The Respiratory System - Workings: how the respiratory system functions
The main function of the respiratory system is to provide oxygen for the body's cells and remove the carbon dioxide they produce. Oxygen is the most important energy source for the cells. They need it for cellular respiration: the process by which the simple sugar glucose is oxidized (combined with oxygen) to form the energy-rich compound adenosine triphosphate (ATP). Glucose is produced in cells by the breakdown of more complex carbohydrates, including starch, cellulose, and complex sugars such as sucrose (cane or beet sugar) and fructose (fruit sugar). ATP is the compound used by all cells to carry out their ordinary functions: growth, the production of new cell parts and chemicals, and the movement of compounds through cells and the body as a whole.
The mechanical process by which the body takes in oxygen and then releases carbon dioxide is called breathing or pulmonary ventilation. Inhalation (or inspiration) occurs when air flows into the lungs. Exhalation (or expiration) occurs when air flows out of the lungs. A single breath, called a respiratory cycle, consists of an inhalation followed by an exhalation. Breathing is brought about by the actions of the nervous system and the respiratory muscles.
When Earth was new, its atmosphere was probably composed of hydrogen, methane, and ammonia gases—much like the other planets in our solar system. Over billions of years, the composition of the atmosphere has changed considerably. Scientists theorize that a series of events that began when gases were released by early volcanic activity led to the formation of Earth's current atmosphere.
The air humans breathe in is Earth's atmosphere. The air humans breathe out, however, has a different composition. The following list breaks down the major components of those two types of air and their approximate percentages:
Nitrogen: 78% (inhaled air)/ 78% (exhaled air)
Oxygen: 21% (inhaled air)/ 16% (exhaled air)
Carbon dioxide: 0.04% (inhaled air)/ 4.5% (exhaled air)
Although most of Earth's atmosphere is composed of nitrogen, the human body cannot utilize this gas, so it is simply exhaled. Exhaled air has a decreased amount of oxygen and an increased amount of carbon dioxide. These amounts show how much oxygen is retained within the body for use by the cells and how much carbon dioxide is produced as a by-product of cellular metabolism.
The respiratory muscles are the diaphragm and the intercostal muscles. When the diaphragm (the dome-shaped sheet of muscle beneath the lungs that separates the thoracic chest cavity from the abdominal cavity) contracts, it flattens and moves downward. The intercostal muscles are found between the ribs. When the external intercostal muscles contract, they pull the ribs upward and outward. When the internal intercostal muscles contract, they pull the ribs downward and inward. The actions of all these muscles produce changes in the pressure within the alveoli and the bronchial tree.
All forms of matter—solid, liquid, and gas—exert pressure. In the case of a gas (like air), that pressure is caused by the motion of the gas particles. Gas particles have a tendency to fly away rapidly from each other and fill any container in which they are placed. As they do so, they constantly collide against the walls of that container and each other. The collisions of the gas particles causes gas pressure. In a large container, the gas particles in a certain amount of gas will be far apart and less collisions will occur. As a result, the gas pressure will be low. In a smaller container, the gas particles in that same amount of gas will be closer together and more collisions will occur. This will result in high gas pressure.
Inhalation occurs when motor nerves from the medulla oblongata in the brain carry impulses to the diaphragm and intercostal muscles, stimulating them to contract. When the diaphragm is stimulated to contact, it moves downward. Its dome is flattened and the size of the chest cavity is increased. The external intercostal muscles are also stimulated to contract, and they move the ribs up and outward. This also increases the size of the chest cavity. Since the lungs are attached to the chest (thoracic) walls, as the chest expands, so do the lungs. This action reduces the pressure inside the lungs relative to the pressure of the outside atmospheric air. As a consequence, a partial vacuum is created in the lungs and air rushes in from the outside to fill them. The quantity of fresh air taken in during an inhalation is referred to as tidal air.
