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THE RESPIRATORY SYSTEM

INTRODUCTION

1. The metabolic operations of living cells consume oxygen and liberate carbon dioxide.

2. The respiratory system includes the nose, nasal cavities and sinuses, the pharynx, larynx, trachea, conducting passageways, and exchange surfaces.

3. The respiratory system provides an extensive area for gas exchange and moves filtered, humidified air to and from those surfaces. The system also plays a role in social communication through the production of sounds.

FUNCTIONAL ANATOMY AND ORGANIZATION

1. The conducting passageways of the upper respiratory tract provide initial filtration and humidify the incoming air. The lower respiratory tract includes delicate conduction passages and the alveolar exchange surfaces.

The Upper Respiratory Tract

The Nose:

1. Air normally enters the respiratory system via the external nares that open into the nasal cavity.The vestibule is guarded by coarse hairs that deny access to large particles.

2. The maxillary, nasal, frontal, ethmoid, and sphenoid bones for the lateral and superior walls of the nasal cavity. The nasal septum divides it into left and right sides, and the hard and soft palates form the floor. Posteriorly the internal nares open into the nasopharynx.

3. The nasal conchae project towards the septum from the lateral walls of the nasal cavity. The spaces between the conchae are the meatuses. These grooves create turbulence that swirls the air against the mucous epithelium lining the nasal cavity.

4. The respiratory epithelium consists of a pseudostratified, ciliated, columnar epithelium with goblet cells. The moving carpet of mucus acts as a particle trap, capturing dust, debris, and microorganisms that might otherwise damage the respiratory surfaces.

5. In the nasal cavity the lamina propria contains abundant blood vessels, and the epithelium and lamina propria form the respiratory mucosa.

The Pharynx:

1. The pharynx is a chamber shared by the digestive and respiratory systems. It extends between the internal nares and the entrances to the larynx and esophagus.

2. The nasopharynx lies superior to the soft palate. It contains the entrances to the Eustachian tubes and the adenoid tonsil.

3. The oropharynx is continuous with the oral cavity. It extends between the soft palate and the base of the tongue at the level of the hyoid bone. The palatine tonsils lie in its lateral walls.

4. The laryngopharynx includes the narrow zone between the hyoid and the entrance to the esophagus.

5. A stratified squamous epithelium lines the pharynx and provides mechanical protection from abrasion, pathogenic organisms, and chemical attack.

The larynx:

1. Inspired air passes through the glottis en route to the lungs. The larynx surrounds and protects the glottal opening.

2. The larynx consists of the thyroid, cricoid, arytenoid, corniculate, and cuneiform cartilages.The epiglottis projects into the pharynx and has a flexible attachment to the thyroid cartilage.

3. Intrinsic ligaments bind the cartilages to one another. Extrinsic ligaments attach the laryngeal cartilages to the hyoid bone and trachea.

4. Two pairs of folds span the glottal opening. The ventricular folds are inelastic but the tension in the vocal folds can be adjusted by voluntary muscular contractions.

5. During expiration air flowing through the larynx vibrates the vocal folds and produces sound waves. The muscles that control the tension in the vocal folds alter the frequency of the sound produced.

6. The larynx performs phonation, while the pharynx, oral cavity, nasal cavity, sinuses, tongue, lips, and cheeks participate in articulation.

7. The intrinsic laryngeal musculature regulates tension in the vocal folds and opens and closes the glottis. The extrinsic laryngeal musculature positions and stabilizes the larynx.

8. During swallowing the epiglottis folds back over the entrance to the glottis.

9. Coughing and laryngeal spasms are protective reflexes that protect the glottis and trachea from foreign objects and irritants.

10. Pharyngeal surfaces of the larynx that are subject to abrasion are covered by a stratified squamous epithelium; other laryngeal surfaces bear a typical respiratory epithelium.

The Trachea:

1. The trachea extends from the level of the sixth cervical vertebra, at the base of the larynx, to the level of the fifth thoracic vertebra.

2. At its caudal limit the trachea divides to form the primary bronchi.

3. The trachea is a tough, flexible tube with a diameter of roughly 2.5 cm (1 inch) and a length of around 11 cm (4.25 inch).

4. The submucosa contains C-shaped tracheal cartilages that encircle the lateral and anterior surfaces of the trachea. The posterior wall is unsupported and tolerates considerable distortion as food passes along the esophagus.

The Primary Bronchi:

1. The trachea branches within the mediastinum, forming the left and right primary bronchi. This histological organization of the bronchi resembles that of the trachea.

2. Each bronchus enters a lung at a groove, the hilus. The root of the lung consists of a connective tissue mass including the bronchus and the pulmonary vessels and nerves.

