Download Mechanics of Ventilation (cont`d)

Chapter 6: Respiratory System
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Structure and Function of the Respiratory
System
• Structures
–
Nose/nostrils
–
Nasal cavity
–
Pharynx
–
Larynx
–
Trachea
–
Bronchi
–
Bronchioles
–
Alveoli
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Structure and Function of the Respiratory
System (cont’d)
• Anatomical structures of respiratory system.
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Structure and Function of the Respiratory
System (cont’d)
• Functions
–
Conducts air into & out of lungs
–
Exchanges gases between air & blood
–
Humidifies air: prevents damage to membranes due to drying
out
–
Warms air: helps maintain body temperature
–
Filters air
• Mucus traps airborne particles
• Cilia move mucus toward oral cavity to be expelled
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Structure and Function of the Respiratory
System (cont’d)
• Alveoli
–
Saclike structures surrounded by capillaries in lungs
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Attached to respiratory bronchioles
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Site of exchange of oxygen & carbon dioxide
–
300 million in lungs
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Provide tremendous surface area where diffusion can take place
–
Respiratory membrane: 2 cell membranes that aid diffusion
• Membrane of alveolar cells
• Membrane of cells of capillary wall
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Structure and Function of the Respiratory
System (cont’d)
• Respiratory membrane.
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Mechanics of Ventilation
• Pleural Sac
–
Double-layered membrane that encases each lung
• Visceral (pulmonary) pleura: outer surface of lungs
• Parietal pleura: inner surface of thoracic cavity & diaphragm
• Pleural fluid: lubricating fluid between 2 membranes
• Intrapleural pressure: pressure in pleural cavity between 2
membranes; less than atmospheric pressure
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Mechanics of Ventilation (cont’d)
• Pressure Changes During Ventilation
–
Increase in volume of intrathoracic cavity:
• Increases lung volume
• Decreases intrapulmonic pressure
• Causes air to rush into lungs (inspiration)
–
Decrease in volume of intrathoracic cavity:
• Decreases lung volume
• Increases intrapulmonic pressure
• Causes air to rush out of lungs (expiration)
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Mechanics of Ventilation (cont’d)
• Inspiration
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Inspiratory muscles increase intrathoracic cavity volume
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Diaphragm: most important inspiratory muscle
• Flattens as it contracts
• Puts in motion pressure changes that cause inspiration
• Contraction moves abdominal contents forward & downward
–
Muscles that elevate ribs: external intercostals, scalenes,
sternocleidomastoid, pectoralis minor
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Mechanics of Ventilation (cont’d)
• Expiration
–
No muscular effort needed at rest
–
Passive recoil of diaphragm & other muscles decreases
intrathoracic cavity volume
–
During exercise or voluntary forced expiration, accessory
muscles of expiration contract, pulling ribs downward:
• Internal intercostals
• Rectus abdominis
• Internal oblique muscles of abdominal wall
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Mechanics of Ventilation (cont’d)
• Inspiration and expiration.
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Mechanics of Ventilation (cont’d)
• Muscles involved in inspiration & expiration.
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Mechanics of Ventilation (cont’d)
• Airflow Resistance
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Airflow = P1 − P2/Resistance
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Where P1 − P2 is pressure difference between 2 areas &
Resistance is resistance to airflow between 2 areas
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Thus, airflow can be increased by:
• Amplifying pressure difference between 2 areas
• Decreasing resistance to airflow
–
Diameter of airway is biggest factor affecting airflow at rest
–
In exercise, bronchodilation decreases resistance to airflow
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Mechanics of Ventilation (cont’d)
• Pulmonary Ventilation
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Amount of air moved in & out of lungs in given time period
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Tidal volume: amount of air moved per breath
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Volume of air moved per minute can be calculated as:
• VE = VT × f
• Where VE = volume of air expired per minute; VT = tidal
volume; f = breathing frequency per minute
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Greater in trained athletes
–
Pulmonary ventilation = anatomical dead space + alveolar
ventilation
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Mechanics of Ventilation (cont’d)
• Lung Capacities and Volumes
–
Determined using spirometry equipment
–
Reserve of tidal volume at rest allows increase in tidal volume
during maximal exercise
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Residual volume: air left in lungs after max. exhalation
• Frequency and Depth of Breathing
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Increase in depth of breathing occurs first after onset of exercise
–
If increase in depth not sufficient, rate of breathing will increase
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Mechanics of Ventilation (cont’d)
• Lung volumes and capacities.
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Diffusion at the Lungs
• Factors Promoting Diffusion
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Large surface area of alveoli
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Thinness of respiratory membrane (2 cells thick)
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Pressure differences of oxygen & carbon dioxide between air in
alveoli & blood
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Partial pressure: portion of pressure due to a particular gas in
a mixture of gases
–
Dalton’s law: total pressure of gas mixture = sum of partial
pressures of each gas
–
Henry’s law: amount of gas dissolved in any fluid depends on
temperature, partial pressure of gas, & solubility of gas
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Diffusion at the Lungs (cont’d)
• Oxygen Diffusion
–
Partial pressure of oxygen (PO2) must be > in alveoli than in
blood & > in blood than in tissue
• PO2 at sea level = 159.1 mm Hg
• PO2 in alveoli = 105 mm Hg
• PO2 in arterial blood entering lungs = 40 mm Hg
• PO2 in blood leaving lungs = 100 mm Hg
• PO2 in tissues = 40 mm Hg
–
Thus, differences between PO2 in alveoli & blood (65 mm Hg)
and between blood & tissue (60 mm Hg) provide driving force for
diffusion of oxygen
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Diffusion at the Lungs (cont’d)
• Capillary gas exchange at lungs & tissue.
