The Respiratory System

The respiratory system is a vital key in maintaining homeostasis, and also serves as the human body's lifeline (Patton PhD & Thibbodeau PhD, 2008). This system is composed of numerous substances, including erythrocytes, the diaphragm, and the trachea, that work together in order for the system to function properly.

Erythrocytes, or more commonly referred to as red blood cells (RBCs), are found in every system of the body (Patton PhD & Thibbodeau PhD). In adults, they are produced by the bone marrow, and live for about 120 days (Brewer, 2002). In the relatively short amount of time that they survive, they travel approximately 930 miles around your body (Abrahams et al., 1993).

The term "blood" does not refer to only one type of cell. RBCs make up about 41% of the blood in the human body, and outnumber white blood cells by 500 to 1, but blood is also made up of platelets, dissolved salts, hormones, fats, sugars, and proteins. Under high pressure, blood flows away from the heart through the arteries and most arteries carry bright red, oxygenated blood, or RBCs. Erythrocytes play an extremely important role because of the job that they do: transporting oxygen (Brewer). Once oxygen has been inhaled into the lungs, RBCs transport the oxygen (O₂) from the lungs to the other cells in the body. While
making their drop-offs, RBCs also pick up carbon dioxide (CO₂) disposes it into the lungs. From the lungs, the CO₂ is exhaled back into the air (Patton PhD & Thibbodeau PhD).

Hemoglobin is a protein that is found within the red blood cells. It contains iron, which it uses to link with oxygen molecules, and combines with the oxygen when the RBCs travel through the lungs. As the blood travels through the body, it is actually the hemoglobin of the RBCs that carries and transfers oxygen where it is needed (Brewer).

Red blood cells also have proteins called antigens on their surface. Each person's antigens are unique in that they categorize the type of blood a person has. The main types of blood are called A, B, AB, and O (Brewer). It is important to know a person's blood type for things such as a blood transfusion or an organ implantation, because if the antigens in the foreign blood are not recognized by the red blood cells, antibodies will be produced and reject the new blood or organ.

While all other cells have a nucleus, red blood cells do not. With the lack of a nucleus, hemoglobin is able to do a fantastic job of boosting the amount of oxygen that the blood can carry (by more than one hundred times!) (Brewer). RBCs are also structured in an interesting way. They are caved in on both sides, which maximize their oxygen-carrying capacity. The shape also helps make these cells capable of squeezing through the narrowest of blood vessels. (Brewer) (Eyewitness Visual Dictionaries, 1991) (Patton PhD & Thibbodeau PhD)

The shape, function, and sheer number of red blood cells, all make it quickly clear just how important these cells play in contributing to homeostasis. Without them, and the work that they do, we would not survive.

While it is now obvious that RBCs are vital in order to maintain life, the body houses many different structures that are just as important. In inspiration, the most important muscle is the diaphragm (Patton PhD & Thibbodeau PhD). Other important muscles involved in inspiration include the intercostals and abdominal muscles, and it is interesting to note than when breathing hard, we sometimes use muscles in the neck to help expand the chest (Fishbein MD, 2007). The diaphragm is fastened to the bottom edge of the rib cage and arches up under the lungs, giving it a dome shape (Patton PhD & Thibbodeau PhD) (it has also been described as shaped like a parachute). The diaphragm contracts and flattens when air is breathed in, making the chest deeper. As the chest expands, the lungs expand as well, drawing in air through the nose to fill the space in the lungs (Solway, 2007). Inhaling is an active process that takes work by the breathing muscles, but exhaling is automatic. In exhalation, the diaphragm and the rib muscles relax, and the diaphragm is returned to its original shape, which shrinks the lungs and pushes air out of the body - much the same way that air blows out of a balloon when untied (Patton PhD & Thibbodeau PhD).

The brain plays a key role in the mechanics of the diaphragm. When the blood that reaches the brain carries a certain amount of CO₂, a message is sent to the "breathing center," which then sends a signal to the diaphragm and rib muscles telling them to contract. The nerves controlling the contracting run through a spot called the solar plexus, which is located on the front of the abdomen. If there is a sudden blow to the solar plexus, it can stop the nerve messages for a moment, causing a person to feel "winded" and unable to breathe until the nerves recover. (Silverstein et al.)

