Unit 20 - The Respiratory System

 

The respiratory system consists of the lungs and the tubes which connect the lungs to the atmosphere. The respiratory system is another one of the homeostatic systems. It works in conjunction with the circulatory system to supply the cells with oxygen and remove carbon dioxide from them.

 

A. Stages of respiration - There are five distinct stages of respiration that can be recognized.

 

1. Pulmonary ventilation or the movement of air into and out of the lungs.

 

2. External respiration - Exchange of oxygen and carbon dioxide between the blood and the lungs.

 

3. Transport of oxygen and carbon dioxide in the blood.

 

4. Internal respiration - Exchange of oxygen and carbon dioxide between the blood and tissues.

 

5. Cellular respiration - Utilization of oxygen by the cells and the production of carbon dioxide by the cells.

 

B. Anatomy of the respiratory system The respiratory system consists of the nose and nasal cavity, pharynx, larynx, trachea, bronchi, and lungs. It is divided into two functional areas. The respiratory zone is the site of gas exchange and consists of the respiratory bronchiole, alveolar ducts, and alveoli. The conducting zone includes all other respiratory passageways.

 

1. Nasal cavity - The cavity begins with the external nares through which air enters into the cavity proper (vestibule). The vestibule is divided into right and left halves by the nasal septum.

 

a. The floor of the cavity is composed of the anterior hard palate, and the more posterior soft palate. The palate separates the nasal cavity from the mouth.

 

b. The lateral walls are formed by the irregular nasal conchae bones. The superior and middle conchae are part of the ethmoid bone while the inferior conchae are separate bones. The entire nasal cavity is lined by moist mucous membrane which warms, moistens, and cleans the air. The nasal conchae increase the surface area to which the air is exposed.

 

c. Paranasal sinuses - These are membrane lined cavities found in the frontal, sphenoid, and maxillae bones. They open and drain into the nasal cavity.

 

d. Nasolacrimal ducts drain tears from the surface of the eyes into the nasal cavity.

 

e. Air exits the nasal cavity into the pharynx via the internal nares.

 

2. Pharynx - This is a muscular tube (throat region) which is used by both the respiratory and digestive tracts. It is divided into three regions.

 

a. Nasopharynx - This portion lies immediately behind the nasal cavity.

 

(1) The auditory (eustachian) tubes from the middle ear open into it.

 

(2) The pharyngeal tonsils (lymphoid tissue) are located in its posterior wall.

 

(3) The soft palate and its extension, the uvula, form its floor.

 

b. Oropharynx - This part of the pharynx extends from the soft palate to the laryngopharynx.

 

(1) The mouth opens into the oropharynx.

 

(2) The lingual tonsils are found in this region at the base of the tongue.

 

(3) The palatine tonsils are found on the sidewalls of the oropharynx.

 

c. Laryngopharynx - This extends from the oropharynx to the larynx and esophagus. Like the oropharynx it is a common passageway for both air and food.

 

3. Larynx - This structure connects the laryngopharynx to the trachea below. It is composed of nine cartilages. One of these is the epiglottis, a leaf shaped cartilage that guards the opening into the larynx. During swallowing the larynx moves upward toward the epiglottis, causing it to tilt and thereby deflect food into the esophagus behind and below the larynx. Other points of interest on the larynx include the following.

 

a. Glottis - This is the opening into the larynx. It is flanked by two horizontal folds of the mucous membrane. The upper folds form the false vocal cords and the lower fold form the true vocal cords.

 

b. Vocal cords - These folds vibrate when air moves across them and produce sound. These cords are thicker in males than in females and therefore the male voice has a lower pitch. The pitch of the vocal cords can be altered by adjusting the diameter of the opening of the glottis or by altering the tension on the cords by muscle contraction.

 

(1) Laryngitis - This is inflammation of the mucous membrane which covers the cords. It may be due to infection or overuse. It can result in hoarseness or total voice loss.

