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Chapter 13The Respiratory System
Respiration
Respiration has two functions:
obtains O2for use by the bodys cells and
eliminates CO2 produced by body cells
Encompasses 2 separate but related processes
Internal respiration aka Cellular Respiration
Metabolic processes carried out within themitochondria, which use O2 and produce CO2, whilederiving energy (ATP) from nutrient molecules
External respiration
Sequence of events involved in the exchange of O2and CO2 between the external environment and thecells of the body
Nonrespiratory Functions of RespiratorySystem
Route for water loss and heat elimination
Enhances venous return
Helps maintain normal acid-base balance
Enables speech, singing, other vocalizations Defends against inhaled foreign matter
Removes, modifies, activates, or inactivatesvarious materials passing through the pulmonarycirculation
Nasal passages include the receptors for smell
External Respiration
4 steps
1. Ventilation movement of air into and out of thelungs
2. Pulmonary Gas Exchange - O2 and CO2 diffusebetween air in alveoli and blood within the pulmonary
capillaries3. Transport - Blood transports O2 and CO2 between
lungs and tissues
4. Systemic Gas Exchange - O2 and CO2 areexchanged between blood and tissues and bydiffusion across systemic (tissue) capillaries
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External andInternal
Respiration
Respiratory System
Consists of three major types of structures:
Respiratory airways leading to the lungs
Lungs
Structures of the thorax involved in producingmovement of air through the airways into and outof the lungs
Respiratory Airways Tubes that carry air between the atmosphere and
the air sacs
Nasal passages (nose)
Pharynx (common passageway for resp & dig)
Trachea (windpipe)
Larynx (voice box)
Right and left
bronchi
Bronchioles Alveoli (air sacs)
are clustered at
ends of terminal
bronchioles
Respiratory Airways
Trachea and larger bronchi
Fairly rigid, nonmuscular tubes
Rings of cartilage prevent collapse
Bronchioles
No cartilage to hold them open
Walls contain smooth muscle innervated byautonomic nervous system
Sensitive to certain hormones and localchemicals
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Alveoli Thin-walled
inflatable sacs
Function in gasexchange
Pulmonarycapillariesencircle eachalveolus
Walls consist of asingle layer of flat
Type I alveolar cells
Type II alveolar cells secrete pulmonary surfactant
Alveolar macrophages guard lumen
Lungs
Occupy much of thoracic cavity
Heart, associated vessels, esophagus, thymus,
and some nerves also occupy space
Two lungs
Each is divided into several lobes
Tissue consists of highly branched airways, thealveoli, the pulmonary blood vessels, and largequantities of elastic connective tissue
Thoracic Cavity
Outer chest wall (thorax)
Formed by 12 pairs of ribs which join sternum anteriorlyand thoracic vertebrae posteriorly
Diaphragm
Dome-shaped sheet of skeletal muscle
Separates thoracic cavity from the abdominal cavity
Pleural sac
Double-walled, closed sac that separates each lungfrom the thoracic wall
Pleural cavity interior of plural sac
Intrapleural fluid
Lubricant, secreted by surfaces of the pleura
Pleural Sac
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Respiratory Mechanics
Interrelationships among pressures inside andoutside the lungs are important in ventilation
3 different pressure considerations important inventilation
Atmospheric (barometric) pressure
Intra-alveolar (intrapulmonary) pressure
Intrapleural (intrathoracic) pressure
Pressures Important in Ventilation
Respiratory Mechanics
Changes in intra-alveolarpressure produce flow of airinto and out of the lungs
If intra-alveolar pressure is lessthan atmospheric pressure, airenters lungs.
If the opposite occurs, air exitslungs.
Boyles law states that at anyconstant temperature, thepressure exerted by a gas variesinversely with volume of a gas.
