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Anatomy of the Respiratory System (p. 842)

1. Respiration includes ventilation of the lungs, gas exchange with the blood, and oxygen use by the tissues.

2. The conducting division of the respiratory system consists of the nose, pharynx, larynx, trachea, bronchi, and most bronchioles; it serves only for airflow.

3. The respiratory division consists of the alveoli and other distal gasexchange regions of the lungs.

4. The nose extends from the anterior nares to the posterior nares and is divided by the nasal septum into right and left nasal fossae.

5. Each fossa has three scroll-like nasal conchae covered with a ciliated mucous membrane. The conchae warm, humidify, and cleanse the air flowing over them.

6. The pharynx is a muscular passage divided into nasopharynx, oropharynx, and laryngopharynx.

7. The larynx is a cartilaginous chamber beginning superiorly at the glottis and ending about 4 cm below this at the trachea. It contains the vocal cords and keeps food and drink out of the airway. Intrinsic muscles of the larynx function in speech and its extrinsic muscles help to close off the larynx during swallowing.

8. The trachea is a 12-cm tube, supported by cartilaginous rings, ending where it branches inferiorly into the two primary bronchi. The ciliated mucosa of the trachea acts as a mucociliary escalator to remove inhaled debris, stuck in the tracheal mucus, from the respiratory tract.

9. Each lung is a conical organ extending from the superior apex to the inferior, broad base. The left lung is divided into two lobes and the right lung into three.

10. One primary bronchus supplies each lung; it divides into one secondary bronchus for each lobe of the lung, and this divides into smaller tertiary bronchi.

11. Bronchioles are finer divisions of the airway lacking cartilage. The smallest members of the conducting division are the terminal bronchioles. Beyond this, thin-walled respiratory bronchioles begin the respiratory division. Respiratory bronchioles have alveoli along their walls and branch distally into alveolar ducts.

12. An alveolus is a thin-walled sac surrounded by a basket of blood capillaries. It is composed of squamous and great alveolar cells and contains alveolar macrophages, the last line of defense against inhaled debris. Alveoli are the primary site of gas exchange with the blood.

13. The surface of each lung is a serous membrane called the visceral pleura. It continues as the parietal pleura, which lines the inside of the rib cage. The space between the pleurae is the pleural cavity, and is lubricated with pleural fluid. The pleurae reduce friction during breathing, contribute to the pressure gradients that move air into and out of the lungs, and help compartmentalize the thoracic cavity.

Mechanics of Ventilation (p. 850)

1. Airflow is governed by the relationship of pressure and resistance.

2. The average atmospheric (barometric) pressure at sea level is 760 mmHg

3. Inspiration is achieved by a muscular effort that increases the volume of the lungs. This reduces the intrapulmonary pressure (according to Boyle's law) to a few mmHg below atmospheric pressure, and air flows down its pressure gradient from the atmosphere into the lungs.

4. Expiration occurs when elastic recoil of the thoracic cage reduces lung volume and increases intrapulmonary pressure to a few mmHg above atmospheric pressure. Air then flows down its pressure gradient from the lungs into the atmosphere.

5. Inspiration is achieved primarily by contractions of the diaphragm and external intercostal muscles. Other muscles aid in deep inspiration. Expiration is achieved primarily by elastic recoil of the thoracic cage, but the internal intercostals, abdominal muscles, and other muscles aid in deep or rapid expiration.

6. Airflow is inversely related to resistance in the airway. High pulmonary compliance means that the lungs expand easily and resistance is minimal. Compliance is reduced in such diseases as tuberculosis and black lung disease, which stiffen the lungs.

7. Resistance also varies with the diameter of the airway. Bronchoconstriction increases resistance and reduces airflow; bronchodilation increases airflow. Asthma and anaphylaxis can cause fatal bronchoconstriction.

8. Alveolar surface tension also affects resistance by tending to cause alveolar collapse during expiration and resisting inflation during inspiration; but surface tension is normally minimized by a lipoprotein pulmonary surfactant secreted by the great alveolar cells. Surfactant deficiency is the cause of respiratory distress syndrome in premature infants.

