Transcript Physiology of the Respiratory System

Physiology of the Respiratory
System
Pulmonary Ventilation

Breathing, 2 phases
Inspiration: air moves into the lungs
 Expiration: air moves out of the lungs

Gas moves down a pressure gradient
 Air in the atmosphere exerts pressure
of 760 mm Hg

Inspiration
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Diaphragm contracts, it flattens,
which makes thoracic cavity
longer
Intercostals muscles contract,
elevated sternum & ribs, which
enlarges thoracic cavity
Lungs pulled out because of
cohesion of the pleura
Air pressure in alveoli & tubes
decrease & air moves into
lungs
Elastic recoil

Tendency of the thorax & lungs to
return to their preinspiration volume
Expiration

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Inspiratory muscles
relax, decreasing size
of thorax
Alveolar pressure
increases thus
positive pressure
gradient from alveoli
to atmosphere &
expiration occurs
Pulmonary Volumes
Tidal volume= volume of air exhaled after a
typical inspiration; normal TV=500 ml
 Expiratory reserve volume= largest additional
volume that can be forcibly expired after
expiring tidal air; normal ERV=1000-1200 ml
 Inspiratory reserve volume= amount of air
that can be forcibly inspired over and above
normal inspiration; normal IRV=3300 ml
 Residual volume= air that can not be forcibly
expired but is trapped in alveoli, RV=1200 ml
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Vital capacity
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Largest volume of
air that an
individual can
move in and out of
the lungs
VC=IRV=TV=ERV
Alveolar Ventilation

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Volume of inspired air
that actually reaches
the alveoli
Part of air inspired fills
our air passageways,
this is the anatomical
dead space
Anatomical dead space
is approximately 30% of
TV, thus alveolar
ventilation is 70 % of
TV
Pulmonary Gas Exchange

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A gas diffuse “down” its
pressure gradient
Concentration of O2 in air is
about 21% thus the partial
pressure of O2 is about 160
mmHg

21% x 760 mm Hg = 160 mm
Hg
Amount of Oxygen that diffuses
into blood depends on:
Oxygen pressure gradient
 Total functional surface area of
alveolus
 Respiratory minute volume
 Alveolar ventilation
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Hemoglobin
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4 polypeptide chains
(2 alpha & 2 beta)
each with an iron
containing heme
molecule
Oxygen can bind to
iron in heme group
CO2 can bind to
amino acids in chain
Transport of Oxygen

Oxygen travels in two
forms in blood:
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Dissolved in plasma
Associated with
hemoglobin as
oxyhemoglobin (most)
Increasing PO2 in
blood accelerates
hemoglobin
association with O2
Transport of Carbon Dioxide
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Dissolved carbon dioxide
(10%)
Bound to amine (NH2)
groups of amino acids to
form
carbaminohemoglobin
(20%)
In the form of bicarbonate
ions (more than 2/3)

CO2 + H20 H2CO3 
H + HCO3

Catalyzed by carbonic
anhydrase
Carbon Dioxide and pH

Increasing carbon dioxide content of blood
increases H ion concentration thus increases
the acidity and decrease the pH
Respiratory Control Centers

Main integrators that control
nerves that affect inspiratory &
expiratory muscles are located
in brainstem
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Medullary rhythmicity center
generates basic rhythm of
respiratory cycle
Can be altered by input inputs from:
 Apneustic center in pons
stimulates to increase length and
depth of respiration
 Pneumotaxic center in pons
inhibits apneustic center to
prevent overinflation of the lungs
Factors that influence breathing
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PCO2 acts on chemoreceptors in medulla:
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Increasing PCO2 increases RR
Decreasing PCO2 decreases RR
Decrease in blood pH stimulates
chemoreceptors in carotid & aortic bodies
Arterial blood PO2 has little influence if it
stays above a certain level
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Decrease in PO2 below 70 mmHg increases RR
Arterial blood pressure & breathing
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Sudden rise in blood pressure results in
reflex slowing of respirations
Hering-Breuer reflexes
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Help control respiratory depth &
volume of tidal air
Miscellaneous factors
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Sudden painful stimulations produces reflex
apnea (no respirations) but continued painful
stimulus cause faster & deeper respirations
Sudden cold stimuli on skin causes reflex
apnea
Stimulation of pharynx or larynx by irritating
chemicals or touch causes temporary apneachoking reflex