Question B
Describe and contrast the respiratory systems of mammals and birds
The respiratory systems of birds and mammals are similar as they both involve the use
of lungs in the pleural cavity to extract oxygen from the air for the synthesis of ATP by
respiration. Both systems work in cycles, however the avian respiration requires two
cycles ( inhalation, exhalation, inhalation, exhalation) to move the air through the entire
system while mammals require one cycle. For both, respiration is an active process
involving the contraction of muscles, however the muscles in each system vary.
In mammals during inhalation the diaphragm and the external intercostal muscles
contract, causing the ribs to move up and outwards. This increased the thorasic volume,
decreases air pressure inside the lungs causing fresh air to move into the lungs. During
exhalation the diaphragm and external intercostal muscles relax, causing the lungs to
return to normal size by elastic recoil, the thorasic volume decreases, internal pressure
increases and air leaves the lung. The intercostal muscles are only used during forced
exhalation such as during exercise. This is called tidal ventilation as the air enters and
leaves the organism through the same opening and the air cannot flow through the
alveoli, which are dead end sacs, only in and out the same entrance so the air flows
through the bronchi both ways. This means the air in the alveoli can become “stagnant”.
At rest only a very small volume, around 1/12, of the air in the lung is replaced. This is
due to the structure of the lung as if a higher volume was expelled, the alveoli would
collapse, which is a difficult biological process to reverse. This means there is a low
turnover of fresh air per inhalation causing there to be a smaller change in oxygen and
carbon dioxide concentrations in the alveoli. Following on there is a lower average
oxygen content and higher carbon dioxide concentration in the alveoli which is why the
mammals have a relatively low extraction efficiency of around 25%.
Birds have an incomplete diaphragm and the lungs cannot inflate as are attached to
the ribcage, making them rigid. But birds have seven or nine air sacs depending on the
species which carry out the ventilation. Inhalation happens by the muscles in the
sternum contacting causing negative pressure in the air sacs and air to move in through
the nostrils into the trachea. This means that although air enters and leaves the
respiratory system along the trachea, it can achieve unidirectional air flow across the
lungs with the use of the air sacs, so is non-tidal. During primary inhalation air travels
down the trachea to the posterior air sacs and a small amount into the lungs. Then in
primary exhalation the air travels from the posterior air sacs into the ventrobrochi and
dorsobronchi in the lungs which are divided up to increase the surface area for gas
exchange. The parabronchi are made up of air capillaries which the air flows through in
one direction and blood capillaries which is where the gas exchange takes place in a
cross current direction to help maintain a concentration gradient. Then during secondary
inhalation the air travels through to the cranial air sacs and then in secondary exhalation
the air moves back into the trachea and out of the nostrils. Primary and secondary
inhalation and primary and secondary exhalation happen at the same time so that birds
can maintain an almost constant supply of fresh air over the lungs which leads to their
high oxygen extraction of around 50%.
Both systems are protected by cilia and mucus produced in the trachea. The mucus
keeps the trachea moist, preventing it from drying out and catches any foreign particles
that would enter the lungs and possibly cause damage. The cilia, tiny hair like cell
structures beat upwards, moving the mucus away from the lungs and allows for it to be
coughed up and disposed of.
Also the gas exchange surfaces for both classes are adapted similarly for high rates of
diffusion with good blood supply, movement of external and internal mediums through
ventilation and pumping the blood and a short diffusion pathway of the lung epithelium,
and the capillary wall being a very short distance to cross. As we can see the main
difference is the structure of the lung. In birds ventilation is carried out by the air sacs at
oxygen absorption is done by the lungs, so the two processes are separated. In
mammals the lungs carry out both ventilation and oxygen absorption, having one organ
carrying out both processes makes it less specialised and so less efficient at doing both

Describe and contrast the respiratory systems of mammals and birds