The Respiratory system

Question 1

Why do we need a respiratory system?

Answer 1

The respiratory system creates an interface between the
environment and the tissues (via the blood) to overcome the
limitation of distance in the properties of diffusion.
Problem 1 – dehydration
For effective diffusion between air (atmosphere) and liquid
(blood), the surface of the respiratory system must not only be
thin, but it must also be moist. This requires the respiratory
system to operate in an enclosed space.
Problem 2 – air movement requires a pump
Air movement is initiated by a muscular pump (compare with
the heart in the cardiovascular system). Air moves from an
area of higher pressure to an area of lower pressure (very
much like blood flow in the cardiovascular system

Question 2

The lungs

Answer 2

The lungs are the major organ of the respiratory system.
They are large, spongy, and elastic “bags” that they fill
up with air with every breath.
Each lung is divided in compartments called lobes. The right
lung has three lobes, but the left lung has only two.

Lung tissue fills the entire thoracic cavity with the
exception of the mid-sternal line where the heart,
major vessels, and the oesophagus are.

Question 3

The pleural sac

Answer 3

The pleural sac is a double-walled
enclosure of the lungs filled with fluid,
the pleural fluid

Reduces friction from movement on
the surface of the lungs
* Fixes lungs firmly on the thoracic wall
without any physical attachments

Question 4

bones and muscles

Answer 4

The lungs are located inside the thoracic cavity
which is made up by bones and muscles.
* The bones (rib cage and spine) offer rigid
protection to sensitive organs (heart and lungs).
* The muscles (diaphragm, intercostals,
abdominals) support the rib cage and turn the
chest into a pump that drives air flow

Question 5

airways

Answer 5

upper respiratory tract- mouth nose

lower respiratory tract - trachea
bronchial tree
alveoli

Question 6

The airways – divisions

Answer 6

With a design philosophy similar to that of the cardiovascular system, the bronchial tree undergoes a
number of divisions (23 generations). At the end of the tree the surface area is enormous (~ 100 m 2

Question 7

Functions of the airways

Answer 7

• Warming up of inspired air.
• Humidification of dry inspired air.
• Filtration of inhaled foreign materials

Question 8

nasal cavity

Answer 8

Turbinates (or conchae)
Bony dividers that increase the
surface area of the nasal cavity.
The surface of the nasal cavity has
a very high blood supply, which
helps warm up and humidify the
inspired air as it flows over it.
The entrance to the nasal cavity (called the
vestibule) is the first line of airway defence.
Small hairs and mucus in the vestibule help
trap inhaled particles so that they can be
blown back out.

Question 9

Alveoli

Answer 9

Blood in the pulmonary capillaries is separated from the
air in the alveoli by a single layer of capillary endothelial
cells and a single layer of alveolar epithelial cells.
The thin barrier and enormous surface area provides
ideal conditions for diffusion of gases

Question 10

Mechanisms of ventilation

Question 11

Pressure-volume relationships of gases

Answer 11

Boyle’s Law: P 1x V 1 = P 2xV 2

Question 12

Flow properties in gases

Answer 12

Movement of air is governed by the same basic rules that
we saw with blood flow in the cardiovascular system

Airflow is directly proportional to the pressure difference between two points
(the grater the pressure difference between two points, the greater the airflow)
* Airflow is inversely proportional to the resistance that is created by the airways
(the greater the resistance the smaller the airflow)

In the respiratory system, flow of gases during inspiration and expiration
depends on the creation of a pressure difference between the lungs and the
environment (atmospheric pressure is relatively stable, ~760 mmHg)

Question 13

Airway diameter and flow resistance

Answer 13

Viscosity plays a small role in the resistance of the airways to airflow. In the
context of the respiratory system, viscosity is affected by humidity and the
concentration of the air (high altitude).
Airway radius is the main factor affecting resistance in the respiratory system.
Airways that contribute to variable resistance are the ones that do not have a
rigid cartilage frame.

Factors affecting airway radius:
Bronchodilation: Carbon dioxide, epinephrine (via b 2 receptors)

Bronchoconstriction: Parasympathetic stimulation (via muscarinic receptors),
histamine

Question 14

the muscular pump that initiates breathing- Inspiration

Answer 14

During normal breathing, contraction of the diaphragm and a
moderate expansion of the thoracic cavity (external intercostal
muscles contract) is enough to create adequate airflow

As breathing rate increases the expansion of the thoracic cavity is
assisted further by the contribution of further contraction of the
diaphragm and contraction of the accessory inspiratory muscles

Question 15

The muscular pump that initiates breathing- expiration

Answer 15

Normal expiration is a passive process that does not require
the contraction of any muscles. The relaxation of the
inspiratory muscles and the recoil of the lungs cause the
thoracic cavity to return to its original volume.

Heavy breathing or forceful expiration requires the
contribution of the expiratory muscles (internal intercostal and
abdominal muscles) in order to compress the thoracic cavity
faster and further than restful breathing.

