Transcript The Cardiac Cycle

Pulmonary Ventilation 1
David Taylor
[email protected]
http://www.liv.ac.uk/~dcmt
Pulmonary ventilation 1
 When you have worked through this you should be
able to
 Describe the relationships between airflow, pressure
gradients and resistance – gas laws
 Describe the mechanics of breathing – compare quiet
versus forced
 Reflect upon the integration of heart and lungs working
together
 Explain the nervous control of breathing
Resources
 These slides are available with all my other lectures on
my website http://www.liv.ac.uk/~dcmt
 In the text books:
Chapters 22,23 and 24 in Preston and Wilson (2013)
Chapter 13 in Naish and Court (2014)
Airflow, pressure gradients and
resistance – gas laws
 As in so much physics and physiology:
Δ𝑃
𝑉=
𝑅
So the pressure gradient and the resistance determine the flow per unit time.
 A word about resistance (Poiseuille’s Law)
8𝐿𝜂
𝑅= 4
𝜋𝑟
L is airway length, ƞ is viscosity (dependent on density) – Poiseuille’s law is for laminar flow
 Paradoxically the Pharynx is the site of greatest resistance
 Ask yourself, why not the bronchioles….
What affects resistance?
 Changes in diameter of the airways
 Autonomic control
 Parasympathetic: ACh binds to M3 receptors –
bronchoconstriction
 Sympathetic: Mainly by inhibiting ACh release but also
through Noradrenaline on β2 receptors
 Endocrine: β2 receptors are also sensitive to adrenaline
 Irritants and allergens or inflammatory agents
Chapter 22 p. 273-274 in Preston and Wilson (2013)
Chapter 13 p. 633 in Naish and Court (2014)
Mechanics of breathing
“Passive”
 The diaphragm contracts and this pulls down on the
lungs – air enters.
 The diaphragm lifts the lower ribs
 External intercostal muscles contract also raising ribs
 Normally breathing out is passive.
Chapter 22 p. 268 in Preston and Wilson (2013)
Chapter 13 p. 635 in Naish and Court (2014)
Mechanics of breathing
 “Forced”
 Inspiration can also use the accessory muscles
 scalenes raise first two ribs,
 sternomastoids raise the sternum
 Additionally there is dilation of the upper airways
 Expiration also uses the abdominal muscles
 Rectus abdominis, transversus abdominis, internal and
external oblique muscles
 Internal intercostals pull ribs downward and inward
Chapter 22 p. 268 in Preston and Wilson (2013)
Chapter 13 p. 635 in Naish and Court (2014)
Some words and numbers
 Ventilation (alveolar ventilation)
 The volume of air (L/min) entering and leaving an
alveolus (4L/min)
 Perfusion
 In this case, pulmonary blood flow (5L/min)
 Both change with position in the lung
 (range 3.3-0.8L/min)
Ventilation/perfusion ratio (V/Q)
Distance above bottom of lung
30
No blood flow
V/Q=3.3
20
Flow is
regulated by
alveolar
pressure
10
V/Q=0.8
0
Relative blood flow
Pulmonary venous
pressure above
atmospheric
pressure
Top
Left atrium
The importance of matching ventilation
and perfusion
 Take the obvious extremes
 Lung is well ventilated but there is no perfusion
 Lung is well perfused, but there is no ventilation
 Both would be fatal.
The (local) ways in which perfusion is
controlled
 If ventilation is poor
 Local hypoxia (low O2)
 Leads to vasoconstriction
 So blood is diverted to other (better perfused regions of
the lung)
The inputs to the pathways which
control breathing
 Receptors:
 Central chemoreceptors (ventral surface of medulla) –
increase rate and depth with ↑ CO2
 Peripheral chemoreceptors (carotid bodies, aortic arch)
– increase rate and depth with ↓O2 or pH
 Stretch receptors (bronchi) - inhibit rate
 There are several others, but these are the most
important for now.
The physiological control of breathing
Emotions etc.,
CO2
Higher centres
Central
chemoreceptors
Limbic system
O2 and pH
Peripheral
chemoreceptors
Afferent sensory
neurones
Medulla oblongata
and pons
Inspiratory
(dorsal)
motor neurones
Scalene and
sternomastoid
External
intercostals
Expiratory
(ventral)
motor neurones
Diaphragm
Internal
intercostals
Abdominal
Chapter 24 p.298 and following in Preston and Wilson (2013)
Chapter 13 p. 642-645 in Naish and Court (2014)