BIOL 1412 Lecture Notes - Lecture 13: Thoracic Cavity, External Intercostal Muscles, Thoracic Wall
1 May 2018
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TOPIC XIII: RESPIRATORY PHYSIOLOGY
A) Overview
3 processes involved in exchange of air:
1) Pulmonary ventilation
a) inspiration
b) expiration
2) External Respiration taking the oxygen from the bloodstream into the lungs
3) Internal Respiration gas exchange the the capillaries?
B) Air Exchange
1) Pulmonary Ventilation
result of pressure gradients caused by changes in thoracic cavity volume
o Boyle’s La think of a container; the more that you are squeezing the
container, the less room that there is so the volume will increase
gas volume is inversely proportional to pressure
as ֛ vol, ֝ P (+ vice versa) - for the same # of molecules of air (gas
amount remains constant)
pressures involved:
a) Atmospheric pressure (Patm) = 760 mmHg (sea level)
b) Intrapulmonary Pressure (Ppul) = air pressure inside lungs (between breaths
is = Patm) fig. 22.9
c) Intrapleural Pressure (Pip) = fluid pressure in pleural cavity fig. 22.11
always < Ppul always less than the pulmonary pressure
usually < Patm = ~ 4 mmHg less = 756 mmHg (at rest) usually less
than the atmospheric pressure
thoracic wall recoils out, lungs recoil in – but fluid holds them
together ∴ Pip slightly ֝
types:
a) Quiet Inspiration fig. 22.15 when you think of quite inspiration think of what
will you normal breath be?
active process (muscles contract)
at start Patm = Ppul (760 mmHg) – no air moves, then:
i. diaphragm , external intercostals contract, ֛ volume of
thoracic cavity
ii. lungs resist expansion ∴ Pip ֝ (756 ֜ 754 mmHg)
iii. higher pressure difference between Ppul and Pip pushes lungs
out ֜ lungs expand ∴ Ppul ֝ (760 ֜ 758 mmHg)
iv. air moves in down P gradient (until Ppul = Patm)
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b) Forced Inspiration involves more muscles – we have these two main muscle
that we use during quite and then we just add on more muscles to it
involves diaphragm, external intercostals and sternocleidomastoids,
pectoralis minors, scalenes contract (∴ active)
֛֛ vol of thoracic cavity ∴ pressure gradient ֛, and more air moves
in
c) Quiet Expiration
relax muscles ֜ lungs to resting size ∴ ֝ thoracic cavity size
(passive process)
vol ֝ Pip ֛ (754 ֜756 mmHg)
∴ Ppul ֛ (760 ֜ 762 mmHg) ֜ air moves out down pressure
gradient
d) Forced Expiration you can utilize muscles
laboured or impeded (e.g. asthma) breathing
relax diaphragm, ext. intercostals and contract internal intercostals,
abdominals (ACTIVE process)
Pip ֛ - lung volume ֝ ∴ Ppul ֛ and air moves out
Stretch in lungs determined by:
o Compliance = effort needed to stretch lungs; low = much effort
o Recoil = ability to return to resting size after stretch
both = result of elastic CT + surfactant
Lung collapse prevented by:
a) Pip is always below Ppul
pneumothorax = air into pleural cavity
o Patm = Ppul = Pip ∴ lungs collapse, thoracic wall expands
b) Presence of surfactant
= lipoprotein/phospholipid mixture
o in watery film coating alveoli - ֝ surface tension
o allows easier stretch of lungs (֛ compliance)
o prevents alveolar collapse
Respiratory distress syndrome
o newborns < 7 months gestation
o inadequate surfactant ∴ alveoli tend to collapse (low
compliance)
o ∴ effort high ֜ exhaustion, death
Air Flow and Airway Resistance:
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o where: F = air flow
ΔP = Patm – Ppul
R = airway resistance
o Resistance determined by diameter of bronchi/bronchioles
Asthma, bronchitis, emphysema ֛ airway R
o more difficult to expire than to inspire
inspiratory mechanics open airways/expiratory close airways
SNS - dilates bronchiolar smooth muscle (bronchodilation)
PSNS – contracts it (bronchoconstriction)
Respiratory Volumes:
o measured using a spirometer
1 respiration = 1 inspiration + 1 expiration
o volumes:
a) Tidal Volume (TV) – inspired or expired air during quiet respiration
(~ 500 ml)
b) Inspiratory Reserve Volume (IRV) – excess air over TV taken in on a
max inspiration (~ 3000 ml)
c) Expiratory Reserve Volume (ERV) – excess air over TV pushed out
on max expiration ~ 1200 ml
d) Residual Volume (RV) – volume of air in lungs after max expiration
(~ 1200 ml)
e) Minute Respiratory Volume = Tidal Volume x Respiratory Rate
e.g. = 500mL x 12 breaths/minute
= ~ 6 L/min (on average)
f) Forced Expiratory Volume in 1 second (FEV1) –
volume expired in 1 second, with max effort, following
maximum inspiration
Respiratory Capacities:
o 2 or more volumes
a) Inspiratory Capacity (IC) = TV + IRV
b) Vital Capacity (VC) = TV + IRV + ERV
largest volume in/out of lungs
c) Total Lung Capacity (TLC) – max amount of air lungs can hold =
TV + IRV + ERV + RV (=VC + RV)
Clinical Application:
o FEV1 is measured while measuring VC and expressed as % VC (allows
correction for body size)
usually FEV1 = ~80% VC
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Document Summary
3 processes involved in exchange of air: pulmonary ventilation, inspiration, expiration, external respiration taking the oxygen from the bloodstream into the lungs, internal respiration gas exchange the the capillaries, air exchange, pulmonary ventilation. Result of pressure gradients caused by changes in thoracic cavity volume: boyle"s la(cid:449) think of a container; the more that you are squeezing the container, the less room that there is so the volume will increase. Gas volume is inversely proportional to pressure. As (cid:1435) vol, (cid:1437) p (+ vice versa) - for the same # of molecules of air (gas amount remains constant) Pressures involved: atmospheric pressure (patm) = 760 mmhg (sea level, intrapulmonary pressure (ppul) = air pressure inside lungs (between breaths is = patm) fig. 22. 9: intrapleural pressure (pip) = fluid pressure in pleural cavity fig. Always < ppul always less than the pulmonary pressure. Usually < patm = ~ 4 mmhg less = 756 mmhg (at rest) usually less types: quiet inspiration fig.
