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Physiology of Organisms
2/1/15
Compare respiration in birds to respiration in mammals. In comparing mammals
and birds with similar ecological niches (insectivore bats and insectivore birds,
for example), are there apparent advantages one class of animal has over the
other when it comes to respiration?
Respiration in physiology is defined as the process of gas exchange to transfer
oxygen and carbon dioxide between the atmosphere and metabolically active
cells. In both mammals and birds respiration is carried out by the lungs and
associated structures, which differ between the two groups. The mammalian lung
consists of branching tubes, from the trachea to bronchi to bronchioles, which
become increasingly narrow and more numerous as they penetrate deeper into
the lung. These do not carry out gas exchange, but acts as airways through which
air moves from the exterior to the alveoli; small sacs in which gas exchange
occurs. The equivalent sites of gas exchange in birds are 10 μm in diameter air
capillaries that stem from the parabronchi, tubes that branch between
dorsobronchi and ventrobronchi, which are connected to the larger mesobroncus.
These tubes comprise the bird lung, and are supplied with air through the
trachea. The differences in structure of the lung and associated elements have
implications for the mechanism of ventilation and for the efficiency of gas
exchange.
Despite differences in the structures in which gas exchange between the
atmosphere and the blood occurs in mammals and birds, this element of
respiration is fairly similar in both groups. In both mammals and birds oxygen
and carbon dioxide move between the air and the blood by simple diffusion, the
driving force for this movement being the partial pressure gradients between the
blood and the air. Fick’s law of diffusion states that diffusion of gas across a sheet
of tissue is proportional to the area of the sheet and inversely proportional to the
thickness of the tissue, therefore efficiency of this transfer is increased by greater
surface area and decreased thickness of the gas exchange surfaces. In mammals
the alveoli make up a very large surface area. For example; a human adult lung
contains 300-500 million alveoli, with a surface area of 50-100 m2 over which gas
exchange can occur. Additionally there are very short distances for diffusion of
gases between the blood and the air. The pulmonary circulation is closely
associated with the alveoli; a network of capillaries wrap around the alveoli
minimising the distance for diffusion of oxygen into the blood and carbon dioxide
out into the alveoli. This diffusion distance can be as small as 0.5μm. The large
surface area and short diffusion distances improve the efficiency of gas exchange
in the mammalian lung, following Fick’s law. The same is true in birds. The air
capillaries in the parabronchi where gas exchange occurs have a large surface
area, and are closely associated with blood capillaries to create short diffusion
distances. Additionally the air capillaries and blood capillaries are arranged so
that flow is crosscurrent; air passing through the air capillaries in the
parabronchi and blood through the blood capillaries travel at right angles to one
another. This increases the efficiency of gas exchange, since oxygen and carbon
dioxide pressure gradients are maintained. In both birds and mammals, the
pulmonary circulation receives the entire cardiac output, so flow is high despite
low pressure, meaning red blood cells can pass through the circulation in less
than a second while passing numerous alveoli or air capillaries, maximising gas
Physiology of Organisms
2/1/15
exchange. This also ensures that gas exchange occurs with the blood every time
that it is circulated through the body. Both mammalian and avian lungs have large
surface areas and small diffusion distances resulting in efficient gas exchange, and
despite different structures (gas exchange occurs across the walls of tubes in
birds, and sacs in mammals) gas exchange takes place by the same process of
diffusion. However, this gas exchange is more efficiently in birds due to the crosscurrent system, creating an advantage in birds occupying a similar ecological
niche to certain mammals. The way in which gas exchange occurs is far more
similar in birds and mammals than their mechanisms of ventilation.