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Diffusion at the Lungs (cont’d)
• Carbon Dioxide Diffusion
–
Partial pressure of carbon dioxide (PCO2) must be > in blood
than in alveoli & > in tissue than in blood
• PCO2 in atmospheric air = 0.2 mm Hg
• PCO2 in alveoli = 40 mm Hg
• PCO2 in arterial blood entering lungs = 46 mm Hg
• PCO2 in blood leaving lungs = 40 mm Hg
• PCO2 in tissues = 46 mm Hg
–
Thus, differences between PCO2 in alveoli & blood (6 mm Hg)
and between blood & tissue (6 mm Hg) provide driving force for
diffusion of carbon dioxide
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Diffusion at the Lungs (cont’d)
• Lung Blood Flow
–
Determines velocity at which blood passes through pulmonary
capillaries
–
Increased blood flow during exercise results in increased gas
diffusion
–
Blood pressure in pulmonary circulation is low compared with
systemic
–
Equilibration of oxygen between alveoli air & lung capillary blood
takes 0.25 seconds
–
As blood flow increases with exercise, less time is available for
this equilibration
–
However, increased capillary blood volume slows blood flow
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Blood Gas Transport
• Oxygen Transport
–
Only 9 to 15 mL of oxygen can be dissolved in plasma, which is
insufficient to meet needs of body
–
RBCs containing hemoglobin transport 98% of oxygen
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Oxyhemoglobin: oxygen bound to hemoglobin
–
Deoxyhemoglobin: hemoglobin not bound to oxygen
–
Concentration of hemoglobin determines amount of oxygen that
can be transported
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Blood Gas Transport (cont’d)
• Oxyhemoglobin disassociation curve.
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Blood Gas Transport (cont’d)
• Oxyhemoglobin Disassociation Curve
–
Temperature effect
• Increase in temp.
• Shifts curve to right
• Decreases affinity of hemoglobin for oxygen
• Decrease in temp.
• Shifts curve to left
• Increases affinity of hemoglobin for oxygen
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Blood Gas Transport (cont’d)
• Effect of temperature & acidity on hemoglobin disassociation
curve.
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Blood Gas Transport (cont’d)
• Oxyhemoglobin Disassociation Curve (cont’d)
–
pH effect (Bohr effect)
• Increase in acidity
• Shifts curve to right
• Decreases affinity of hemoglobin for oxygen
• Decrease in acidity
• Shifts curve to left
• Increases affinity of hemoglobin for oxygen
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Blood Gas Transport (cont’d)
• Oxyhemoglobin Disassociation Curve (cont’d)
–
2,3-Diphosphoglycerate (2,3 DPG) effect
• Increase in 2,3 DPG
• Shifts curve to right
• Decreases affinity of hemoglobin for oxygen
• Decrease in 2,3 DPG
• Shifts curve to left
• Increases affinity of hemoglobin for oxygen
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Blood Gas Transport (cont’d)
• Carbon Dioxide Transport
–
3 methods
• 7% to 10% is dissolved in plasma
• 20% is bound to hemoglobin
• 70% is transported as bicarbonate
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Blood Gas Transport (cont’d)
• Ability of hemoglobin to bind oxygen & carbon dioxide.
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Gas Exchange at the Muscle
• Occurs due to partial pressure differences between
oxygen & carbon dioxide between tissue & blood
• Myoglobin
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Oxygen transport molecule similar to hemoglobin
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Found in skeletal & cardiac muscle
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Reversibly binds with oxygen
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Assists in passive diffusion of oxygen from cell membrane to
mitochondria
–
Functions as oxygen reserve at start of exercise
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Control of Ventilation
• Respiratory Control Center
–
Portion of medulla oblongata & pons
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Serves as pacemaker, generating a rhythmical breathing pattern
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Rate & depth of breathing can be modified by:
• Higher brain centers
• Chemoreceptors in medulla
• Other peripheral inputs
–
Pulmonary ventilation is generally involuntary, but can changed
voluntarily
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Control of Ventilation (cont’d)
• The respiratory control center in the medulla.
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Control of Ventilation (cont’d)
• Central Chemoreceptors
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Located in medulla, separate from respiratory control center
–
Respond to changes within CSF, esp. in H+ concentration or pH
• Peripheral Chemoreceptors
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Located in carotid arteries & aortic arch
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Respond to changes in blood PCO2 & H+ concentration
• Other Neural Input
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Stretch receptors in lungs & respiratory muscles
–
Proprioceptors & chemoreceptors in skeletal muscle & joints
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Effects of Exercise on Pulmonary
Ventilation
• Three phases of changes in pulmonary ventilation.
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Ventilation Is Associated With Metabolism
• Ventilatory Equivalents
–
Amount of air ventilated needed to obtain 1 L of oxygen or
expire 1 L of carbon dioxide
–
Ventilatory equivalent of oxygen: ratio of pulmonary
ventilation (VE) to oxygen (VO2): VE/VO2
–
Ventilatory equivalent of carbon dioxide: ratio of pulmonary
ventilation (VE) to carbon dioxide (VCO2): VE/VCO2
• Ventilatory Threshold (VT)
–
Technique using ventilatory equivalents to estimate lactate
threshold
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Ventilation is Associated With Metabolism
(cont’d)
• VT and RCP.
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Ventilation Is Associated With Metabolism
(cont’d)
• Respiratory Compensation Point (RCP)
–
The work intensity at which both VE/VO2 & VE/VCO2 increase
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Characterized by a decrease in end-trial partial pressure of O2
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Indicates end of control of VE by PCO2
–
VT & RCP can be used to create 3 training zones of exercise
intensity, based on heart rate:
• Light-intensity: <VT
• Moderate-intensity: between VT & RCP
• High-intensity: >RCP
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