What is respiration?

The Mechanics of Respiration

The primary function of respiration is performed by the
lungs and their associated tissues. Air must be breathed in through the mouth and nose through the larynx (voice box) into the main airway, the
trachea (windpipe). Inside the chest, the trachea branches into the two main airways called bronchi, which in turn successively branch many times into small bronchi called bronchioles. These airways end in very small sacs called alveoli. These alveoli have a very thin membrane separating the air space from the blood in the capillaries. Oxygen (O2) diffuses through the alveolar membrane across the capillary membrane and into the blood to be taken to all the tissues of the body. Tissues excrete carbon
dioxide (CO2) into the blood that is carried back to the lungs. Carbon dioxide diffuses from the blood into the alveoli and is carried back through the airways and out of the lungs with the exhaled air. The mouth and nose humidify dry air to ensure that the linings of the lower airways do not dry out. The main airway divides to supply the left and right lungs. These large airways are cylindrical. Their circular shape is maintained by C-shaped cartilage in the walls. The stiff walls prevent collapse of the airways and the loss of gases through the walls of these “conducting” airways. The airways branch repeatedly into smaller airways. As the airways become smaller, they have less cartilage, until, in the very smallest airways, the cartilage is absent. These thin airways, which are called respiratory bronchioles, have alveoli budding from their walls. Gases may diffuse through the walls of these airways. These bronchioles become alveolar ducts and then erupt into lobular sacs of alveoli. There are about 300 million alveoli in an individual’s lungs, which provide about 70 square meters of extremely thin membrane through which most gas exchange occurs.

The lungs are elastic in nature and have a tendency to collapse. They do not because they adhere to the inside surface of the chest wall in much the same way that a moist suction cup adheres to a smooth surface. The two surfaces may slide against each other without separating. The chest wall has a tendency to expand because the rib cage and the muscles between the ribs (the intercostal muscles) tend to pull the chest out and up. The balance of these forces (elastic recoil and chest wall expansion) keeps the lungs slightly expanded at all times. Like balloons, the alveolar sacs have a tendency to collapse. The lungs produce a substance called surfactant that keeps the small air sacs from collapsing.

Air gets into and out of the lungs in the following way. The lungs expand, drawing air into them. The lungs adhere to the diaphragm in the same way that they adhere to the chest wall. When the muscles of the diaphragm contract, the lungs are pulled down. At the same time, the intercostal muscles contract slightly, making the chest wall rise up and out. These actions cause the lungs to expand. This expansion causes the pressure inside the lungs to decrease, sucking in air. When the intercostal muscles and diaphragm relax, the elasticity of the lungs causes them to deflate again to their resting state. This passive recoil of the lungs causes the pressure inside them to increase and pushes air out of the lungs.

The amount of air breathed in each breath is called the tidal volume. Each breath contains about 500 milliliters of air. Normally, a human being breathes in and out about twelve times in one minute. This results in about 6,000 milliliters of air being breathed each minute. Not all air that enters the mouth or nose reaches the area of the lung where gases are exchanged. About 150 milliliters of each breath stay in the larger airways. Therefore, 4,200 milliliters of air reach the alveolar space each minute. Since the chest wall and the chemical surfactant tend to keep the lungs partially inflated even after the breath is normally exhaled, additional air can be blown from the lungs, if one exhales consciously and forcefully. The volume of air blown out in this manner, which is called the expiratory reserve volume, is normally about 1,000 milliliters. Additional air can also be drawn into the lungs after a normal inspiration. This volume, the inspiratory reserve volume, is normally about 3,000 milliliters. The sum of the tidal, inspiratory reserve, and expiratory reserve volumes, which is called the vital capacity of the lungs, is about 4,500 milliliters. If these reserves are called into play, tidal volume can be increased almost tenfold. The breathing rate can also be increased at least twofold. Therefore, total alveolar ventilation can be as
great as 120,000 milliliters per minute.

The oxygen that is drawn into the lungs diffuses into the blood, and carbon dioxide diffuses out of the blood into the alveolar spaces to be exhaled. Fresh air exerts a pressure (barometric pressure) of 760 millimeters of mercury (mmHg). Oxygen is 21 percent of air; therefore, it has a partial pressure of about 160 mmHg. It mixes with air in the lungs that has lost oxygen to the blood. This results in a reduction in the partial pressure to about 100 mmHg by the time the breathed air reaches the alveoli. Blood pumped by the right ventricle of the heart into the lungs to be oxygenated has only 40 mmHg of oxygen. Oxygen diffuses from an area of high concentration in the alveoli to the blood, which has a low concentration. This
diffusion process is rapid enough that the partial pressure in the blood becomes equal to that of the alveoli before it courses one-half the distance through the lung capillary. Although the partial pressure is 100 mmHg, the amount of oxygen carried in blood fluids (plasma) is low. Therefore, without red blood
cells containing hemoglobin, blood cannot carry much oxygen to the tissues.

