1. Unit II: Transport Breathing Mechanism
Chapter 20 pp

2. Pulmonary Ventilation
Breathing – repeating cycle of inspiration and expiration
quiet respiration – at rest
forced respiration – during exercise
Flow of air in and out of lungs requires a pressure difference between air pressure within lungs and outside body

3. Pulmonary Ventilation
Accessory
Inspiratory
Muscles
Primary Inspiratory Muscle
External intercostal muscles
Sternocleido-
mastoid muscle
Expiratory
Scalene
muscles
Pectoralis
minor muscle
Serratus
anterior muscle
Primary
Muscle
Diaphragm
Internal
intercostal
Transversus
thoracis
muscle
External
oblique
Rectus
abdominis

4. Pressure and Flow Atmospheric pressure drives respiration
1 atmosphere (atm) = 760 mmHg
Boyle’s Law:
pressure is inversely proportional to volume
for a given amount of gas, as volume , pressure  and as volume , pressure 
Charles’s Law:
Volume is directly proportional to its absolute temperature
for a given amount of gas, if absolute temperature is doubled, so is the volume

5. Inspiration Phrenic nerves stimulate diaphragm to flatten and drop &
intercostal muscles cause ribs to move slightly up and out
 volume of thoracic cavity
 intrapleural pressure
visceral pleura clings to parietal pleura 
lungs expand with visceral pleura -  intrapulmonary pressure
net result: 757 mmHg
Inflation aided by warming of inhaled air
500 ml of air flows with a quiet breath

6. Passive Expiration Expiration achieved by elasticity of thoracic cage
After inspiration, phrenic nerves continue to stimulate diaphragm to produce a braking action to elastic recoil
As volume of thoracic cavity , intrapulmonary pressure  and air is expelled
During quiet breathing: +3mmHg
During forced breathing: +30mmHg

7. Respiratory Cycle

8. Resistance to Airflow Diameter of bronchioles Surface Tension
Thin film of water needed for gas exchange
creates surface tension that would collapse alveoli
Pulmonary surfactant (great alveolar cells)
an agent that disrupts the H+ bonds of water
decreases surface tension

9. Alveolar Ventilation Anatomical dead space
conducting division of airway
Physiological dead space
sum of anatomical dead space + any pathological alveolar dead space
Alveolar ventilation rate
directly relevant to ability to exchange gases

10. Lung Volumes and Capacities

11. Neural Control of Breathing
Breathing depends on repetitive stimuli from brain
Controlled at 2 levels:
Voluntary control provided by motor cortex
Neurons in medulla oblongata and pons control unconscious breathing

12. Respiratory Control Centers
Medulla oblongata:
Ventral respiratory group (VRG)
primary generator of respiratory rhythm
Dorsal respiratory group (DRG)
modifies respiratory rhythm
Integrating center

13. Respiratory Control Centers
Pons:
Pontine respiratory group (PRG) or Pneumotaxic center
regulates shift from inspiration to exhalation
as frequency rises, breathe shorter and shallower

14. Respiratory Control Centers
Quiet Breathing
INHALATION
(2 seconds)
Diaphragm and external
intercostal muscles
contract and inhalation
occurs.
Activity in the
VRG
stimulates the
inspiratory
muscles.
Neurons in the VRG
become inactive. They
remain quiet for the
next 3 seconds and
allow the inspiratory
muscles to relax.
intercostal muscles relax
and passive exhalation
EXHALATION
(3 seconds)
Forced Breathing
INHALATION
EXHALATION
Inspiratory muscles
contract, and expiratory
muscles relax. Inhalation
occurs.
Increased activity in the
DRG stimulates neurons of
the VRG that in turn activate
the accessory muscles
involved in inhalation. The
expiratory center of the
VRG is inhibited.
DRG and
inspiratory
center of VRG
are inhibited.
Expiratory
is active.
After each inhalation,
active exhalation occurs
as the neurons of the
VRG stimulate the
appropriate accessory
muscles. Inspiratory
muscles relax.

15. Air-Water Interface Gases diffuse down their pressure gradients
Henry’s law
amount of gas that dissolves in water is determined by its solubility in water and its partial pressure in air
Temperature of liquid
Example:
Soda is put into the can under
pressure, and the gas (carbon
dioxide) is in solution at
equilibrium.
Open the can:
internal pressure falls
gas molecules come out of solution
Volume difference is so great
that within a
half hour you are
left with “flat”
soda.

16. Systemic
circuit
Pulmonary
capillary
Interstitial fluid
PO2 = 40
PCO2 = 45
PO2 = 95
PCO2 = 40
Internal Respiration
Respiratory
membrane
Alveolus
PO2 = 100 mm Hg
PCO2 = 40 mm Hg
PO2 = 100
External Respiration
Alveolar Gas Exchange
Exchange across the water film covering alveolar epithelium and respiratory membrane
Factors affecting gas exchange:
Pressure gradients of gases

17. Other Factors Affecting Gas Exchange
Gas solubility
CO2 20 times as soluble as O2
Membrane thickness - only 0.5 m thick
Membrane surface area ml blood in alveolar capillaries, spread over 70 m2
Ventilation-perfusion coupling - areas of good ventilation need good perfusion (vasodilation)

18. Oxygen Transport Concentration in arterial blood
98.5% bound to hemoglobin
1.5% dissolved
Binding to hemoglobin
each heme group of 4 globin chains may bind O2
oxyhemoglobin (HbO2 )
deoxyhemoglobin (HHb)
Protein subunits
Iron ion
Heme unit

19. Oxyhemoglobin Dissociation Curve
Steep slope = a large change in
amount of oxygen associated with Hb.
Blood entering the systemic circuit
has a PO2 of 95 mm Hg.
Blood leaving peripheral tissues has
an average PO2 of 40 mm Hg.
The PO2 in active muscle tissue may
drop to 15–20 mm Hg.
Oxyhemoglobin (% saturation)
PO2 (mm Hg)

20. Oxyhemoglobin Dissociation Curve
Bohr Effect
PO2 (mm Hg)
Oxyhemoglobin (% saturation)
pH increase = curve shifts to the left
Hemoglobin releases less oxygen.
pH decrease = curve shifts to the right
Hemoglobin releases more oxygen.
7.4
7.6
7.2

21. Carbon Dioxide Transport
H2CO3 H+ + HCO3–
CO2 diffuses
into the
bloodstream.
93% diffuses
into RBCs.
~7% remains
in solution.
~23% CO2 is carried as
Carbaminohemoglobin
(HbCO2)
~70% is converted to
carbonic acid by
carbonic anhydrase
Hydrogen ions bind
to hemoglobin acting as
pH buffers.
Bicarbonate ions move into
the surrounding plasma in
exchange for extracellular
chloride ions (Cl–).
Chloride shift

22. Summary of Transport Alveolar air space Pulmonary capillary Systemic
Plasma
Red blood cell
Chloride
shift
Cells in
peripheral
tissues
CO2 delivery
O2 delivery
O2 pickup
CO2 pickup