Chapter 23

I. INTRODUCTION

A.   The two systems that cooperate to supply O2 and eliminate CO2 are the cardiovascular and the respiratory system.

1.     The respiratory system provides for gas exchange.

2.     The cardiovascular system transports the respiratory gases.

3.     Failure of either system has the same effect on the body: disruption of homeostasis and rapid death of cells from oxygen starvation and buildup of waste products.

B.    Respiration is the exchange of gases between the atmosphere, blood, and cells. It takes place in three basic steps: ventilation (breathing), external (pulmonary) respiration, and internal (tissue) respiration.

C.    The purpose of delivering oxygen to the cells is to enable the cells to carry on aerobic cellular respiration which uses oxygen, glucose, ADP and enzymes to produce ATP (36 – 38 per molecule of glucose).This is accomplished through glycolysis, Krebs cycle, and the electron transport chain.  The byproducts CO2 and H2O are delivered to the lungs by the blood and exhaled. ATP is essential for many enzymatic reactions necessary for life.  Thus, without sufficient oxygen, the decrease in ATP production would not support human life.

II.   RESPIRATORY SYSTEM ANATOMY

A.   The respiratory system consists of the nose, pharynx, larynx, trachea, bronchi, and lungs.

1.     The upper respiratory system refers to the nose, pharynx, and associated structures. The lower respiratory system refers to the larynx, trachea, bronchi, and lungs.

2.     The conducting system consists of a series of cavities and tubes - nose, pharynx, larynx, trachea, bronchi, bronchiole, and terminal bronchioles - that conduct air into the lungs. The respiratory portion consists of the area where gas exchange occurs - respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

B.    The external portion of the nose is made of cartilage and skin and is lined with mucous membrane. Openings to the exterior are the external nares.

1.     The external portion of the nose is made of cartilage and skin and is lined with mucous membrane.

2.     The bony framework of the nose is formed by the frontal bone, nasal bones, and maxillae .

3.     The interior structures of the nose are specialized for warming, moistening, and filtering incoming air; receiving olfactory stimuli; and serving as large, hollow resonating chambers to modify speech sounds.

4.     The internal portion communicates with the paranasal sinuses and nasopharynx through the internal nares.

5.     The inside of both the external and internal nose is called the nasal cavity. It is divided into right and left sides by the nasal septum. The anterior portion of the cavity is called the vestibule.

C.    Pharynx

1.     The pharynx (throat) is a muscular tube lined by a mucous membrane.

2.     The anatomic regions are the nasopharynx, oropharynx, and laryngopharynx.

3.     The nasopharynx functions in respiration. Both the oropharynx and laryngopharynx function in digestion and in respiration (serving as a passageway for both air and food).

D.   Larynx

1.     The larynx (voice box) is a passageway that connects the pharynx with the trachea.

2.     It contains the thyroid cartilage (Adam’s apple); the epiglottis, which prevents food from entering the larynx; the cricoid cartilage, which connects the larynx and trachea; and the paired arytenoid, corniculate, and cuneiform cartilages.

 

E.    The Structures of Voice Production

1.     The larynx contains vocal folds (true vocal cords), which produce sound. The thinner vocal folds of females produce high pitches, and the thicker vocal folds of males produce low pitches. The ventricular folds (false vocal cords) are superior to the vocal folds and function in holding the breath against pressure in the thoracic cavity when they are brought together.

2.     Sound originates from the vibration of the vocal folds, but other structures are necessary for converting the sound into recognizable speech.

F.    Trachea

1.     The trachea (windpipe) extends from the larynx to the primary bronchi.

2.     It is composed of smooth muscle and C-shaped rings of cartilage and is lined with pseudostratified ciliated columnar epithelium.

a.     The cartilage rings keep the airway open.

b.     The cilia of the epithelium sweep debris away from the lungs and back to the throat to be swallowed.

G.   Bronchi

1.     The trachea divides into the right and left pulmonary bronchi.

2.     The bronchial tree consists of the trachea, primary bronchi, secondary bronchi, tertiary bronchi, bronchioles, and terminal bronchioles.

a.     Walls of bronchi contain rings of cartilage.

b.     Walls of bronchioles contain smooth muscle.