The reverse occurs in exhalation. In healthy people, exhalation is mostly a passive process that depends more on the elasticity of the lungs than on muscle contraction. During exhalation, motor nerve stimulation from the brain decreases. The diaphragm relaxes and its dome curves up into the chest cavity, while the external intercostal muscles relax and the ribs move back down and inward. As the chest cavity decreases in size, so do the lungs. The air in the lungs is forced more closely together and its pressure increases. When that pressure rises to a point higher than atmospheric pressure, the air is expelled or forced out of the lungs until the two pressures are equal again.
Under normal circumstances, energy is expended during inhalation, but not during exhalation. However, air can be forcefully expelled, such as during talking, singing, or playing a musical wind instrument. Forced exhalation is an active process that requires muscle contraction. In such a case, the internal intercostal muscles are stimulated to contract, pulling the ribs down and in. This forces more air out of the lungs. The abdominal muscles (rectus abdominis) may also be stimulated to contract, compressing the abdominal organs and pushing the diaphragm upward. This action forces even more air out of the lungs.
A healthy adult at rest breathes in and out—one respiratory cycle—about twelve to sixteen times per minute (children breathe more rapidly, about eighteen to twenty times per minute). Exercise and other factors can change this rate. Total lung capacity is about 12.5 pints (6 liters). Under normal circumstances, an individual inhales and exhales about 1 pint (475 milliliters) of air in each respiratory cycle. Only about three-quarters of this air reaches the alveoli. The rest of the air remains in the respiratory tract. Regardless of the volume of air breathed in and out (called the tidal volume), about 2.5 pints (1200 milliliters) remains in the respiratory passageways and alveoli. This amount of air, called the residual volume, keeps the alveoli inflated and allows gas exchange between the lungs and blood vessels to go on continuously.
Once air has filled the lungs, the oxygen in that air must be transported to all the cells in the body. In return, all cells in the body release carbon dioxide that must be transported back to the lungs to be exhaled. The exchanges of gases in the body is known as respiration. External respiration is the exchange of gases through the thin membranes of the alveoli and those of the blood capillaries surrounding them. Internal respiration is the exchange of gases between the blood capillaries and the tissue cells of the body. Within the body, all gases are exchanged through the process of diffusion.
Diffusion is the movement of molecules from an area of greater concentration (existing in greater numbers) to an area of lesser concentration (existing in lesser numbers). Diffusion takes place because molecules have free energy, meaning they are always in motion. This is the case especially with molecules in a gas, which move quicker than those in a solid or liquid. Oxygen and carbon dioxide, the gases that pass between the alveoli and their capillaries and between the blood and the interstitial fluid (fluid surrounding cells of the body), move by diffusion.
In 1943, French oceanographer Jacques-Yves Cousteau (1910–1997) and French engineer Emile Gagnan developed the aqualung or scuba gear. This scuba (an acronym for s elf- c ontained u nderwater b reathing a pparatus) system not only benefitted recreational divers, but scientists as well. It has become an indispensable tool in the study of marine biology.
The aqualung allows a diver to swim freely down to about 180 feet (55 meters). Recordsetting dives of over 300 feet (91 meters) have been made with scuba gear. It consists of a canister or canisters of highly compressed air that the diver wears on his or her back. The unit is connected to a demand regulator that automatically supplies air at the same pressure as that of the surrounding water. A mouthpiece attached to the regulator allows the diver to breathe.
EXTERNAL RESPIRATION. After inhalation, the air in the alveoli contains a high concentration of oxygen and a low concentration of carbon dioxide. Conversely, the blood in the pulmonary capillaries surrounding the alveoli (which has come from the body) has a low concentration of oxygen and a high concentration of carbon dioxide. Following the law of diffusion, oxygen molecules in the air in the alveoli flow into the pulmonary capillaries. Carbon dioxide molecules flow in the opposite direction, from the blood in pulmonary capillaries into the air in the alveoli.
