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chapter
6
The Respiratory
System and Its
Regulation
Learning Objectives
• Find out how the respiratory system brings oxygen to
muscles and tissues and rids the body of carbon
dioxide
• Learn the steps involved in respiration and gas
exchange
• Discover how your respiratory system regulates your
breathing and gas exchange
Anatomy of the Respiratory System
Transportation of Oxygen and
Carbon Dioxide
• Pulmonary ventilation (breathing): movement of air into
and out of the lungs
• Pulmonary diffusion: the exchange of O2 and CO2
between the lungs and the blood
• Transport of O2 and CO2 via the blood
• Capillary diffusion: the exchange of O2 and CO2
between the capillary blood and the metabolically
active tissue
Boyle’s Law
If temperature is constant:
Pressure1 x Volume1 = Pressure2 x Volume2
Pulmonary Ventilation
Inspiration: active process involving the diaphragm and
the external intercostal muscles
–Pressure in the lung is less than the air pressure
outside the body, air following the pressure gradient
coming into the lung
Expiration: usually a passive process involving
relaxation of the inspiratory muscles; pressure
increases in the lungs and air is forced out
–Active process during forced breathing
Process of Inspiration and Expiration
Lung Volumes Measured by
Spirometry
Reprinted, by permission, from J. West, 2000, Respiratory physiology: The essentials (Baltimore, MD: Lippincott,
Williams, and Wilkins), 14.
Pulmonary Ventilation
Key Points
• Pulmonary ventilation is the process by which air is
moved into and out of the lung (inspiration, expiration)
• Inspiration is an active process in which the diaphragm
and intercostal muscles contract, increasing
dimensions and volume of the thoracic cage
• Expiration at rest is normally passive; the inspiratory
muscles relax, decreasing the thoracic cage
• Forced inspiration and expiration are active processes
involving accessory muscles
• Lung volumes and capacities are measured by
spirometry
Pulmonary Diffusion
• Replenishes blood's oxygen supply that has been
depleted for oxidative energy production
• Removes carbon dioxide from returning venous blood
Blood Flow to the Lungs at Rest:
Pulmonary Hemodynamics
• Low pressure and resistance circulation compared to
the systemic circulation
• Lungs receive 4-6 L/min of blood flow
• Pulmonary artery mean pressure = 15 mmHg
(aortic pressure = 95 mmHg)
• Left atrial pressure = 5 mmHg
• Pulmonary vessels are thin walled
Pressures in the Pulmonary and
Systemic Circulations
Reprinted, by permission, from J. West, 2000, Respiratory physiology: The essentials (Baltimore, MD: Lippincott,
Williams, and Wilkins), 36.
Respiratory Membrane:
Gas Exchange
•
•
•
•
Alveolar wall
Capillary wall
Basement membranes (0.5-4.0 mm)
Gases will move along a concentration gradient based
on partial pressures
Anatomy of the Respiratory
Membrane
Partial Pressures of Air
Standard atmospheric pressure (at sea level) = 760 mmHg
Gas Percent of Air Partial Pressure
N2
79.04%
600.7 mmHg
O2
20.93%
159.1 mmHg
CO2
0.03%
0.2 mmHg
Partial Pressures and Gas Exchange
in the Lung and Tissues
Fick’s Law of Diffusion
Reprinted, by permission, from J. West, 2000, Respiratory physiology: The essentials (Baltimore, MD:
Lippincott, Williams, and Wilkins), 26.
Uneven Distribution of Blood Flow
in the Lung
Reprinted, by permission, from J. West, 2000, Respiratory physiology: The essentials (Baltimore, MD: Lippincott,
Williams, and Wilkins), 44.
Pulmonary Diffusion
Key Points
• Pulmonary diffusion is the process by which gases are
exchanged across the respiratory membrane in the
alveoli
• The amount and rate of gas exchange depends on the
partial pressure of each gas
• Gases diffuse along a pressure gradient, moving from
an area of higher pressure to lower pressure
.
• Vgas A / T x D x (P1 - P2)
(continued)
Pulmonary Diffusion (continued)
Key Points
• Oxygen diffusion rate increases as you move from rest
to exercise
• Exercising muscle requires more oxygen for
metabolism; when venous oxygen is depleted, oxygen
exchange at the alveoli is facilitated due to an
increased pressure gradient
• The pressure gradient for carbon dioxide exchange is
less than for oxygen exchange, but carbon dioxide’s
membrane solubility is 20 times greater than oxygen,
so CO2 crosses the membrane readily
Oxygen Transport
• Oxygen is transported bound to hemoglobin (>98%) or
dissolved in plasma (<2%)
• Hemoglobin concentration largely determines the
oxygen-carrying capacity of blood
• Increased H+ (acidity) and temperature of a muscle
favors oxygen unloading in the muscle
• Oxygen carrying capacity seldom limits performance in
healthy individuals
Oxyhemoglobin Dissociation Curve
Reprinted, by permission, from S.K. Powers and E.T. Howley, 2004, Exercise physiology: Theory and application to
fitness and performance, 5th ed. (New York: McGraw-Hill Companies), 205. With permission from The McGraw-Hill
Companies.
Effects of pH and Temperature on the
Oxyhemoglobin Dissociation Curve
Reprinted, by permission, from S.K. Powers and E.T. Howley, 2004, Exercise physiology: Theory and application to
fitness and performance, 5th ed. (New York: McGraw-Hill Companies), 206. With permission from The McGraw-Hill
Companies.
Carbon Dioxide Transport
• Bicarbonate ions
Muscle: CO2 + H2O → H2CO3 → H+ + HCO3Lung: H+ + HCO3- → H2CO3 → CO2 + H2O
• Dissolved in blood plasma
• Bound to hemoglobin (carbaminohemoglobin)
Gas Exchange
Key Points
•
•
O2 is transported in the blood primarily bound to
hemoglobin
Hemoglobin unloading of O2 in tissues is enhanced by:
1. Decreased PO2
2. Decreased pH
3. Increased temperature
•
•
Hemoglobin is ~98% saturated with oxygen, and O2
carrying capacity typically does not limit performance
CO2 is primarily transported as bicarbonate ion in the
blood
Gas Exchange at the Muscles:
Arterial–Venous Oxygen Difference
Oxygen Transport in the
Muscle: Myoglobin
Reprinted, by permission, from S.K. Powers and E.T. Howley, 2004, Exercise physiology: Theory and application to
fitness and performance, 5th ed. (New York: McGraw-Hill Companies), 207. With permission from The McGraw-Hill
Companies.
Factors Affecting Oxygen Uptake
and Delivery
1. Oxygen content of blood
2. Amount of blood flow
3. Local conditions within the muscle
Gas Exchange at the Muscle
Key Points
• (a-v)O
2 difference is the difference in the oxygen
content of the arterial and mixed venous blood
throughout the body
• O2 delivery to the tissues depends on the O2 content in
the blood, blood flow to the tissues, and local
conditions
• CO2 exchange at the tissues is similar to O2 exchange
except CO2 leaves the muscle to be transported to the
lungs
Regulation of Pulmonary Ventilation
• Higher brain centers
– Expiratory centers
– Inspiratory centers
• Chemoreceptors
• Mechanoreceptors in the active muscles and the lung
muscles
• Hypothalamic input
• Conscious control
Central and Peripheral Regulators
of Respiration