Which Of The Choices Below Determines The Direction Of Respiratory Gas Movement?

Air is a mixture of gases (including nitrogen, which makes up 78% of air) and small particles called aerosols. There are two kinds of gasses in the atmosphere: inert and reactive. Inert gasses include nitrogen, helium, argon and carbon dioxide; they have no active chemical properties or biological effects when inhaled by humans. Reactive gasses are made up of oxygen, water vapor, ozone, fluorine, chlorine, bromine and iodine. Some of these react with normal atmospheric conditions to form acids that can irritate the lungs, while others are toxic if breathed at high concentrations.
Aerosol particles can be very fine (less than 0.1 microns) or coarse (5-10 microns). They may consist of liquid droplets, solid dusts, or both. Aerosols can also contain bacteria, fungi, pollen, and other organic matter as well as metal oxides and salts. When aerosols enter our body through the nose or mouth, they cause irritation and inflammation due to their corrosive nature. Fine aerosols, however, are not usually harmful because they quickly become entrapped within mucus membranes, where they cannot exert their irritating qualities on the tissues. Large aerosols, however, do reach the blood stream and produce more serious symptoms. This is why it is important for people who work around chemicals or fire extinguishers to use appropriate protective equipment such as face masks and respirators.
The most common type of respiration involves inhalation and exhalation of air through the nostrils. Air enters the lungs via the trachea (windpipe), where the airflow is regulated by muscles attached to the bronchial tubes. These tubes act like one-way valves, allowing air into the lungs but preventing its escape from them. A person exhales through his/her mouth or nose because he has less lung capacity than an animal does. Humans breathe faster than animals, and we take smaller breaths than they do. We also tend to hold our breath longer between breathes. Breathing is controlled by the medulla oblongata in the brainstem, and the chemical messages carried by the nerves stimulate the diaphragm and intercostal muscles surrounding the windpipes (the “bronchi”) to contract. As the diaphragm contracts, the chest expands, and the abdomen falls. During inspiration, the diaphragm relaxes and the stomach sinks down, expanding the chest cavity. At the end of inspiration, the chest begins to expand again, and the stomach rises. The entire process takes about 2 seconds.
When you inhale, oxygenated blood flows into your lungs from the pulmonary arteries. It then passes through capillaries to exchange waste products for new oxygen. Oxygen diffuses across the thin walls of the capillary endothelial cells. Carbon dioxide and water vapor diffuse out of the capillaries and into the alveoli. Alveolar tissue consists of a network of extremely fine air sacs separated by tiny elastic fibers. Each alveolus contains millions of microscopic pores called fenestrations. Fenestrations provide the only direct passageway between the external environment and the internal surfaces of the alveolar epithelium.
Each fenestration is surrounded by a special membrane layer composed mainly of proteins known as surfactants. Surfactants reduce surface tension so that the alveolar fluid will spread over the entire inner wall surface of the fenestration. This reduces the negative charge of the outer surface of the cell membrane, making it easier for ions to cross the barrier. Water molecules move easily inside the alveoli because of hydrogen bonds formed between water and the lipids in the phospholipid bilayer of the cell membrane. The presence of surfactant also prevents leakage of liquid from the alveoli into the spaces outside the cells.
Once inside the alveolus, oxygen dissolves readily in the alveolar fluid. However, the dissolved oxygen is unable to get inside the interior of the red blood cells, since the outer lipid layer of the cell membrane prevents it from crossing. Therefore, all the oxygen available for consumption after breathing must first diffuse across the outer lining of the alveolar epithelium. After passing through the fenestrations, the oxygen reaches the inner side of the red blood cell membrane. Here, it combines with hemoglobin to form oxyhemoglobin. Hemoglobin binds four times as much oxygen as it would without the binding sites. Thus, even though there is plenty of oxygen present in the bloodstream, it is bound tightly enough to prevent any significant amount from reaching the tissues. Once inside the red blood cells, the oxygen can travel directly to the mitochondria of the muscle cells. Mitochondria convert glucose into ATP (adenosine triphosphate), the primary source of energy used by the heart, smooth muscles, and skeletal muscles.
Other gases besides oxygen and carbon dioxide are found in the Earth’s atmosphere. For example, methane, nitrous oxide, ammonia, and sulfur hexafluoride are also found in the atmosphere. Most of these are considered greenhouse gases, meaning that they absorb heat radiation emitted from the sun. Methane contributes to global warming, and other gases contribute to smog formation.
As mentioned earlier, nitrogen is a major component of the Earth’s atmosphere. Nitrogen comprises 78 percent of air, and it occurs naturally in three different forms: N2 (78%), NOx (nitric acid), and NH3 (ammonia). Atmospheric nitrogen reacts chemically with O2 to form nitrogen oxides (NO and NO2). These compounds combine with sunlight to form ground level ozone. Ground level ozone causes eye damage and lung irritation. Other nitrogen species, notably N2O4 and NO2, also occur naturally in trace amounts.
Carbon monoxide (CO) is another product of combustion processes. It is produced during incomplete burning of fuels containing carbon. It is odorless and colorless. Exposure to CO may result in death if sufficient quantities are absorbed. CO is highly poisonous.
Argon, He, Kr, Xe, and Ne are noble gases. Argon is used primarily as a coolant gas in various industries. Helium is used as a lifting gas in balloons. Krypton, xenon, and neon are used as flammable gases in industry and as components of certain welding rods. Neon is excited by electric discharge and gives off visible light.
Water vapor is another constituent of the atmosphere. It constitutes approximately 18 percent of the total volume of air in the Earth’s atmosphere. About half of the mass of the human body is comprised of water. Water vapor is invisible and odorless. It is not normally hazardous unless large volumes are inhaled. Under certain circumstances, such as exposure to excessive moisture, water vapors can cause health hazards.
Exposure to cigarette smoke, radon, asbestos, chloroform, and benzene can affect the overall health of people. The International Agency for Research on Cancer classifies many occupational exposures as carcinogens. Occupational diseases caused by workplace exposures range from simple skin disorders to cancer.
This article was written by Thomas J. Mancini, MS, MA, EMRPHS, and edited by Shirley Salyer. Special thanks to Dr. Frank Cibelli of the National Safety Council.

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