Download Chapter 7 Body Systems

Chapter 24
Physiology of the
Respiratory System
1
Respiratory Physiology
• complex processes that help maintain
homeostasis
• Respiratory function includes the following:
– External respiration
• Pulmonary ventilation (breathing)
• Pulmonary gas exchange
– Transport of gases by the blood
– Internal respiration
• Systemic tissue gas exchange
• Cellular respiration
– Regulation of respiration
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Pulmonary Ventilation (breathing)
•
Respiratory cycle
–
–
•
Inspiration— moves air into the lungs
Expiration— moves air out of the lungs
Mechanism of pulmonary ventilation
–
Pulmonary ventilation mechanism must establish
two gas pressure gradients:
1. pressure within alveoli of lungs is lower than atmospheric
pressure to produce inspiration
2. pressure in alveoli of lungs is higher than atmospheric
pressure to produce expiration
–
Pressure gradients are established by changes in
size of thoracic cavity
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Pulmonary Ventilation (breathing)
Inspiration— contraction of
diaphragm produces
inspiration— as it contracts, it
makes thoracic cavity larger
• Expansion of thorax results
in decreased intrapleural
pressure (Pip), leading to a
decreased alveolar
pressure (Pa)
• Air moves into lungs when
alveolar pressure (Pa)
drops below atmospheric
Intrapleural is the space surrounding
pressure (Pb)
the lungs
• Compliance— ability of
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pulmonary tissues to
Pulmonary Ventilation (breathing)
Expiration— a passive process that begins
when inspiratory muscles are relaxed,
decreasing size of thorax
• Decreasing thoracic volume increases intrapleural
pressure (Pip) and thus increases alveolar
pressure (Pa) above atmospheric pressure (Pb)
• Air moves out of lungs when Pa > Pb
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Pulmonary Volumes
• Pulmonary volumes— the amounts of air moved in and out and
remaining are important to the normal exchange of oxygen and
carbon dioxide
– FYI:
• Spirometer— instrument used to measure volume of air
(Figure 24-10)
• Tidal volume (TV)— amount of air exhaled after normal
inspiration
• Expiratory reserve volume (ERV)— largest volume of
additional air that can be forcibly exhaled (between 1.0 and
1.2 liters is normal ERV)
• Inspiratory reserve volume (IRV)— amount of air that can be
forcibly inhaled after normal inspiration (normal IRV is 3.3
liters)
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Pulmonary Volumes
• Residual volume (RV)— amount of air that
CANNOT be forcibly exhaled (1.2 liters)
– “wind knocked out of you” = your residual volume
(along with your expiratory reserve) is forced out
of your airways  alveoli collapse
• Between breaths, an exchange of O2 and
CO2 occurs between this trapped air in the
alveoli and the blood.
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Pulmonary Capacities
• Pulmonary capacities— the sum of two or more pulmonary
volumes
– Vital capacity— the sum of IRV + TV + ERV
• Represents the largest volume of air one can move in
and out of lungs
• A person’s vital capacity depends on many factors,
including the size of the thoracic cavity and posture
• Larger person has larger vital capacity
• Excess fluid in abdominal cavities
• Emphysema: alveolar walls stretched too much and
unable to recoil  increased RV (air trapped)
– Functional residual capacity— amount of air
at the end of a normal respiration
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Pulmonary Capacities
Alveolar ventilation— volume of inspired air that reaches the
alveoli
• Only this volume of air takes part in exchange of gases
– Anatomical dead space— “dead” air in passageways that do not
participate in gas exchange (nose, pharynx, larynx, trachea,
bronchi)
– Physiological dead space— anatomical dead space plus the
volume of any nonfunctioning alveoli (caused by disease)
– Alveoli must be properly ventilated for adequate gas exchange
• Chronic obstructive pulmonary disease (COPD): chronic
bronchitis and emphysema
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Pulmonary Gas Exchange
• Partial pressure of gases— pressure exerted
by a gas in a mixture of gases
– Dalton’s law of partial pressures— the partial pressure
of a gas in a mixture of gases is directly related to the
concentration of that gas in the mixture and to the total
pressure of the mixture
Arterial blood Po2 + Pco2 = Alveolar Po2 + Pco2
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Pulmonary Gas Exchange
•
Exchange of gases in the lungs takes place between
alveolar air and blood flowing through lung
capillaries
–
Four factors determine the amount of oxygen that
diffuses into blood:
1. The oxygen pressure gradient between
alveolar air and blood
2. The total functional surface area of the
respiratory membrane
3. The respiratory volume
4. Alveolar ventilation
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Pulmonary Gas
Exchange
Structural factors that facilitate
oxygen diffusion from alveolar
air to blood:
• Walls of the alveoli and
capillaries form only a
very thin barrier for
gases to cross
• Alveolar and capillary
surfaces are large
• Blood is distributed
through the capillaries in
a thin layer so each red
blood cell comes close
to alveolar air
Alveolar blood supply. rich blood
supply to alveoli (which have been
removed). The numerous, narrow
branches ensure that each red
blood cell is exposed to the
alveolar air.