Hemoglobin is a very efficient carrier of oxygen. Each molecule of hemoglobin can carry four molecules of oxygen. In the same way that a disposable diaper absorbs water, hemoglobin absorbs oxygen from the plasma, allowing more oxygen to diffuse into the blood from the alveoli. As hemoglobin absorbs oxygen, it turns from a bluish purple to red. Between partial pressures of 20 and 100 mmHg, hemoglobin can absorb a large amount of oxygen. Hemoglobin does have a maximum capacity for oxygen that is reached at about 100 mmHg. Hemoglobin is called saturated at this point, and it can hold no more even if the partial pressure of oxygen increases. Hemoglobin is filled to half capacity by the time the plasma partial pressure reaches 30 mmHg. When the partial pressure increases from 20 to 100 mmHg, the increase in the amount carried by the plasma is 2.1 milliliters of oxygen per liter of blood. With the same change in partial pressure, hemoglobin increases the amount of oxygen carried by approximately 150 milliliters per liter of blood. Blood can carry more
than seventy times the amount that plasma alone can carry at this range of partial pressure. If the partial pressure does increase beyond 100 mmHg, little more oxygen is added to the blood. Oxygen is added to plasma in the dissolved form at a rate of 0.03 milliliters of oxygen per liter of plasma for each 1 mmHg change in the partial pressure.

Oxygen is carried to the tissues by the blood, where it is efficiently removed from hemoglobin. The partial pressure in the tissues is between 20 and 60 mmHg, depending upon the particular tissue and the rate at which the tissue uses oxygen. Inside the tissues, the partial pressure can be as low as 1 mmHg, providing a large difference to stimulate diffusion into the tissues. Oxygen is quickly absorbed by the tissue. Just as rapidly, carbon dioxide diffuses out of the cells and into the blood. There are also special ways in which the blood carries carbon dioxide to increase its capacity.

Carbon dioxide dissolves in plasma in much the same way that oxygen does, but supplemental mechanisms are required to carry the large amounts of carbon dioxide produced by the body. Carbon dioxide is also absorbed by red blood cells. It diffuses into the red blood cells, where it is changed in chemical form. Stimulated by an enzyme, carbonic anhydrase, carbon dioxide is combined with water and converted to a new chemical (the bicarbonate ion). The bicarbonate ion can attach to hemoglobin in this form. This, in effect, keeps the concentration of carbon dioxide in the plasma low, allowing more to diffuse. In the normal range of operation (40 to 50 mmHg), blood can absorb about 470 milliliters of carbon dioxide per liter of blood. With this large capacity, the partial pressure need change only a few mmHg to carry all the carbon dioxide that is produced by the tissues.

The anatomy of the lungs and the functioning of the respiratory system are well suited to meet most of the challenges that life presents. Exercise is a good example of how the respiratory system can handle a challenge. At rest, a fit young man breathes 6,000 milliliters of air per minute and uses about 250 milliliters of oxygen per minute to supply his body’s needs. When exercising to his maximal capacity, the same individual may use as many as 4,000 milliliters of oxygen per minute. To supply this increased demand, the respiratory system must utilize all the reserve volumes discussed above and increase the breathing rate to a total of 120,000 milliliters of air per minute. The brain senses the movement of the arms and the legs. It also senses the greatly increased amount of carbon dioxide produced by the exercising muscles. In turn, the brain sends signals to the chest and diaphragm to breathe much deeper and faster.

Another example of the large reserve capacity of the human lungs is the ability to hold the breath. Since only a small amount of oxygen from each breath is used, a person can take a deep breath and hold it easily for nearly one minute. Some pearl divers can take a deep breath and swim underwater for four minutes or longer. The urge to breathe that one experiences while holding one’s breath is produced when the brain senses the buildup of carbon dioxide and the decrease of oxygen in the blood.

The brain also uses its ability to sense the oxygen in the blood to adjust to unusual environments. At high altitudes, there is less oxygen in the air. With less oxygen in the air, less gets into the blood. The brain senses this condition and signals the respiratory system to breathe more air. Therefore, when one travels into the mountains, one will breathe slightly deeper and faster. One is not aware of the increased breathing until fairly high altitudes are reached (above 10,000 feet). If one begins to exercise, however, performing even mild exercise such as brisk walking, one will be very aware of breathing heavily. This situation is greatly intensified if the person has diseased lungs. With some severe lung diseases, even people living at low altitudes (sea level) have shortness of breath, and some need to breathe air supplemented with extra oxygen.

Disorders and Diseases

The major type of lung disease, which is called obstructive disease, has three subclasses. The first is general obstruction, a disease in which material is abnormally present in an airway. The second is disease in which the large airways are narrowed. The third is disease in which the small airways and alveoli are diseased.

The case of general airway obstruction is simple. The simplest form is one in which a foreign body such as food or part of a child’s toy is lodged in a large airway, such as the trachea or a main bronchus. The Heimlich maneuver (standing behind the affected individual, clasping the hands in a fist just below the rib cage, and thrusting up and in with the fist) is very effective in dislodging food caught in the trachea or larynx. An object that is small enough (such as a peanut), however, can get farther into the lung, in which case special instruments or surgery are necessary to remove the object. Tumors can also grow into the opening of an airway and obstruct it. Severe cases of tonsillitis are examples of this type of obstruction. Surgery is sometimes necessary to remove such a tumor if it limits airflow.

