LESSON 7 The Human Respiratory System and Breathing.

LESSON ASSIGNMENT Paragraphs 7-1 through 7-41.

LESSON OBJECTIVES After completing this lesson, you should be able to:

7-1. Match characteristics and processes of breathing and respiration with their descriptions.

7-2. Given a list of sentences about respiration or breathing, select the false statement.

7-3. Complete incomplete sentences about breathing or respiration.

SUGGESTION After completing the assignment, complete the exercises at the end of this lesson. These exercises will help you to achieve the lesson objectives.

a. The processes of respiration and breathing serve to provide oxygen to the body cells. This oxygen is used in the process of metabolic oxidation. In metabolic oxidation, the energy trapped in glucose molecules is released for use in the body's activities.

b. Also, the carbon dioxide (CO₂) produced during metabolic oxidation and any other unwanted gases are removed from the body.

 7-2. DEFINITIONS

a. Respiration.  In general, respiration is the exchange of gases. In the human body, two kinds of respiration take place.

(1) External respiration.  In external respiration, gases are exchanged between the blood and the surrounding air.

(2) Internal respiration.  In internal respiration, gases are exchanged between the blood and the individual cells of the body.

b. Breathing.  On the other hand, breathing is the process by which air is moved into and out of the lungs.

(1) Types.  In humans, there are two types of breathing. In costal breathing, the rib cage is used. In diaphragmatic breathing, there is reciprocal interaction between the diaphragm and the abdominal wall.

(2) Direction of air flow.  When the air flows inward, we call it inhalation (inspiration). When the air flows outward, we call it exhalation (expiration).

7-3. PHYSICAL PRINCIPLES

Both respiration and breathing are essentially physical processes. Air and/or various gases are moved from one place to another. Their movement is because of differences in their relative pressures from one space to another.

a. Pressure Gradient.  Consider a situation in which there are two separate but connected spaces. If the concentration or pressure of that substance is greater in one space than the other, then there is a pressure gradient for that substance. As a result, the substance will move from the area of higher pressure to the area of lower pressure.

b. Boyle's Law.  Assume that we have a container and we can change the volume of the container without allowing a gas to escape. Boyle's law tells us that if we increase the volume, the pressure inside will decrease. Likewise, if we decrease the volume, the pressure inside will increase.

c. Pascal's Law. If a closed container is filled with a fluid, a pressure applied to the fluid will produce an equal pressure at each and every point on the inner surface.

d. Surface Area.  Most phenomena in breathing and respiration take place at one surface or another. As surface area increases, more gases can be exchanged or treated.

7-4. GENERAL ANATOMY AND CONSTRUCTION OF THE HUMAN TRUNK

The human trunk (Figure 7-1) can be considered a hollow cylinder. A muscular membrane, the thoracic diaphragm, extends across this hollow and divides the trunk into upper and lower cavities.

Figure 7-1. Schematic frontal section of the human trunk.

a. Thoracic Cavity.  The thoracic cavity is the space of the trunk above the diaphragm. It is open to the outside by way of the neck and head. Since the wall of the thorax is reinforced by special muscles, bones, and cartilages, we can consider the thorax to be a "solid-walled container" filled with gas.

b. Abdominopelvic Cavity.  The abdominopelvic cavity is the rest of the trunk cavity below the diaphragm. The abdominopelvic cavity is a closed system. Its walls are "elastic" since they are made up of musculature. The abdominopelvic cavity is filled with a fluid continuum. This fluid continuum consists primarily of water contained in the soft tissues of the abdomen and the pelvis.

Breathing is basically the process of moving air into and out of the lungs.

Breathing is accomplished by manipulating the pressure gradient between the surrounding atmosphere and the thoracic cavity. For all practical purposes, the pressure of the surrounding atmosphere can be considered a constant. Thus, the desired pressure gradients are achieved by changing the pressure within the thoracic cavity. The pressure in the thoracic cavity alternates so that it is less and then greater than the pressure of the surrounding atmosphere.

