The human respiratory system, working in conjunction with the cardiovascularsystem, supplies oxygen to, and removes carbon dioxide from, the cells of thebody. The respiratory system conducts air to the respiratory surfaces of thelungs. There, the blood in the lung capillaries readily absorbs oxygen and gives off carbon dioxide gathered from the body cells. The circulatory systemtransports oxygen-laden blood to the body cells and picks up carbon dioxide.The term respiration describes the exchange of gases across cell membranes both in the lungs (external respiration) and in the body tissues (internal respiration). Pulmonary ventilation, or breathing, exchanges volumes of air withthe external environment.
The human respiratory system consists of the respiratory tract and the lungs.The respiratory tract can be divided into an upper and a lower part. The upper part consists of the nose, nasal cavity, pharynx (throat), and larynx (voicebox). The lower part consists of the trachea (windpipe), bronchi, and bronchial tree. The respiratory tract cleans, warms, and moistens air during its trip to the lungs. The nose has openings to the outside that allow air to enter. Hairs inside the nose trap dirt and keep it out of the respiratory tract.The nose leads to a large cavity within the skull. This cavity and the spaceinside the nose make up the nasal cavity. A nasal septum, supported by cartilage and bone, divides the nasal cavity into a right and left side. Epithelium, a layer of cells that secrete mucus and cells equipped with cilia, lines the nasal passage. Mucus moistens the incoming air and traps dust. The cilia move pieces of the mucus with its trapped particles to the throat, where it isspit out or swallowed. Stomach acids destroy bacteria in swallowed mucus. Sinuses, epithelium-lined cavities in bone, surround the nasal cavity. Blood vessels in the nose and nasal cavity release heat and warm the entering air.
Air leaves the nasal cavity and enters the throat or pharynx. From there it passes into the larynx, which is located between the pharynx and the trachea or windpipe. A framework of cartilage pieces supports the larynx, which is covered by the epiglottis, a flap of elastic cartilage that moves up and down like a trap door. When we breathe, the epiglottis stays open, but when we swallow, it closes. This valve mechanism keeps solid particles and liquids out ofthe trachea. If we breathe in something other than air, we automatically cough and expel it. Should these protective mechanisms fail, allowing solid foodto lodge in and block the trachea, the victim is in imminent danger of asphyxiation.
Air enters the trachea in the neck. Epithelium lines the trachea as well as all the other parts of the respiratory tract. C-shaped cartilage rings reinforce the wall of the trachea and all the passageways in the lower respiratory tract. Elastic fibers in the trachea walls allow the airways to expand and contract when we inhale and exhale, while the cartilage rings prevent them fromcollapsing. The trachea divides behind the sternum to form a left and right bronchus, each entering a lung. Inside the lungs, the bronchi subdivide repeatedly into smaller airways. Eventually they form tiny branches called terminalbronchioles. Terminal bronchioles have a diameter of about 0.02 in (0.5 mm).The branching air-conducting network within the lungs is called the bronchial tree.
The lungs are two cone-shaped organs located in the thoracic cavity, or chest, and are separated by the heart. The right lung is somewhat larger than theleft. The pleural membrane surrounds and protects the lungs. One layer of thepleural membrane attaches to the wall of the thoracic cavity, and the otherlayer encloses the lungs. A fluid between the two membrane layers reduces friction and allows smooth movement of the lungs during breathing. The lungs aredivided into lobes, each one of which receives its own bronchial branch. Thebronchial branch subdivides and eventually leads to the terminal bronchi. These tiny airways lead into structures called respiratory bronchioles.
The respiratory bronchioles branch into alveolar ducts that lead into outpocketings called alveolar sacs. Alveoli, tiny expansions of the wall of the sacs, form clusters that resemble bunches of grapes. The average person has a total of about 300 million gas-filled alveoli in the lungs. These providean enormous surface area for gas exchange. Spread flat, the average adult male's respiratory surface would be about 750 sq ft (70 m2), approximately the size of a handball court. Arterioles and venules make up a capillary network that surrounds the alveoli. Gas diffusion occurs rapidly across the walls of the alveoli and nearby capillaries. The alveolar-capillary membrane together is extremely thin, about 0.5 in (6-37mm) thick.
