Topics to Review

1
Topics to Review

• pH
• Buffers
• Diffusion
• Law of mass action (chemistry)

2
Functions of the Respiratory System

• Provides a way to exchange O2 and CO2 between the
  atmosphere and the blood
• oxygen is used by the cells of the body solely
  for the process of aerobic respiration
• carbon dioxide is a waste product of aerobic
  respiration and must be removed from the body
• Regulation of body pH
• Protection from inhaled pathogens and irritating
  substances
• Vocalization

3
The Thorax and Respiratory Muscles

• The bones of the spine and ribs and their
  associated skeletal muscles form the thoracic
  cage
• Contraction and relaxation of these muscles alter
  the dimensions of the thoracic cage which
  promotes ventilation
• 2 sets of intercostal muscles connect the 12
  pairs of ribs
• additional muscles (sternocleidomastoid and
  scalenes) connect the head and neck to the
  sternum and the first 2 ribs
• a dome-shaped sheet of skeletal muscle called the
  diaphragm forms the floor
• the abdominal muscles also participate in
  ventilation

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Organization of the Respiratory System

• Anatomically, the respiratory system can be
  divided into the
• upper respiratory tract which includes the mouth,
  nasal cavity, pharynx and larynx
• lower respiratory tract which includes the
  trachea, 2 primary bronchi, the branches of the
  primary bronchi and the lungs
• also called the thoracic portion of the
  respiratory system because it is enclosed within
  the thorax

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The Pleural Membranes and Fluid

• Within the thorax are 2 double layered pleural
  sacs surrounding each of the 2 lungs
• Parietal pleura
• lines the interior of the thoracic wall and the
  superior face of the diaphragm
• Visceral pleura
• covers the external surface of the lungs
• A narrow intrapleural space between the pleura is
  filled with 25 mL of pleural fluid which holds
  the 2 layers together by the cohesive property of
  water
• serves to lubricate the area between the thorax
  and the outer lung surface
• holds the lungs tight against the thoracic wall
• prevents lungs from completely emptying even
  after a forceful exhalation

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The Respiratory System

• Together, the respiratory system and the
  circulatory system deliver O2 to cells and remove
  CO2 from the body through 4 processes
• Pulmonary ventilation (breathing)
• movement of air into and out of the lungs
• Inspiration/inhalation and expiration/expiration
• External respiration
• gas exchange between the lungs and blood
• Transport
• movement of O2 and CO2 between the lungs and
  cells
• Internal respiration
• gas exchange between blood and the cells

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Organization of the Respiratory System

• Functionally, the respiratory system can be
  divided into the
• the conducting zone (semi-rigid airways) lead
  from the external environment of the body to the
  exchange surface of the lungs
• the exchange surface (respiratory zone) consists
  of the alveoli which are a series of
  interconnected sacs (surrounded by pulmonary
  capillaries) that form that allows oxygen from
  inhaled air to enter blood and carbon dioxide to
  exit blood and enter air that is to be exhaled

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Upper Respiratory Tract

• Air enters the upper respiratory tract through
  either the mouth or nose and passes through the
  pharynx
• warms and humidifies (adds H2O) inspired air
• hair in the nose filters inspired air of any dust
• Air then passes through the larynx or voice box
  
• contains the vocal cords (bands of connective
  tissue) which vibrate and tighten to produce
  sound

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Lower Respiratory Tract (Conducting Zone)

• Air continues into the lower respiratory tract
  through a series of progressively branching tubes
  beginning with the trachea which is a
  semi-flexible tube held open by C-shaped rings of
  cartilage
• The distal end of the trachea splits into 2
  primary bronchi (division 1) which lead to the 2
  lungs
• Within each lung the bronchi branch repeatedly
  (divisions 2-11) into progressively smaller
  bronchi
• the walls of the bronchi are supported by
  cartilage
• Bronchi send air into the bronchioles (divisions
  12-23)
• these airways are supported by smooth muscle only
• contraction causes the diameter to decrease
• bronchoconstriction
• relaxation causes the diameter to increase
• bronchodilation

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Trachea
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Lower Respiratory Tract (Conducting Zone)