Large-airway narrowing is another type of related airway obstructive disease. Asthma and bronchitis are examples of this type of disease. The walls of the trachea and larger bronchi become thickened and thus make the passageway for air smaller. In addition, the specialized muscle (smooth muscle) surrounding the large airways has a tendency to contract, making the opening in the airway even smaller. These conditions result in difficulty of breathing, particularly when inhaling. Relatively rapid airway narrowing caused by smooth muscle contraction is called an
asthma attack. Irritants such as air pollution, tobacco smoke, and pollen can start an asthma attack. Exercise, particularly in cold weather, can also stimulate an attack in some asthmatics. Asthma attacks can last for hours and sometimes days. There are some drugs, frequently taken in an inhaled form, that help relieve the symptoms by relaxing the smooth muscle. Many cases of asthma, however, are resistant to these drugs. Some asthmatics have benefited from drugs that help decrease the frequency and severity of
attacks. Asthma usually begins in childhood and has a tendency to run in families.

Chronic obstructive pulmonary disease is the term that characterizes obstructive disease of the smallest airways and alveoli. Emphysema and chronic bronchitis belong to this class of lung disease. Emphysema consists of enlargement of the smallest bronchioles and the alveolar sacs. The walls of the alveoli disappear, and with them the capillaries. Therefore, the area previously used to exchange oxygen and carbon dioxide is lost. Since the air sacs are enlarged, the oxygen must travel farther to diffuse into the blood. Emphysema can be indicated by chest X rays and pulmonary function tests but cannot be definitely identified until after death. Emphysema is frequently associated with chronic bronchitis. Chronic bronchitis is characterized by enlargement of the mucous glands and by excessive mucus (sputum) production in the bronchial tree. The enlargement of the mucous glands alone can increase the resistance to airflow. Bronchitis is considered chronic when mucus is produced for three months of the year for at least two years.
The sputum can be very thick and may form into plugs to completely block off areas of the lung from airflow. Chronic obstructive pulmonary disease is generally a combination of both emphysema and chronic bronchitis of varying degrees. Persistent cough with expectoration is a normal symptom of this lung disease. With the destruction of airways and alveoli, some of the elastic recoil of the lungs is lost. As a result, exhalation is very laborious. Excess air is left in the lungs at the end of the exhalation, causing the chests of sufferers to be enlarged. Chronic obstructive disease is commonly found in long-term smokers.

Restrictive lung disease is another major classification of lung diseases. The main general feature of this class is primary changes in respiratory system tissues that restrict the movement of the lungs and thus respiration. Cystic fibrosis is the primary example of this disease. Cystic fibrosis appears to be caused by a malfunction of the immune system that produces a thick scarlike substance in the walls of the alveoli. The walls of the alveoli become thick and very stiff (fibrous). In some cases, the scar tissue grows across the small airway opening and closes off the airway. These closed air pockets are called cysts. The stiffness of the airways increases the elastic recoil of the lungs, making it very difficult to inhale.

Perspective and Prospects

Hippocrates (c. 460-c. 370 b.c.e.) recognized the breathing of air as an important function. He believed, however, that the function of breathing was to cool the generator of heat, the heart. Aristotle (384-322 b.c.e.) believed that air was breathed into the arteries, which carried it in the gaseous form to the rest of the body. Galen (129-c. 199 c.e.) transformed medicine from a hypothetical (philosophical) science into an experimental science by performing the first experiments on animals. He found that the arteries did not contain air, and he deduced that a quality of air (oxygen had not yet been discovered), not air itself, was important to life. In the seventeenth century, William Harvey discovered that blood circulated from arteries to veins in both the lung and the rest of the body, and oxygen was identified at the end of the eighteenth century by Joseph Priestly. Claude Bernard described the union of oxygen and hemoglobin at the end of the nineteenth century.

Many major technological advances have been made. Machines have been developed to assist and in some cases completely take over the function of respiration. Respirators can assist patients who have difficulty breathing on their own. Victims of poliomyelitis whose muscles for respiration are no longer functional, as well as paralyzed patients, have been greatly helped by respirators. Respirators maintain breathing during surgery when the patient receives general anesthesia. They also assist premature babies whose lungs are not fully developed. Scientists can now make the chemical surfactant that helps keep the lungs open. Premature babies frequently do not make enough surfactant; therefore, administration of synthetic surfactant can be lifesaving. Some machines can completely assume the function of the lung. These machines, called extracorporeal membrane oxygenators, can do the job of both the heart and the lungs. They are used in heart transplantation operations. They are also used to function in the place
of severely damaged lungs of newborns until those lungs can repair themselves.

Knowledge of the functioning of the respiratory system has allowed humans to function in unusual environments. Humans are able to travel to high altitudes (for example, the top of Mount Everest) with the assistance of supplemental oxygen. Travel into outer space, where there is no oxygen, is now possible because an atmosphere can be created that is suitable for long-term living in space. Experimental work is being performed with the breathing of special liquids instead of air. Success with liquid breathing may allow humans to exist in different environments, such as the deep sea, and may also have therapeutic value.

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