1. Chapter 24:
Physiology of the Respiratory System
1

2. RESPIRATORY PHYSIOLOGY
• Definition: complex, coordinated processes that help
maintain homeostasis (Figure 24-1)
• 4 major functions:
1. External respiration
• Pulmonary ventilation (breathing)
• Pulmonary gas exchange
1. Transport of gases by the blood
2. Internal respiration
• Systemic tissue gas exchange
• Cellular respiration
1. Regulation of respiration
2

3. 3

4. PULMONARY VENTILATION
• Respiratory cycle (ventilation, breathing)
– Inspiration: movement of air into the lungs
– Expiration: movement of air out of the lungs
• Mechanism of pulmonary ventilation
– The pulmonary ventilation mechanism must establish two
gas pressure gradients (Figures 24-2 and 24-3)
• One in which the pressure within the alveoli (Pₐ) of the
lungs is lower than atmospheric pressure (Pb) to
produce inspiration
• One in which the pressure in the alveoli (Pₐ) of the
lungs is higher than atmospheric pressure (Pb) to
produce expiration
4

5. Surface Tension
• Force exerted by fluid in alveoli to resist
distension.
• Lungs secrete and absorb fluid, leaving a very thin film of fluid.
– This film of fluid causes surface tension.
• H20 molecules at the surface are attracted to
other H20 molecules by attractive forces.
– Force is directed inward, raising pressure in alveoli.

6. Surfactant
• Phospholipid produced by
alveolar type II cells.
• Lowers surface tension.
Insert fig. 16.12
– Reduces attractive forces of
hydrogen bonding by
becoming interspersed
between H20 molecules.
• As alveoli radius
decreases, surfactant’s
ability to lower surface
tension increases.
Figure 16.12

7. Surfactant
is a substance produce by type II alveolar epithelial cells
(~ 10% of the surface area of the alveoli) which reduce
the surface tension of the fluid in the inner surface of the
alveoli
it is a mixture of phospholipids, proteins, and ions, the
most important component is phospholipid dipalmitoyl
phosphatidylcholine which is responsible for reducing the
surface tension (formed of 2 parts, hydrophilic part
dissolves in the water lining the alveoli and hydrophobic
part directed toward the air)
the alveolar collapse pressure in an average-sized
alveolus with radius of about 100µm and lined with
surfactant, is about 4cm of H2O, but if it is lined with pure
water is about 18cm of H2O  important of surfactant in
reducing the amount of transpulmonary pressure required
to keep

8. Boyle’s Law
• Changes in intrapulmonary pressure occur as a
result of changes in lung volume.
– Pressure of gas is inversely proportional to its volume.
• Increase in lung volume decreases intrapulmonary
pressure.
– Air goes in.
• Decrease in lung volume, raises intrapulmonary
pressure above atmosphere.
– Air goes out.

9. Quiet Inspiration
• Active process:
– Contraction of diaphragm, increases thoracic volume
vertically.
– Contraction of parasternal and internal intercostals,
increases thoracic volume laterally.
– Increase in lung volume decreases pressure in alveoli,
and air rushes in.
• Pressure changes:
– Alveolar changes from 0 to –3 mm Hg.
– Intrapleural changes from –4 to –6 mm Hg.
– Transpulmonary pressure = +3 mm Hg.

10. Expiration
• Quiet expiration is a passive process.
– After being stretched, lungs recoil.
– Decrease in lung volume raises the pressure within alveoli
above atmosphere, and pushes air out.
• Pressure changes:
– Intrapulmonary pressure changes from –3 to +3 mm Hg.
– Intrapleural pressure changes from –6 to –3 mm Hg.
– Transpulmonary pressure = +6 mm Hg.

11. 11

12. 12

13. PULMONARY VENTILATION: MECHANISM
– Pressure gradients are established by changes in the size of
the thoracic cavity that are produced by contraction and
relaxation of muscles (Figures 24-4 and 24-5)
– Boyle’s law: the volume of gas varies inversely with pressure
at a constant temperature
– Inspiration: contraction of the diaphragm and external
intercostals produces inspiration; as they contract, the
thoracic cavity becomes larger (Figures 24-6 and 24-7)
• Expansion of the thorax results in decreased intrapleural
pressure, leading to decreased alveolar pressure
• Air moves into the lungs when alveolar pressure drops
below atmospheric pressure
• Compliance: ability of pulmonary tissues to stretch, thus
making inspiration possible
13

14. 14

15. 15

16. 16

17. 17

18. PULMONARY VENTILATION:
MECHANISM (cont.)
– Expiration: a passive process that begins when the
inspiratory muscles are relaxed, which decreases
the size of the thorax (Figures 24-8 and 24-9)
• Decreasing thoracic volume increases the intrapleural pressure and
thus increases alveolar pressure above the atmospheric pressure
• Air moves out of the lungs when alveolar pressure exceeds the
atmospheric pressure
• The pressure between parietal and visceral pleura is always less
than alveolar pressure and less than atmospheric pressure; the
difference between intrapleural pressure and alveolar pressure is
called transpulmonary pressure, always negative to avoid “collapse
tendency of the lungs”
• Elastic recoil: tendency of pulmonary tissues to return to a smaller
size after having been stretched; occurs passively during expiration
18

