The essay elucidates how substances are transported over extended distances within animals, addressing the limitation of diffusion over longer ranges. It dissects the requisite mechanisms: a pump generating pressure gradients, bulk flow systems to carry substances, and uptake systems for their retr...
Diffusion is slow except over very short distances. How are
substances moved over longer distances within animals?
The process of diffusion is involved in almost all animal functions, but suffers
the problem of being incredibly slow except over short distances. For
example, it is no problem for oxygen to diffuse across a capillary, taking just
0.005 seconds, but for oxygen to diffuse a whole 1cm would take 15.5 hours.
Therefore, more complex systems are needed to more substances over
longer distances. Firstly, these systems will require a pump in order to
generate a pressure gradient. Secondly, these systems will require a bulk
flow mechanism to carry what has been pumped, and finally, an uptake
system will be required to get the substances out of bulk flow.
A pump is simply a structure that converts one form of energy into
mechanical energy in order to generate a pressure (figure 1). The most
obvious one in the case of animals is the heart, with chemical energy in the
form of glucose is converted into ATP, which is then used to drive the
contraction of cardiac muscle. In mammals, this heart is divided into four
chambers; two atria and two ventricles, along with a series of valves.
Critically, this setup allows isovolumetric contraction to occur. Thus, the
ventricles can contract without any change in ventricular volume, as both the
atrioventricular and semi-lunar valves are closed, causing a near vertical rise
in ventricular pressure. When this pressure exceeds that in the arteries, it
forces the semi-lunar valves open (aortic valve in left ventricle, pulmonary
valve in right ventricle), driving a flow of blood into the pulmonary and aortic
arteries. The heart therefore forms the start of the bulk flow system that will
serve the lungs and rest of the body.
A contractile pump is also required to move another substance, air, in
animals that have lungs. Given the incredibly slow rate of diffusion, it would
take an unrealistic time for oxygen containing air to diffuse all the way down
the trachea and bronchioles into the alveoli. Therefore, a pressure gradient
needs to be created to ‘suck’ air into the lungs. This is done by the
contraction of the diaphragm, which expands the thorax in the vertical axis. In
addition the external and anterior internal intercostal muscles also contract,
raising the sternum and widening the thorax. Essentially, this decreases the
intrapleural pressure, causing the lungs to expand, and therefore decreasing
the pressure inside the lungs. There is now a pressure gradient, and so
atmospheric air can flow passively into the lungs.
The digestive system of many animals is incredibly long, in humans 9
metres, and so moving food and waste also requires the generation of
pressure. It often occurs via peristalsis. This involves small contractions of
smooth muscle that make up structures such as the esophagus. The
pressure gradients generated here are comparatively minute, but lots of them
enable the food and waste to be shifted.
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