The result of external respiration is that blood leaves the lungs laden withoxygen and cleared of carbon dioxide. When this blood reaches the cells of the body, internal respiration takes place. Under a higher partial pressure inthe capillaries, oxygen breaks away from hemoglobin, diffuses into the tissuefluid, and then into the cells. Conversely, concentrated carbon dioxide under higher partial pressure in the cells diffuses into the tissue fluid and then into the capillaries. The deoxygenated blood carrying carbon dioxide then returns to the lungs for another cycle.
Pulmonary ventilation, or breathing, exchanges gases between the outside airand the alveoli of the lungs. Ventilation, which is mechanical in nature, depends on a difference between the atmospheric air pressure and the pressure inthe alveoli. When we expand the lungs to inhale, we increase internal volumeand reduce internal pressure. Lung expansion is brought about by two important muscles, the diaphragm and the intercostal muscles. The diaphragm is a dome-shaped sheet of muscle located below the lungs that separates the thoracicand abdominal cavities. When the diaphragm contracts, it moves down. The domeis flattened, and the size of the chest cavity is increased, lowering pressure on the lungs. When the intercostal muscles, which are located between theribs, contract, the ribs move up and outward. Their action also increases thesize of the chest cavity and lowers the pressure on the lungs. By contracting, the diaphragm and intercostal muscles reduce the internal pressure relative to the atmospheric pressure. As a consequence, air rushes into the lungs. When we exhale, the reverse occurs. The diaphragm relaxes, and its dome curvesup into the chest cavity, while the intercostal muscles relax and bring theribs down and inward. The diminished size of the chest cavity increases the pressure in the lungs, thereby forcing out the air.
Physicians use an instrument called a spirometer to measure the tidal volume,that is, the amount of air we exchange during a ventilation cycle. Under normal circumstances, we inhale and exhale about 500 ml, or about a pint, of airin each cycle. Only about 350 ml of the tidal volume reaches the alveoli. The rest of the air remains in the respiratory tract. With a deep breath, we can take in an additional 3,000 ml (3 liters or a little more than 6 pints) ofair. The total lung capacity is about 6 liters on average. The largest volumeof air that can be ventilated is referred to as the vital capacity. Trainedathletes have a high vital capacity. Regardless of the volume of air ventilated, the lung always retains about 1200 ml (3 pints) of air. This residual volume of air keeps the alveoli and bronchioles partially filled at all times.
A healthy adult ventilates about 12 times per minute, but this rate changes with exercise and other factors. The basic breathing rate is controlled by breathing centers in the medulla and the pons in the brain. Nerves from the breathing centers conduct impulses to the diaphragm and intercostal muscles, stimulating them to contract or relax. There is an inspiratory center for inhaling and an expiratory center for exhaling in the medulla. Before we inhale, theinspiratory center becomes activated. It sends impulses to the breathing muscles. The muscles contract and we inhale. Impulses from a breathing center inthe pons turn off the inspiratory center before the lungs get too full. A second breathing center in the pons stimulates the inspiratory center to prolong inhaling when needed. During normal quiet breathing, we exhale passively asthe lungs recoil and the muscles relax. For rapid and deep breathing, however, the expiratory center becomes active and sends impulses to the muscles tobring on forced exhalations.
The normal breathing rate changes to match the body's needs. We can consciously control how fast and deeply we breathe. We can even stop breathing for a short while. This occurs because the cerebral cortex has connections to the breathing centers and can override their control. Voluntary control of breathing allows us to avoid breathing in water or harmful chemicals for brief periods of time. We cannot, however, consciously stop breathing for a prolonged period. A buildup of carbon dioxide and hydrogen ions in the bloodstream stimulates the breathing centers to become active no matter what we want to do. Forthis reason, people cannot kill themselves by holding their breath.
The respiratory system is open to airborne microbes and to outside pollution.It is not surprising that respiratory diseases occur, in spite of the body'sdefenses. Some respiratory disorders are relatively mild and, unfortunately,very familiar. We all experience the excess mucus, coughing, and sneezing ofthe common cold from time to time. The common cold is an example of rhinitis, an inflammation of the epithelium lining the nose and nasal cavity. Viruses, bacteria, and allergens are among the causes of rhinitis. Other respiratorydisorders include laryngitis, pneumonia, bronchitis, chronic obstructive pulmonary disease, and lung cancer.