 

4. Trachea - This is the beginning of the lower respiratory tract. The stratified squamous epithelium that characterized the tract in the nasal cavity, pharynx, and larynx, gives way to pseudostratified ciliated columnar epithelium. The trachea is about eleven centimeter long and is reinforced by "C" shaped rings of cartilage which prevent its collapse. The trachea divides into the left and right primary bronchi.

 

5. Bronchi, bronchioles, and alveoli

 

a. The primary bronchi divide to form the secondary bronchi, one for each lobe of the lung.

 

b. Secondary bronchi divide to form the tertiary bronchi. Each tertiary bronchus supplies a specific region of the lung termed a bronchopulmonary segment. There are ten such segments in the right lung and eight or nine in the left lung. The tertiary bronchi keep branching within each bronchopulmonary segment and ultimately give rise to the bronchioles. All of the bronchi contain cartilage. The bronchioles do not contain cartilage, and the pseudostratified ciliated columnar epithelium that lines the bronchi is replaced in the bronchioles by ciliated columnar epithelium, and then by cuboidal epithelium.

 

c. Each bronchiole enters into a lobule of the lung. The lobule may be thought of as a respiratory unit composed of bronchioles. air sacs, pulmonary blood vessels, and separated from other lobules by connective tissue partitions known as septa. The bronchiole that enters the lobule is known simply as a bronchiole. It give rise to still smaller bronchioles known as the terminal bronchioles. The epithelium lining the terminal bronchiole is cuboidal and lacks cilia. Each terminal bronchiole gives rise to respiratory bronchioles.

 

d. Respiratory bronchioles terminate in alveolar ducts. These are shaped like long hallways with many openings off the side. These openings are the alveoli or air sacs. Exchange can occur across the respiratory bronchioles, alveoli and the alveolar ducts.

 

(1) Respiratory membrane - The wall of each alveolus is composed of the respiratory membrane. It is across here that exchange of gases between the blood and atmosphere occurs. The respiratory membrane has the following parts.

 

(a) A simple squamous epithelium. This epithelium consists of type I or standard epithelial cells and a lesser number of type II or septal cells. Septal cells are also termed surfactant cells after the substance which they produce. In addition, roaming alveolar macrophages wander around the epithelium phagocytizing particulate matter.

 

(b) A basement membrane upon which the epithelium rests.

 

(c) An endothelium from the pulmonary capillary.

 

e. The entire bronchial tree up to the bronchioles is reinforced by cartilage. As one moves deeper into the tree the plates of cartilage become smaller and fewer, and the smooth muscle component becomes greater. The bronchioles lack cartilage completely.

 

6. Lungs - The lungs consist of all of the alveoli, bronchial tubes up to, and including, the secondary bronchi, and blood vessels. In addition there is a large amount of connective tissue which supports all of these structures. Much of the connective tissue is rich in elastic fibers and this gives the lungs an elastic recoil that is important in their functioning.

 

(1) Lobes - The lungs are divided into lobes with each lobe being supplied by a secondary bronchus. The right lung has three lobes and the left lung has two lobes.

 

(2) Pleura - Each lobe is enclosed in a double walled serous membrane termed the pleural membrane. The visceral pleura adheres tightly to the lung while the parietal pleura lines the pleural cavity. Between the membranes is a thin layer of serous fluid. This fluid lubricates during lung movements and also couples the two plural membranes together by surface tension. This effectively couples the lungs to the wall of the thoracic cavity which aids during ventilation.

 

C. Mechanism of ventilation - Ventilation is broken down into inspiration (inhalation) and expiration (exhalation). The movement of air into and out of the lungs is due to pressure differences between the lungs and the atmosphere. Air, like all fluids, moves from areas of high pressure to areas of lower pressure.

 

1. Atmospheric pressure - This is the pressure hat the weight of the air produces. It varies depending upon altitude, but at sea level it is equal to 760mm/Hg. For purposes of discussion, the normal atmospheric pressure will be considered to represent a base line of 760mm/Hg.

