The Mechanism of Breathing Out Diaphragm The diaphragm is a thin, dome-shaped sheet of muscle, approximately level with the bottom of the ribs, which separates the thoracic cavity from abdominal cavity. The sheet of muscle is curved upward in the middle, like an upside-down saucer. As you breathe in, the sheet is pulled downward, which makes the cavity larger. This decreases air pressure in the lungs, so air rushes into the lungs. The intercostals muscles also contract causing the rib cage to move upward and outward, which causes air to enter the lungs. As you breath out (expire) the ribs and diaphragm return to their relaxed positions, causing a decrease in thoracic volume. Lung Capacity Tidal Volume = the amount of air moved by a normal individual at rest. Expiration reserve volume = after a noraml breath, force out as much extra air as you can. Inspiratory reserve volume = the amount of extra air that you can force in at the end of a normal inhalation. The total of these = TOTAL LUNG CAPACITY The Respiratory Capacities of an Average Adult Expiratory Reserve 1500 cm3 Volume Tidal Volume 500 cm3 Inspiratory reserve 2000 cm3 volume Vital Capacity 4000 cm3 Exchanges of Gases As air enters the alveoli in the lungs, oxygen gets dissolved in water and crosses the membranes to enter the blood stream. This occurs by the process of diffusion. As oxygen enters the capillaries, hemoglobin molecules in the red blood cells pick up oxygen. Each hemoglobin molecule has four sites of attachment. Carbon dioxide is released in the opposite direction and passes into the alveoli for expiration Blood now rich in oxygen is then transported via the pulmonary vein back to the left side of the heart where it will be pumped out to every cell in the body for cellular respiration. While hemoglobin transports oxygen, there is no carrier for carbon dioxide and travels the blood as carbonic acid.