• The inner (mucosal) surface of the trachea and
  bronchi consists of epithelial tissue that
  functions as the mucocilliary escalator to trap
  and eliminate inhaled debris
• Goblet cells
• secrete mucus to trap debris in inspired air
• Pseudostratified ciliated columnar epithelium
• move debris trapped in mucus up towards the mouth
  for expectoration/swallowing

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Airway Resistance

• The amount of energy (work) necessary for
  ventilation is partly determined by the
  resistance (opposition) of the airways to airflow
• The key factor in determining the airway
  resistance is the radius of the airways
  (Resistance 1/r4)
• The bronchioles which have smooth muscle in the
  walls can readily change their radii resulting in
  significant changes in airflow resistance
• bronchoconstriction increases airway resistance
  (decreases airflow)
• bronchodilation decreases airway resistance
  (increaes airflow)

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Lower Respiratory Tract (Respiratory Zone)

• Bronchioles branch into respiratory bronchioles
  which begins the respiratory zone (exchange
  surface)
• Respiratory bronchioles move air into blind sacs
  called alveoli (terminus of the airways)
• approximately 150 300 million per lung
• exterior surface is surrounded by large numbers
  of blood vessels for gas exchange
• exterior surface is surrounded by large numbers
  of elastic fibers to aid in lung recoil during
  exhalation
• white blood cells (macrophages) are found in the
  lumen of the alveoli to protect against inhaled
  pathogens
• interior (luminal) surface covered with a thin
  film of water

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Respiratory Zone
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Anatomy of Alveoli

• Composed of very thin (simple) epithelial tissue
  consisting of 2 predominant alveolar cell types
• Type I (squamous) alveolar cells (95 of alveolar
  surface area)
• allows for very rapid exchange (short diffusional
  distance) of O2 and CO2 with blood
• Type II or great (cuboidal) alveolar cells
• secrete surfactant into the alveolar lumen
• Alveoli represents an enormous surface area for
  gas exchange (2800 square feet or half of a
  football field)

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Properties of Alveoli

• Compliant
• easily stretched or deformed
• attributed by the very thin Type I alveolar cells
• Elastic
• ability to recoil after being stretched
• resistance to being stretched
• attributed by
• the elastic fibers surrounding the alveoli
• surface tension of fluid molecules at the
  air-fluid interface (surface) within the alveoli

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Compliance vs. Elastance

• Lung (alveolar) tissue is half way between
  being perfectly elastic and perfectly compliant
• easily stretched and recoils readily
• There is an inverse relationship between
  elastance and compliance
• if a material (lungs) becomes more elastic, it
  becomes less compliant
• not easily stretched, but recoils very readily
• if a material (lungs) becomes more compliant, it
  becomes less elastic
• very easily stretched, but does not recoil much

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Alveolar Surface Tension

• During inhalation the alveoli expand and adjacent
  water molecules on the luminal surface are pulled
  apart from one another causing the H-bonds
  between them to be stretched (like a spring)
  creating tension
• During exhalation the tension within the H-bonds
  is released which returns the water molecules to
  their original spacing pulling the alveoli inward
  allowing them to recoil

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Surfactant

• Type II alveolar cells secrete surfactant
  (surface active agent) which is a fluid
  consisting of amphiphilic molecules into the
  lumen of the alveoli
• These molecules disrupt the cohesive forces
  between water molecules by inserting themselves
  between some of the water molecules preventing
  H-bonds from forming and thus decreases the
  surface tension of the water on the luminal
  surface
• Reducing surface tension simultaneously increases
  compliance and reduces elasticity of the alveoli
  which greatly decreases the amount of work needed
  to expand the alveoli during inspiration (easily
  inflated) while retaining the ability to recoil
• In a lung without surfactant the surface tension
  is so high that it is impossible to be inflated
  using the muscles of respiration

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Lung Diseases of Elastance and Compliance

• Certain diseases change the balance between
  elastic and compliant properties of the lungs
• the development of tough scar tissue within
  alveoli in pulmonary fibrosis reduces the
  compliance (increases the elastance) of lung
  tissue
• Individuals have more difficulty (exert more
  effort) during inspiration resulting in lower
  lung volumes, while expiring more air than normal
  