 

2. Anatomy of the pleural cavity - In order to understand the ventilation process it is necessary to understand the structure of the pleural cavity. The walls of this cavity are composed of the chest wall which is lined by the pleura. The floor of the cavity is composed of the diaphragm, a broad, flat muscle used in breathing. The space inside of the cavity is referred to as the intrapleural space. The pleural cavity is sealed away from the atmosphere. The lungs are contained within the pleural cavity. The space inside of the lungs is referred to as the intrapulmonary space. The space communicates with the atmosphere.

 

3. Inspiration - Normally the intrapleural pressure is slightly negative with respect to atmospheric pressure. The range during normal quite breathing is about 754 to 756 mm/Hg (- 6 to -4 mm/Hg. During inspiration the following events occur.

 

a. The volume of the pleural cavity increases. This is brought about the contraction of the diaphragm, which pulls the pleural cavity downward. If breathing is forced then the external intercostal muscles contract and raise the rib cage, increasing the volume even more.

 

b. As the pleural cavity does not communicate to the outside, the increase in volume causes a decrease in intrapleural pressure.

 

c. The reduction in intrapleural pressure on the outside of the lungs causes a corresponding decrease in intrapulmonary pressure within the lungs. The intrapulmonary pressure drops below atmospheric pressure and air flows into the lungs from the higher atmospheric pressure to the lower intrapulmonary pressure.

 

4. Expiration - During quite breathing this is a passive process and is basically the reverse of inspiration.

 

a. The inspiratory muscles relax thereby returning the pleural cavity to its original volume.

 

b. Decreased volume leads to increased intrapleural pressure which in turn leads to increased intrapulmonary pressure. The elastic recoil of the lungs adds to this pressure. The overall result is that the intrapulmonary pressure swings positive (about 763mm/Hg or +3mm/Hg). Air then moves to the lower pressure outside of the lungs.

 

c. Forced expiration can occur by contraction of the internal intercostal muscles. This forces the rib cage downward, decreasing the pleural cavity volume even more.

 

5. Summary of pressure changes during the respiratory cycle

 

 

INSPIRATION EXPIRATION

 

Intrapleural Intrapulmonary Intrapleural Intrapulmonary

 

754 mm/Hg 758 mm/Hg 756 mm/Hg 763 mm/Hg

-6 -2 -4 +3

 

6. Prevention of lung collapse - The alveoli have a layer of moisture on their inner surface and it would be expected that during expiration the alveoli would collapse and stick together due to surface tension. This normally does not happen for two reasons.

 

a. The lungs are usually partially inflated even during expiration. The adherence of the visceral pleural membrane to the parietal pleural membrane insures a partial inflation.

 

b. There is a chemical substance known as surfactant which reduces the surface tensions with the alveoli. This is produced by the septal cells. Chemically, surfactant is a phospholipoprotein.

 

D. Lung volumes - The air volumes associated with the respiratory cycle have been compartmentalized. These volumes can be used for diagnosis of respiratory difficulties. The volumes along with their typical values are listed below.

 

1. Total lung capacity - This is the total volume of air that the lungs can hold. Typically it is 5900 ml in men and 4400 ml in women.

 

2. Residual air volume - The amount of air left following a maximum expiration. This is the volume of air that cannot be exhaled. It is usually 1200 ml in males and 1000 ml in females.

 

3. Vital capacity - The total amount of air that can be moved in a respiratory cycle. It is equal to the total capacity minus the residual air volume. In males it is about 4700 ml and 3400 ml in females. Vital capacity can be subdivided into three components.

 

a. Tidal volume - The amount of air normally inspired during quiet breathing. It is usually about 500 ml in both sexes.

 

b. Inspiratory reserve - This is the volume that can be forcefully inspired following normal inspiration. It is about 3000 ml in men and 2100 in women.

 

c. Expiratory reserve - This is the volume that can be forcefully expired following a normal expiration. It is about 1200 ml in men and 800 ml in women.