The Lower Respiratory Tract

1. The lobes of the lung are separated by fissures.The right lung has three lobes and the left has two.

2. The connective tissues of the root extend into the parenchyma of the lung as a series of trabeculae that branch to form the septa that divide the lung into lobules. Each lobule receives tributaries of the pulmonary arteries and veins, and it contains the terminal branches of a respiratory passageway.

Bronchi and Bronchioles:

1. The primary or extrapulmonary bronchi branch to form secondary bronchi. The intrapulmonary bronchi are surrounded by bands of smooth muscle, and the cartilaginous supports are irregular and confined to the larger tributaries.

2. Bronchioles are small passageways with diameters under 1 mm. Each bronchiole supplies a single lobule with terminal and respiratory bronchioles that deliver air to the respiratory surfaces.

Alveolar Ducts and Alveoli:

1. The respiratory bronchioles open into alveolar ducts. Many alveoli are interconnected at each alveolar duct.

2. Alveoli have thin, membranous walls. Septal cells produce surfactant that keeps the alveoli from collapsing. Alveolar macrophages patrol the alveolar epithelium engulfing debris or potential pathogens. A network of capillaries surrounds each alveolus.

3. The respiratory defense system includes the filtration and mucus secreting components of the nasal cavity, the mucus escalator of the upper and lower respiratory tracts, and the alveolar phagocytes.

4. Overloading the respiratory defenses can lead to epithelial changes that promote lung cancer or other conditions.

The Pleural Cavities

1. The thoracic cavity is bounded by the ribcage and the muscular diaphragm. The mediastinum divides the region into two pleural cavities.

2. The visceral pleura covers the surfaces of the lungs, while the parietal pleura lines the opposing surfaces of the body wall, diaphragm, and mediastinum. In life the pleural cavities contain a small quantity of fluid that lubricates these surfaces.

3.Damage to the pleura may reduce the lubrication, producing a pleural rub. The accumulation of excess pleural fluid represents a pleural effusion.

Respiratory Changes at Birth

1. Prior to delivery the fetal lungs are fluid-filled and collapsed. At the first breath the lungs fill, and they never completely empty thereafter. Cartilages keep the conducting passageways open and the surfactant secreted by the septal cells prevents alveolar collapse.

RESPIRATORY PHYSIOLOGY

1. Pulmonary ventilation refers to the movement of air in and out of the lungs. External respiration involves the exchange of gases between the alveolar air and the pulmonary capillaries. Internal respiration occurs when gas diffuses between peripheral capillaries and the interstitial fluids. Cellular respiration refers to the exchange underway between the interstitial fluids and living cells.

2. Hypoxia refers to an inadequate supply of oxygen within peripheral tissues. If the oxygen supply is completely shut off, anoxia results, leading to the death of the tissues. Pulmonary Ventilation

Gas Pressure, Volume, and Temperature:

1. As the pressure increases, the volume decreases; when the pressure decreases, the volume expands. Increased temperature elevates the pressure; decreased temperature reduces it.

2. The inverse relationship between pressure and volume is called Boyle's law. The pressure changes that occur when flying or diving affect air spaces in the body.

Pressure and Air Flow:

1. Atmospheric pressure results from the weight of the air, and one atmosphere is approximately 15 psi, or 760 mm Hg.

2. A single respiratory cycle consists of an inspiration and an expiration.

3. The volume of the lungs changes as the dimensions of the pleural cavities are altered. Inspiration is an active process, requiring the contraction of skeletal muscles.Expiration may be active or passive.

4. The intrapulmonary (intra-alveolar) pressure is measured at the alveoli. The intrapleural pressure is measured inside the pleural cavity. The intrapleural pressures are usually lower than the intrapulmonary pressure, so the lungs are held open.

5. When the intrapleural pressures rise above the intrapulmonary pressures, the lungs collapse.Introducing air into a pleural cavity (a pneumothorax) leads to the collapse of the lung, a condition known as atelectasis.

Respiratory Mechanics:

1. In quiet breathing inspiration is active and expiration passive. During diaphragmatic breathing the movements of the diaphragm provide the necessary changes to thoracic volume. In costal breathing the ribcage moves to alter thoracic volume.

2. During forced breathing both inspiration and expiration are active.

Respiratory Performance:

1. The total vital capacity includes the tidal volume plus the expiratory and inspiratory reserves. The air left in the lungs at the end of maximum expiration constitutes the residual volume, and the minimal volume represents the air retained even after atelectasis.

2. The respiratory minute volume can be calculated if you know the tidal volume and the respiratory rate.

3. Pulmonary function tests assess the vital capacity and permit diagnosis based upon characteristic deviations form normal values.