Boyles Law
Respiratory Mechanics: Inspiration
Major inspiratory muscles:
Diaphragm
External intercostal muscles
During Quiet Respiration, 75% of thoracic cavityenlargement is due to diaphragm contraction
Diaphragm contraction decreases intrapleural pressure
The lungs expand, increasing volume
Increased volume lowers the intra-alveolar pressure toa level below atmospheric pressure.
Air enters the lungs.
Accessory inspiratory muscles can further enlarge thethoracic cavity.
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Anatomy of theRespiratory
Muscles
Respiratory Mechanics: Expiration
Onset of expiration begins with relaxation ofinspiratory muscles
Relaxation of diaphragm and muscles of chestwall, plus the elastic recoil of the alveoli,
decrease the size of the chest cavity Intrapleural pressure increases; lungs arecompressed
Intra-alveolar pressure increases When pressure increases to level above atmosphericpressure, air is driven outexpiration occurs
Forced expiration can occur by contraction ofexpiratory muscles Abdominal wall muscles
Internal intercostal muscles
Respiratory Muscle ActivityDuring Inspiration
Respiratory Muscle ActivityDuring Expiration
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Airway Resistance
Primary determinant of resistance to airflow is theradius of the conducting airway
Autonomic nervous system controls contraction ofsmooth muscle in bronchioles (changes radii)
Chronic obstructive pulmonary disease (COPD)
abnormally increases airway resistance
Expiration is more difficult than inspiration
Diseases
Chronic bronchitis
Asthma
Emphysema
Compliance
Lungs have elastic recoil rebound if stretched
Compliance
Refers to how much effort is required to stretch
or distend the lungs
The less compliant the lungs are, the morework is required to produce a given degree ofinflation
Decreased by factors such as pulmonaryfibrosis
Elastic Recoil
Refers to how readily the lungs rebound afterhaving been stretched
Responsible for lungs returning to theirpreinspiratory volume when inspiratory muscles
relax at end of inspiration Depends on 2 factors
Highly elastic connective tissue in the lungs
Alveolar surface tension
Thin liquid film lines each alveolus
Reduces tendency of alveoli to recoil
Helps maintain lung stability
Newborn respiratory distress syndrome
Work of Breathing
Normally requires 3% of total energy expenditurefor quiet breathing
Lungs normally operate at about half full
Work of breathing is increased in the following
situations: When pulmonary compliance is decreased
When airway resistance is increased
When elastic recoil is decreased
When there is a need for increased ventilation
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Variations inLung Volume
Lung Volumes and Capacities
Can be measured by a spirometer
Spirogram is a graph that records inspiration
and expiration
Lung Volumes and Capacities
Description Average
Value
Tidal volume (TV) Volume of air entering or leaving lungs during a
single breath
500 ml
Inspiratory reserve
volume (IRV)
Extra volume of air that can be maximally
inspired over and above the typical resting tidal
volume
3000 ml
Inspiratory capacity
(IC)
Maximum volume of air that can be inspired at
the end of a normal quiet expiration (IC =IRV
+ TV)
3500 ml
Expiratory reservevolume (ERV)
Extra volume of air that can be actively expiredby maximal contraction beyond the normal
volume of air after a resting tidal volume
1000 ml
Residual volume
(RV)
Minimum volume of air remaining in the lungs
even after a maximal expiration
1200 ml
Lung Volumes and Capacities
Description Average Value
Functional residual
capacity (FRC)
Volume of air in lungs at end of normal
passive expiration (FRC = ERV +
RV)
2200 ml
Vital capacity (VC) Maximum volume of air that can be
moved out during a single breath
following a maximal inspiration (VC =IRV + TV + ERV)
4500 ml
Total lung capacity (TLC) Maximum volume of air that the lungs
can hold (TLC = VC + RV)
5700 ml
Forced expiratory volume
in one second (FEV1)
Volume of air that can be expired
during the first second of expiration ina VC determination