The diaphragm is considered the most important muscle of inspiration because without it, the ability for O₂ to get to and CO₂ to get from the lungs is difficult. It is also an important part of the body because it is what separates the thoracic cavity, which houses the respiratory system, from the abdominal cavity, which houses the digestive system. Unfortunately, this can also cause a slight problem. With its location between the two cavities, an opening for the esophagus to pass through is necessary. This opening can become enlarged and sometimes a part of the stomach can move upward through the diaphragm and into the chest. Referred to as a hiatal hernia, this can result in GERD (gastroesophageal reflux disease). (Patton PhD & Thibbodeau PhD)

An interesting thing to learn about the diaphragm is that it is what causes hiccups. Hiccups occur when the diaphragm contracts rapidly which forces air to rush into the lungs and through the vocal cords. The "hiccup" sound is produced when the glottis snaps shut (Silverstein et al.). Another interesting thing in relation to the diaphragm is that when pushing on the diaphragm from behind, it forces it upward, which increases the pressure inside the chest, making whatever may be blocking the airways pop out. Henry Heimlich created this maneuver, thus its name "The Heimlich Maneuver." (Patton PhD & Thibbodeau PhD)

The respiratory tract is divided into two main parts: the upper respiratory tract and the lower respiratory tract. The upper respiratory tract is located outside of the chest cavity and contains three main organs which are the nose, pharynx, and larynx. The lower respiratory tract is located inside of the chest cavity and contains three main organs which are the trachea, the bronchial tree, and the lungs. (Patton PhD & Thibbodeau PhD)

The trachea, most commonly known as the windpipe, is one of the most important organs in the respiratory system because it is the main airway to the lungs. Its two main functions are to furnish an open passage for air to get to and from the lungs, along with serving a protective function by way of the mucous membrane and the tiny hair-like structures called cilia that are on its surface, which traps airborne particles and microorganisms that could lead to illness and/or infection (Patton PhD & Thibbodeau PhD). Particles that are trapped in the mucus get swept along in the current of the cilia and up into the throat where they are either spat out or swallowed and emptied into the stomach where any germs are killed and passed harmlessly out of the body (Silverstein et al.). Mucus that is secreted from the mucous membrane also moistens the inhaled air (Brewer).

When looking at the structure of the respiratory system, it looks like an upside down tree (thus the name "bronchial tree." The trachea is in effect the trunk of the tree. Air moves down the trachea and then branches into the two main bronchi, then into smaller and smaller bronchi, eventually ending in the alveoli (Williams, 1997).

The trachea itself is a strong and flexible tube measuring approximately four to five inches long and up to an inch in diameter. The walls are thick and muscular and are strengthened by C-shaped rings of almost non-collapsible, tough, rubbery cartilage. The cartilage rings are what keep the airway open even when you turn your head or twist your neck. There are muscles at the back of the tube which can pull together to narrow the opening (Silverstein et al.). This action of the muscles comes in handy if something solid accidentally gets into the trachea because it keeps the object from slipping farther down. If ever there is something caught in the trachea, it is imperative for it to come out. The reason for this is that there is no other way for the air to get to the lungs. Here is where use of the Heimlich maneuver could be life-saving. Obstruction of the trachea is the fifth major cause of accidental death in the US. (Patton PhD & Thibbodeau PhD)

The opening of the trachea, located at the lower part of the larynx (the voice box), is covered by a small flap of tissue called the epiglottis, which acts as a lid to the trachea and protects it by preventing food and liquids from entering it. Without the protection of the epiglottis, food and liquid would not be able to reach the digestive system properly. (Patton PhD & Thibbodeau PhD)

It is difficult to grasp the vast importance that the respiratory system holds since the act of breathing is such a natural one. However, the truth is that without this lifeline that supplies us with oxygen, removes carbon dioxide, and provides a stable environment in which cells can function, we would die. Mary Kittredge describes it well when she says to:"Imagine a large factory that performs many tasks and produces many different products. At the core of this factory is a delicate, complex system of machinery [that runs] day and night, 7 days a week, as many as 17,380 times a day. Over its [up to 100 year] lifetime, this system will perform its essential task an astonishing 650 million times. Yet it rarely malfunctions or needs a tune-up and in fact automatically does almost all of its own maintenance, cleanup, and repair, which is fortunate since it's parts are nearly all hidden and difficult to reach, and within three minutes of a breakdown will cause the entire factory to fall into total ruin, never to run again." (Kittredge, 1989)

From the nostrils that we use to breathe and help purify the air, to the tiny alveoli which make it possible for air to get close enough to blood for the gas exchange, every part of the respiratory system plays a role in our survival. But there are, of course, two structures that are most important in this system: the lungs.

The lungs contain nearly 1500 miles of airways and each has a total surface area of 646 square feet. They are shaped like cones with a wide base and a narrow top and have different sections called lobes. The right lung has three lobes, but to make room for the heart, the left lung has only two. Together, the lungs contain over 300 million alveoli, hold an average of 3 liters of air (in adults), and house approximately 6½ liters every minute. (Brewer)

The respiratory system, like all systems of the body, act as a part of a whole in order for us to be able to walk, talk, and live. Imagine trying to ride a bike without the chain, or the tires, or the pedals, or even the handles. Each part of the bike is used for something entirely different, and yet without one essential part, the entirety of the bike is useless. The body is the same way. As already stated, the respiratory system is what supplies O2 and removes CO2 from all cells of the body, but it also has importance elsewhere, such as helping keep airborne contaminants from getting to the rest of the body, assisting with hearing, and assisting with speech and sound production. It also works closely with the circulatory system, and is partly controlled by the nervous system. (Patton PhD & Thibbodeau PhD)

While the respiratory system is what transports all of the blood from the heart to the lungs where it is oxygenated and loses CO2, the circulatory system is what supplies the blood to all cells of the body. The contraction and relaxation of the respiratory musculature, which is what enables us to breathe, is controlled by the nervous system (Patton PhD & Thibbodeau PhD). The respiratory center is located in the medulla oblongata of the brain and its task is to regulate the activity of the respiratory muscles, ensure correct ventilation of the lungs, and to adapt the rate of respiration to the various conditions it might experience (Roca & Serrano, 1996).