9. At rest, an average adult inhales about 500 mL of air in one inspiration. About 150 mL of this is dead air, filling the conducting division (anatomic dead space) where no gas exchange occurs; 350 mL ventilates the alveoli. This quantity times the respiratory rate is the alveolar ventilation rate (for example, 350 mL/breath X 12 breaths/min = 4,200 mL/min).

10. In addition to gas exchange, breathing serves the purposes of speaking, laughing, crying, yawning, hiccuping, expelling noxious fumes, coughing, sneezing, and expelling abdominal contents (by means of the Valsalva maneuver).

Saladin: Anatomy & I 22. The Respiratory System I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 22 The Respiratory System 875

11. Pulmonary function is measured with a spirometer, which quantifies various respiratory volumes and capacities (see table 22.2) and can help the clinician assess the severity of restrictive and obstructive disorders of the respiratory system. Restrictive disorders reduce pulmonary compliance and obstructive disorders reduce the speed of airflow.

Neural Control of Ventilation (p. 857)

1. The respiratory rhythm is governed by pacemakers in the brainstem which control the respiratory muscles.

2. The medulla oblongata contains two respiratory nuclei. One of these, the inspiratory center, consists mostly of inspiratory (I) neurons. Firing of these neurons ultimately stimulates the diaphragm (via the phrenic nerves) and external intercostal muscles (via the intercostal nerves) and causes inspiration.

3. The expiratory center of the medulla has both I neurons and expiratory (E) neurons. It is not employed in normal relaxed breathing (eupnea), but inhibits the inspiratory center when deep expiration is needed.

4. The pons contains an apneustic center which seems to prolong inspiration, and a pneumotaxic center which acts on the inspiratory center of the medulla to vary the rate and depth of breathing.

5. The brainstem respiratory centers receive input from the limbic system, hypothalamus, and frontal lobe of the cerebrum, enabling mental states to affect breathing.

6. They also receive input from chemoreceptors in the arteries and from receptors in the airway and lungs that respond to airborne irritants, stretching of the lungs, and other stimuli.

Gas Exchange and Transport (p. 859)

1. Air is composed of nearly 79% N2, 21% O2, 0.5% H2O, and 0.04% CO2. The concentrations of these gases are also expressed as partial pressures, the fraction that each contributes to the total atmospheric pressure (see Dalton's law).

2. Expired air shows changes that result from what the body adds to and takes from the inhaled air: it is about 75% N2, 14% O2, 6% H2O, and 5% CO2.

3. At the air-water interface in the alveoli, gases diffuse down their concentration gradients at rates determined by their solubility in water and partial pressures in the alveolar air and blood (see Henry's law). The blood thus unloads CO2 into the alveolus, to be expired, and loads O2 to be carried to other tissues of the body.

4. The efficiency of alveolar gas exchange depends on the concentration gradients of the gases (air vs. blood), solubility of the gases in water, thickness of the respiratory membrane between the blood and alveolar air, and ventilation-perfusion coupling.

5. Ventilation-perfusion coupling is the tendency of the lungs to direct the most blood to the best-ventilated parts of the lungs, and direct the most air to the best-perfused parts of the lung. The lungs thus minimize wasteful ventilation of poorly perfused areas of the lung and wasteful blood circulation to poorly ventilated areas.

6. About 1.5% of the O2 in the blood is dissolved in the plasma, and 98.5% is bound to hemoglobin in the RBCs. Each hemoglobin can carry up to 4 O2. It is called oxyhemoglobin (HbO2) if it carries one or more O2 molecules.

7. The relationship between oxygen concentration (PO2) and percent HbO2 is the oxyhemoglobin dissociation curve (fig. 22.21). It shows that binding of the first oxygens to hemoglobin accelerates the binding of more O2, until the Hb becomes saturated.

8. About 90% of the CO2 in the blood is carried as bicarbonate (HCO3~) ions, 5% is bound to proteins as carbamino compounds, and 5% is dissolved in the blood plasma.