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Respiratory Dysfunction
2 general categories of dysfunction that yieldabnormal results during spirometry
Obstructive lung disease
Restrictive lung disease
Additional conditions affecting respiratory function
Diseases affecting diffusion of O2 and CO2 acrosspulmonary membranes
Reduced ventilation due to mechanical failure
Failure of adequate pulmonary blood flow
Ventilation/perfusion abnormalities involving a poormatching of air and blood so that efficient gasexchange does not occur
Abnormal Spirograms Associated with Obstructive and
Restrictive Lung Diseases
Pulmonary Ventilation
Volume of air breathed in and out in one minute
Pulmonary ventilation = tidal volume x respiratory rate
(ml/min) (ml/breath) (breaths/min)
Alveolar Ventilation
More important than pulmonary ventilation
Volume of air exchanged between theatmosphere and the alveoli per minute
Less than pulmonary ventilation due to
anatomic dead space Volume of air in conducting airways that isuseless for exchange
Averages about 150 ml in adults
Alveolar ventilation =
(tidal volume dead space) x respiratory rate
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Effect of Different Breathing Patterns onAlveolar Ventilation
Alveolar Ventilation Alveolar dead space
Quite small and of little importance in healthy people
Can increase to lethal levels in several types ofpulmonary disease
Local controls act on smooth muscle of airways andarterioles to match airflow to blood flow
Accumulation of CO2 in alveoli decreases airwayresistance leading to increased airflow
Increase in alveolar O2 concentration brings aboutpulmonary vasodilation which increases blood flow tomatch larger airflow
Gas Exchange
At both pulmonary capillary and tissue capillary levels,gas exchange involves simple diffusion of O2 and CO2down partial pressure gradients
Partial pressure exerted by each
gas in a mixture equals total
pressure times the fractionalcomposition of this gas in mixture
O2 is 21% of atmosphere, so
21% of the atmospheric
pressure is due to O2
Oxygen and CarbonDioxide Exchange AcrossPulmonary and Systemic
Capillaries Caused byPartial Pressure Gradients
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Gas Exchange
Factors that affect the rate of gas transfer acrossalveolar membrane:
As surface area increases, diffusion rate increases
Differences in thickness of barrier separating airand blood
Rate of gas exchange is directly proportional to thediffusion coefficient for the gas
Partial pressure gradients of O2 and CO2
Diffusion Constant
Gas Exchange
Exchange across systemic capillaries also occursdown partial pressure gradients
O2 equilibrates to 100 (partial pressure) in alveoli; O2in body tissues is ~40 (cells are using O2)
The PCO2 in systemic capillaries is low (e.g., 40)compared to tissue cells (e.g., 46), (cells are makingCO2 by metabolism)
O2 diffuses from the systemic capillaries into the tissuecells (higher concentration to lower = diffusion)
CO2 diffuses from tissues to capillaries Overall: Blood equilibrates with tissue cells, so blood
leaving systemic capillaries is low in O2 and high in CO2.
The blood returns to the heart and then the lungs. At the pulmonary capillaries, the blood acquires O2 and
releases some CO2.
Gas Transport Most oxygen in the blood is transported bound to
hemoglobin.
Hb + O2 HbO2(reduced hemoglobin or (oxyhemoglobin)
deoxyhemoglobin)
Gas Transport Hemoglobin and O2 combine to form oxyhemoglobin
This is a reversible process, favored to form oxyhemoglobinin the lungs.
Hemoglobin tends to combine with O2 as it diffuses from the
alveoli into the pulmonary capillaries.
A small percentage of O2 is dissolved in plasma.
The dissociation of oxyhemoglobin into hemoglobinand free O2 occurs at the tissue cells.
The reaction is favored in this direction as O2 leaves thesystemic capillaries and enters tissue cells.
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Gas Transport
Partial pressure of O2 (PO2) is the main factor determiningpercent hemoglobin saturation
The percent saturation is high where the PO2 is high (lungs).
The percent saturation is low where the PO2 is low (tissues).
