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Chapter 23
The Respiratory System: An Introduction
▪The respiratory system has five basic functions:
▪Organization of the respiratory system- can divided into upper and lower
respiratory systems
▫The upper respiratory system▫The lower respiratory system▫The surface area of the lungs is very large, 35 times that of the body.
▫Respiratory mucosa▫Respiratory Defense SystemThe Upper Respiratory System
▪Nose and nasal cavity- the nose is the primary passageway for air entering the
respiratory system
▫External nares▫Vestibule▫Nasal septum▫Olfactory region▫Nasal conchae▫Nasal meatuses▫Palates-
▫Nasal mucosa▪The Pharynx- the chamber shared by the digestive and respiratory systems, is
divided into three sections, nasopharynx, oropharynx, laryngopharynx.
The Larynx- a cartilaginous structure that surrounds and protects the glottis
▪Cartilages and Ligaments of the Larynx
▫Thyroid cartilage▫Cricoid cartilage▫Epiglottis▫Smaller cartilages; arytenoids, corniculate, and cuneiform
▫Vestibular and vocal ligaments▫Vestibular and vocal folds▪Sound Production- air passing through the glottis vibrates the vocal folds and
produces sound waves.
▪Laryngeal musculatureThe Trachea and Primary Bronchi
▪The Trachea- Windpipe, a tough, flexible tube, epithelia is continuous with the
▫Tracheal cartilages- 15-20, serve to stiffen the tracheal walls and protect
the airway.
▫Trachealis muscle2
▪The Primary Bronchi- initial branches of the trachea, left and right branches
▫Right bronchus is larger in diameter than the left, and descends into the
right lung at a steep angle.
The Lungs
▪Lobes and surfaces of the lungs- the lungs have distinct lobes that are
separated by deep fissures.
▪The Bronchi- the bronchi and their branches form the bronchial tree.
▪The Bronchioles- Each tertiary bronchus branches several times within the
bronchopulmonary segment, giving rise to multiple bronchioles.
▫The autonomic nervous system regulates the activity of these smooth
-Bronchodilation-Bronchoconstriction▫Pulmonary lobules-Each terminal bronchiole delivers air to a single pulmonary lobule
-Within each lobule, the terminal bronchiole branch to form several
respiratory bronchioles, the thinnest and most delicate branches of
the bronchial tree.
▪Alveolar Ducts and Alveoli
▫Alveolar ducts▫Alveolar sacs▫Each alveolus is surrounded by an extensive network of capillaries
▫The capillaries are surrounded by a network of elastic fibers
▫Alveolar epithelium contains simple squamous cells (pneumocytes type I)
with roaming alveolar macrophages and pneumocytes type II cells.
▫Respiratory membrane- a composite structure containing:
-Gas exchange occurs across the respiratory membrane
▪Blood Supply to the Lungs
▫The respiratory exchange surfaces receive blood from arteries of the
pulmonary circuit, continues to arterioles and capillaries that surround the
▫The conducting portions of the respiratory tract receive blood from the
bronchiole arteries.
▪The Pleural cavities and Pleural membranes
▫The two pleural cavities are separated by the mediastinum
▫Each lung occupies a single pleural cavity, which is lined by a serous
membrane called the pleura, consisting of visceral and parietal layers.
▫Pleural fluid fills the pleural cavity, providing lubrication between
An Overview of Respiratory Physiology
▪Respiration usually refers to two integrated processes; external respiration and
internal respiration
▫Internal respiration▫External respiration▫Three integrated steps involved in external respiration are:
Pulmonary Ventilation
▪Pulmonary ventilation is the physical movement of air into and out of the
respiratory tract.
▪The movement of air- air moves from place to place in response to pressure
▫Atmospheric pressure-
▫Boyle’s Law-
▫Air will flow from an area of higher pressure to an area of lower pressure
▫During inhalation:
▫During exhalation:.
▫Pleural fluid within the pleural cavity creates a relatively strong bond
between the visceral and parietal pleura.
▫Compliance▪Pressure changes during inhalation and exhalation
▫Intrapulmonary pressure▫Intrapleural pressure▫Respiratory cycle-
▪The mechanics of breathing
▫Muscles of inhalation- inhalation is an active process, involving the
contraction of one or more of these muscles:
▫Muscles of exhalation- exhalation can be passive or active, when active,
can include one or more of these muscles:
▫Modes of breathing- respiratory muscles are used in various
combinations, depending on the volume of air that must be moved in or
out of the system.
-Quiet breathing- (eupnea),
-Diaphragmatic breathing-Costal breathing-Forced breathing- (hyperpnea),
-Accessory muscles assist with inhalation and exhalation
involves contraction of the internal intercostal muscles
▪Respiratory rates and volumes
▫Respiratory rate▫Respiratory minute volume▫Aveolar ventilation▫Relationships among VT, VE, and VA- The respiratory rate and tidal
volume together determine the alveolar ventilation rate:
▫Respiratory Performance and Volume Relationships- only a small
proportion of the air in the lungs is exchanged during a single quiet
respiratory cycle.