7-7. TYPES OF HUMAN BREATHING

The two types of human breathing are costal and diaphragmatic. They may be used individually and independently, or they may be used in combination.

7-8. LUNG CAPACITIES

a. Total Lung Capacity.  From the instant of the "first breath," the lungs have a certain total volume called the total lung capacity. This is the entire volume of air in the lungs after one inhales as much as one can. Total lung capacity equals the sum of the residual volume and the vital capacity.

b. Residual Volume.  After the "first breath," the lungs are never completely emptied. Thus, there is a certain portion of air that is always present in the lungs. After one exhales as much air as possible, the portion remaining in the lungs is called the residual volume. In actuality, this is not "dead air," because air circulation continually refreshes the air of the residual portion.

c. Vital Capacity.  The vital capacity of the lung is the total amount of air that can be exchanged during total filling and emptying of the lung. For example, if one inhales as much air as one can and then exhales as much as possible, the volume exhaled would be the vital capacity.

7-9. BREATHING CYCLES

A breathing (respiratory) cycle is a sequence in which the lungs are filled and emptied to produce an exchange of the air in the lungs. The cycle includes an inhalation of air (filling of the lung with air), then a rapid exhalation (emptying), and then a short rest period. See Figure 7-2 for a representation of the "filling" of the lungs.

Figure 7-2. "Filling" of the lungs.

a. Volume Exchanges During Breathing.  The amount of air exchanged in a given period depends upon the rate and depth (volume) of breathing. Rate and depth are adjusted according to physiological demand. The rate of respiration is the number of breathing cycles per minute.

b. Some Types of Breathing Cycles.

(1) Quiet ("tidal") breathing.  As one takes part in ordinary, low-level activity, the breathing cycles are of the quiet type. This type involves only a minimal exchange of air.

(2) Complementary cycle.  Over a period of time, quiet breathing may not totally satisfy the oxygen requirements of the body. Thus, we can observe a breathing cycle with a slightly greater volume exchange called the complementary cycle. It provides a little extra oxygen to make up the difference.

(3) Forced breathing.  In forced breathing, the volumes of air exchanged are much greater than in quiet breathing. The actual volume exchanged depends upon the oxygen demand.

(4) Holding of breath.  One can inhale a volume of air and hold it for a period. If one makes an exhalation effort but still holds the air inside the lungs, it is called Valsalva's maneuver (forced expiration against a closed glottis).

(5) Cough.  If one suddenly releases the air, terminating Valsalva's maneuver, the result is a cough. If the musculature of a patient's abdominal wall is paralyzed, the patient cannot execute the Valsalva's maneuver and cannot produce a cough.

(6) Speech.  During speech or vocalization, the breathing cycles overlap. That is, the subsequent cycle begins before the previous one is ended. The purpose of this is to maintain a continuous outflow of air.

Costal breathing is breathing accomplished by moving of the rib cage as a whole.

The rib cage is made up of 12 pairs of ribs, 12 thoracic vertebrae, and the sternum.

a. Ribs.

(1) Structure of a "typical" rib.  Each rib is a flat-type bone that is curved laterally. Along its inferior margin is a subcostal groove.

(2) Attachments.

(a) All 12 pairs of ribs are attached posteriorly to the thoracic vertebrae.

(b) Anteriorly, the upper 10 pairs of ribs are attached directly or indirectly to the sternum. The indirect attachments are made through costal cartilages to the ribs above.

(c) It is important to note that both the posterior and anterior articulations are located essentially in the midline of the body, back and front.

(3) Costal cartilages.  The costal cartilages are bars of cartilage of varying lengths. Since costal cartilages are elastic, they can be twisted (deformed) and returned to their original shape.

b. Sternum.  The sternum is located in the midline anteriorly, immediately beneath the skin. (Since the sternum is a flat bone with hematopoietic (blood-forming) red marrow and is so close to the surface of the body, it is a convenient location for taking a sample of hematopoietic tissue for clinical examination--the sternal punch.)

