text: describe the rspiration of human structural characteristics?   the respiratory system consists of organs that deliver oxygen to the circulatory system for transport to all body cells. the goals of the respiratory system are to provide oxygen to the tissues and to remove carbon dioxide.  respiration includes 2 processes: external respiration and internal respiration. the former involves the absorption of o2 and removal of co2 from the body as a whole preventing the lethal buildup of this waste product in body tissues. this process involves two major functional events: (1) pulmonary ventilation, which means the inflow and outflow of air between the atmosphere and the lung alveoli; (2) diffusion of oxygen and carbon dioxide between the alveoli and the blood (guyton, 1991). the internal respiration entails the utilization of o2, which is essential for cells to liberate the energy needed for cellular activities and production of co2 and the gaseous exchanges between the cells and their fluid medium.  it is divided into two major functional events: (3) transport of oxygen and carbon dioxide in the blood and body fluids to and from the cells; and (4) regulation of ventilation and other facets of respiration (guyton, 1991).  the respiratory and circulatory systems work together to deliver oxygen to cells and remove carbon dioxide in following the two-phase processes as mentioned in the former paragraph. the first phase of respiration begins with breathing in, or inspiration. inspiration brings air from outside the body into the lungs. oxygen in the air moves from the lungs through blood vessels to the heart, which pumps the oxygen-rich blood to all parts of the body (microsoft, 2003). oxygen then moves from the bloodstream into cells, which completes the first phase of respiration. in the cells, oxygen is used in a separate energy-producing process called cellular respiration, which produces carbon dioxide as a byproduct (microsoft, 2003). the second phase of respiration begins with the movement of carbon dioxide from the cells to the bloodstream. the bloodstream carries carbon dioxide to the heart, which pumps the carbon dioxide-laden blood to the lungs (microsoft, 2003). in the lungs, breathing out, or expiration, removes carbon dioxide from the body, thus completing the respiration cycle.   the respiratory system is made up of a gas-exchanging organ (the lungs) and a pump that ventilates the lungs. the pump consists of: (1) the chest wall; (2) the respiratory muscles, which increase and decrease the size of the thoracic cavity; (3) the areas in the brain that control the muscles; and (4) the tracts and nerves that connect the brain to the muscles (ganong, 1995). at rest, a normal person breaths 12-15 times a minute. five hundred milliliters of air per breath, or 6-8 l/min, is inspired and expired (ganong, 1995). this air mixes with the gas in the alveoli, and by simple diffusion, o2 enters the blood in the pulmonary capillaries while co2 enters the alveoli.  in this manner, 250 ml of o2 enters the body per minute and 200 ml of co2 is excreted (ganong, 1995). during quiet breathing, the decrease in pressure in the chest starts about 2.5 mm hg (compared to the outside pressure), decreasing to around 6 mm hg towards the end of inspiration. strong breathing efforts can produce a pressure decrease (vacuum) in the chest as high as 30 mm hg. day-in and day-out, without the prompt of conscious thought, the respiratory system carries out its life-sustaining activities. if the respiratory system’s tasks are interrupted for more than a few minutes, serious, irreversible damage to tissues occurs, followed by the failure of all body systems, and ultimately, death.  the respiratory system also plays other important roles in the body aside from the intake of oxygen and removal of carbon dioxide. it helps regulate the balance of ph (acidity and basicity) in tissues, a process crucial for the normal functioning of cells. it protects the body against disease-causing organisms and toxic substances inha