H.   Lungs

1.     Lungs are paired organs in the thoracic cavity; they are enclosed and protected by the pleural membrane.

a.     The parietal pleura is the outer layer which is attached to the wall of the thoracic cavity.

b.     The visceral pleura is the inner layer, covering the lungs themselves.

c.     Between the pleurae is a small potential space, the pleural cavity, which contains a lubricating fluid secreted by the membranes.

d.     The lungs extend from the diaphragm to just slightly superior to the clavicles and lie against the ribs anteriorly and posteriorly.

e.     The lungs almost totally fill the thorax.

2.     The right lung has three lobes separated by two fissures (oblique and inferior); the left lung has two lobes separated by one fissure (oblique) and a depression, the cardiac notch.

a.     The secondary bronchi give rise to branches called tertiary (segmental) bronchi, which supply segments of lung tissue called bronchopulmonary segments.

b.     Each bronchopulmonary segment consists of many small compartments called lobules, which contain lymphatics, arterioles, venules, terminal bronchioles, respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

3.     Alveoli

a.     Alveolar walls consist of type I alveolar (squamous pulmonary epithelial) cells, type II alveolar (septal) cells, and alveolar macrophages (dust cells).

b.     Type II alveolar cells secrete alveolar fluid, which keeps the alveolar cells moist and which contains a component called surfactant. Surfactant lowers the surface tension of alveolar fluid, preventing the collapse of alveoli with each expiration.

c.     Gas exchange occurs across the alveolar-capillary membrane.

 

 

 

3.     The lungs have a double blood supply.

a.     Blood enters the lungs via the pulmonary arteries (pulmonary circulation) and the bronchial arteries (systemic circulation). Most of the blood leaves by the pulmonary veins, but some drains into the bronchial veins.

b.     In the lungs vasoconstriction in response to hypoxia diverts pulmonary blood from poorly ventilated areas to well ventilated areas.  This phenomenon is known as ventilation – perfusion coupling.

II.   PULMONARY VENTILATION

A.   Respiration occurs in three basic steps: pulmonary ventilation, external respiration, and internal respiration.

B.    Inspiration (inhalation) is the process of bringing air into the lungs.

1.     The movement of air into and out of the lungs depends on pressure changes governed in part by Boyle’s law, which states that the volume of a gas varies inversely with pressure, assuming that temperature is constant.

2.     The first step in expanding the lungs involves contraction of the main inspiratory muscle, the diaphragm.

3.     Inhalation occurs when alveolar (intrapulmonic) pressure falls below atmospheric pressure. Contraction of the diaphragm and external intercostal muscles increases the size of the thorax, thus decreasing the intrapleural (intrathoracic) pressure so that the lungs expand. Expansion of the lungs decreases alveolar pressure so that air moves along the pressure gradient from the atmosphere into the lungs. Because of contraction of muscles, inhalation is an active process.

4.     During forced inhalation, accessory muscles of inspiration (sternocleidomastoids, scalenes, and pectoralis minor) are also used.

 

 

C.    Expiration (exhalation) is the movement of air out of the lungs.

1.     Exhalation occurs when alveolar pressure is higher than atmospheric pressure. Relaxation of the diaphragm and external intercostal muscles results in elastic recoil of the chest wall and lungs, which increases intrapleural pressure, decreases lung volume, and increases alveolar pressure so that air moves from the lungs to the atmosphere. There is also an inward pull of surface tension due to the film of alveolar fluid. Quiet exhalation is a passive process because it simply involves the relaxation of muscles that contracted during inhalation.

2.     Exhalation becomes active during labored breathing and when air movement out of the lungs is impeded. Forced expiration employs contraction of the internal intercostals and abdominal muscles.

D.   Alveolar Surface Tension

1.     In the lungs, surface tension causes the alveoli to assume the smallest diameter possible.

a.     During breathing, surface tension must be overcome to expand the lungs during each inspiration. It is also the major component of lung elastic recoil, which acts to decrease the size of the alveoli during expiration.

b.     The presence of surfactant, a phospholipid produced by the type II alveolar (septal) cells in the alveolar walls, allows alteration of the surface tension of the alveoli and prevents their collapse following expiration.

E.    Compliance is the ease with which the lungs and thoracic wall can be expanded. Any condition that destroys lung tissue causes it to become filled with fluid, produces a deficiency in surfactant, or in any way impedes lung expansion or contraction, decreases compliance.

F.    The walls of the respiratory passageways, especially the bronchi and bronchioles, offer some resistance to the normal flow of air into the lungs. Any condition that obstructs the air passageway increases resistance, and more pressure to force air through is required.

IV. LUNG VOLUMES AND CAPACITIES

A.   Air volumes exchanged during breathing and rate of ventilation are measured with a spiromometer, or respirometer, and the record is called a spirogram.