-Pulmonary Volumes include the following:
-Resting Tidal Volume-Expiratory Reserve-Residual Volume-Inspiratory Reserve Volume-Pulmonary Capacities can be determined by adding the values of
various volumes, they include:
-Inspiratory capacity-Functional residual capacity-Vital capacity-Total lung capacity-
Gas Exchange
▪To understand the events of gas exchange, we need to consider
1) the partial pressures of the gases involved
2) the diffusion of molecules between a gas and a liquid
▪The gas laws- principles that govern the movement and diffusion of gas
▫Daltons’ law-
▫Partial Pressures-
▫Henry’s Law-
▪Diffusion and respiratory function
▫The composition of alveolar air-
▫Efficiency of diffusion at the respiratory membrane, there are five reasons
gas exchange is efficient at the respiratory membrane:
▫Partial pressures in alveolar air and alveolar capillaries:
▫Partial pressures in the systemic circuit
Gas Pickup and Delivery
▪RBC continually remove O2 and CO2 from blood plasma, so the plasma
continually absorbs gases.
▪The reactions between the plasma and RBC are temporary and reversible
▪Oxygen transport
▫Hemoglobin saturation- the percentage of heme units containing bound
oxygen at any given moment.
▫Oxygen-hemoglobin saturation curve-
▫Hemoglobin and pH▫Hemoglobin and temperature▫Hemoglobin and BPG▫Fetal hemoglobin-
▪Carbon dioxide transport
▫CO2 is generated by aerobic metabolism in peripheral tissues, after
entering the bloodstream, a CO2 molecule is:
▫Carbonic acid formation- 70% of absorbed CO2 is transported as
carbonic acid.
▫Hemoglobin binding- roughly 23% of the CO2 carried by the blood is
bound to the globular protein portions of the HB molecules inside RBC’s
▫Plasma transport- only about 7% of carbon dioxide is transported
dissolved in the plasma.
Control of Respiration
▪Local regulation of gas transport and alveolar function
▫The rate of oxygen delivery in each tissue and the efficiency of oxygen
pickup at the lungs are largely regulated at the local level.
▪Respiratory centers of the brain
▫Respiratory control has both involuntary and voluntary components
▫Respiratory centers in the medulla oblongata
-respiratory rhythmicity centers- paired centers that set the pace of
-divided into a dorsal respiratory group (DRG) and a ventral
respiratory group (VRG)
-There is reciprocal inhibition between neurons involved with
inhalation and exhalation.
-Patterns of interaction between these groups of neurons differs
between quiet and forced breathing.
-During quiet breathing:
-During forced breathing:
▫The apneustic and pneumotaxic centers- are paired nuclei of the pons
that adjust the output of the respiratory rhythmicity centers.
-Their activities regulate the respiratory rate and depth of
respiration in response to sensory stimuli or input from other
centers in the brain.
-Each apneustic center provides continuous stimulation to the DRG
on that side of the brain stem.
-The pneumotaxic centers inhibit the apneustic centers and
promote passive or active exhalation
-Interactions between the DRG and VRG establish the basic pace
and depth of respiration.
▪Respiratory reflexes
▫Activities of the respiratory centers are modified by sensory information
from several sources:
▫Information from these receptors alters the pattern of respiration,
the induced changes have been called respiratory reflexes.
▫Chemoreceptor reflexes
-Chemoreceptor inputs come from cranial nerves IX and X and
receptors that monitor the composition of the cerebrospinal fluid:
-stimulation of these chemoreceptors leads to an increase in the
depth and rate of respiration.
▫Hypercapnia and hypocapnia
-Hypercapnia- an increase in the PCO2
-Hypocapnia- an abnormally low PCO2, brought about by
hyperventilation (rate and depth of respiration exceed the demand
for oxygen delivery and CO2 removal)
▫Baroreceptor reflexes
-When blood pressure falls, the respiratory rate increases
-When blood pressure rises, the respiratory rate declines
▫Hering-Breuer reflexes
-The sensory information from these reflexes is distributed to the
apneustic centers and the VRG.
-The Hering-Breuer reflexes are not involved in normal quiet
breathing or in tidal volumes under 1000 ml, there are two such
-Inflation reflex-Deflation reflex▫Protective reflexes- operate when there is an exposure to toxic vapors,
chemical irritants, or mechanical stimulation of the respiratory tract.
-Receptors are located in the epithelium of the respiratory tract
-Sneezing, coughing, laryngeal spasms are examples of protective
▪Voluntary control of respiration
▫Conscious thought processes tied to strong emotions affect the
respiratory rate by stimulating centers in the hypothalamus
▫Emotional states can affect respiration through the activation of the
sympathetic or parasympathetic division of the ANS
▫Anticipation of strenuous exercise can trigger an automatic increase in
the respiratory rate
▫Conscious control has a limit; you can not “hold your breath ‘til death”