Respiratory Physiology I

1
Respiratory Physiology I

• Ankit Patel

2
Function

• Exchange oxygen and carbon dioxide between
  environment and cells of the body
• Oxygen and carbon dioxide are exchanged between
  inspired air and pulmonary capillary blood

3
Structure

• Includes the lungs and series of airways that
  connect lungs to external environment
• Conducting zone (or conducting airways)
• Brings air into and out of lungs
• Respiratory zone
• Lined with alveoli allowing for gas exchange

4
Structure
5
Structure
6
Conducting Zone

• Includes the nose, nasopharynx, larynx, trachea,
  bronchi, bronchioles, and terminal bronchioles
• Bring air into and out of respiratory zone
• Warm, humidify, and filter the air before it
  reaches gas exchange region
• Trachea ? 2 bronchi ? 2 smaller bronchi ?
  bronchioloes
• Lined with mucus-secreting and ciliated cells
  that function to remove inhaled particles

7
Trachea
8
Conducting Zone

• Walls contain smooth muscle
• Autonomics nervous system affect airway diameter
• Sympathetic neurons ? ß2 receptors ? relaxation
  and dilation of airways
• Epinephrine (from adrenal gland) also activates
  ß2 receptors
• Parasympathetic neurons ? muscarinic receptors
  ?contraction and constriction of airways
• ß2-adrenergic agonists (e.g., epinephrine,
  isoproterenol, and albuterol) used to dilate
  airways in treatment of asthma

9
Respiratory Zone

• Includes respiratory bronchioles, alveolar ducts,
  and alveolar sacs (lined with alveoli)
• Alveoli are pouchlike evaginations of the walls
• Each lung has 300 million alveoli
• Exchange of gases occurs rapidly and efficiently
  across alveoli because walls are thin and have
  large surface area for diffusion

10
Alveoli
11
Pulmonary Blood Flow

• Cardiac output of the right heart
• Right Ventricle ? Pulmonary Artery ? Arterioles ?
  Pulmonary Capillaries (form networks around
  alveoli) ? Venules ? Pulmonary Vein ? Left Atrium
• Regulation of pulmonary blood flow is
  accomplished by altering resistance of pulmonary
  arterioles
• Controlled by local factors (mainly O2)

12
Pulmonary Blood Flow
13
Lung Volumes

• Measured by a spirometer
• Tidal Volume (VT)
• Volume of air that fills both alveoli and airways
  with a normal breath (500 mL)
• Inspiratory Reserve Volume
• Additional volume inspired above tidal volume
  (3000 mL)
• Expiratory Reserve Volume
• Additional volume expired below tidal volume
  (1200 mL)
• Residual Volume (RV)
• Volume remaining in lungs after maximal forced
  expiration (1200 mL)

14
Lung Volumes
15
Lung Capacities

• Inspiratory Capacity (IC)
• Tidal Volume Inspiratory Reserve Volume (3500
  mL)
• Functional Residual Capacity (FRC)
• Expiratory Reserve Volume Residual Volume
  (2400 ml)
• Volume remaining in lungs after a normal tidal
  volume is expired (equilibrium volume)
• Vital Capacity (VC)
• Inspiratory Capacity Expiratory Reserve Volume
  (4700 mL)
• Volume that can be expired after maximal
  inspiration
• Total Lung Capacity (TLC)
• Vital Capacity Residual Volume (5900 ml)

16
Volumes Capacities
17
Dead Space

• Volume of airways and lungs that does not
  participate in gas exchange
• Anatomic Dead Space (150 ml)
• Volume of conducting airways because NO alveoli
• TV of 500 ml is inspired, but 500 ml does not
  reach alveoli for gas exchange because portion
  fills conducting airways
• Physiologic Dead Space
• Anatomic dead space functional dead space in
  alveoli
• Functional dead space refers to alveoli that do
  not participate in gas exchange because of
  mismatch in ventilation and perfusion
• Normally, Anatomic Physiologic Dead Space

18
Ventilation Rate

• Volume of air moved into and out of lungs per
  unit time
• Minute ventilation total rate of air movement
  into and out of lungs
• Alveolar ventilation corrects for physiologic
  dead space

19
Forced Expiratory Volumes

• Vital Capacity volume expired following maximal
  inspiration
• Forced Vital Capacity (FVC) volume forcibly
  expired after maximal inspiration
• FEV1 Volume forcibly expired in the 1st second
• FEV2 Cumulative volume expired in 2 seconds
• FEV3 Cumulative volume expired in 3 seconds
• FVC and FEV1 are useful indices of lung disease
• FEV1/FVC can be used to differentiate among
  diseases

