The two pleura have a small fluid filled space between them called the pleural cavity. The pleura help the lungs expand and contract without friction. The walls of the thoracic cavity are formed by the ribs and two sets of muscles between the ribs, the internal and external intercostal muscles. The floor of the thoracic cavity is formed by the diaphragm . The diaphragm and the intercostal muscles are skeletal muscles.
Blood flows to the lungs via branches of the pulmonary artery .
A network of capillaries encloses each alveolus. It is between the capillaries and the alveoli that gas exchange occurs. Blood returns to the heart in venules and veins which will give rise to the pulmonary veins. B. Respiratory cycle To inhale normally, the external intercostals contract , lifting the rib cage up and out. The diaphragm contracts , losing its domed shape and flattening out.