Summary of the respiratory system lectures, including pre-session and post-session activities
The main function of the respiratory system is gas exchange: Oxygen (O2) from the air we inhale is
passed on to blood and carbon dioxide (CO2) in the blood is passed on to the air we exhale. To
optimise gas exchange the design of the lungs is such that we have 1) an as large as possible area of
gas exchange, 2) an as small as possible diffusion distance and 3) an as steep as possible diffusion
gradient.
Ventilation is the mechanical process of breathing in and out: refreshing the air in the lungs to
maintain steep diffusion gradients for O2 and CO2. Don’t mix it with cellular aerobic respiration,
which is the process whereby cells use fuel (usually glucose) and O2 to produce the energy carrier
ATP, and the waste products water and CO 2.
The two lungs are supplied with air through the larynx, trachea, main bronchi, secondary and
tertiary bronchi that are all reinforced by cartilage rings to prevent collapse. After many branching
points air flows through bronchioles that can vary their airflow resistance due to broncho-
constriction/dilation. The diameter of the bronchioles can be adjusted by smooth muscle. At the
end of terminal bronchioles are alveoli, grouped in alveolar sacks. The alveolar epithelium cells are
very thin and surrounded by a capillary network with very thin endothelium cells. The total diffusion
distance from air to blood, through capillary endothelium and alveolar epithelium cells (together
forming the respiratory membrane) is <1 µm, aiding gas exchange. The total surface of alveolar
walls is huge, ~70 m2, aiding gas exchange. The deoxygenated blood entering these capillaries
becomes saturated with oxygen, which is then transported from the pulmonary circulation back to
the heart, which pumps the oxygenated blood to all tissues. There the oxygen diffuses down its
diffusion gradient from blood into cells and the CO 2 produced in the cell, moves down its diffusion
gradient into blood. In the lungs CO 2 diffuses down its gradient into the air in the alveoli.
To inspire, the intercostal muscles and the diaphragm muscles contract, which increases the volume
of the chest cavity, and with it, the lungs will expand. The resulting drop in pressure inside the lung
will make air flow down the pressure gradient into the lungs. To exhale you just relax these muscles
and the elastic recoil of the lungs will reduce its volume, increasing the pressure inside that will
cause air to flow out.
The tidal volume is the amount in-and-expired at rest is only around half a litre in an adult. If the
breathing rate is 12 breaths per minute that will give a respiratory minute volume of 6 litre/min.
When necessary, forced inhaling and exhaling can increase the lung volume used (e.g. during
exercise) to 4-5 litres to keep the air in the alveoli fresh and the diffusion gradient steep to wash out
CO2 more efficiently.
The breathing rate and depth are controlled by the respiratory centre in the medulla of the brain
stem, part of the autonomic nervous system. The respiratory centre controls the rhythm and
contraction force of the intercostal muscles and the diaphragm. It receives information from chemo-
receptors in the brain, the carotid sinus and aortic arch that sense levels of O2, CO2 and pH (measure
of acidity. Low pH (high CO2 levels) will accelerate the breathing and increase the tidal volume,
increasing the freshness of the air in the alveoli and so increase gas exchange. High pH (low CO 2
levels) will decelerate the breathing and reduce tidal volume, and so air freshness and gas exchange.
The main function of the respiratory system is gas exchange: Oxygen (O2) from the air we inhale is
passed on to blood and carbon dioxide (CO2) in the blood is passed on to the air we exhale. To
optimise gas exchange the design of the lungs is such that we have 1) an as large as possible area of
gas exchange, 2) an as small as possible diffusion distance and 3) an as steep as possible diffusion
gradient.
Ventilation is the mechanical process of breathing in and out: refreshing the air in the lungs to
maintain steep diffusion gradients for O2 and CO2. Don’t mix it with cellular aerobic respiration,
which is the process whereby cells use fuel (usually glucose) and O2 to produce the energy carrier
ATP, and the waste products water and CO 2.
The two lungs are supplied with air through the larynx, trachea, main bronchi, secondary and
tertiary bronchi that are all reinforced by cartilage rings to prevent collapse. After many branching
points air flows through bronchioles that can vary their airflow resistance due to broncho-
constriction/dilation. The diameter of the bronchioles can be adjusted by smooth muscle. At the
end of terminal bronchioles are alveoli, grouped in alveolar sacks. The alveolar epithelium cells are
very thin and surrounded by a capillary network with very thin endothelium cells. The total diffusion
distance from air to blood, through capillary endothelium and alveolar epithelium cells (together
forming the respiratory membrane) is <1 µm, aiding gas exchange. The total surface of alveolar
walls is huge, ~70 m2, aiding gas exchange. The deoxygenated blood entering these capillaries
becomes saturated with oxygen, which is then transported from the pulmonary circulation back to
the heart, which pumps the oxygenated blood to all tissues. There the oxygen diffuses down its
diffusion gradient from blood into cells and the CO 2 produced in the cell, moves down its diffusion
gradient into blood. In the lungs CO 2 diffuses down its gradient into the air in the alveoli.
To inspire, the intercostal muscles and the diaphragm muscles contract, which increases the volume
of the chest cavity, and with it, the lungs will expand. The resulting drop in pressure inside the lung
will make air flow down the pressure gradient into the lungs. To exhale you just relax these muscles
and the elastic recoil of the lungs will reduce its volume, increasing the pressure inside that will
cause air to flow out.
The tidal volume is the amount in-and-expired at rest is only around half a litre in an adult. If the
breathing rate is 12 breaths per minute that will give a respiratory minute volume of 6 litre/min.
When necessary, forced inhaling and exhaling can increase the lung volume used (e.g. during
exercise) to 4-5 litres to keep the air in the alveoli fresh and the diffusion gradient steep to wash out
CO2 more efficiently.
The breathing rate and depth are controlled by the respiratory centre in the medulla of the brain
stem, part of the autonomic nervous system. The respiratory centre controls the rhythm and
contraction force of the intercostal muscles and the diaphragm. It receives information from chemo-
receptors in the brain, the carotid sinus and aortic arch that sense levels of O2, CO2 and pH (measure
of acidity. Low pH (high CO2 levels) will accelerate the breathing and increase the tidal volume,
increasing the freshness of the air in the alveoli and so increase gas exchange. High pH (low CO 2
levels) will decelerate the breathing and reduce tidal volume, and so air freshness and gas exchange.