Question 16

The pleural sac

Question 17

Intrapleural pressure

Answer 17

responsible for keeping the lungs inflated.

Question 18

The breathing cycle and pressure/flow relationships

Answer 18

Basic principles:
* Inspiration occurs when Palv is below Patm and
expiration occurs when Palv is above Patm
* For the lungs to remain inflated P tp must be
a positive number.
* During inspiration P ip becomes more
negative and P tp increases. This forces the
lungs to overcome the elastic recoil and
follow the chest expansion.
* During expiration P ip becomes less negative
but P tp remains positive which prevents the
lungs from collapsing.

Question 19

Legend

Answer 19

Patm : The ambient atmospheric pressure.
Palv : The pressure inside the lungs (at the alveoli) relative to Patm
P ip : The pressure inside the pleural cavity.
P tp : The difference between Palv and P ip (P tp = Palv - Pip)

Question 20

Surfactant stabilises alveoli and increases lung compliance

Answer 20

T=surface tension
P= pressure
r=radius

𝐿𝑎𝑤 𝑜𝑓 𝐿𝑎𝑃𝑙𝑎𝑐𝑒: 𝑃 = 2𝑇/𝑟

According to the law of Laplace, if two
bubbles have the same surface tension,
the smaller bubble will have the higher pressure.

Surfactant contains proteins that disrupt
the forces between water molecules and
the result is a reduction in the surface
tension of the alveolar walls. Reduced surface tension means:
* The alveolar spaces are less prone to collapsing
* The lung is more compliant and is inflated easier

Question 21

Lung volumes

Question 22

Spirometry trace

Answer 22

Tidal volume: The amount of air that is moved in and out of the lungs with every breath during normal breathing

Reserve volumes: The amount of additional air that can be moved in and out of the lungs during heavier breathing

Vital capacity: The total amount of air that can be moved in and out of the lungs during maximal respiratory effort

Functional residual capacity : The amount of air that is left in the lungs at the end of a normal expiration

Residual volume: The amount of air that is left in the lungs at the end of a maximal expiration

The functional residual capacity
and the residual volume cannot
be measured using a spirometer.

Question 23

Respiratory “dead space”

Answer 23

Anatomical dead space refers to the part of
the airways where gas exchange does not
take place and is a fixed volume ~150 ml.
Alveolar dead space refers to areas of the
lungs where gas exchange can take place but
that are not properly perfused with blood
(e.g. apex of upright lung).

Question 24

Minute ventilation

Answer 24

Minute ventilation (VE) is the amount of air that is moved by the lungs in one minute·
V E = Tidal volume x breathing frequency·
Normal V E (rest) ~ 6 l/min
(0.5 l x 12 br/min)
·
Maximal VE (exercise) ~ 150 l/min !
(3 l x 50 br/min)
·
Hyperpnoea = V E increases in proportion to metabolic rate·
Hyperventilation = V E increases more than metabolic rate does

Question 25

Lecture summary

Answer 25

• The driving pressure difference that causes airflow is created by manipulating the volume of
  the thoracic cavity (Boyle’s law).
• Inspiration is caused by the contraction of the inspiratory muscles that increases the volume of
  the thoracic cavity. Expiration is mainly a passive process but during heavy or forced breathing
  it is assisted by the contraction of the expiratory muscles.
• Intrapleural pressure is sub-atmospheric and this prevents the lungs from collapsing by
  maintaining a positive transpulmonary pressure.
• Surfactant ensures that small alveoli do not not collapse, and it makes the lungs easier to
  inflate by increasing their compliance.
• Spirometry is used to measure lung volumes and assess pulmonary function.
• Gas exchange happens only in alveoli (exchange zone). All other airways do not contribute to
  gas exchange and are acting only as conduits (conductive zone, or “dead space”).

Question 26

Diffusion and
gas carriage in blood

Question 27

Partial pressure

Answer 27

Dalton’s Law
The pressure of a gas mixture is equal to the sum of the pressures of the individual gases.

Composition of air in atmosphere:
* 78.1% Nitrogen (N 2)
* 20.9% Oxygen (O 2)
* 0.033% Carbon dioxide (CO2

l pressures of atmospheric gases:
N 2 = 760 mmHg x 78.1% = 594 mmHg
O 2 = 760 mmHg x 20.9% = 159 mmHg
CO 2 = 760 mmHg x 0.033% = 0.25 mmHg

Partial pressures of atmospheric gases in the lungs:
N 2 = (760 mmHg – 46 mmHg) x 78.1% = 558 mmHg
O 2 = (760 mmHg – 46 mmHg) x 20.9% = 149 mmHg
CO 2 = (760 mmHg – 46 mmHg) x 0.033% = 0.24 mmHg

Question 28

Fick’s law of diffusion

Answer 28

Rate of diffusion ∝ A
T × (P1 − P2 ) × D

From Fick’s law it is obvious that greater solubility for a particular gas means a greater rate of diffusion
for that gas (CO2 is a lot more soluble than O2 and it diffuses across the membrane a lot easier)

