Transcript 11. Structure and function of the respiratory systemx

Section A: Applied Anatomy and
Physiology
11. Structure and function of the
respiratory system
Syllabus
• Structure of the nasal passages, trachea, bronchii, bronchioles, and
alveoli
• Lobes of the lung and pleural membrane
• Mechanics of breathing at rest and during exercise
• Respiratory muscles, to include: diaphragm, external intercostals,
sternocleidomastoid, pectoralis minor, internal intercostals, and
abdominal muscles
• Control of ventilation
• Definitions, values and measurement of respiratory volumes at rest
and during exercise
• Effect of exercise on respiratory volumes and pulmonary ventilation
• Gaseous exchange, partial pressures and tissue respiration
• The effect of altitude on the respiratory system
External Respiration
• Involves the movement of gases into and out
of the lungs.
• The exchange of gases between the lungs and
blood is known as pulmonary diffusion.
Nasal Passages
• Nasal cavity is divided by a cartilaginous
septum that forms the passages.
• Interior structures aid the process by:
– Mucous membranes and blood capillaries moisten
and warm the inspired air
– Ciliated epithelium filters and traps dust particles
– Small bones (chonchae) increase the surface area
to improve efficiency
Oral Pharynx and Larynx
• Air entering the larynx passes over the vocal
chords and into the trachea
• Swallowing – the larynx is drawn upwards and
forwards against the base of the epiglottis
(preventing entry of food)
Trachea
• Approx. 10cm in length and lies in front of the
oesophagus.
• Composed of 18 ‘rings’ of cartilage, which are
also lined by a mucous membrane and ciliated
cells.
• Extends from larynx and directs air into the
right and left primary bronchi.
Bronchi and Bronchioles
• Trachea divides into right and left bronchi,
which further subdivide into lobar bronchi
– Three feeding the lobes of the right lung
– Two feeding the lobes of the left lung
• Further subdivision of these form bronchioles
• Bronchioles enable the air to pass into the
alveoli via the alveolar ducts
Alveoli
• Responsible for the exchange of gases between
the lungs and the blood
• The alveolar wall are extremely thin, are lined by
a film of water
– Essential for dissolving oxygen from inspired air
– Alveoli walls also contain elastic fibres further
increasing surface area
– Surrounding each alveolus is an extensive capillary
network
Mechanics of Breathing
Breathing
• Lungs are surrounded by pleural sacs
containing pleural fluid, which reduces friction
• Sacs are attached to both the lungs and the
thoracic cage, which enables the lungs to
inflate and deflate as the chest expands and
flattens
Inspiration
• Active
• Occurs as a result of contraction of;
– External intercostals
– Diaphragm
• As the chest expands through these muscular
contractions, the surface tension created by
the film of pleural fluid causes the lungs to be
pulled outwards
Inspiration During Exercise
• Additional muscles;
– Sternocleidomastoid
– Scalenes
– Pectoralis major
Expiration
• Generally a passive process
• During exercise, the process becomes more
active;
– The internal intercostals
– Abdominals
– Latissimus dorsi
Respiratory Regulation
• Controlled by nervous system
• Basic rhythm is governed and co-ordinated by
the respiratory centre (medulla)
• Inspiration generally lasts up to 2 seconds
after which impulses cease and expiration
occurs by elastic recoil of lungs
Factors Controlling Rate of Breathing
• Chemoreceptors
– CO2 levels
• Proprioceptors and Mechanoreceptors
• Stretch receptors
– Hering-Breur reflex (prevents overinflation)
• Thermoreceptors
– Temperature of blood
–
• Baroreceptors
– State of lung inflation
Neural Control
The respiratory centre in the medulla of the brain
controls breathing.
It is made up of two main areas:
• The inspiratory centre is responsible for the
rhythmic cycle of inspiration and expiration
• The expiratory centre is inactive during quiet
ventilation. When the rate and depth of
breathing increases (detected by stretch
receptors in the lungs) the expiratory centre
inhibits the inspiratory centre and stimulates
expiratory muscles.
Cont.
• In most circumstances the neural control of
breathing is involuntary
• The resp centre sends out impulses via the
phrenic and intercostal nerves to the
respiratory muscles
• The muscles are stimulated for a short period,
causing insipration
• Then when the stimulus stops, expiration
occurs
Other factors influencing the neural
control of breathing include:
1. A large drop in oxygen tension.
This is monitored by chemoreceptors in the aorta
and carotid arteries and results in an increase in the
rate and depth of breathing.
2. A rise in blood pressure, monitored by baroreceptors
in the aorta and carotid arteries, resulting in a
decrease in ventilation rate
3. Proprioceptors in the muscles responding to
movement stimulate the respiratory centre,
increasing the rate and depth of breathing
4. The respiratory centre can also be affected by higher
centres in the brain, e.g. emotional influences
Chemical Control
• The respiratory centre responds mainly to changes in
the chemistry and temperature of the blood
• The most significant factor is a lowering in pH
• This occurs when there is an increase in the amount of
CO2 being produced by the cells
• The increase is detected by the respiratory centre (in
the brain)
• It results in an increase in the rate and depth of
breathing
• A rise in body temperature will cause an increase in the
rate but not the depth of breathing
Reminder
• Pulmonary diffusion – gaseous exchange at
the lungs
• Its functions
– Replenish oxygen
– Remove carbon dioxide
Partial Pressure of Gases
• Central to understanding of gaseous exchange is
the concept of partial pressure
• “the individual pressure that the gas exerts when
it occurs in a mixture of gases”
• The pressure is proportional to its concentration
• Partial pressures added together = total pressure
of gas
Composition of Air
• Nitrogen = 79%
• Oxygen = 20.9%
• Carbon dioxide = 0.03%
• The percentages are obviously the relative
concentrations!