• in emphysema, the destruction of elastic fibers
  and alveolar tissue (reduction in surface area
  (tension)) reduces the elastance (increases the
  compliance) of lung tissue
• patients have little difficulty (exert less
  effort) during inspiration resulting greater
  lungs volumes, while expiring less air than normal

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Pulmonary Ventilation

• The movement of air into and out of the airways
  occurs as a result of increasing and decreasing
  the dimensions of the thoracic cavity through the
  contraction and relaxation of the skeletal
  muscles of respiration
• Since the alveoli are stuck to the interior
  surface of the thorax via the pleura, dimensional
  changes in the thoracic cavity result in the same
  dimensional changes in the alveoli
• Dimensional changes in the alveoli create air
  pressure changes in the alveoli as expressed by
  Boyles Law

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Pulmonary Ventilation

• Changes in the pressure in alveolar air (alv)
  create air pressure gradients between the air in
  the alveoli and the atmospheric air that
  surrounds our bodies (atm) which drive air flow
  into and out of the lungs
• Air always flows from an area of higher pressure
  to an area of lower pressure
• When alv lt atm inspiration occurs
• air flows into the lungs
• When alv gt atm expiration occurs
• air flows out of the lungs
• When alv atm no air flow occurs
• at transition between inspiration and expiration

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Boyles Law

• The mathematical inverse relationship that
  describes what happens to the pressure of a gas
  or fluid in a container following a change in the
  volume (dimensions) of the container
• V1 x P1 V2 x P2
• V volume of a container
• P pressure within the container
• force of collisions between molecules within the
  container and the wall of the container
• determined by the concentration of molecules
  within the container
• If the volume of a container increases, then
  pressure within the container must decrease
• If volume of a container decreases, then pressure
  within the container must increase

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Inspiration

• Before inspiration (at end of previous
  expiration), the alv (0 mm Hg) atm (0 mm Hg)
  (no air movement)
• Expansion of the thoracic cavity (by the
  contraction of the diaphragm, the external
  intercostals, the scalenes and the
  sternocleidomastoid) pulls the alveoli open which
  increases their volume and decreases their
  pressure (-1 mm Hg)
• the alveolar pressure decreases below atmospheric
  pressure, creating a pressure gradient resulting
  in inspiration
• As the alveoli fill with air (more molecules),
  the alv pressure increases until it equals atm
  pressure
• Inspiration ends when alv (0 mm Hg) atm (0 mm
  Hg)

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Quiet Expiration

• Before expiration, (at end of previous
  inspiration), the alv (0 mm Hg) atm (0 mm Hg)
  (no air movement)
• Expiration begins as action potentials along the
  nerves that innervate the muscles of inspiration
  cease allowing these muscles to relax returning
  the diaphragm and ribcage to their relaxed
  positions
• allows the alveoli to collapse which decreases
  their volume and increases their pressure (1 mm
  Hg)
• the alveolar pressure increases above atmospheric
  pressure, creating a pressure gradient resulting
  in quiet (passive) expiration
• As the alveoli empty with air, the alv pressure
  decreases until it equals atm pressure
• Expiration ends when alv (0 mm Hg) atm (0 mm Hg)

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Forced Expiration

• Forced expiration requires an additional decrease
  in thoracic and lung volume over what passive
  expiration can provide
• Accomplished through the contraction of the
  internal intercostals (pull ribs inward) and the
  abdominals (decrease abdominal volume and
  displace the liver and intestines upward)

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• The amount (volume) of air that enters or exits
  the lungs during either quiet or forced breathing
  can be plotted on a graph called a spirogram

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Lung Volumes

• Tidal volume (TV)
• volume of air that moves into and out of the
  lungs with each breath during quiet ventilation
  (500 ml)
• Inspiratory reserve volume (IRV)
• additional volume of air that can be inspired
  forcibly into the lungs after a tidal inspiration
• Expiratory reserve volume (ERV)
• additional volume of air that can be expired
  forcibly from the lungs after a tidal expiration
• Residual volume (RV)
• volume of air left in the lungs after forced
  expiration
• this air can NEVER be expired

