chapter
6
The Respiratory
System and Its
Regulation
External Respiration
Pulmonary ventilation involves inspiration and
expiration.
Pulmonary Diffusion is the exchange of oxygen and
carbon dioxide between the lungs and the blood.
RESPIRATORY SYSTEM
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 Diffusion
• Replenishes blood's oxygen supply that has
been depleted for oxidative energy production
• Removes carbon dioxide from returning venous
blood
• Occurs across the thin respiratory membrane
Laws of Gases
Dalton's Law: The total pressure of a mixture of
gases equals the sum of the partial pressures
of the individual gases in the mixture.
Henry's Law: Gases dissolve in liquids in
proportion to their partial pressures, depending
on their solubilities in the specific fluids and
depending on the temperature.
Partial Pressures of Air
• Standard atmospheric pressure (at sea level) is
760 mmHg.
• Nitrogen (N2) is 79.04% of air; the partial
pressure of nitrogen (PN2) = 600.7 mmHg
• Oxygen (O2) is 20.93% of air; PO2 = 159.1
mmHg.
• Carbon dioxide (CO2) is 0.03%; PCO2 = 0.2
mmHg.
Did You Know…?
The solubility of a gas in blood and the temperature of
blood are relatively constant. Differences in the partial
pressures of gases in the alveoli and in the blood create a
pressure gradient across the respiratory membrane. This
difference in pressures leads to diffusion of gases across
the respiratory membrane. The greater the pressure
gradient, the more rapidly oxygen diffuses across it.
Comparison of Pressure (mmHg) in the
Pulmonary and Systemic Circulations
Reprinted, by permission, from J. West, 2000, Respiratory physiology: The essentials (Baltimore, MD:
Lippincott, Williams, and Wilkins), 36.
Anatomy of the Respiratory
Membrane
Partial Pressures of Respiratory
Gases at Sea Level
Partial pressure (mmHg)
Gas
% in
dry air
Dry
air
Alveolar
air
Venous
blood
Diffusion
gradient
Total
100.00
760.0
760
760
0
H2O
0.00
0.0
47
47
0
20.93
159.1
104
40
64
0.03
0.2
40
45
5
79.04
600.7
569
573
0
O2
CO2
N2
Key Points
Pulmonary Diffusion
• Pulmonary diffusion is the process by which gases are
exchanged across the respiratory membrane in the
alveoli to the blood and vice versa.
• The amount 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.
(continued)
Key Points (continued)
Pulmonary Diffusion
• Oxygen diffusion capacity increases as you move from
rest to exercise.
• 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 easily.
Oxygen Transport
• Hemoglobin concentration largely determines
the oxygen-carrying capacity of blood.
• Increased H+ (acidity) and temperature of a
muscle allow more oxygen to be unloaded
there.
• Training affects oxygen transport in muscle.
Oxyhemoglobin Dissociation Curve
Carbon Dioxide Transport
• Dissolved in blood plasma (7% to 10%)
• As bicarbonate ions resulting from the
dissociation of carbonic acid (60% to 70%)
• Bound to hemoglobin
(carbaminohemoglobin) (20% to 33%)
Arterial–Venous Oxygen Difference
Did You Know…?
The increase in (a-v)O2 difference during
strenuous exercise reflects increased oxygen use
by muscle cells. This use increases oxygen
removal from arterial blood, resulting in a
decreased venous oxygen concentration.
Factors Affecting Oxygen Uptake
and Delivery
1. Oxygen content of blood
2. Amount of blood flow
3. Local conditions within the muscle
Key Points
External and Internal Respiration
• Oxygen is largely transported in the blood bound to
hemoglobin and in small amounts by dissolving in
blood plasma.
• Hemoglobin saturation decreases when PO2 or pH
decreases or if temperature increases. These factors
increase oxygen unloading in a tissue that needs it.
• Hemoglobin is usually 98% saturated with oxygen,
which is higher than what our bodies require, so the
blood's oxygen-carrying capacity seldom limits
performance.
(continued)
Key Points (continued)
External and Internal Respiration
• Carbon dioxide is transported in the blood as
bicarbonate ion, in blood plasma or bound to
hemoglobin.
• The (a-v)O2 difference—a difference in the oxygen
content of arterial and venous blood—reflects the
amount of oxygen taken up by the tissues.
• Carbon dioxide exchange at the tissues is similar to
oxygen exchange except that it leaves the muscles and
enters the blood to be transported to the lungs for
clearance.
Regulators of Pulmonary Ventilation
at Rest
•
•
•
•
•
•
Higher brain centers
Chemical changes within the body
Chemoreceptors
Muscle mechanoreceptors
Hypothalamic input
Conscious control
Pulmonary Ventilation
Ventilation (VE) is the product of tidal volume (TV)
and breathing frequency (f):
VE = TV x f