Download pg3essay2

Essay A, Student 2, Marker 3
Describe and contrast the gas exchange system of fish and mammals
Word Count: 799
Respiration is the metabolic process within an organism’s cells by which energy is
released (and ATP generated) from organic molecules (such as glucose) by the
addition of oxygen through a condensation reaction to produce carbon dioxide and
water. In order to maintain respiration the cells must be provided with a continual
supply of oxygen, as well as the capacity to remove waste carbon dioxide. The gas
exchange system of Eukarya allows such means of provision and removal through
diffusion, though its methods differ significantly across phyla. There is an obvious
variance in gas exchange systems between fish and mammals, the main reason for
this being the differing respiratory mediums by which the O₂ is provided i.e. water
and air, and therefore the varying physiology that is required for extraction/excretion.
The respiration system of fish is based around the gas exchange surface membrane
evaginations (eversion of membranous tissue to the outside of the body) known as
gills, the blood, and the branchial system. The gas permeable membranes are
approximately 1 cell thick (~5µm) to allow a short diffusion pathway for the O₂ and
CO₂ to cross, and due to their constant submersion in water are kept moist which is
essential to their function. The huge surface area provided through stacks of gill
filaments and their uniformly folded lamellae extensions means that large quantities
of red blood cells flowing both to and from the gills through a network of capillaries,
are in very close proximity to the gas exchange surface at any one time. The unidirectional flow of water through the mouth and ultimately over the gills and out of the
body helps maintain the partial pressure gradient required to make the exchange of
O₂ and CO₂ gases between blood and water. The concurrent flow of blood through
the capillaries in the opposite direction to the water-flow also ensures the maximum
partial pressure gradient is sustained and preserves optimum efficiency of the
exchange of gases by carrying away O₂ to the tissues as soon as it is absorbed.
Fishes maintain ventilation of water over their gills by either the two-pump
mechanism (opening of the mouth to allow water to rush in then the subsequent
closing and contraction to increase pressure and force water over the gills) or the
ram-breathing mechanism (utilising energy used in locomotion to force water over
the lungs i.e. swimming forwards with mouth open).
Essay A, Student 2, Marker 3
The respiration system of mammals is based around the gas exchange surface
invaginations (inversion of membranous tissue to the inside of the body) known as
lungs, the blood, and the pulmonary system. The gas permeable membranes in this
case are elastic and able to expand and contract as ventilation of air occurs. They
are of a similar thickness i.e. ~1 cell thick (~0.5–2.5µm) to that of fish gills in order to
minimise the diffusion pathway of the gases. Moisture is also essential in keeping the
membrane functional but as mammals do not breathe water, alternative means must
be used to keep them wet. Mucus is secreted within the lungs to maintain moisture
levels and a surfactant is also exuded to prevent surface tension on the mucus and
allow the lungs to inflate. Again a large surface area is provided within the lung but
the membrane structure is that of thousands of tiny, individually inflatable air sacs
called alveoli that are surrounded directly by the capillaries containing red blood
cells. Unlike fish ventilation, mammalian lungs perform both inhalation (through the
lowering and contraction of the diaphragm and the opening of the ribs to decrease
pressure and take air and fresh O₂ into the lungs) and exhalation (the relaxation and
raising of the diaphragm and the closing of the ribs to increase pressure and rid the
lungs of stale air and waste CO₂). The red blood cells moving through the capillary
network across the alveoli, carry waste CO₂ from and fresh O₂ to the body’s tissues.
The relative low dissolved O₂ content of water compared to that of air (4-8ml in
contrast to 160-320ml) means that water breathers must be considerably more
efficient at removing O₂ from the respiratory medium than air breathers. Aquatic
vertebrates are able to remove around 90% of O₂ from water whilst mammals only
extract around 25%. The constant, uni-directional water flow over the gills of fish
helps maintain this level of efficiency, along with the counter-current flow of blood as
previously mentioned.
Respiratory membranes in both fish and mammals have many necessary similarities,
such as maintained moisture levels to ensure functionality; large surface area; close
networks of capillaries; and a thin epithelial surface to maximise the exchange of
gases by diffusion. They also have many required differences to cope with the
varying levels of O₂ availability.
Essay A, Student 2, Marker 3
References
PURVES, W K et al. (2004). Life, the Science of Biology (7th edition). USA: Sinauer
Associates.
CAMPBELL, N A et al. (2008). Biology (8th edition). San Francisco: Pearson Benjamin
Cummings.
MAINA, J N et al. (2010). Recent Advances into Understanding Some Aspects of the
Structure and Function of Mammalian and Avian Lungs. Physiological and Biochemical
Zoology, 83, 5, 792-807.