Question 29

Gas partial pressure in solution

Answer 29

Gas molecules move between air and liquid in order to achieve equilibrium of partial pressures.
Movements of molecules depend on:
* Temperature
* Pressure difference
* Solubility

Question 30

Oxygen is carried in blood mainly by haemoglobin

Answer 30

Haemoglobin is an oxygen binding protein contained within the red blood cells.
Each haemoglobin molecule contains four polypeptide chains (globins), and four haeme groups.
In the centre of each haeme group is a ferrous atom (Fe2+) that binds oxygen

Question 31

control of breathing

Question 32

Ventilation is involuntary

Answer 32

The respiratory muscle groups (and therefore ventilation) are controlled by the CNS

Question 33

Ventilation can be voluntary too!

Answer 33

Breathing can be altered voluntarily:
* Hyperventilation
* Breath holding
* Speaking, swallowing

Question 34

Protective interruption of respiratory rhythm

Answer 34

Slowly adapting pulmonary stretch receptors
These receptors are located within airway smooth muscle
and they are active only when tidal volume approaches
the physical limitations of the lung tissue for expansion.
This mechanism, called the Hering-Breuer reflex, protects
the lungs by initiating a signal that inhibits inspiration (the
switch-off point).

Question 35

Protective interruption of respiratory rhythm

Answer 35

Rapidly adapting pulmonary stretch receptors (irritant)
These receptors are dense in the trachea and large airways.
They respond to stimuli such as cigarette smoke, inhaled
particles and cold air, by initiating reflexes such as coughing
and sneezing and by causing bronchoconstriction.

Question 36

What creates the ventilatory drive?

Answer 36

Peripheral chemoreceptors
(main stimulus is PaO2 )

Carotid chemoreceptors (carotid bodies)
Aortic chemoreceptors (aortic arch)

Question 37

What creates the ventilatory drive?

Answer 37

Central chemoreceptors
Respond to changes in PaCO2
(through changes in concentration of H + )

Although these have been defined
chemically, their anatomical identity
within the medulla is not well defined

Only carbon dioxide can cross the very selective
blood-brain barrier but the central chemoreceptors
are stimulated by the resulting increase in [H + ] and
not by carbon dioxide itself.

Question 38

Control by PaO2

Answer 38

Reduced levels of arterial oxygen affect only the peripheral chemoreceptors, and ventilation
increases substantially only after arterial oxygen reaches a value of ~60 mmHg (compare this with
the oxyhaemoglobin dissociation curve).
Note, that it is the PO 2 that stimulates the peripheral chemoreceptors and not the content of oxygen.

Question 39

Control by PaCO2/[pH]

Answer 39

Carbon dioxide is a very potent stimulus for ventilation and it exerts its effects with smaller
deviations from normal arterial values.
Both central and peripheral chemoreceptors are involved in the response to increased PCO 2 , but
central chemoreceptors are the more important of the two.
Carbon dioxide exerts its effects by altering the concentration of H + in the brain extracellular fluid.

Question 40

Control of ventilation in steady state exercise

Answer 40

phase 1- Ventilation increases abruptly and as
early as in the first breath after exercise
has commenced. It is thought to be
controlled by neural mechanisms

phase 2/3- Ventilation increases gradually (phase II)
until steady state is achieved (phase III).
Chemical stimuli are more likely to
contribute to this part of the response

Question 41

PARADOX
Ventilation during exercise increases in the absence of a clear stimulus

Answer 41

So, what drives the exercise-induced
increase in ventilation?
Phase I (neural mechanisms)
* Muscle afferents
* Central command
* Learnt response
Phase II (chemical mechanisms)
* P CO 2 and pH oscillations
* Plasma K+ , catecholamines
* Temperature
* Hypoglycaemia, metabolic rate

Question 42

Lecture summary

Answer 42

Neural control of breathing
Ø Breathing is involuntary and it is controlled by the respiratory rhythm centre.
Ø Breathing can also be voluntary and it can be interrupted for speaking, swallowing and in order to
protect the lungs.
* Chemoreceptors
Ø Peripheral chemoreceptors respond to changes in PO 2 . PCO 2 and [H + ] also stimulate the
peripheral chemoreceptors but their role is mainly to modulate the response to PO 2 .
Ø Central chemoreceptors respond only to arterial PCO 2 but the stimulus within the CSF is the [H+ ].
* Ventilatory responses.
Ø Oxygen causes ventilation to rise exponentially after a slow start until PO2 drops to ~60 mmHg..
Ø Carbon dioxide is a very potent stimulus and stimulates breathing in a linear fashion.
* Breathing during exercise.
Ø The quick phase I response appears to be neural in origin. Phases II and III appear to be chemical.
Ø The increase in ventilation happens without any obvious stimuli to the chemoreceptors.