20
FEV Lung Disease

• Normal Person
• FEV1/FVC is 0.8 (80 of vital capacity can be
  forcibly expired in 1st second)
• Obstructive Lung Disease (Asthma)
• FVC and FEV1 are decreased, but FEV1 is decreased
  more than FVC
• FEV1/FVC is decreased
• Restrictive Lung Disease (Fibrosis)
• FVC and FEV1 are decreased, but FEV1 is decreased
  less than FVC
• FEV1/FVC is actually increased

21
FEV Lung Disease
22
Mechanics of Breathing

• Inspiration
• Diaphragm contracts ? abdominal contents pushed
  downward and ribs are lifted upward and outward
• Produces ? intrathoracic volume ? ? intrathoracic
  pressure and initiates flow of air into the lungs
• Expiration
• Normally a passive process as diaphragm relaxes
• Air is driven out of lungs by reverse pressure
  gradient between lungs and the atmosphere
• Abdominal muscles can assist during exercise or
  disease states

23
Mechanics of Breathing
24
Mechanics of Breathing
25
Compliance Pressure

• Distensibility of a system
• Measure of how volume changes as a result of a
  pressure change
• Compliance of lungs and chest wall is inversely
  correlated with elastance
• Transmural pressure is pressure across a
  structure
• Transpulmonary pressure is difference between
  intra-alveolar pressure and intrapleural pressure
• Lung pressures always referred to atmospheric
  pressure, which is called "zero"

26
Compliance of Lungs

• Sequence of inflation followed by deflation
  produces a pressure-volume loop
• Slope of pressure-volume loop is compliance of
  lung
• Pressure outside lung made more negative ? lung
  inflates and volume increases until alveoli are
  filled and become stiffer and less compliant
• Pressure outside lungs less negative ? lung
  volume to decrease during expiration

27
Compliance of Lung
28
Compliance of Chest Wall

• Negative Intrapleural Pressure created by 2
  opposing elastic forces
• Lungs tend to collapse
• Chest Wall tends to spring out
• Therefore, negative intrapleural pressure
  prevents lungs from collapsing and chest wall
  from springing out
• Pneumothorax (introducing air into intrapleural
  space) causes intrapleural pressure to become
  equal to atmospheric pressure (intrapleural
  pressure 0)
• No longer negative intrapleural pressure so
• Lungs collapse and chest wall springs out

29
Compliance of Chest Wall
30
Changes in Compliance

• Emphysema (increased lung compliance)
• Associated with loss of elastic fibers in lungs
• At FRC, tendency of lungs to collapse is less
  than tendency of chest wall to expand
• To reestablish balance, volume must be added to
  the lungs to increase their collapsing force
• Thus, combined lung and chest-wall system seeks
  new higher FRC
• Fibrosis (decreased lung compliance)
• Associated with stiffening of lung tissues
• At FRC, tendency of lungs to collapse is greater
  than tendency of chest wall to expand
• To reestablish balance, the lung and chest-wall
  system will seek a new lower FRC

31
Surface Tension of Alveoli

• Alveoli are lined with film of fluid
• Surface Tension
• Attractive forces between adjacent molecules of
  liquid are stronger than attractive forces
  between molecules of liquid and molecules of gas
• As molecules of liquid are drawn together by
  attractive forces, surface area becomes smaller
• Surface tension generates a pressure that tends
  to collapse alveolus
• P 2T/r

32
Surface Tension of Alveoli

• P 2T/r
• Large alveolus has low collapsing pressure
• Small alveolus has high collapsing pressure
• However, alveoli need to be small to increase
  total surface area for gas exchange
• Fundamental conflict is solved by Surfactant

33
Surfactant

• Mixture of phospholipids that line alveoli and
  reduce their surface tension
• Reduces collapsing pressure for a given radius
• Intermolecular forces between surfactant
  molecules break up the attracting forces between
  liquid molecules lining the alveoli
• Surfactant also increases lung compliance
• Reduces work of expanding the lungs during
  inspiration

34
Surfactant
35
Case 7 Chief Complaint

• Within an hour of being born, a 2 week premature
  baby starts breathing very rapidly and grunting
• Skin appears cyanotic