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Lung Capacities

• The addition of 2 or more specific lung volumes
  is referred to as a capacity
• Inspiratory capacity (IC)
• total amount of air that can be inspired after a
  tidal expiration (IRV TV)
• Functional residual capacity (FRC)
• amount of air remaining in the lungs after a
  tidal expiration (RV ERV)
• Vital capacity (VC)
• the total amount air capable of entering/exiting
  the airways (TV IRV ERV) (4600 ml)
• Total lung capacity (TLC)
• sum of all lung volumes (5800 ml)

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Control of Ventilation

• Breathing occurs automatically whereby the
  contraction of the skeletal muscles of
  respiration are controlled by a spontaneously
  firing network of neurons in the brainstem but
  can be controlled voluntarily up to an extent

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Control of Ventilation

• The most current (albeit incomplete) model of the
  control of ventilation states that
• the rhythmic pattern arises from a neural network
  with spontaneous firing neurons in the medulla
  oblongata that control inspiratory and expiratory
  muscles
• neurons in the pons integrate sensory information
  and interact with medullary neurons to influence
  ventilation
• ventilation is subjected to continuous modulation
  by various chemoreceptor and mechanoreceptor
  linked reflexes and higher brain centers
  (emotion)

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Respiratory Centers of the Medulla

• The dorsal respiratory group (DRG), or
  inspiratory center is the pacesetter for
  ventilation
• spontaneously initiates a short burst of action
  potentials every 5 seconds
• these action potentials travel down the phrenic
  nerve which stimulates the contraction of the
  diaphragm and travel down intercostal nerves
  which stimulates the contraction of the external
  intercostals resulting in inspiration
• sets a quiet ventilation rate at 12
  breaths/minute
• When the DRG is not firing action potentials,
  these muscles are NOT STIMULATED and relax
  resulting in expiration

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Depth and Rate of Ventilation

• The rate and depth of ventilation is controlled
  by the frequency and duration of the action
  potential bursts initiated by the DRG
• Various chemoreceptors and mechanoreceptors in
  response to certain stimuli alter the pattern of
  action potential generation by the DRG
• can either excite OR inhibit the DRG
• influence the contraction and relaxation pattern
  of the respiratory muscles changing rate and/or
  depth of breathing
• results in either hyperventilation or
  hypoventilation

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Respiratory Centers of the Medulla

• The ventral respiratory group (VRG), or
  expiratory center is a group of neurons that fire
  action potentials only during forced expiration
• send action potentials to the internal
  intercostal muscles and abdominal muscles causing
  their contraction
• increases the amount of air that exits the lungs

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Respiratory Centers of the Pons

• Pneumotaxic center
• sends action potentials every 5 seconds to the
  DRG which inhibits the DRG
• ending inspiration
• providing a smooth transition between inspiration
  and expiration

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Receptors of Respiration

• Chemoreceptors respond to changes in PCO2 or PO2
• The MAJOR factor that influences respiration rate
  and depth is the PCO2 in the body
• Central chemoreceptors are found in the brain and
  respond to changes in levels of CO2 in the
  cerebral spinal fluid
• Peripheral chemoreceptors are found in arteries
  near the heart and neck and respond to changes in
  levels of CO2 or O2 in blood
• Mechanoreceptors in the walls of the airways,
  will respond to mechanical stimuli such as
  stretching of the lungs and thoracic cavity
  during inspiration and the presence of irritants

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Reflexes by Chemoreceptors

• An increase in PCO2 (hypercapnia) will stimulate
  the DRG and result in an increase in respiration
  rate and depth
• hyperventilation
• a decrease in PCO2 will inhibit the DRG and
  result in a decrease in respiration rate and
  depth
• hypoventilation
• Only a substantial decrease in PO2 of blood
  leaving the lungs (lt60 mm Hg) will stimulate the
  DRG and result in an increase in respiration rate
  and depth
• an increase in O2 will inhibit the DRG and result
  in a decrease in respiration rate and depth