Calculating Partial Pressure
Effect of Altitude
Effect of Altitude
• With altitude there is a decrease in
atmospheric pressure BUT the percentages of
gases within the air remain identical to those
found at sea level
• It is the partial pressure of the gases that
changes in direct proportion to an increase in
altitude
Effect of Altitude cont.
• E.G. – at rest the pO2 of arterial blood is approx
100mmHg, while in the resting muscles and
tissues it is 40mmHg.
• The difference between the two indicates the
pressure gradient.
• The pO2 of arterial blood at an altitude of 8000ft
drops to 60mmHg, while that in the muscles
remains at 40mmHg!
Altitude Training
• The principle:
– With an increase in altitude, the partial pressure
of oxygen in the atmosphere decreases by about a
half, causing the body to adapt by INCREASING
RED BLOOD CELL MASS AND HAEMOGLOBIN
LEVELS to cope with a lower pO2
Altitude Training
• It is widely used by endurance athletes to
enhance their oxygen-carrying capacity
• Recent evidence:
– Living at altitude and training at sea level
produces the greatest endurance performance
– Can increase the oxygen-carrying capacity of the
blood by up to 150%
Altitude Training
• Disadvantages:
– Expensive
– Can cause altitude sickness
• Detraining!
– Due to lack of oxygen , training at higher
intensities is difficult
– Any benefits are soon lost on return to sea level
Oxyhaemoglobin Dissociation Curve
http://www.bio.davidson.edu/Courses/anphys/
1999/Dickens/Oxygendissociation.htm
Transport of Oxygen
• Each molecule of haemoglobin can combine with
4 molecules of oxygen
• The amount of oxygen that can combine with
haemoglobin is determined by the partial
pressure of oxygen
• High pO2 = complete saturation
• Low pO2 = saturation decreases
Transport of Oxygen and Dissociation
• Hb is totally saturated at the lungs (alveoli)
• As the pO2 is reduced, Hb saturation
decreases accordingly.
– This is largely due to the increased acidity of the
blood (decrease in blood pH), caused by an
increase in CO2 or LA and the increase in body
temperature, which causes a shift to the right in
the haemoglobin saturation curve.
The Release of Oxygen from
Haemoglobin
• At rest
– The pO2 in the alveoli is approx 100mmHg
– 100% saturation
– In resting muscle and tissue the pO2 is 40mmHg
– 75% saturation
– Means that 25% of the oxygen picked up at the
lungs is released into the muscle to help in energy
production
The Release of Oxygen from
Haemoglobin
• During exercise
– The pO2 in the alveoli remains at approx
100mmHg
– 100% saturation
– In working muscles the pO2 can be greatly
reduced, up to 15mmHg
– 25% saturation
– Means that 75% of the oxygen picked up at the
lungs is released into the muscle to help meet the
extra energy demands
The Bohr Shift
• Increased oxygen released to tissues!
Gas Exchange at Muscles and Tissues
• High pO2 in arterial blood and relatively low pO2 in
muscles causes a pressure gradient
• High pCO2 in tissues and low pCO2 in arterial blood
causes a movement of CO2 in opposite direction
• Production of CO2 stimulates the dissociation of
oxygen from haemoglobin
• Myoglobin has a much higher affinity for oxygen than
haemoglobin
a-VO2 difference
• The arterial-venous oxygen difference is the
difference in oxygen content of the blood in
the arteries and the veins.
• It is a measure of the amount of oxygen
consumed by the muscles
Lung Volumes
LUNG VOLUME
DEFINITION
TYPICAL REST
VALUE
CHANGE DURING
EXERCISE
Tidal volume (TV)
Volume inspired or
expired per breath
500ml
Increase
Inspiratory reserve
volume
Maximal volume
inspired following
end of resting
inspiration
3100ml
Decrease
Expiratory reserve
volume
Maximal volume
expired following
end of resting
expiration
1200ml
Decrease
Residual volume
(RV)
Volume of air
remaining in the
lungs at the end of
maximal expiration
1200ml
Remains the same
Lung Capacities
LUNG CAPACITIES
DEFINITION
TYPICAL REST
VALUE
CHANGES DURING
EXERCISE
Inspiratory capacity
(TV + IRV)
Maximum volume
of air inspired from
resting expiratory
levels
3600ml
Increase
Vital capacity
(TV + IRV + ERV)
The maximum
volume forcibly
expired following
maximal inspiration
5000ml
Slight decrease
Total lung capacity
(VC + RV)
The volume of air
that is in the lungs
following maximal
inspiration
6000ml
Slight decrease
Minute ventilation
(TV * f)
The volume of air
inspired or expired
per minute
7500ml
Dramatic increase
Minute Ventilation
TIDAL VOLUME (TV) *
FREQUENCY
(BREATHS/MIN)
= MINUTE VENTILATION
REST
500ml * 15
= 7.5L / min
MAXIMAL WORK
4,000ml * 50
= 200L / min
Adaptive Responses of Respiratory
System to Training
1.
SMALL INCREASES IN LUNG VOLUMES
1.
2.
IMPROVED TRANSPORT OF RESPIRATORY GASES
1.
2.
3.
Increased amount of RBC’s (haemoglobin)
Increased blood plasma reduces viscosity
ENHANCED GASEOUS EXCHANGE AT THE ALVEOLI AND TISSUES
1.
4.
Result from increased strength in respiratory muscles
Increased capillary density
GREATER UPTAKE OF OXYGEN BY THE MUSCLES
1.
2.
Increased myoglobin and mitochondrial density
Increase in a-VO2 difference