Unit II: Transport
Breathing Mechanism
Chapter 20
pp 736-759
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
Pulmonary Ventilation
Accessory
Inspiratory
Muscles
Primary Inspiratory Muscle
External intercostal muscles
Sternocleidomastoid muscle
Scalene
muscles
Pectoralis
minor muscle
Serratus
anterior muscle
Primary
Inspiratory
Muscle
Diaphragm
Accessory
Expiratory
Muscles
Internal
intercostal
muscles
Transversus
thoracis
muscle
External
oblique
muscle
Rectus
abdominis
Internal
oblique
muscle
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
Inspiration
1. Phrenic nerves stimulate diaphragm to flatten and drop &
intercostal muscles cause ribs to move slightly up and out
–  volume of thoracic cavity
2.  intrapleural pressure
3. visceral pleura clings to parietal pleura 
4. 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
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
Respiratory Cycle
Resistance to Airflow
1. Diameter of bronchioles
2. 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
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
Lung Volumes and Capacities
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
Respiratory Control Centers
• Medulla oblongata:
– Ventral respiratory group (VRG)
• primary generator of respiratory rhythm
– Dorsal respiratory group (DRG)
• modifies respiratory rhythm
• Integrating center
Respiratory Control Centers
• Pons:
– Pontine respiratory group (PRG) or Pneumotaxic center
• regulates shift from inspiration to exhalation
• as frequency rises, breathe shorter and shallower
Respiratory Control Centers
Quiet Breathing
Forced Breathing
INHALATION
(2 seconds)
INHALATION
Inspiratory muscles
contract, and expiratory
muscles relax. Inhalation
occurs.
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.
Diaphragm and external
intercostal muscles relax
and passive exhalation
occurs.
EXHALATION
(3 seconds)
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
center of VRG
is active.
After each inhalation,
active exhalation occurs
as the neurons of the
expiratory center of the
VRG stimulate the
appropriate accessory
muscles. Inspiratory
muscles relax.
EXHALATION
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.
Alveolar Gas Exchange
• Exchange across the
water film covering
alveolar epithelium
and respiratory
membrane
• Factors affecting gas
exchange:
− Pressure gradients
of gases
External Respiration
PO2 = 40
PCO2 = 45
Pulmonary
circuit
Systemic
circuit
Alveolus
Respiratory
membrane
PO2 = 100
PCO2 = 40
PO2 = 100 mm Hg
PCO2 = 40 mm Hg
Pulmonary
capillary
Internal Respiration
Interstitial fluid
Systemic
circuit
PO = 95
PCO22 = 40
PO2 = 40
PCO2 = 45
PO = 40
PCO22 = 45Systemic
capillary
Other Factors Affecting Gas Exchange
• Gas solubility
– CO2 20 times as soluble as O2
• Membrane thickness - only 0.5 m thick
• Membrane surface area - 100 ml blood in alveolar capillaries,
spread over 70 m2
• Ventilation-perfusion coupling - areas of good ventilation need
good perfusion (vasodilation)
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
Oxyhemoglobin Dissociation Curve
Steep slope = a large change in
amount of oxygen associated with Hb.
Oxyhemoglobin (% saturation)
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.
PO2 (mm Hg)
Oxyhemoglobin Dissociation Curve
Bohr Effect
pH increase = curve shifts to the left
Hemoglobin releases less oxygen.
Oxyhemoglobin (% saturation)
7.6
7.4
7.2
pH decrease = curve shifts to the right
Hemoglobin releases more oxygen.
PO2 (mm Hg)
Carbon Dioxide Transport
CO2 diffuses
into the
bloodstream.
~7% remains
in solution.
93% diffuses
into RBCs.
~23% CO2 is carried as ~70% is converted to
carbonic acid by
Carbaminohemoglobin
carbonic anhydrase
(HbCO2)
Hydrogen ions bind
to hemoglobin acting as
pH buffers.
H2CO3
H+ + HCO3–
Bicarbonate ions move into
the surrounding plasma in
exchange for extracellular
chloride ions (Cl–).
Chloride shift
Summary of Transport
O2 pickup
O2 delivery
Pulmonary Systemic
capillary capillary
Plasma
Red blood cell
Red blood cell
Cells in
peripheral
tissues
Alveolar
air space
Chloride
shift
Alveolar
air space
Pulmonary
capillary
CO2 delivery
Systemic
capillary
CO2 pickup