The mechanism of ventilating the lung is different in birds and mammals, largely
due to differences in lung structure, although the underlying principles involved
in ventilation are the same. Flow of air is defined by the equation: Flow = Δ
Pressure/Resistance. The pressures differences between any two regions causes
flow between those regions, therefore to have flow of air between the atmosphere
and the alveoli, and the atmosphere and the parabronchi there must be a
pressure gradient generated between the two. Both mammalian and avian lungs
are contained in the airtight thoracic cavity, but in mammals are separated from
the abdomen by the diaphragm, which is not present in birds. In the mammalian
lung the pressure gradient required to induce air flow from the atmosphere to the
alveoli relies on the low volume intrapleural space between the parietal pleura
lining the thoracic cage and the visceral pleura encasing the lungs. These are
separated by a 10μm thick layer of fluid, the interactive forces in which hold the
surfaces together. When mammals inspire, the diaphragm contracts, lowering and
compressing the contents of the abdomen to expand the thoracic cavity vertically.
The intercostal muscles that lie between the ribs contract to raise the ribcage. The
volume of the thoracic cavity increases, but since it is airtight subatmospheric
pressure is generated in the intrapleural space. This causes the lungs to expand,
alveolar volume increases, until the negative pressure is equal to the outward
elastic recoil forces of the chest wall, and inward elastic recoil forces of the lungs.
This leads to the generation of a pressure differential, since the increased volume
causes a fall in pressure inside the lungs to below atmospheric pressure, resulting
in flow of air into the lungs, followed by a return to atmospheric pressure. The
same principle of flow of air = Δ Pressure/Resistance is important for ventilation
of the avian lung, although birds’ lungs are ventilated by a different mechanism to
mammals’. Although the bird’s lung is also located inside the thoracic cavity, the
ribs move forward only very slightly during inspiration, and the volume of the
lung does not change greatly during breathing. Instead volume changes
predominately occur in the associated air sacs, which penetrate between organs
and into bones; the cranial air sacs that are connected to ventrobronchi, and the
caudal air sacs connected to the mesobronchus. The air sacs are expanded during
inspiration by lowering of the sternum and lateral movement of the posterior
ribs, increasing the volume and creating subatmospheric pressure, resulting in
airflow into the lungs as in the mammalian lung. During inspiration air flows into
the air sacs, passing through the parabronchi when flowing to the cranial air sacs.
Then during expiration compression of the air sacs occurs by motion of the
sternum dorsally, which forces air from the caudal air sacs through the
parabronchi, and from the cranial air sacs through the ventrobronchi to the
trachea. Airflow is directed in this manner through the parabronchi,
Physiology of Organisms
2/1/15
unidirectionally, due to the openings of the ventrobronchi and dorsobronchi into
the mesobronchus having variable resistance to airflow dependent on its
direction. This mechanism results in oxygenated air passing through the
parabronchi continually during both inspiration and expiration. It maximises the
amount of oxygen extracted from and carbon dioxide lost to the air, as there is no
mixing of oxygenated air with air that has already undergone gas exchange,
therefore only oxygenated air undergoes gas exchange, meaning large partial
pressure gradients are maintained. In comparison, in mammals there is
bidirectional airflow; fresh air moving into the lungs is mixed with air that has
undergone gas exchange, meaning the air in the alveoli has a lower partial
pressure of oxygen. Therefore gas exchange is even more efficient in birds than in
mammals, creating an even more significant advantage in terms of their
respiration to birds occupying the same ecological niche as mammals.
In conclusion the main difference between birds and mammals in respiration is
their mechanism of ventilation of the lungs, where birds utilise air sacs while
mammals use movement of the diaphragm and ribcage to create pressure
differences to induce airflow into the lungs. The process of gas exchange occurs
by diffusion in both groups, both mammals and birds maximise gas exchange
through use of large surface areas and short diffusion distances. However, gas
exchange occurs far more efficiently in birds due to the cross-current system and
greater partial pressure differences from the unidirectional flow of air through
the parabronchi. The implication of this in birds and mammals with similar
ecological niches, such as insectivore birds and insectivore bats, is that the birds
have an advantage in respiration over the mammals. With more efficient
respiration, the birds, for example during hunting where a greater level of
respiration is required due to an increased level of activity, will be able to
oxygenate their blood and remove carbon dioxide at a faster rate. The mammals
at the same level of activity would need to increase their ventilation rate in order
to achieve the same rate of gas exchange, resulting in greater energy costs.
Bibliography
‘Respiration in air and water’ Lecture notes Michael J. Mason, Ph.D.
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