The reason the respiratory system has an effect on speech and sound production is because we use our breathing equipment to make sounds. Air flows into and out of the system via the larynx, and stretched across the inside of the larynx is where the vocal cords are housed. When air flows through the opening, and the vocal cords are stretched tight, they vibrate. The muscles in the larynx vary the volume of sound by adjusting the size of the puffs of air coming from the vocal cords. An interesting little tidbit about sound is that men typically have lower pitched voices than women. This is because they have a larger larynx and longer vocal cords. (Silverstein et al.)

It is the mucus of the respiratory system that assists with smell. The mucus protects the sensitive smelling apparatus, the olfactory epithelium, and carries odor particles to the olfactory membrane. (Silverstein et al.)

Other ways that the respiratory system is involved in maintaining homeostasis include the following: the tonsils, which are also part of the respiratory system (although mostly part of the lymphatic system), produce disease-fighting white blood cells. Pleural membranes protect the lungs so that they can move while we breathe without being damaged by the ribs (Silverstein et al.). The nasal cavity is surrounded by sinuses, which help lighten the skull, act as resonance chambers of speech, and help warm and moisten the inhaled air (which is important because if this did not happen, the drying effect of the air could cause infections in different parts of the system (Roca & Serrano)) (Whittemore, 2004). The respiratory system also helps maintain the body's temperature, plays a part in balancing the blood's acid-base alkaline composition, and some parts of the system build blockades that prevent certain diseases from spreading elsewhere (Kittredge). Also, without the respiratory system, the food we eat would do no good because the O2 supplied plays a vital role in enabling the body's cells to turn food into energy (Petechuk, 2004).

With all of its abilities and wonder, the respiratory system is still like everything else in the human body - fragile. When we are no longer able to breathe on our own, whether by a debilitating accident, or a respiratory disease (i.e. asthma, emphysema, pneumonia, tuberculosis), we can thank the scientists who, with their research, have made possible the life-saving measures that we use so frequently today. People such as Antoine Laurent Lavoisier (known as the father of modern chemistry), August Krogh (who discovered passive diffusion) (Petechuk), and Sir Joseph Barcroft (who proved that oxygen is diffused from the lung into the blood) (Kittredge), are three of the most significant names in respiratory research.

All human tissues require oxygen and will eventually die without it. Oxygen is also important, because we need an abundant supply of it in order for cells to create an abundant supply of ATP (Whittemore). Along with the creation of CPR, where the extra CO2 in the rescuers' breath helps stimulate the victim's breathing center (Silverstein et al.), machines that we are all at least vaguely familiar with help save lives every single day. There are three primary types of systems that supply oxygen. Concentrators are machines that plug into an outlet and take the oxygen from the room air. Compressed gas is what is used when we see a person using an oxygen tank - this is a steel or aluminum cylinder that contains oxygen. Liquid machines are large stationary containers that have a refillable, portable unit with a lightweight tank (Petechuk). All of these use a version of an oxygen mask which delivers a gas mixture with a higher than usual concentration of O2 and the pressure delivered by the masks help the gas pass more easily into the blood capillaries (Silverstein et al.).

With all of the scientific breakthroughs, there is still a lot to understand. Currently, researchers are learning about what controls the timing and pattern of cellular production, movement, and separation. While doing their research, scientists can be surprised by finding an answer to a question not yet asked. For example, in 2000, "researchers at the Massachusetts institute of technology discovered that a chemical receptor found in the brain may be key to fetal development of the respiratory system. When scientists "knocked out" this receptor during prenatal development to see how it affected learning and memory in mice, they discovered that it also led to fatal respiratory distress. (Petechuk)

Another respiratory-related subject still not understood is SARS (severe acute respiratory syndrome). SARS was first reported in 2003 in Asia, and although researchers believe that it spreads mainly by close person-to-person contact, there is still a lot of uncertainty on the topic. Scientists are working on testing treatments and vaccines, but for now there is no treatment or cure. (CDC, n.d.)

Of the top ten worldwide causes of death, respiratory diseases make up almost half (Brewer). This statistic, all that we have covered in the previous paragraphs, and so much more, make it apparent that taking care of your body and making sure your respiratory system is working properly, is a vital key for a long and healthy life. From the small cells to the large organs, this system is complex and beautiful - and worth every bit of care we can give it.

References

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