B.    Among the pulmonary air volumes exchanged in ventilation are tidal (500 ml), inspiratory reserve (3100 ml), expiratory reserve (1200 ml), residual (1200 ml) and minimal volumes. Only about 350 ml of the tidal volume actually reaches the alveoli, the other 150 ml remains in the airways as anatomic dead space.  Refer to your notes for further detail.

C.    Pulmonary lung capacities, the sum of two or more volumes, include inspiratory (3600 ml), functional residual (2400 ml), vital (4800 ml), and total lung (6000 ml) capacities (Figure 23.17).

D.   The minute volume of respiration is the total volume of air taken in during one minute (tidal volume x 12 respirations per minute = 6000 ml/min).

V.   EXCHANGE OF OXYGEN AND CARBON DIOXIDE

A.   To understand the exchange of oxygen and carbon dioxide between the blood and alveoli, it is useful to know some gas laws.

1.     According to Dalton’s law, each gas in a mixture of gases exerts its own pressure as if all the other gases were not present.

a.     The partial pressure of a gas is the pressure exerted by that gas in a mixture of gases. The total pressure of a mixture is calculated by simply adding all the partial pressures. It is symbolized by P.

b.     The partial pressures of the respiratory gases in the atmosphere, alveoli, blood, and tissues cells are shown in the text.

c.     The amounts of O2 and CO2 vary in inspired (atmospheric), alveolar, and expired air.

Refer to your notes for further detail.

2.     Henry’s law states that the quantity of a gas that will dissolve in a liquid is proportional to the partial pressure of the gas and its solubility coefficient (its physical or chemical attraction for water), when the temperature remains constant.

a.     Nitrogen narcosis and decompression sickness (caisson disease, or bends) are conditions explained by Henry’s law. Refer to your notes for further detail.

b.     A major clinical application of Henry’s law is hyperbaric oxygenation. This technique uses pressure to cause more oxygen to dissolve in the blood and is used to treat anaerobic bacterial infections (such as tetanus and gangrene) and a number of other disorders and injuries.

B.    External and Internal Respiration

1.     In internal and external respiration, O2 and CO2 diffuse from areas of their higher partial pressures to areas of their lower partial pressures and results in the conversion of deoxygenated blood (more CO2 than O2) coming from the heart to oxygenated blood (more O2 than CO2) returning to the heart.

2.     It depends on partial pressure differences, a large surface area for gas exchange, a small diffusion distance across the alveolar-capillary (respiratory) membrane, and the solubility and molecular weight of the gases.

3.     Internal (tissue) respiration is the exchange of gases between tissue blood capillaries and tissue cells and results in the conversion of oxygenated blood into deoxygenated blood.

4.     At rest only about 25% of the available oxygen in oxygenated blood actually enters tissue cells. During exercise, more oxygen is released.

 

 

 

 

VI. TRANSPORT OF OXYGEN AND CARBON DIOXIDE IN THE BLOOD

A.   Oxygen Transport

1.     In each 100 ml of oxygenated blood, 1.5% of the O2 is dissolved in the plasma and 98.5% is carried with hemoglobin (Hb) inside red blood cells as oxyhemglobin (HbO2)

a.     Hemoglobin consists of a protein portion called globin and a pigment called heme.

b.     The heme portion contains 4 atoms of iron, each capable of combining with a molecule of oxygen.

2.     Hemoglobin and Oxygen Partial Pressure

a.     The most important factor that determines how much oxygen combines with hemoglobin is PO2.

b.     The relationship between the percent saturation of hemoglobin and PO2 is illustrated in the oxygen-hemoglobin dissociation curve.

c.     The greater the PO2, the more oxygen will combine with hemoglobin, until the available hemoglobin molecules are saturated.

3.     Other Factors Affecting Hemoglobin Affinity for Oxygen

a.     In an acid (low pH) environment, O2 splits more readily from hemoglobin. This is referred to as the Bohr effect.

b.     Low blood pH (acidic conditions) results from high PCO2.

c.     Within limits, as temperature increases, so does the amount of oxygen released from hemoglobin. Active cells require more oxygen, and active cells (such as contracting muscle cells) liberate more acid and heat. The acid and heat, in turn, stimulate the oxyhemoglobin to release its oxygen.

d.     BPG (2, 3-biphosphoglycerate) is a substance formed in red blood cells during glycolysis. The greater the level of BPG, the more oxygen is released from hemoglobin.