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How Blood Transports Oxygen
– Hemoglobin
• made up of four polypeptide chains, each with an
iron-containing heme group
• Some CO2 can bind to amino acids in the chains
• O2 can bind to iron in the heme groups
– Oxygenated blood contains about 0.3 ml of dissolved O2
per 100 ml of blood
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How Blood Transports CO2
– A small amount of CO2 dissolves in plasma
and is transported as a solute (10%)
– < 1/4 of blood CO2 combines with amino
groups of hemoglobin (20%)
• CO2 association with hemoglobin is accelerated
by an increase in blood Pco2
– > 2/3 of blood CO2 is carried in plasma as
bicarbonate ions (70%)
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Systemic Gas Exchange
• Exchange of gases takes place
between capillaries and cells
– Oxygen diffuses out of arterial
blood because the oxygen
pressure gradient favors its
outward diffusion
– As oxygen diffuses out of
blood, blood Po2 decreases,
which accelerates
oxyhemoglobin dissociation to
release more oxygen to
plasma for diffusion to cells
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•
Oxygen-hemoglobin dissociation curve. The graph represents the
relationship between PO2 and O2 saturation of hemoglobin (Hb-O2 affinity).
The inset shows how the graphed curve relates to oxygen transport by the
blood. Notice that at high plasma PO2 values (point A), hemoglobin (Hb) is
fully loaded with oxygen. At low plasma PO2 values (point B), Hb is only 19
partially loaded with oxygen.
Systemic Gas Exchange
• Carbon dioxide exchange
between tissues and blood takes
place in the opposite direction
from oxygen exchange
– Bohr effect— increased Pco2
decreases the affinity between
oxygen and hemoglobin
– Haldane effect— increased carbon
dioxide loading caused by a
decrease in Po2
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•
The increased PCO2 in systemic tissues decreases the affinity between Hb
and O2, shown as a right shift of the oxygen-hemoglobin dissociation curve.
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This phenomenon is known as the Bohr effect. A right shift can also be
caused by a decrease in plasma pH.
•
At the same time, the decreased PO2 commonly observed in systemic tissues
increases the CO2 content of the blood, shown as a left shift of the CO2 dissociation
curve. This phenomenon is known as the Haldane effect.
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Regulation of Pulmonary Function
• Respiratory control centers— the main integrators
that control the nerves that affect inspiratory and
expiratory muscles are located in the brainstem
– Medullary rhythmicity center— generates the basic
rhythm of respiratory cycle
– Basic breathing rhythm can be altered by different
inputs to medullary rhythmicity center
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Regulation of Pulmonary Function
• Factors that influence breathing— sensors from the nervous
system provide feedback to medullary rhythmicity center
– Changes in the Po2, Pco2 and pH of arterial blood influence
medullary rhythmicity area
• Pco2 acts on central chemoreceptors in medulla
– if it increases, result is faster breathing
– if it decreases, result is slower breathing
– Arterial blood pressure controls breathing through
respiratory pressoreflex mechanism
– Hering-Breuer reflexes help control respirations by
regulating depth of respirations and volume of tidal air
– Cerebral cortex influences breathing by increasing or
decreasing rate and strength of respirations
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Regulation of Pulmonary Function
• Ventilation and perfusion
– Alveolar ventilation— air flow to the alveoli
– Alveolar perfusion— blood flow to the alveoli
– Efficiency of gas exchange can be maintained
by limited ability to match perfusion to
ventilation— for example, vasoconstricting
arterioles that supply poorly ventilated alveoli
and allow full blood flow to well-ventilated
alveoli
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The Big Picture
• The internal system must continually get new oxygen and
rid itself of carbon dioxide because each cell requires
oxygen and produces carbon dioxide as a result of cell
respiration
• Specific mechanisms involved in respiratory function:
– Blood gases need blood and the cardiovascular system to be
transported between gas exchange tissues of lungs and various
systemic tissues of body
– Regulation by the nervous system adjusts ventilation to compensate for
changes in oxygen or carbon dioxide levels in internal environment
– Skeletal muscles of the thorax aid airways in maintaining flow of fresh air
– Skeleton houses the lungs, and the arrangement of bones facilitates the
expansion and recoil of the thorax
– Immune system prevents pathogens from colonizing the respiratory26
tract
and causing infection