36
Case 7 Test Results

• Pulse 160 (normal 140) and Respiratory Rate 70
  (normal 50)
• Arterial CO2 is elevated and arterial O2 is
  depressed
• pH is low

37
Case 7 Diagnosis

• Baby has Neonatal Respiratory Distress Syndrome
• Lungs lack surfactant
• Premature infants are less likely to have
  surfactant present
• Small alveoli have ? surface tension and ?
  pressures causing them to collapse (atelectasis)
• Collapsed alveoli are not ventilated and cannot
  participate in gas exchange
• Consequently, hypoxemia (low oxygen within blood)
  and respiratory acidosis develop (elevated CO2,
  decreased pH)
• ? lung compliance ? ? work of inflating lungs
  during breathing

38
Case 7 Treatment

• Baby is placed under continuous positive airway
  pressure (splint airways open) and surfactant is
  administered
• Respiratory rate returns to normal as does
  arterial blood gas values
• 1 week later baby is discharged home with family

39
Airflow, Pressure, Resistance

• Q ?P/R
• Q airflow (ml/min or L/min)
• ?P pressure gradient (mm Hg or cm H2O)
• R airway resistance (cm H2O/L/sec)
• Between breaths, alveolar pressure atmospheric
  pressure
• No pressure gradient, no driving force, and no
  airflow
• During inspiration, alveolar pressure lt
  atmospheric pressure (b/c increase lung volume)
• Pressure gradient drives airflow into lungs

40
Airway Resistance

• R 8 ? l/p r4
• R resistance
• ? viscosity of inspired air
• l length of the airway
• r radius of the airway
• Medium-sized bronchi are sites of highest airway
  resistance

41
Changes in Airway Resistance

• Change airway diameter to alter resistance and
  airflow
• Smooth muscle in walls of conducting airways is
  innervated by autonomic nerve fibers
• Parasympathetic stimulation ? constriction of
  bronchial smooth muscle ? ? airway diameter ? ?
  resistance to airflow
• Sympathetic stimulation ? relaxation of bronchial
  smooth muscle via ß2 receptors ? ? airway
  diameter ? ? resistance to airflow
• ß2 agonists such as epinephrine used for treating
  asthma

42
Breathing Cycle

• 3 Phases
• rest, inspiration, and expiration
• Transmural pressure airway or alveolar pressure
  - intrapleural pressure
• transmural pressure is an expanding pressure on
  the lung
• - transmural pressure is a collapsing pressure on
  the lung

43
Breathing Cycle Rest

• Alveolar pressure atmospheric pressure 0
• No airflow because no pressure difference
• Intrapleural pressure is negative
• Opposing forces of lungs trying to collapse and
  chest wall trying to expand create a negative
  pressure
• Transmural pressure across the lungs and airways
  at rest is 5 cm H2O which means these structures
  will be open
• Volume present in lungs equilibrium volume FRC

44
Breathing Cycle Inspiration

• Diaphragm contracts ? ? thorax volume ? ? lung
  pressure
• Airway and alveolar pressure become negative
• Pressure gradient drives airflow into lungs
• Intrapleural pressure becomes even more negative
• ? lung volume ? ? elastic recoil of lungs ? pull
  more forcefully against intrapleural space
• Airway and alveolar pressures become negative as
  ? thorax volume

45
Breathing Cycle Expiration

• Expiration is passive process
• Alveolar pressure becomes positive
• Elastic forces of lungs compress the greater
  volume of air in the alveoli
• Alveolar pressure gt atmospheric pressure so air
  flows out of lungs and volume returns to FRC
• Intrapleural pressure returns to resting value of
  5 cm H2O
• As ? lung volume, ? elastic recoil of lungs

46
Forced Expiration Normal

• Expiratory muscles make lung and airway pressures
  very positive
• Expiratory muscles raise intrapleural pressure
• Will lungs/airways collapse under positive
  intrapleural pressure?
• No, because transmural pressure remains positive
• Expiration is rapid and forceful because pressure
  gradient between alveoli and atmosphere is much
  greater than normal

47
Forced Expiration COPD

• ? Lung Compliance because ? elastic fibers
• ? Intrapleural pressure to same value as in
  normal
• ? Alveolar pressure and airway pressure because
  lungs have ? elastic recoil
• Negative transmural pressure across large airways
  
• Airways collapse ? ? resistance to airflow and
  expiration is difficult
• Persons with COPD expire slowly with pursed lips
  ? ? airway pressure ? prevents negative
  transmural pressure ? prevents collapse