The respiratory system shares the same responsibility as the cardiovascular system in that they both supply oxygen to, and expel carbon dioxide from, the body. Oxygen is vital because it provides the fuel for all of the body's functions. The respiratory system also eliminates toxic waste, regulates temperature and ensures the stability of the blood's acid-alkaline balance. Divided into the upper and lower respiratory systems, the upper consists of the nose and throat; the lower, refers to the larynx, trachea, bronchi and lungs. The inner surface area of the Lungs is approximately 35 times larger than the surface area of your skin. This huge surface allows the body to effectively absorb oxygen and expel carbon dioxide. Breathing begins in the nose, which alerts you to potentially toxic smells. The nose also warms and moistens the air which enters the body, and filters out airborne germs which can get trapped in the mucous membranes. The air passes through the throat and enters the trachea through the larynx. The trachea and bronchial tubes channel air into the lungs, and protect them from particles and germs which become ensnared in their mucous membranes. Before inhalation begins, the air pressure inside the lungs is equal to the atmospheric pressure outside the body. When the diaphragm contracts to increase the volume of the lungs, the pressure inside the lungs drops and air rushes in until the pressure is equal again. Exhalation begins when the inspiratory muscles relax, the diaphragm pushes up towards the lungs and the surface tension of the lungs pulls inward, decreasing lung volume and pushing air out. Blood and air exchange oxygen and carbon dioxide through the membrane of the bronchial tubes, and the Bronchioles, alveolar ducts and alveoli.The blood, now enriched with oxygen, returns to the heart to be pumped through the rest of the body. And lack of iron or lack of blood in the body causes insufficient oxygen to the transported to the cells, resulting in symptoms of breathlessness, weakness, coughing and fatigue.