(1) The sternum is made up of three parts--the manubrium above, the body as the main portion, and the xiphoid process below.

(2) Where the manubrium articulates with the top of the body of the sternum is a sternal angle (Louis' angle). The sternal angle is important in costal breathing, since it allows for greater expansion of the rib cage. (In the clinic, the sternal angle is important as a landmark. It marks the site of the second rib and is used to identify locations on the chest wall.)

c. Thoracic Vertebrae.  Posteriorly, there are 12 thoracic vertebrae, joined by intervertebral discs. Their curvature, the thoracic curvature, is concave anteriorly. During breathing, this curvature straightens and thus increases the expansion of the rib cage.

d. Segmentation.  The segmentation of the thorax is produced by both the intervertebral discs and the intercostal spaces between adjacent ribs. Such segmentation of the rib cage allows motion to take place, especially bending to the right or left.

e. Intercostal Muscles. The intercostal spaces are filled by two layers of intercostal muscles. The intercostal muscles extend from the vertebrae behind to the sternum in front. A strengthening "plywood effect" is created by the arrangement of the two layers at a right angle to each other. Therefore, these muscles help to maintain the "solid-wall" condition of the thorax. For this reason, a pressure gradient can be maintained between the inside and outside of the thorax.

f. Skeletal Muscles Attached to the Rib Cage.  Various skeletal muscles are attached to the rib cage. Some extend from above and draw the rib cage upward. Others extend from below and draw the cage downward.

In costal inhalation, the lungs are expanded and inflated with air because of upward movement of the rib cage. The expansion of the rib cage is sufficient to allow the needed volume of air to enter the lungs. There are two different types of movements of the ribs that produce this expansion of the rib cage.

a. One type of movement involves the so-called "bucket handle" effect. As each rib swings upon its ends, like a bucket handle swinging up from the sides of the bucket, the rib moves upward and outward laterally. As this type of movement occurs on both sides of the rib cage, the transverse diameter of the rib cage increases from side to side.

b. The second type of movement is described as follows: The lowest points of the ribs are their front ends at the sternum. During inhalation, these front ends move upward and forward along with the sternum. This increases the diameter of the thoracic cavity from front to back (anterior-posterior (A-P) diameter).

c. The increases in the transverse and A-P diameters enlarge the volume of the thoracic cavity and thus decrease the pressure of the air inside (Boyle's law). Thus, there is a relatively higher atmospheric pressure outside. This pushes air into the respiratory passageways and into the alveoli of the lungs. The alveoli are inflated by this inflowing air.

a. The lungs empty during costal exhalation, a process that is essentially the reverse of costal inhalation. The rib cage moves downward as a whole.

(1) In small-volume exchanges, the costal cartilages are sufficiently resilient (elastic or springy) to pull the rib cage downward.

(2) With greater-volume exchanges, musculature can be recruited to aid in lowering the rib cage.

(3) Gravity may also play a role.

b. As the transverse and A-P diameters decrease, the volume of the thoracic cavity also decreases. This increases the pressure of the air inside (Boyle's law). Thus, there is a relatively lower atmospheric pressure outside, and air is forced out of the lungs. (The elasticity (springiness) of tissues within the thoracic cavity also helps to push the air out.)

a. The abdominopelvic cavity is a closed system filled with a fluid (water) continuum.

b. The abdominopelvic cavity is inclosed by essentially muscular barriers.

(1) The inferior end is closed off by the pelvic diaphragm.

(2) The cylindrical walls of the abdomen are composed of three muscular sheets. Their orientation is similar to plywood. These muscles are kept taut by their intrinsic tone, but they are capable of additional contraction.

(3) Forming the top of the abdominopelvic cavity is the thoracic diaphragm. We discuss the thoracic diaphragm in the next paragraph.