 

4. Dead air space - All exchange occurs across the alveoli. The remainder of the bronchial tree and other parts of the respiratory system constitute dead air space, usually about 150 ml.

 

a. This volume must be subtracted from the tidal volume to get a true picture of how much atmospheric air enters the alveoli.

 

500 ml (tidal volume) - 150 ml (dead air space) = 350 ml

 

5. Minute respiratory volume - This is the amount of air moved into the respiratory tract in one minute. It is equal to the tidal volume times the respiratory rate. An average or typical value would be as follows.

 

500 ml (tidal volume) X 12 (respiratory rate) = 6000 ml

 

6. Alveolar ventilation - This is the actual amount of atmospheric air which enters the alveoli per minute. As it is necessary to subtract the dead air space from the tidal volume to determine this, the typical alveolar ventilation becomes as follows.

 

350 ml X 12 = 4200 ml (quiet breathing)

 

a. During exercise it is necessary to increase the alveolar ventilation. This is accomplished by increasing both rate and depth of breathing, but the increase in depth is more important.

 

E. Respiratory terminology - There are six terms that are frequently used by both respiratory physiologists and clinicians. They and their meanings are as follows.

 

1. Pnea - breathing

 

2. Eupnea - normal breathing

 

3. Apnea - no breathing

 

4. Dyspnea - labored breathing

 

5. Hyperpnea - increased depth of breathing

 

6.          Hyperventilation rapid deep breathing

 

7. Hypopnea slow, shallow breathing

 

F. Control of respiration - The rate and depth of ventilation varies with the oxygen demands of the body. As we have previously seen with both cardiac output and blood pressure, there are both neural and chemical control mechanisms.

 

1. Neural control - The respiratory muscles which are responsible for inspiration and expiration are under the control of the nervous system. These muscles are striated or voluntary, but the integrating centers are only partially under conscious control. The integrating centers are located in the medulla and pons regions of the brain.

 

a. Major control centers - There are two physiological centers located in the reticular formation of the medulla. These are the inspiratory and expiratory areas. Anatomically they may be the same structure and are frequently referred to collectively as the medullary rhythmicity area.

 

(1) Inspiratory area (Dorsal respiratory group, DRG) - This center initiates inspiration by sending nerve impulses to the diaphragm and external intercostal muscles. It also sends a signal which stimulates the expiratory center during forced breathing.

 

(2) Expiratory area (Ventral respiratory group, VRG) - This center is normally not active during normal breathing. Expiration is a passive process during quiet breathing and results as the cessation of activity of the inspiratory center. During forced or deep breathing the expiratory center becomes active due to stimulation by the inspiratory center. It sends signals to the internal intercostals forcing expiration.

 

b. Modulating subcenters - There are two centers located in the pons that act as modifiers of the medullary rhythmicity area.

 

(1) Pneumotaxic center - This sends inhibitory signals to the inspiratory center that limit the period of inspiration.

 

(2) Apneustic center - This center sends excitatory signals to the inspiratory center which prolongs inspiration. It is normally overridden b the pneumotaxic center.

 

c. Stretch receptors (Hering-Breuer reflex) - These are located in the lungs and respond when the lungs expand and contract . During deep breathing they send back impulses that inhibit the inspiratory center (inflation reflex) and prohibit over inflation. During exhalation they inhibit the expiratory center and stimulate the inhibitory center preventing lung collapse (deflation reflex). They do not seem to play a major role during normal breathing.

 

d. Cerebral effects - The cerebral cortex can modify the basic respiratory rate established by the medullary centers, but it cannot override them. Although you can consciously hold your breath, you cannot commit suicide that way because you will reach what is known as the "breaking point" and will then breath regardless of your desires.

 

2. Chemical control - Although it is the nervous center operating the respiratory muscles that actually cause ventilation to occur, ultimately the center is under chemical control. There are two major chemical factors involved in the regulation of respiration. They are the concentrations of carbon dioxide and oxygen in the arterial blood.