19. 19

20. Quiet Inspiration
• Active process:
– Contraction of diaphragm, increases thoracic volume
vertically.
– Contraction of parasternal and internal intercostals,
increases thoracic volume laterally.
– Increase in lung volume decreases pressure in alveoli,
and air rushes in.
• Pressure changes:
– Alveolar changes from 0 to –3 mm Hg.
– Intrapleural changes from –4 to –6 mm Hg.
– Transpulmonary pressure = +3 mm Hg.

21. Expiration
• Quiet expiration is a passive process.
– After being stretched, lungs recoil.
– Decrease in lung volume raises the pressure within alveoli
above atmosphere, and pushes air out.
• Pressure changes:
– Intrapulmonary pressure changes from –3 to +3 mm Hg.
– Intrapleural pressure changes from –6 to –3 mm Hg.
– Transpulmonary pressure = +6 mm Hg.

22. 22

23. PULMONARY VENTILATION
• Pulmonary volumes: we must have normal volumes of air moving in
and out as well as remaining in the lungs for normal exchange of
oxygen and carbon dioxide to occur (Figure 24-11)
– Spirometer: instrument used to measure the volume of air (Figure
24-10)
– Tidal volume (TV): amount of air exhaled after normal inspiration,
500 ml
– Expiratory reserve volume (ERV): largest volume of additional air
that can be forcibly exhaled past the TV(1000-1200 ml is normal
ERV)
– Inspiratory reserve volume (IRV): amount of air that can be
forcibly inhaled after normal inspiration (normal IRV is 3300ml)
– Residual volume: amount of air that cannot be forcibly exhaled
(1200 ml)
23

24. 24

25. 25

26. PULMONARY VENTILATION (cont.)
• Pulmonary capacities: the sum of two or more pulmonary
volumes
– Vital capacity (VC): the largest volume of air that can be moved in and
out of the lungs; IRV + TV + ERV
• A person’s VC depends on many factors, including the size of the thoracic cavity
and posture
– Functional residual capacity (FRC): the amount of air at the end of a
normal respiration; ERV + RV
– Total lung capacity (TLC): the sum of all four lung volumes; the total
amount of air a lung can hold
– Inspiratory capacity (IC): the maximal amount of air an individual can
inspire after a normal expiration; TV + IRV
– Alveolar ventilation: volume of inspired air that reaches the alveoli
– Anatomical dead space: air in passageways that do not participate in
gas exchange (Figure 24-6)
– Physiological dead space: anatomic dead space plus the volume of
any nonfunctioning alveoli (as in pulmonary disease)
– Alveoli must be properly ventilated for adequate gas exchange
26

27. PULMONARY VENTILATION (cont.)
• Pulmonary air flow: rates of air flow into and out of
the pulmonary airways
– Total minute volume: volume moved per minute
(ml/min); TV x RR; 6000ml
– Forced expiratory volume (FEV) or forced vital capacity
(FVC): volume of air expired per second during forced
expiration (as a percentage of VC) (Figure 24-12)
– Flow-volume loop: graph that shows flow (vertically) and
volume (horizontally), with the top of the loop
representing expiratory flow volume and the bottom of
the loop representing inspiratory flow volume relations
(Figure 24-13)
27

28. • Restrictive
disorder:
– Vital capacity is
reduced.
– FVC is normal.
• Obstructive
disorder:
– VC is normal.
– FEV1 is < 80%.
28

29. TIDAL
BREATHING
FORCED
EXPIRATION
NORMAL
FEV1
FEV1
FEV1 = 3.0L
FVC = 4.2L
FEV1/FVC = 80%
OBSTRUCTIVE
FEV1 = 0.9L
FVC = 2.3L
FEV1/FVC = 40%
RESTRICTIVE
FEV1
1 SECOND
FEV1 =1.8L
FVC = 2.3L
FEV1/FVC = 90%

30. Pulmonary Disorders
• Dyspnea:
– Shortness of breath.
• COPD (chronic obstructive pulmonary
disease):
– Asthma:
• Obstructive air flow through bronchioles.
– Caused by inflammation and mucus secretion.
» Inflammation contributes to increased airway
responsiveness to agents that promote bronchial
constriction.
» IgE, exercise.

31. Pulmonary Disorders
(continued)
– Emphysema:
• Alveolar tissue is destroyed.
• Chronic progressive condition that reduces surface area for gas
exchange.
– Decreases ability of bronchioles to remain open during expiration.
» Cigarette smoking stimulates macrophages and leukocytes to
secrete protein digesting enzymes that destroy tissue.
• Pulmonary fibrosis:
– Normal structure of lungs disrupted by accumulation of
fibrous connective tissue proteins.
• Anthracosis.