9. The loading of CO2 from the tissue fluids is promoted by carbonic anhydrase, an enzyme in the RBCs that promotes the reaction of CO2 and water to form carbonic acid. The carbonic acid breaks down to HCO3~ and H+. Most of the H+ binds to hemoglobin, while the HCO3~ is exchanged for Cl~ from the blood plasma.

10. This binding of H+ to hemoglobin promotes the unloading of O2 to the systemic tissues. In one pass through the capillaries of a resting tissue, the blood gives up about 22% of its O2 to the tissue (the utilization coefficient).

11. In the alveoli, Hb unloads O2. This unloading causes H+ to dissociate from the Hb and recombine with HCO 3 to produce carbonic acid. The carbonic acid is then broken down by carbonic anhydrase into water and CO2. The latter is exhaled.

12. Hemoglobin unloads varying amounts of O2 to different tissues according to their needs. Hemoglobin adjusts O2 unloading in response to variations in the tissue's PO2, temperature, and pH (the Bohr effect), and the RBC's own temperature- and hormone-sensitive concentration of bisphosphoglycerate (BPG).

Blood Chemistry and the Respiratory

1. Breathing is stimulated especially by the pH of the body fluids, but also by the PCO2 and to some extent PO2. These conditions are monitored by central chemoreceptors in the brainstem and peripheral chemoreceptors in the aortic arch and carotid arteries.

2. Ultimately, breathing is adjusted to maintain a stable pH in the brain. Blood pH is determined largely by PCO2 because of the reaction of CO2 and water: CO2 + H2O ^ H2CO3 ^ HCO3~ + H+. The more CO2 is present, the more H+ is generated and the lower the pH is; the less CO2, the higher the pH.

3. Normally, the blood pH ranges from 7.35 to 7.45. A pH below this range is called acidosis and a pH above this range is alkalosis.

4. Acidosis is usually caused by a CO2 excess (hypercapnia) and can therefore be corrected by increasing pulmonary ventilation to expel more CO2.

5. Alkalosis is usually caused by a CO2 deficiency (hypocapnia) and can therefore be corrected by reducing pulmonary ventilation to allow for metabolically generated CO2 to build up in the blood.

6. PCO2 can also have a direct effect on ventilation even when the pH is stable.

7. Po 2 normally has little effect on ventilation, but long-term hypoxemia (Po 2 < 60 mmHg) can trigger hypoxic drive, in which ventilation is driven more by O2 than CO2 levels. This can occur in such conditions as emphysema and mountain climbing.

Saladin: Anatomy & I 22. The Respiratory System I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

876 Part Four Regulation and Maintenance

Respiratory Disorders (p. 868)

1. Hypoxia, a deficiency of O2 in the tissues, can be of hypoxemic, ischemic, anemic, or histotoxic origin. It can cause cyanosis and, if severe and prolonged, tissue necrosis.

2. Oxygen excess can generate hydrogen peroxide and free radicals that cause oxygen toxicity.

3. The chronic obstructive pulmonary diseases (COPDs) are asthma, chronic bronchitis, and emphysema. Asthma is an allergic disease while the others are usually caused by tobacco smoke. Chronic bronchitis entails congestion of the airway with thick mucus, and susceptibility to respiratory infection. Emphysema entails destruction of pulmonary alveoli and air retention in expiration.

4. Lung cancer also is usually caused by tobacco smoke. Its variations are squamous-cell carcinoma, adenocarcinoma, and small-cell carcinoma. It tends to metastasize rapidly.

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A Disquistion On The Evils Of Using Tobacco

A Disquistion On The Evils Of Using Tobacco

Among the evils which a vitiated appetite has fastened upon mankind, those that arise from the use of Tobacco hold a prominent place, and call loudly for reform. We pity the poor Chinese, who stupifies body and mind with opium, and the wretched Hindoo, who is under a similar slavery to his favorite plant, the Betel but we present the humiliating spectacle of an enlightened and christian nation, wasting annually more than twenty-five millions of dollars, and destroying the health and the lives of thousands, by a practice not at all less degrading than that of the Chinese or Hindoo.

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