The respiratory system is essential for life. Normal lung function requires a balanced interrelationship between the respiratory, nervous, and cardiovascular systems. The brain receives and sends out stimuli to maintain a normal oxygen and carbon dioxide balance. The medulla and the pons are the major brain centers that affect respiration. The body's respiratory center in the medulla is normally stimulated by an increased concentration of carbon dioxide, and to a lesser extent, by decreased levels of oxygen in arterial blood. Stimulation of the respiratory center causes an increase in the rate and depth of breathing, thus blowing off excess carbon dioxide and reducing blood acidity. The heart pumps oxygenated blood from the left side of the heart through the arterial circulation to all parts of the body. Oxygen is transported primarily in red blood cells. As oxygen is used by cells, unoxygenated blood containing carbon dioxide returns to the heart and lungs through the venous circulation.
The thoracic cavity is made up of 12 pairs of ribs that connect in the posterior thorax to the vertebral bodies of the spinal column. The lungs lie within the thoracic cavity on either side of the heart, extending from the diaphragm to just above the clavicles or collarbones. Light, spongy and elastic structures, the lungs inflate with inspiration and deflate, but do not completely collapse, with expiration. The right lung is shorter and wider than the left lung. Each lung is divided into lobes the right lung has three lobes; the left lung has two lobes. The lung's lobes are further divided into segments. The pleurae are membranes that cover each lung and line the thoracic cavity. The visceral pleura are serous membranes that cover the outer surface of each lung. The parietal pleura line the inner rib cage and upper surface of the diaphragm. The smooth opposing surfaces of the pleura, lubricated by pleural fluid, allow the lungs to move easily within the rib cage during inspiration and expiration. The pleural space is the potential space between the visceral and parietal pleurae.
The illustration below is a posterior view, which identifies structures of the thoracic cavity.