Inspiration: air cavities that increase in volume will decrease in air pressure (low pressure). likewise, air cavities that decrease in volume will increase in air pressure (high pressure). gases move or diffuse from regions of high pressure towards regions of low pressure. this movement is how air enters and exits the lungs. due to surface tension between the two layers of pleura, the volume inside the lungs increases when the size of the thoracic cavity increases. besides the diaphragm, what other muscles are involved in expanding the thoracic cavity? what happens to the air pressure inside the lungs when the size of the thoracic cavity increases? explain why outside air enters the lungs when the size of the thoracic cavity increases. what keeps the alveoli from sticking to each other? what is pneumothorax? how does pneumothorax cause atelectasis?