After gas exchange occurs in the lungs, the pulmonary capillaries carry the oxygenated (carrying oxygen) blood toward the heart. They merge to form venules, which merge to form larger and larger veins. Finally, the oxygenated blood reaches the left atrium of the heart through the four pulmonary veins. After flowing into the left ventricle, the blood is pumped out to the rest of the body.
Almost all the oxygen that diffuses into the pulmonary capillaries attaches to red blood cells in the blood. The primary element of red blood cells is a protein pigment called hemoglobin. Hemoglobin molecules account for one-third the weight of each red blood cell. At the center of each hemoglobin molecule is a single atom of iron, which gives red blood cells their color. The oxygen molecules bond to the iron atoms to create compounds called oxyhemoglobins. The main function of red blood cells is to transport this form of oxygen to the cells throughout the body.
INTERNAL RESPIRATION. Internal respiration occurs between the cells in the body and the systemic capillaries (capillaries in the body outside of the lungs). The bond between the oxygen molecules and the iron atoms of hemoglobin is not a very strong or stable one. When red blood cells enter tissues in the body where the concentration of oxygen is low, the bond is readily broken and the oxygen molecules are released.
Fish and most other aquatic animals use gills for respiration. In fish, these external respiratory organs are located in gill chambers at the rear of the mouth. Gills are specialized tissues with many infoldings. Each gill is covered by a thin layer of cells and filled with blood capillaries.
Water taken in through a fish's mouth is forced through openings called gill slits. It then washes over the delicate gills. The exchange of gases—oxygen and carbon dioxide—occurs through diffusion, much like in human lungs. Oxygen that is dissolved in the water diffuses through the thin membranes of the gills and passes into the capillaries. Carbon dioxide, produced as a waste product by the fish's cells, diffuses from the capillaries through the gills into the passing water.
All higher vertebrates or animals that have a backbone or spinal column (including humans) have immature gill slits when they are in an embryo stage or initially developing. However, these gill slits never fully mature and become functional. They disappear as the vertebrate embryo develops.
This occurs when the systemic capillaries pass among the body cells. The blood in the systemic capillaries has a high concentration of oxygen molecules and a low concentration of carbon dioxide molecules. The body cells and the interstitial fluid surrounding them have just the opposite: a low concentration of oxygen molecules and a high concentration of carbon dioxide molecules (because cells use oxygen to create energy, giving off carbon dioxide as the waste product of human metabolism).
Thus, in internal respiration, oxygen diffuses from the capillaries into the interstitial fluid to be taken up by the cells. At the same time, carbon dioxide diffuses from the interstitial fluid into the capillaries. Red blood cells in the now deoxygenated (carrying very little oxygen) blood then transport the carbon dioxide molecules back to the heart through ever larger veins. Finally, the blood returns to the right atrium of the heart via the venae cavae. After flowing into the right ventricle, the deoxygenated blood is pumped through the pulmonary arteries to the lungs, where the cycle of respiration begins once again.
Plants do not "breathe" like animals. All animals have some mechanism for removing oxygen from the air and transmitting it into their bloodstreams, while expelling carbon dioxide from their bloodstreams in the process. Plants exchange oxygen and carbon dioxide with Earth's atmosphere, but in a different process.
Plants create energy for their cells through the process known as photosynthesis. Simply put, a plant absorbs sunlight into chlorophyll (green pigment located in plant cells called chloroplasts) and takes in carbon dioxide from the air through stomata (microscopic openings on the underside of its leaves). It also absorbs water from the soil through its roots. Using the energy from sunlight, the plant combines carbon dioxide and water to create the simple sugar glucose (which is later used to form more complex carbohydrates such as starch and cellulose). Oxygen is a by-product of this process.
In the second phase of photosynthesis, called respiration, the plant combines glucose and oxygen with enzymes to create adenosine triphosphate (ATP), a high-energy molecule used by cells of all organisms to store energy. Since plants use less oxygen during respiration than is created during photosynthesis, they expel that oxygen through their stomata. This action occurs mainly at night when photosynthesis cannot take place.