The thoracic diaphragm is attached to the inferior margin of the rib cage and to the bodies of the lumbar vertebrae behind. As a muscular membrane, it domes upward into the thoracic cavity. Upon contraction, the fibers of the thoracic diaphragm shorten and pull downward. This downward motion produces a piston-like pressure on the contents of the abdominopelvic cavity.

a. As the thoracic diaphragm contracts and lowers, the vertical diameter of the thoracic cavity is increased. This increases the volume of the thoracic cavity. Thus, according to Boyle's law, the pressure of the air in the lungs decreases. The relatively higher atmospheric pressure outside pushes the air into the lungs, and the alveoli are inflated.

b. At the same time, the thoracic diaphragm produces a pistol-like pressure upon the noncompressible fluid continuum in the abdominopelvic cavity. By Pascal's law, the resulting pressure is distributed equally to the elastic walls of the cavity. As these walls are stretched by the added pressure, they "store" potential energy.

a. When the thoracic diaphragm relaxes, it no longer pushes down upon the contents of the abdominopelvic cavity. The potential energy stored in the stretched muscular walls becomes kinetic energy, and the walls rebound. This energy is sufficient for exhalation during quiet breathing.

b. However, during forced breathing, the muscles of the abdominal wall will contract in accordance with the amount of air to be pushed out.

c. As the muscles in the abdominal wall rebound (and contract in forced breathing), pressure is applied to the fluid continuum in the abdominopelvic cavity. By Pascal's law, this pressure is transferred to the underside of the thoracic diaphragm. The relaxed thoracic diaphragm is thus pushed up into the thoracic cavity. This decreases the vertical diameter and the volume of the thoracic cavity. The decreased volume results in increased pressure within the lungs (Boyle's law). Since the air pressure in the lungs is relatively greater than the outside atmospheric pressure, air is forced out through the respiratory passageways. (This is aided by the elastic rebound of tissues in the thoracic cavity.)

The human respiratory system consists of a series of organs that form a passageway for the air flowing to and from the alveoli of the lungs. The lungs themselves are discrete organs of the body containing the alveoli and are located in individual serous cavities.

The air passageway can be conveniently divided into three groups of structures. The larynx is the central portion. The other organs are grouped as supra laryngeal or infra-laryngeal.

The general functions of the supra laryngeal structures (Figure 7-3) are to condition the in flowing air and to test it. Conditioning includes cleansing, warming, and moistening.

Figure 7-3. Supra laryngeal structures.

The (external) nose is the beginning of the respiratory system in humans. It is located in the center of the front of the face. It is pyramid shaped, with the base facing inferiorly. The base consists of two openings called the nares or nostrils. These open into a pair of vestibules, one on each side. The nares are guarded by stiff nasal hairs. These nasal hairs serve to remove the larger particles (such as lint and cinders) from the inflowing air.

The vestibules of the nose are continuous posteriorly with the right and left nasal chambers.

a. Nasal Septum.  Like the vestibules, the nasal chambers are separated by a nasal septum, a vertical wall from front to back. Constructed of bone and cartilage, the nasal septum extends from the floor to the roof and from front to back.

b. Mucoperiosteal Lining.  Each nasal chamber is lined with a mucoperiosteal lining. This mucoperiosteal lining is a special combination of tissues, which are rich in blood vessels. This excellent supply of blood furnishes moisture and heat. On the surface of the mucoperiosteum are minute hair-like processes called cilia. The cilia continuously drive fluids on the surface to the rear. A part of the fluids secreted on the surface is a mucous material. As a part of the continuous process of cleansing the inflowing air, finer particles are trapped by the mucus.

c. Conchae.  Thus, the conditioning of the inflowing air depends upon direct contact with the mucoperiosteum. The greater the surface area, the more efficient will be the conditioning. The conchae are three shelf-like projections that extend from the lateral wall of each nasal chamber. Thus, a superior, a middle, and an inferior concha are found on each side. During ordinary breathing, the air enters the vestibules of the nose and passes through the lower portions of the nasal chambers in direct contact with the inferior and middle conchae.

d. Olfactory Epithelium.  As the air passes through the nasal chambers, some of the air reaches the superior recesses of the nasal chambers. In these superior recesses is found the olfactory epithelium. The olfactory epithelium contains special hair cells that can detect individual molecules found in the air. Thus, the sense of smell (olfaction), tests the quality of inflowing air.

e. Paranasal Sinuses.  Connected with each nasal chamber are cavities found in the middle layer of various skull bones. These cavities are the paranasal sinuses. Like the nasal chambers, they are lined with a continuation of the mucoperiosteum. Each paranasal sinus is named according to the bone in which it is located. The function of the paranasal sinuses is unknown.