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Respiratory Acidosis

• Normal body pH is 7.4
• CO2 H2O ? H2CO3 ? H HCO3
• An increase in the PCO2 of the body will drive
  the above reaction to the right
• results in the synthesis excessive amounts of H
  causing the body pH to decrease (acidic)
• respiratory acidosis (pH lt 7.4) which can
  denature proteins and depress the CNS
• Chemoreceptors will stimulate the DRG to increase
  the ventilation rate and depth (hyperventilation)
  
• removes CO2 from the body faster resulting in a
  decrease in CO2 levels
• causes the above reaction to proceed to the left
  decreasing the amount of H
• increasing the pH of the body back to 7.4

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Respiratory Alkalosis

• Normal body pH is 7.4
• CO2 H2O ? H2CO3 ? H HCO3
• A decrease in the PCO2 of the body will drive the
  above reaction to the left
• results in a decrease in the amount of H causing
  the body pH to increase basic (alkaline)
• respiratory alkalosis (pH gt 7.4)
• Chemoreceptors will inhibit the DRG to decrease
  the ventilation rate and depth (hypoventilation)
• removes CO2 from the body more slowly resulting
  in an increase in CO2 levels
• causes the reaction to proceed to the right
  increasing the amount of H
• decreasing the pH of the body back to 7.4

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Reflexes by Mechanoreceptors

• Pulmonary irritant reflexes
• irritants in the conducting zone of the
  respiratory tract, stimulate mechanoreceptors to
  which
• stimulate contraction of bronchiolar smooth
  muscle (bronchoconstriction)
• stimulate the VRG to cause a forced expiration
  (cough)
• Hering-Breuer reflex (Inflation reflex)
• during deep inspirations, mechanoreceptors in the
  lungs are stimulated as the lungs are stretched,
  which inhibits the DRG to stop further
  inspiration (preventing possible damage to
  alveoli)

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Daltons Law of Partial Pressures

• The air that we inspire and expire has a pressure
  of 760 mmHg (at sea level) and is a mixture of 4
  gasses
• N2, O2, H2O and CO2 each making up a different
  proportion of the total mixture and provides a
  proportional contribution to the total pressure
  of the air that is ventilated
• the pressure of air is equal to the sum of the
  individual (partial) pressures of each gas in the
  mixture
• Alv air is
• 75.4 N2 Pnitrogen 573 mmHg
• 13.2 O2 Poxygen 100 mmHg
• 6.2 H2O Pwater 47 mmHg
• 5.2 CO2 Pcarbon dioxide 40 mmHg
• Total 100 760 mmHg

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Respiration Gas Exchange

• The diffusion of O2 and CO2 between alveolar air
  and capillary blood and between capillary blood
  and cells occur simultaneously and depend on the
  concentration (partial pressure) gradient of each
  gas
• LUNGS (external respiration)
• O2 exits the alveoli and enters the blood
• the amount of O2 in the blood increases
• CO2 exits the blood and enters the alveoli
• the amount of CO2 in the blood decreases
• CELLS of the body (internal respiration)
• O2 exits the blood and enters the cells
• the amount of O2 in the blood decreases
• CO2 exits the cells and enters the blood
• the amount of CO2 in the blood increases

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External Respiration

• Blood that is flowing towards the lungs is
• low in O2 (PO2 40 mmHg)
• high in CO2 (PCO2 46 mmHg)
• O2 diffuses from the alveoli into the blood
  because
• the PO2 in the alveolus is greater (104 mmHg)
  than the PO2 in the blood (40 mmHg)
• CO2 diffuses from the blood into the alveoli
  because
• the PCO2 in the blood is greater (46 mmHg) than
  the PCO2 in the alveolus (40 mmHg)
• Both gasses diffuse until they reach equilibrium
  with the partial pressures in the alveoli which
  DO NOT CHANGE
• After gas exchange at the lungs has been
  completed, the blood leaving the lungs has a PO2
  of 100 mm Hg and a PCO2 of 40 mm Hg