 

a. Carbon dioxide - the direct effect - Carbon dioxide in the arterial blood diffuses into the cerebrospinal fluid. In this fluid it undergoes the following reaction.

 

C02 + HOH = H2C03 = H+ + HCO3-

 

The hydrogen ion produced by this reaction reacts with pH sensitive cells in the medulla which then excite the inspiratory center. This is the source of excitation for the inspiratory center. Consequently, carbon dioxide is considered to be the major factor controlling respiration.

 

(1) When you hold your breath carbon dioxide levels in the body begin to rise. At such time as the carbon dioxide levels reach a critical point, the "breaking point," you will automatically breath.

 

(2) It is possible to extend the time period before the breaking point by hyperventilating for a short period of time prior to hold your breath. The rapid deep breathing effectively reduces carbon dioxide below normal levels and it therefore takes longer to build up to the breaking point. This is also why when people become agitated and start hyperventilating, they often have difficulty breathing.

 

b. Oxygen - indirect effect - There are two chemoreceptors known as the aortic and carotid bodies (located in the walls of the respective arteries) which are sensitive to arterial oxygen concentration. If the oxygen concentration drops below normal then a nervous reflex from these receptors will speed up the activity of the inspiratory center and therefore respiration.

 

(1) Under normal conditions the arterial blood is saturated with oxygen and the receptors play no role in regulating respiration. At high altitude where oxygen levels become low, the blood will no longer be saturated and the receptors will cause a speed up in ventilation rate.

 

c. pH - If the arterial blood pH decreases this will stimulate the aortic and carotid bodies which in turn will increase ventilation rate.

 

G. Special reflexes - These are reflexes which involve the respiratory system in some manner.

 

1. Swallowing - The common passage of food and air through the oropharynx and laryngopharynx pose a problem for breathing. There is a special reflex that avoids the passage of food materials into the lower respiratory tract.

 

a. Swallowing activates stretch receptors located in the wall of the pharynx.

 

b. The stretch receptors activate a motor reflex which causes the larynx to be raised until, the epiglottis forms a roof over the glottis.

 

c. The epiglottis deflects food over the glottis and into the esophagus.

 

2. Cough - This a reflex to clear the lower respiratory tract of foreign material. The process is as follows.

 

a. A short inspiration occurs after which the glottis is closed off by the epiglottis.

 

b. A forceful contraction of the expiratory muscles occurs.

 

c. The glottis is released and the air in the lungs is violently expelled, hopefully, removing the foreign material with it.

 

d. A sneeze is basically the same process but directed through the nasal cavity for the clearing of the upper respiratory tract.

 

3. Valsalva - This is a reflex that creates a pneumatic support when strenuous activities such as lifting heavy weights are attempted. The process is as follows.

 

a. A deep breath is taken and held.

 

b. The expiratory muscles are contracted.

 

c. The effect is to create a very high thoracic pressure which may be "pushed" against when doing something strenuous.

 

H. Transport of respiratory gases

 

1. Exchange of gases between blood and alveoli and between the blood and tissues

 

a. Partial pressure - Dalton's law states that the total pressure exerted by a mixture of gases is equal to the sum of the pressure exerted by each gas.

 

(1) Atmospheric pressure at sea level is equal to 760 mm/Hg. Oxygen makes up 21% of the atmosphere and therefore the partial pressure of oxygen is .21 X 760 or l60 mm/Hg.

 

(2) Concentrations of gases are always expressed as partial pressures even when they are in the blood stream. This is expressed as P02 and PC02.

 

(3) The concentration of a gas in a liquid is directly proportional to its partial pressure (Henry's Law). Of course the ultimate concentration also depends upon the solubility constant (how well it dissolves). For example, carbon dioxide dissolves much more readily in water than does oxygen. This means that if carbon dioxide and oxygen have the same partial pressure, there will be a much higher concentration of carbon dioxide in solution than oxygen. However, as it is easier to speak of concentrations than partial pressures, and so for purposes of the discussion the concentration of gases will be used.