The two nasal chambers are continuous posteriorly with a single cavity known as the nasopharynx.

a. Pharyngeal Tonsils ("Adenoids").  The pharyngeal tonsils are a pair of lymphoid aggregates in the upper posterior recess of the nasopharynx.

b. Auditory (Pharyngeotympanic or Eustachian) Tubes.  On each lateral wall of the nasopharynx is a small mound with a slit-like opening. This is the opening of the auditory tube, which passes laterally to the middle ear cavity. Because of this tube, the air pressures are kept equal on the inner and outer sides of the tympanic membrane (eardrum).

c. Soft Palate.  The floor of the nasopharynx is the soft palate. The soft palate is a musculomembranous structure. (Unlike the soft palate, the hard palate is bony. The hard palate forms the floor of the nasal chambers and the roof of the oral cavity.)

The nasopharynx (of the respiratory system) and the oropharynx (of the digestive system) are continuous posteriorly with the pharynx proper. During swallowing, the soft palate is raised like a trap door to close off the upper air passageways. This prevents movement of food into the upper air passageways.

The larynx (voice box; "Adams apple") is located in the lower anterior neck region. In many respects, the larynx is different in men and women (sexual dimorphism).

The larynx is suspended from the hyoid bone by a membrane. The root of the tongue is attached to the top anterior portion of the hyoid bone. These three structures--the larynx, the hyoid bone, and the tongue--are together known as the hyoid complex. They always move together as a unit.

The larynx performs several functions in humans.

a. Its primary function is to control the volume of the air passing through the air passageways, to and from the alveoli of the lungs (para 7-28).

b. The larynx also produces selected vibration frequencies in the moving column of air (para 7-29).

c. During swallowing, the hyoid complex is raised into the oral cavity. As this happens, the epiglottis of the larynx acts like a trap door, turning down to cover the entrance of the larynx. This prevents swallowed items from entering the lower air passageway, altogether forming the glottis.

A pair of folds is found at the bottom of the vestibule of the larynx. These are called the vocal folds or true vocal cords. Extending from front to back, there is one vocal fold on each side. With a special set of muscles, the vocal folds can be drawn apart or pulled together, altogether forming the glottis.

a. Thus, the vocal folds are used to control the size of the opening between them, which is called the rima glottidis. When the rima glottidis is wide, air can flow easily between the upper and lower air passageways. When the vocal cords are drawn so tightly that the rima glottidis is completely closed, no air can flow through.

b. In Valsalva's maneuver (para 7-9b(4), (5)), the lungs are filled with air and the rima glottidis is closed tightly. The muscles of the trunk wall contract strongly to increase the internal pressure of the trunk.

(1) This internal pressure stiffens the trunk into a more rigid structure. Thus, one uses Valsalva's maneuver to provide support for a strenuous effort with the upper members.

(2) When Valsalva's maneuver is followed by a sudden opening of the rima glottidis, the result is a cough. This is used to clear the air passageways.

(3) An individual whose trunk wall muscles are paralyzed cannot do these things.

Human speech is a combination of a number of processes. Essentially, a column of air flows out through the oral cavity, where it is chopped into bits of speech known as phonemes.

a. Speech sounds produced when the oral cavity is not blocked are called vowels. Sounds resulting from the closing or chopping action of the oral cavity are known as consonants.

b. The column of air vibrates at different frequencies (pitch). These vibration frequencies are gained by the air as it passes through the larynx. The pitch is varied by a change in the tension of the vocal cords. The higher the tension, the higher will be the pitch (vibration frequency).