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Internal Respiration

• Blood that is delivered to all the cells of the
  body is
• high in O2 (100 mmHg)
• low in CO2 (40 mmHg)
• O2 diffuses from the blood into the interstitial
  fluid
• PO2 in the blood is greater (100 mmHg) than the
  PO2 in the interstitial fluid (40 mmHg)
• CO2 diffuses from the interstitial fluid to the
  blood
• PCO2 in the interstitial fluid is greater (46
  mmHg) than the PCO2 in the blood (40 mmHg)
• Both gasses diffuse until they reach equilibrium
  with the partial pressures in the cell which DO
  NOT CHANGE
• After gas exchange at the cells has been
  completed, the blood leaving the cells has a PO2
  of 40 mm Hg and a PCO2 of 46 mm Hg

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Hypoxia

• Hypoxia (too little oxygen) occurs when
• not enough O2 reaches alveoli
• low O2 in inspired air (at high altitude)
• inadequate alveolar ventilation (fibrosis,
  asthma, drug overdoses that depress the nervous
  system)
• decreased O2 exchange between alveoli and blood
• physical loss of alveolar surface area
  (emphysema)
• thickened alveoli slow O2 diffusion (fibrosis)
• excess fluid accumulation between alveolar air
  and capillary slow O2 diffusion
• within alveoli (pneumonia)
• between alveoli and capillary (pulmonary edema)
• inadequate O2 transport in blood
• reduction in the amount of the O2 carrying
  protein hemoglobin in blood

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Gas Transport in Blood

• The law of mass action plays an important role in
  how O2 and CO2 are transported
• Changes in O2 and CO2 concentrations in blood
  disturb the equilibrium of reactions, shifting
  the balance between reactants and products

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Oxygen Transport

• Since O2 is only slightly soluble in water,
  plasma (fluid portion of blood) cannot hold
  enough to meet the needs of the body
• carries 2 of the total O2 in blood
• The vast majority of O2 (98) is bound to the
  protein hemoglobin (Hb) found within RBCs
• in pulmonary capillaries when plasma PO2
  increases as O2 diffuses in from alveoli, Hb
  binds to O2
• Hb O2 ? HbO2
• at cells where O2 is being used and plasma PO2
  decreases, Hb gives up its O2
• Hb O2 ? HbO2
• Overall the binding of oxygen to hemoglobin is
  reversible and is expressed as Hb O2 ? HbO2

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Oxygen Transport vs. Oxygen Consumption

• At rest, respiring tissues of the body uses
  approximately 250 mL of O2 per minute
• plasma can only carry 15 mL of O2 per minute to
  these tissues
• at normal Hb levels, RBCs can carry 985 mL of O2
  per minute to these tissues
• The total O2 delivery rate of the blood at rest
  is 1000 mL of O2 per minute (4 times the demand
  of respiring tissues), but drops off only what
  the cells need (250 mL of O2 per minute)
• the remaining oxygen is regarded as a reservoir
  to be dropped off when the consumption of O2 in
  respiring cells increase such as during exercise

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Hemoglobin (Hb)

• Protein made of 4 polypeptide chains (subunits)
  each containing a heme group
• each heme group contains an atom of iron (Fe)
  (makes RBCs/blood red) in the center which is
  capable of binding to one molecule of O2
• A single molecule of hemoglobin can carry up to 4
  O2
• O2 is picked up (loaded) onto Hb at the lungs and
  is dropped off (unloaded) Hb at the cells of the
  body
• therefore, O2-Fe interaction is a weak bond that
  can be easily broken
• Hb O2 ? HbO2
• if O2 increases, then reaction shifts to the
  right
• if O2 decreases, then reaction shifts to the left
• In a body at rest only 1 molecule of O2 is
  unloaded at the cells and 1 molecule of O2 is
  loaded at the lungs per molecule of Hb

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Hemoglobin (Hb)

• Each RBC is filled with 280 million molecules of
  Hb
• can carry 1,120,000,000 molecules of O2
• There are 25,000,000,000,000 RBCs in circulation
• the blood can theoretically transport up to
  28,000,000,000,000,000,000,000 molecules of O2
• The number of O2 molecules that are bound to a
  single Hb is determined by 5 variables
• PO2 of the blood
• temperature of the blood
• H (pH) of the blood
• PCO2 of the blood
• 2,3-DPG in red blood cells
• carbohydrate intermediate of glycolysis
• used as an indicator of metabolic rate