External Respiration

Diffusion Between Liquids and Gases:

1. Differences in pressure move air from one place to another and force gas molecules into solution.

2. The number of gas molecules absorbed by a fluid is determined by their solubility and the pressure of the gas. The relationship between pressure, solubility, and diffusion is known as Henry's law.

3. Decompression sickness occurs when pressure decreases rapidly, and gas molecules leave solution to form bubbles in body fluids. Nitrogen is usually the gas responsible for the observed symptoms.

Mixed Gases and Dalton's Law:

1. In a mixed gas the individual gases exert a pressure that is proportional to their abundance in the mixture. This relationship, called Dalton's law, can be expressed as

pN2 + pO2 + pCO2 + H2O = 760 mm Hg

for air under a pressure of one atmosphere.

Alveolar Ventilation and Gas Exchange:

1. During a normal resting tidal inspiration, 350 ml of air actually transits the conducting passageways and enters the alveolar spaces. The remaining 150 ml does not get farther than the conducting passageways, and stays within the anatomic dead space of the lungs.

2. Blood delivered by the pulmonary arteries has a higher pCO2 and a lower pO2 than alveolar air. Diffusion between the alveolar gas and the blood eliminates these differences. Blood within the pulmonary veins has a pO2 near 95 mm Hg and a pCO2 of around 40 mm Hg.

Internal Respiration

1. Interstitial fluid has a pO2 of 40 mm Hg, and a pCO2 of 45 mm Hg. As a result, blood entering peripheral capillaries delivers and absorbs carbon dioxide.

Gas Pickup and Delivery

1. The transport of oxygen and carbon dioxide within the blood involves reactions that are completely reversible.

Oxygen Transport

1. Around 3 percent of the oxygen transported in plasma is dissolved in solution, and the balance is bound to hemoglobin molecules within red blood cells.

2. Over the range of oxygen pressures normally present in the body, a small change in plasma pO2 will mean a large change in the amount of oxygen bound or released. At alveolar pO2s hemoglobin is almost fully saturated. At the pO2 of peripheral tissues, hemoglobin retains a substantial reserve of oxygen.

3. Adaptations to the lower atmospheric pO2 found at higher elevations include increased respiratory rate, increased heart rate, and elevated hematocrit.

4. Hemoglobin will release those reserves if the pH drops,the temperature or carbon dioxide concentration rises, or the red cells generate DPG.

5. Fetal hemoglobin has a stronger affinity for oxygen than adult hemoglobin. As a result, oxygen can be removed from the maternal bloodstream, even though the maternal hemoglobin is only 70 percent saturated when it arrives at the placenta.

6. Other fetal adjustments include an elevated hematocrit and a rapid heart rate.

Carbon Dioxide Transport

1. Roughly 7 percent of the carbon dioxide transported in the blood is dissolved in the plasma.

2. Another 23 percent is bound as carbaminohemoglobin within the red blood cells.

3. The remaining 70 percent gets converted to carbonic acid by the enzyme carbonic anhydrase. Carbonic acid then dissociates into a hydrogen ion and a bicarbonate ion.

THE CONTROL OF RESPIRATION

1. Autoregulation occurs as tissue metabolism changes the local pO2 and pCO2, thereby changing the binding characteristics of hemoglobin.

The Respiratory Centers of the Brain

1. The respiratory centers consist of three pairs of nuclei in the reticular formation of the pons and medulla.

2. The respiratory rhythmicity center of the medulla sets the pace for respiration.

3. The apneustic center causes strong and sustained inspiratory movements.

4. The pneumotaxic center inhibits the apneustic center and the inspiratory center in the medulla. Respiratory activities change in response to alterations in body temperature, metabolic rate, and CNS-active drugs.

The Reflex Control of Respiration

Mechanoreceptor Reflexes:

1. The inflation reflex limits the duration of inspiration in response to stretching the walls of the lungs.The deflation reflex stimulates inspiration when the lungs are collapsed.

2. Mechanoreceptor reflexes are insignificant during quiet respiration but they are very important during forced respiration.

Chemoreceptor Reflexes:

1. Chemoreceptors monitoring the gas concentration of the blood are serviced by the vagus (X) and glossopharyngeal (IX) cranial nerves. These receptors are most sensitive to elevations in carbon dioxide concentrations. An increased in arterial pCO2s represents a powerful respiratory stimulant.

2. Carbon dioxide levels decline during hyperventilation, reducing the urge to breathe.

Protective Reflexes:

1. Apnea occurs in response to pain, sudden exposure to cold temperatures, or noxious stimuli contacting the glottis or conducting passageways. Sneezing and coughing are other examples of protective reflexes.

Higher Centers and Respiratory Performance

1. Conscious and unconscious thought processes can have a direct effect on respiratory performance via their effects on the respiratory centers or on the respiratory muscles.

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