The infralaryngeal structures (Figure 7-4) include the "respiratory tree" and the lungs. The respiratory tree is so named because it has the appearance of an inverted tree, with its trunk and branches. It is essentially a tubular structure connecting the larynx to the alveoli of the lungs. This tubular structure is lined with a ciliated epithelium. (Remember, cilia are hair-like projections from cells.) The tubes are kept open (patent) by a series of ring-like structures of cartilage.

The "trunk" of the tree is the trachea. The trachea extends from the inferior margin of the larynx, down through the neck, and into the center of the thorax.

Figure 7-4. Infralaryngeal structures.

In the center of the thorax, the trachea divides into right and left primary bronchi. The right is somewhat more vertical than the left. Therefore, when a person accidentally aspirates ("breathes in") a foreign object, it is more likely to be found in the right primary bronchus than the left.

a. Each primary bronchus extends laterally into the substance of the appropriate lung. Within each lung, the tubular structure divides, subdivides, and divides again, up to about 30 times. Thus, the tubes become more and more numerous and smaller and smaller in size. At the terminals of the branching tubes are groups of spherical alveoli. This gives the appearance of a bunch of grapes.

b. A variety of situations may occlude (close or shut off) these tubular air passageways.

(1) A foreign object may be aspirated ("breathed in").

(2) The wall of the tube may constrict in a bronchial spasm.

(3) The lining of the tube may become swollen with fluid and close the passageway.

None of the air found in the upper and lower passageways plays a part in actual respiration. Thus, this air is often referred to as "dead air." During quiet breathing, it amounts to about two-fifths of the total air volume exchanged.

External respiration is the exchange of gases between the air and the blood. External respiration takes place in the alveoli (alveolus, singular). The alveoli are small, spherical sacs that are continuous with the terminal elements of the branches of the respiratory tree. As we indicated earlier, external respiration is a surface phenomenon in which the gases pass through the wall of the alveolus.

a. Since there is a critical relationship between volume and surface area, the inflated alveolus is spherical. The alveolus is also of a particular size that is ideal for the efficiency of external respiration.

b. In each lung, there are billions of alveoli.

c. Numerous blood capillaries are adjacent to the walls of the alveoli.

d. To facilitate the exchange of gases between the air in the alveolus and the blood in the capillaries, the wall of the alveolus contains a special chemical known as surfactant.

e. The inner surfaces of the alveoli must be kept wet to make the transfer of gases possible. Because these surfaces are wet, one of the major fluid losses of the body is with the exhaled air.

In the thoracic cavity is a pair of lungs. Each lung is an individual organ containing the branching elements of one side of the respiratory tree, the connected alveoli, and the corresponding pulmonary NAVL. As with the other organs, the tissues are held together with fibrous connective tissue (FCT).

a. The lungs are located within individual serous cavities, called the pleural cavities. The lungs with their pleural cavities constitute the major contents of the thoracic cavity. The pleural cavities help to provide lubrication.

b. Located in the middle of the thorax, between the two pleural cavities, is the mediastinum ("I stand between"). The mediastinum is a tissue- and organ-filled space. Within it, the heart (of the blood circulatory system) is located at the same level as the lungs.

The two lungs occupy their respective sides of the thoracic cavity.

a. The left lung tends to be smaller. This makes room for the extension of the heart into the left side of the thorax.

b. In general, the right lung is divided into three major lobes. The left lung is in two major lobes.

c. Due to the branching pattern of the respiratory tree (and associated NAVL), each lung consists of broncho pulmonary segments--10 in the right lung and 8 in the left lung.

Surrounding each lung individually is a serous cavity, called the pleural cavity. The minute quantity of serous fluid in the cavity serves as a lubricant. This serves to minimize friction for the expansion and contraction of the lungs during breathing.

a. Each lung is intimately covered with a serous membrane, the visceral pleura.

b. The outer wall of the pleural cavity is lined with another serous membrane known as the parietal pleura. Areas of the parietal pleura are variously named according to their location.

(1) The mediastinal pleura forms the lateral wall of the mediastinum.