 

b. Movement of gases across the alveoli - Gas movement is due to diffusion down concentration gradients.

 

(1) Oxygen diffuse into the pulmonary capillaries from the alveoli because the concentration of oxygen is much higher in the alveoli than in the pulmonary blood.

 

(2) Carbon dioxide diffuses into the alveoli from the pulmonary capillaries because carbon dioxide is in much higher concentration in the pulmonary blood than in the alveoli.

 

c. Exchange at the tissues - Diffusion is again the mechanism, with each gas moving down its own concentration gradient.

 

(1) Oxygen is in higher concentration in the blood than in the tissues where it is consumed. Oxygen therefore diffuses from the blood into the tissues.

 

(2) Carbon dioxide diffuses from the tissues where it is produced and is therefore in higher concentration into the blood.

 

2. Transport of the gases in the blood - Once in the blood, the gases must be transported, either from the lungs to the tissues or from the tissues to the lungs.

 

a. Oxygen transport - The bulk of oxygen (97%) is transported combined with hemoglobin. As hemoglobin is located inside of the red blood cells, oxygen must first diffuse from the alveoli into the plasma and then into the red blood cells.

 

(1) Oxygen combines with hemoglobin to form a compound known as oxyhemoglobin.

 

(2) Each gram of hemoglobin can combine with 1.34 ml of oxygen. On average there is 15 grams of hemoglobin per l00 ml of blood and therefore approximately 20 ml of oxygen can be carried per l00 ml of blood. This is referred to as 20 volumes percent.

 

(3) Oxygen associates with hemoglobin in such a manner that hemoglobin will normally be completely saturated with oxygen at the lungs. The hemoglobin holds on to this oxygen almost completely until it reaches the tissues where oxygen is quite low. There is releases almost all of its oxygen at once which will then diffuse into the tissues.

 

(4) Oxygenated hemoglobin is bright red. Deoxygenated hemoglobin is dark purple. This is why deoxygenated blood that is draw from a vein has a blue-purple color instead of the bright red that we normally associate with blood which has been exposed to the air.

 

b. Carbon dioxide transport - Carbon dioxide produced by the tissues diffuses into the blood where it is transported to the lungs in three different ways.

 

(1) In solution - Carbon dioxide is much more soluble in water than is oxygen. About 5 to 10% is transported in this way.

 

(2) Combined with hemoglobin - About 10 to 20% will combine with amino groups on the hemoglobin molecule to form a compound called carbaminohemoglobin (HbNH2).

 

(3) As bicarbonate - About 60 to 70% of the total carbon dioxide is transported in the form of bicarbonate. Carbon dioxide reacts with water to form carbonic acid which then dissociates to form hydrogen ion and bicarbonate ion. The reaction is as follows.

 

CO2 + HOH = H2CO3 = H+ + HCO3-

 

(a) This reaction occurs very slowly in the plasma but very rapidly inside of the red blood cell due to the presence of the enzyme carbonic anhydrase. Consequently as fast as carbon dioxide diffuses into the RBC it is converted into carbonic acid.

 

(b) The hydrogen ion combines with hemoglobin and is therefore buffered. The bicarbonate associates with potassium ion inside of the cell.

 

(c) Bicarbonate begins to diffuse out of the cell, but the membrane is impermeable to potassium which cannot follow. The bicarbonate is of course negatively charged and as it leaves it creates a deficit of negative charges inside of the RBC.

 

(d) To compensate for the loss of the negative bicarbonate, a negative chloride ion moves into the RBC every time a bicarbonate moves out. This is known as the chloride shift or Hamburger effect.

 

(e) At the level of the lungs the concentration gradients for carbon dioxide are reversed from those of the tissues. All of the reactions are reversible. Bicarbonate begins to diffuse back into the RBC, chloride out, and carbon dioxide is reformed which then diffuses out into the alveoli.