At tissue cells, O2 tends to dissociate from hemoglobin, theopposite of saturation.
This relationship is shown in the oxygen-hemoglobin
dissociation curve.
The plateau of the curve is where the PO2 is high (lungs).
The steep part of the curve exists at the systemic capillaries,where hemoglobin unloads O2 to the tissue cells.
Oxygen-Hemoglobin Dissociation Curve
Gas Transport
Hemoglobin promotes the net transfer of oxygen atboth the alveolar and tissue levels.
Net diffusion of oxygen from alveoli to blood. continuous until hemoglobin is as saturated as possible
(97.5% at 100 mm of Hg). At tissue cells hemoglobin rapidly delivers O
2into blood
plasma and on to tissue cells.
Bohr Effect: shift of the curve to right (more dissociation) isrelated to: high CO2 or acidity (low pH)
Hemoglobin has more affinity for carbon monoxide comparedto O2.
Gas Transport: CO2Most CO2 (about 60%) is transported as Bicarbonate Ion.
CO2 combines with H2O to form carbonic acid (H2CO3).
Carbonic anhydrase facilitates this in the erythrocyte.
Carbonic acid dissociates into H+ and bicarbonate ion (HCO3-).
2-step, reversible process is favored at the tissue cells
The reverse of this process (bicarbonate ions forming freemolecules of CO
2) occurs in the lungs.
30% of the CO2 is bound to hemoglobin in the blood. This is anothermeans of transport.
About 10% of transported CO2 is dissolved in the plasma.
By the chloride shift, the plasma membrane of erythrocytes passivelyfacilitates the diffusion of bicarbonate ions (out of the red cell) andchloride ions.
By the Haldane effect the removal of O2 from hemoglobin at the tissuecells increases the ability of hemoglobin to bind with CO2.
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Abnormalities in Arterial PO2
Hypoxia: Insufficient O2 at the cell level
Categories
Hypoxic hypoxia
Anemic hypoxia Circulatory hypoxia
Histotoxic hypoxia
Hyperoxia: Above-normal arterial PO2
Can only occur when breathing supplemental O2
Scuba divers, hyperbaric chambers, neonates,space exploration
Can be dangerous
Abnormalities in Arterial PCO2
Hypercapnia: excess CO2 in arterial blood
Caused by hypoventilation
Hypocapnia: Below-normal arterial PCO2 levels
Brought about by hyperventilation which can betriggered by
Anxiety states
Fever
Aspirin poisoning
Control of Respiration
Respiratory centers in brain stem establish a rhythmicbreathing pattern
Medullary respiratory center Dorsal respiratory group (DRG)
Mostly inspiratory neurons
Ventral respiratory group (VRG)
Inspiratory neurons
Expiratory neurons
Pre-Btzinger complex Widely believed to generate respiratory rhythm
Pneumotaxic center Sends impulses to DRG that help switch off inspiratoryneurons
Dominates over apneustic center
Control of Respiration
Apneustic center
Prevents inspiratory neurons from being switched off
Provides extra boost to inspiratory drive
Hering-Breuer reflex
Triggered to prevent overinflation of the lungs
Chemical factors that play role in determining
magnitude of ventilation
PO2
PCO2
H+
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Control ofRespiration
Influence of Chemical Factors on Respiration
Peripheral Chemoreceptors
Carotid bodiesare located in thecarotid sinus
Aortic bodies arelocated in theaortic arch
Factors That May Increase VentilationDuring Exercise
Reflexes originating from body movement
Increase in body temperature
Epinephrine release
Impulses from the cerebral cortex
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Influences on VentilationUnrelated to Gas Exchange
Protective reflexes such as sneezing, coughing
Inhalation of noxious agents
can trigger immediate cessation of breathing Pain originating anywhere in body reflexively
stimulates respiratory center
Involuntary modification of breathing occursduring expression of various emotional states
Respiratory center is reflexively inhibited duringswallowing