(2) The diaphragmatic pleura covers the superior surface of the diaphragm.

(3) The costal pleura lines the inner surface of the rib cage.

(4) The cupolar pleura is a dome-like extension into the root of the neck. It contains the apex of the lung.

c. When each lung is in its smaller volume, its corresponding diaphragmatic pleura lies close to the lower costal pleura. The slit-like cavity between them is called the costophrenic sinus. Fluids of each pleural cavity tend to collect in this sinus, since it is the lowest area for each. When the diaphragm contracts and flattens out, each costophrenic sinus opens up and the inferior portion of the expanding lung occupies this space.

As we have seen, breathing is a combination of many factors. These factors are integrated and controlled by the nervous system.

a. Respiratory reflexes are controlled by the respiratory center found in the medullary portion of the hindbrainstem. ( See lesson 12). The level of carbon dioxide (CO₂) in the circulating blood is one of the major influences upon the respiratory reflex.

b. The individual intercostal nerves innervate the intercostal muscles.

c. The muscles attached to and moving the rib cage are innervated by their appropriate nerves. (Ultimately, almost every muscle in the body may be mobilized to assist in breathing.)

d. The diaphragm is innervated by its own individual pair of phrenic nerves.

There are essentially two blood supplies for the lungs--nutrient blood and functional blood. Nutrient blood is carried by the bronchial arteries from the thoracic aorta. Nutrient blood provides nourishment and oxygen to the tissues of the lung. Functional blood is actually involved in the respiratory exchange of gases between the alveoli and the capillaries. Functional blood is brought to and from the lungs by the pulmonary cycle of the cardiovascular system.

a. The pulmonary cycle originates in the right ventricle of the heart. Contraction of the right ventricle forces the blood into the pulmonary arch, which divides into the right and left pulmonary arteries to their respective lungs. Paralleling the branching of the respiratory tree, the arteries divide and subdivide within the lungs. These arteries lead to capillaries in the vicinity of the alveoli. The walls of these capillaries are thin enough to accommodate the passage of gases to and from the alveolus.

b. The blood, now saturated with oxygen, is collected by the pulmonary venous system. The blood is deposited ultimately into the left atrium of the heart.

a. Gases Involved.  Oxygen and carbon dioxide are the primary gases involved in respiration. Under special circumstances, nitrogen may also be of concern.

b. Pressure Gradients.  A gas moves from an area where its pressure is greater to an area where its pressure is less. Thus, the movement of gases depends upon such pressure gradients.

c. External Respiration.  At the alveoli, gases are exchanged between the air inside and the blood in the adjacent capillaries.

d. Internal Respiration.  Within the body, gases are exchanged between the blood of the capillaries and the individual cells of the body.

e. Transportation of Gases.  The gases are transported (Figure 7-5) between the alveoli and the individual cells by the cardiovascular system.

(1) Some of the gases are dissolved directly in the plasma of the blood.

(2) However, in humans, the greater percentage of the gases is carried within the substance of the RBCs (red blood cells, erythrocytes). The RBC, found in great numbers in the blood, is specially constructed for transporting the gases. Hemoglobin, a substance found within RBCs, has a great affinity for oxygen. Yet, the hemoglobin can readily give up the oxygen wherever it is needed.

Figure 7-5. Scheme of the exchange of the gases.

When an individual stops breathing, he will soon die if the tissues of the body, particularly the brain, do not get a fresh supply of oxygen.

a. Various mechanical devices are sometimes used to maintain breathing. One is the pulmotor.

b. In "mouth-to-mouth" resuscitation, the operator forces air from his own respiratory system into the respiratory system of the patient. Fortunately, the initial air forced into the patient is the "dead air" of the operator and still has its full amount of oxygen.

c. There are also various techniques for manipulating the patient's rib cage to simulate normal function.

d. At times, gravity may be used to assist a patient. In particular postures, a patient may find breathing easier. Also, under certain circumstances, a patient may be positioned to drain accumulated fluids from specific parts of the lungs.