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Influence of PO2 on Hemoglobin Saturation

• An oxygen-hemoglobin dissociation curve relates
  hemoglobin O2 saturation and blood PO2
• determines the amount of oxygen that is bound to
  hemoglobin at a particular PO2 in the blood
• Hb at the lungs (PO2 100) is bound to 4 O2
• Hb at respiring tissues (PO2 40) is bound to 3
  O2
• one molecule of O2 moves off of Hb and enters the
  cells of the respiring tissues
• If blood PO2 at the systemic tissues decreases
  below 40 mmHg then more O2 will move off of Hb
  and enter the cells of the respiring tissues
• occurs when tissues become more active or you
  hold your breath

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Other Factors Influencing Hemoglobin Saturation

• bood temperature, blood H, blood PCO2 and
  concentrations of 2,3-DPG in RBCs
• An increase in any of these factors will decrease
  the affinity of Hb for O2 at respiring tissues
• increase O2 drop-off at respiring tissues
• right shift of the O2 -Hb dissociation curve
• these are all increased in blood during exercise
• A decrease in any of these factors will increase
  the affinity of Hb for O2 at respiring tissues
• decrease O2 drop-off at respiring tissues
• left shift of the O2 -Hb dissociation curve
• The change in Hb affinity for O2 due to changes
  in blood pH is also known as the Bohr effect

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Carbon Dioxide Transport

• CO2 that diffuses out of a respiring cell is
  transported in the blood in 3 forms
• as bicarbonate ion (HCO3) in plasma (70)
• CO2 can be converted into bicarbonate ions and
  bicarbonate ions can be converted into CO2
  through the reversible chemical reaction
• CO2 H2O ? H2CO3 ? H HCO3-
• which obeys the laws of mass action
• as carbaminohemoglobin
• bound to amino acids (not heme) of Hb (23)
• as dissolved gas in plasma (7)
• CO2 is 20 times more soluble in plasma than O2
  therefore more can be carried by plasma

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Conversion of CO2 to HCO3

• CO2 H2O ? H2CO3 ? H HCO3-
• CO2 diffuses out of a respiring systemic tissue
  cell and enters a RBC, which increases the amount
  of CO2 in the RBC
• inside the RBC, carbonic anhydrase combines CO2
  and H2O forming carbonic acid (H2CO3)
• H2CO3 quickly dissociates into hydrogen ions (H)
  and bicarbonate ions (HCO3-) in the RBC
• creates a high HCO3- in the RBC
• creates a high H in the RBC

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Transport of CO2 as HCO3

• The high HCO3- in the RBC promotes the
  diffusion of HCO3- out of the RBC into blood
  plasma
• HCO3- is more soluble than CO2 therefore more can
  be carried
• the volume of plasma is greater than the
  collective volume of the cytosol of the RBCs and
  thus has a greater capacity to carry HCO3- (CO2)
• Cl- diffuses from the plasma into the RBC to
  electrically counterbalance the diffusion of
  HCO3- out of the RBC (chloride shift)
• HCO3- circulates back to the lungs in the plasma
• Hb (which just dropped off some of its O2) acts
  as a buffer by binding to the H produced in
  order to prevent a decrease in the pH of the RBC

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Conversion of HCO3 to CO2

• CO2 H2O ? H2CO3 ? H HCO3-
• As the blood flows through the pulmonary
  capillaries, CO2 diffuses out of the plasma and
  RBCs and enters the alveoli, which decreases the
  amount of CO2 in the RBC
• HCO3- diffuses from the plasma into the RBCs
  which increases the amount of HCO3- in the RBC
• Cl- diffuses out of the RBC (reverse chloride
  shift)
• In the RBC, H and HCO3- combines to form H2CO3
• H2CO3 is then converted by carbonic anhydrase to
  CO2 and H2O
• CO2 diffuses out of the RBC and into the alveoli

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