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Gas Exchange in Humans
In humans the gas exchange organ system is the respiratory or
breathing system. The main features are shown in this diagram.
The actual respiratory surface is on the alveoli inside the lungs. An
average adult has about 600 million alveoli, giving a total surface
area of about 100m², so the area is huge. The walls of the alveoli
are composed of a single layer of flattened epithelial cells, as are
the walls of the capillaries, so gases need to diffuse through just
two thin cells. Water diffuses from the alveoli cells into the alveoli
so that they are constantly moist. Oxygen dissolves in this water
before diffusing through the cells into the blood, where it is taken
up by haemoglobin in the red blood cells. The water also contains
a soapy surfactant which reduces its surface tension and stops the
alveoli collapsing. The alveoli also contain phagocyte cells to kill
any bacteria that have not been trapped by the mucus.
The steep concentration gradient across the respiratory surface is
maintained in two ways: by blood flow on one side and by air flow
on the other side. This means oxygen can always diffuse down its
concentration gradient from the air to the blood, while at the same
time carbon dioxide can diffuse down its concentration gradient
from the blood to the air. The flow of air in and out of the alveoli is
called ventilation and has two stages: inspiration (or inhalation)
and expiration (or exhalation). Lungs are not muscular and cannot
ventilate themselves, but instead the whole thorax moves and
changes size, due to the action of two sets of muscles: the
intercostal muscles and the diaphragm.
Inspiratio
n

The diaphragm contracts and flattens downwards

The external intercostal muscles contract, pulling the ribs up and
out

this increases the volume of the thorax

this increases the lung and alveoli volume

this decreases the pressure of air in the alveoli below atmospheric
(Boyle's law)
Normal
expiration

air flows in to equalise the pressure

The diaphragm relaxes and curves upwards

The external intercostal muscles relax, allowing the ribs to fall

this decreases the volume of the thorax

this decreases the lung and alveoli volume

this increases the pressure of air in the alveoli above atmospheric
(Boyle's law)

Forced
expiration
air flows out to equalise the pressure

The abdominal muscles contract, pushing the diaphragm upwards

The internal intercostal muscles contract, pulling the ribs downward

This gives a larger and faster expiration, used in exercise
These movements are transmitted to the lungs via the pleural sac
surrounding each lung. The outer membrane is attached to the
thorax and the inner membrane is attached to the lungs. Between
the membranes is the pleural fluid, which is incompressible, so if
the thorax moves, the lungs move too. The alveoli are elastic and
collapse if not held stretched by the thorax (as happens in stab
wounds or deliberately to rest a lung).
Controlling Breathing Rate
But what controls the breathing rate? It is clearly an involuntary
process (you don’t have to think about it), and like many
involuntary processes (such as heart rate, coughing and sneezing)
it is controlled by a region of the brain called the medulla. The
medulla and its nerves are part of the autonomic nervous system
(i.e. involuntary). The region of the medulla that controls breathing
is called the respiratory centre. It receives inputs from various
receptors around the body and sends output through two nerves to
the muscles around the lungs.
The respiratory centre depends on information relayed via
chemoreceptors that pick up changes in:

carbon dioxide concentration – levels in the blood go up
when the rate of respiration increases and more carbon
dioxide is produced as a waste product.

Oxygen concentration – levels in the blood go down as it is
used in respiration to produce extra ATP as an energy
source for exercise.
The chemoreceptors are stimulated by a rise in carbon dioxide
levels and a fall in pH and oxygen in the blood. The respiratory
centre received the information as a nerve impulse from the
chemoreceptors and uses this to regulate breathing.
Later in this module we will be look in more detail about the effects
of exercise and how the breathing rate and heart rate is controlled
via various changing conditions within the body.
How does the respiratory centre control ventilation? [back to
top]
Unlike the heart, the muscles that cause breathing cannot contract
on their own, but need nerve impulses from the brain for each
breath. The respiratory centre transmits regular nerve impulses to
the diaphragm and intercostal muscles to cause inhalation. Stretch
receptors in the alveoli and bronchioles detect inhalation and send
inhibitory signals to the respiratory centre to cause exhalation. This
negative feedback system in continuous and prevents damage to
the lungs.
One difference between ventilation and heartbeat is that ventilation
is also under voluntary control from the cortex, the voluntary part of
the brain. This allows you to hold your breath or blow out candles,
but it can be overruled by the autonomic system in the event of
danger. For example if you hold your breath for a long time, the
carbon dioxide concentration in the blood increases so much that
the respiratory centre forces you to gasp and take a breath. Pearl
divers hyperventilate before diving to lower the carbon dioxide
concentration in their blood, so that it takes longer to build up.
During sleep there is so little cellular respiration taking place that it
is possible to stop breathing for a while, but the respiratory centre
starts it up again as the carbon dioxide concentration increases. It
is possible that one cause of
cot deaths may be an
underdeveloped respiratory centre in young babies, which allows
breathing to slow down or stop for too long.