The respiratory system
The respiratory system distributes air and exchanges gases. Air travels through the airway from the nostrils to the alveoli, and breathing moves it in and out by changing the pressure in the chest. Oxygen and carbon dioxide cross between the alveolar air and the blood in the lungs, and between the blood and the tissue cells.
What the respiratory system does
The organs of the respiratory system are designed for air distribution and gas exchange. Of all the substances cells need to survive, oxygen is the most crucial: a person can live only a few minutes without it. Removing carbon dioxide from the body is of equal importance. Too much carbon dioxide in the blood alters the pH of interstitial fluid and blood and leads to cell death. The respiratory organs also filter, warm and humidify the air before it reaches the lungs, and they are closely associated with speech or sound production and with olfaction.
The airway
The respiratory system consists of the nose, the nasal cavity, the pharynx or throat, the larynx or voice box, the trachea or windpipe, the bronchi, the bronchioles and the lungs. Its basic structure is a tube that divides and subdivides, ending in thin-walled sacs, the alveoli, which distribute air close enough for gas exchange between air and blood in the lungs to occur by diffusion. The interior surfaces of these structures, from the nasal cavity to the alveoli, are lined with epithelial tissue.
Air enters through the external nostrils, or nares, and flows into the two nasal cavities separated by the nasal septum, where it is filtered, warmed and humidified. The pharynx is divided into three parts, the nasopharynx, the oropharynx and the laryngopharynx, and is also a pathway for the digestive system. The larynx lies below the pharynx and is composed of several pieces of cartilage, the largest being the thyroid cartilage or Adam's apple. Short fibrous bands, the vocal cords, stretch across its interior and produce sound as air passes over them. Another cartilage, the epiglottis, partially covers the opening into the larynx and closes it off during swallowing, preventing food from entering the trachea.
Because the trachea channels air into the lungs, it is vital that it stay open. Its 15 to 20 rings of C-shaped cartilage prevent it from collapsing. The branching that follows can be pictured as an upside-down tree, with the trachea as the main trunk and the right and left primary bronchi entering the right and left lungs. Within each lung the primary bronchi branch into secondary bronchi and then into bronchioles. These progressively narrower branches conduct the air to the alveoli, where the smallest bronchioles end.
The alveoli
The alveoli are tiny air sacs in the lungs where the exchange of oxygen and carbon dioxide occurs. The lungs hold 300 million of them, each approximately 0.2 mm in diameter, sitting like grape clusters on respiratory bronchiole stems in close contact with the pulmonary capillaries. The alveoli are lined with a single layer of flat cells, the type I pneumocytes, interspersed with cuboidal cells, the type II pneumocytes. Type II pneumocytes secrete surfactant, a chemical that reduces surface tension and prevents the alveolar walls from collapsing. Type I pneumocytes create the surface across which oxygen and carbon dioxide diffuse between the alveoli and the adjacent pulmonary capillaries, and their shape optimises this exchange by minimising the distance the gases must travel.
The lungs and the pleura
The lungs are rather large organs whose apices sit under the collarbone and whose bases rest on the diaphragm. Their outer layer, the pleura, is a serosal membrane that also covers the inner surface of the rib cage. The lungs are contained in a closed cavity, which is important for breathing.
Breathing
Breathing involves the respiratory organs together with the brain, spinal cord, nerves, some skeletal muscles and even some bones. It is mechanical: it relies on the law of nature that air moves from an area of higher pressure to an area of lower pressure. Nerve impulses from the brain stem stimulate the diaphragm to contract and flatten and the ribs to move up, which enlarges the pleural cavity and the attached lungs. Air pressure inside the lungs is then lower than the pressure outside, and air rushes in. Air pressure next becomes greater inside the pleural cavity than in the atmosphere. The diaphragm and rib muscles relax, which makes the chest cavity smaller and raises the air pressure, so air rushes out to where the pressure is lower.
Gas exchange in the lungs
Blood flows through thousands of tiny lung capillaries close to the air-filled alveoli, and gases are exchanged between the blood and the alveolar air by diffusion. The amount of oxygen and carbon dioxide in alveolar air and blood is often measured as partial pressure, a unit preferred to concentration because gases dissolve differently in air than in fluid such as blood. Gas exchange, the movement of oxygen from the alveoli into the blood and of carbon dioxide from the blood into the alveoli, is driven by partial pressure gradients. Blood flowing through the lung capillaries is low in oxygen while alveolar air is higher in it, so oxygen moves from the thin-walled alveoli into the blood and is carried to the body's cells. Carbon dioxide, a waste product of cellular metabolism, moves the opposite way, into the alveolar air, and is exhaled.
The heart's right ventricle pumps deoxygenated blood through the pulmonary arteries to the pulmonary capillaries. This blood has oxygen and carbon dioxide levels of 40 mmHg and 46 mmHg, while well-ventilated alveoli have levels of 100 mmHg and 40 mmHg. The blood then flows through the pulmonary veins to the left side of the heart to be pumped throughout the body, and this systemic arterial blood has oxygen and carbon dioxide levels of 100 mmHg and 40 mmHg. Gas pressures equilibrate between the alveoli and the blood, but gas concentrations do not.
Normal oxygen and carbon dioxide levels rely on three conditions: ease of air flow into and out of the lungs, properly distended alveoli, and a short distance between the alveoli and the pulmonary capillaries to optimise gas exchange. These conditions contribute to a respiratory diagnostic tool, the ratio of the partial pressure of oxygen in systemic arteries to the fraction of oxygen in inspired air, PaO2/FiO2. Normal PaO2 is 100 mmHg and FiO2 is approximately 20%, or 0.2, so healthy respiratory conditions yield a PaO2/FiO2 ratio of 500.
Gas exchange in the tissues
Exchange of gases also occurs in the tissue capillaries. There oxyhaemoglobin dissociates, releasing oxygen, which diffuses from red blood cell to plasma to interstitial fluid to tissue cells. Simultaneously, carbon dioxide diffuses in the opposite direction, from tissue cells to interstitial fluid to plasma and finally into red blood cells.