2014) this lowered surface tension stops the alveoli from remaining closed upon expiration so maintains patency. positive pressure is requires upon expiration this is achieved by the elastic fibres that interconnect between alveoli and airways these as well as keeping airways and alveoli patent create a recoiling force that make the lungs recoil into a smaller shape that they are at full inhalation, surface tension within  the alvoli also plays a part in creating positive pressure within the lungs (Marieb & Hoehn 2014). Although drastically reduced by the surfactant it still plays a part in recoiling the lungs back into there resting positions.