I. Pathology

 

1. Hypoxia - This is where there is insufficient oxygen to the tissues. It can be brought about a number of different problems such as heart failure, anemia, impaired respiration, and poisoning. Initial symptoms may include euphoria, loss of coordination, memory loss, and unconsciousness.

 

2. Carbon monoxide poisoning - This gas results from incomplete combustion. It has a much higher affinity for hemoglobin than does oxygen and can therefore tie up all of the hemoglobin, even when present in low concentrations. Early symptoms include headache and dizziness.

 

3. Open pneumothorax - This occurs when the chest cavity is punctured and the atmosphere can communicate into it. The pressure inside the cavity becomes the same as the atmosphere and therefore the ability to inflate the lung is greatly impaired. The end result is often a collapsed lung.

 

4. Asthma - This is an allergic response which when triggered results in spastic contraction of the smooth muscle in the bronchioles. This greatly reduces the air passages to the alveoli and results in dyspnea.

 

5. Bronchitis - This is an inflammation of the bronchi. It results in excess sputum production and pain. Sometimes the bronchi can actually be blocked and when this occurs then that portion of the lung which has been blocked off may collapse.

 

6.  Emphysema - This is a condition where the alveoli enlarge and rupture. Upon rupturing they fuse with each other to form increasingly larger sacs. This has he effect of reducing the total surface area available for exchange. In addition, the fusion results in scar tissue which reduces the available diffusion area even more. Finally, there is a loss of elasticity which makes expiration very difficult. Emphysema results from chronic irritation to the lungs. It is very prevalent in smokers.

 

Emphysema and chronic bronchitis are the two major examples of chronic obstructive pulmonarydisease (COPD). It is a major cause of death and disability which is increasing largely due to smoking and air pollution.

 

7. Pulmonary edema - This is the accumulation of fluid in the alveoli and bronchi. The fluid interferes with gas exchange, making breathing difficult. The condition is usually secondary to some other respiratory problem, but may also be due to failure of the left side of the heart (congestive heart failure). The failing left side does not pump all of the blood which is receives and as a result blood begins to back up in the pulmonary circuit, increasing pulmonary pressure, and therefore greater filtration of fluid into the alveoli.

 

 


8. Cystic fibrosis - This is a genetic disease which is characterized by a viscous mucous that clogs the respiratory passageways and predisposes children to infections. The mechanism of this disease is as follows.

 

a. The cilia which cover the respiratory cilia are embedded in a saline solution and the mucous floats on top. In CF patients, the saline solution is not present and the mucous drops down into the cilia so that they no longer function. This means that mucous is no longer moved upward. This results in accumulation of both mucous and infectious agents.

 

b. The source of the saline solution is active transport of chloride by the epithelial cells. The chloride is actively transported across the membrane into the respiratory tubule lumen. Sodium passively follows the electrical attraction of chloride, and water osmotically follows both, yielding the saline solution on which the mucous floats.

 

c. In CF patients, a defective gene results in the production of a defective chloride channel. The channels are normally produced on the ER and transported to the cell membrane by the Golgi apparatus. In CF individuals, channels are produced, but are not transported to the membrane. Therefore no chloride transport is possible and the saline solution is not produced.

 

d. Recently the CF gene has been identified and isolated. Current experiments in rats use an impotent (non-reproducing influenza virus tagged with DNA for normal chloride channels. This cures the mice, at least until the respiratory epithelium is shed. Human inhalers with this combination are being planned.

 

J. Effects of aging - Loss of elasticity with age causes an increase in rigidity of both the chest wall and the lungs. This causes a decrease in ventilating capacity.

 

1. Between ages 20 and 70, a man may go from a vital capacity of 5900 ml to about 4000 ml.

 

2. There is a corresponding loss in arterial PO2 which becomes more pronounced when lying prone where breathing is more difficult. This is why elderly people tend to become hypoxic during sleep and are better off supported by several pillows.

 

3. There is a loss of phagocytic cell activity, and cilia action of the epithelium becomes diminished. Consequently respiratory tract infections are much more common in the elderly.