4.     Fetal hemoglobin has a higher affinity for oxygen because it binds BPG less strongly and can carry more oxygen to offset the low oxygen saturation in maternal blood in the placenta.

Refer to your notes for further detail.

B.    Carbon Dioxide Transport

1.     CO2 is carried in blood in the form of dissolved CO2 (7%), carbaminohemoglobin (23%), and bicarbonate ions (70%).

2.     The conversion of CO2 to bicarbonate ions and the related chloride shift maintains the ionic balance between plasma and red blood cells. Refer to your notes for further detail.

C.    Summary of Gas Exchange and Transport in Lungs and Tissues

1.     CO2 in blood causes O2 to split from hemoglobin.

2.     Similarly, the binding of O2 to hemoglobin causes a release of CO2 from blood.

VII. CONTROL OF RESPIRATION

A.   Respiratory Center

1.     The area of the brain from which nerve impulses are sent to respiratory muscles is located bilaterally in the reticular formation of the brain stem. This respiratory center consists of a medullary rhythmicity area (inspiratory and expiratory areas), pneumotaxic area, and apneustic area.

2.     Medullary Rhythmicity Area

a.     The function of the medullary rhythmicity area is to control the basic rhythm of respiration.

b.     The inspiratory area has an intrinsic excitability of autorhythmic neurons that sets the basic rhythm of respiration.

c.     The expiratory area neurons remain inactive during most quiet respiration but are probably activated during high levels of ventilation to cause contraction of muscles used in forced (labored) expiration.

3.     Pneumotaxic Area

a.     The pneumotaxic area in the upper pons helps coordinate the transition between inspiration and expiration. It inhibits the inspiratory area of the medullary rhythmicity center and thus shortens the duration of inhalation.  When the pneumotaxic area is more active, breathing rate is more rapid.

b.     The apneustic area in the lower pons sends impulses to the inspiratory area that activate it and prolong inspiration, inhibiting expiration. However, if the pneumotaxic area is active, it overrides the apneustic area.

B.    Regulation of the Respiratory Center

1.     Cortical Influences

a.     Cortical influences allow conscious control of respiration that may be needed to avoid inhaling noxious gasses or water.

b.     Breath holding is limited by the overriding stimuli of increased [H+]  and [CO2].

2.     Chemoreceptor Regulation of Respiration

a.     Central chemoreceptors (located in the medulla oblongata) and peripheral chemoreceptors (located in the walls of systemic arteries) monitor levels of CO2 and O2 and provide input to the respiratory center.

1)    Central chemoreceptors respond to change in H+ concentration or PCO2, or both in cerebrospinal fluid.

2)    Peripheral chemoreceptors respond to changes in H+, PCO2, and PO2 in blood.

b.     A slight increase in PCO2 (and thus H+), a condition called hypercapnia, stimulates central chemoreceptors.

1)    As a response to increased PCO2, increased H+ and decreased PO2, the inspiratory area is activated and hyperventilation, rapid and deep breathing, occurs.

2)    If arterial PCO2 is lower than 40 mm Hg, a condition called hypocapnia, the chemoreceptors are not stimulated and the inspiratory area sets its own pace until CO2 accumulates and PCO2 rises to 40 mm Hg.

c.     Severe deficiency of O2 (decrease in arterial blood PO2 below 50 mm Hg) depresses activity of the central chemoreceptors and respiratory center.

d.     Hypoxia refers to oxygen deficiency at the tissue level and is classified in several ways:

1)    Hypoxic hypoxia is caused by a low PO2 in arterial blood (high altitude, airway obstruction, fluid in lungs).

2)    In anemic hypoxia, there is too little functioning hemoglobin in the blood (hemorrhage, anemia, carbon monoxide poisoning).

3)    Stagnant hypoxia results from the inability of blood to carry oxygen to tissues fast enough to sustain their needs (heart failure, circulatory shock).

4)    In histotoxic hypoxia, the blood delivers adequate oxygen to the tissues, but the tissues are unable to use it properly (cyanide poisoning).

3.     Proprioceptors of joints and muscles activate the inspiratory center to increase ventilation prior to exercise induced oxygen need.

4.     The inflation (Hering-Breuer) reflex detects lung expansion with stretch receptors and limits it depending on ventilatory need and prevention of damage.

5.     Other influences include blood pressure, limbic system, temperature, pain, stretching the anal sphincter, and irritation to the respiratory mucosa.

Refer to your notes for further detail.