Respiration is the process of obtaining sufficient oxygen from an organism’s environment to support its cellular metabolic requirements. Respiration is not just the mechanical process of breathing – the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction. It is also the metabolic process by which an organism obtains energy by reacting oxygen with glucose to give water, carbon dioxide and ATP (energy).
In a typical scenario, organisms find greater difficulty obtaining oxygen (O2) than dealing with the waste gas product of respiration – carbon dioxide (CO2). This is because CO2 is more readily dissolved in plasma and so can be excreted faster than oxygen is obtained.
For oxygen to enter body fluid/plasma it must first dissolve in a fluid layer. There must be a thin layer between the fluid layer and body fluid/plasma.
This comparative article will look at the way different species all tackle the problem of obtaining oxygen for aerobic respiration, starting with insects, then amphibians, fish, mammals and finally birds.
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Insects are able to obtain all the oxygen they need for cellular metabolism without lungs. Instead insects have a hard exoskeleton which contains valve like openings called spiracles. Typically there is one pair of spiracles per body segment. Air flow is regulated by small muscles that operate flap-like valves within each spiracle which contract to close the spiracle, or relax to open it.
Oxygen travels along the spiracles which branch off into a network of tracheal tubes, further branching into tracheoles that terminate as sacs. These sacs provide the thin, moist interface necessary for gaseous exchange.
Oxygen transported this way is delivered directly to the tissues that require it, meaning it is not transported via the circulatory system. Because of this the circulatory system of insects is greatly reduced (as compared to a mammal for example).
Oxygen diffuses into cell cytoplasm along a concentration gradient (having first dissolved in the moist tracheole surface); in a similar manner CO2 diffuses out of the cytoplasm to leave the insect via the spiracles.
Because of the inefficiency of this method of respiration, insects must retain small body sizes. As body size or activity of an insect increases, it may be able to increase oxygen intake by manipulating the spiracle and abdominal muscles to increase ventilation. If activity decreases, due to the toxic nature of elevated oxygen levels, the insect can close spiracles to prevent increased oxygen intake.
Amphibians have lungs which they use to respire, but they are also able to obtain oxygen through their skin. As expected with a gaseous exchange surface, the skin is thin, moist and well vascularised. Oxygen is therefore able to dissolve in the moist layer of the skin and diffuse directly into the blood. When submerged beneath the water surface, amphibians obtain all their oxygen through their skin.
This does however pose a problem, because the skin is a very thin barrier (due to it having evolved as a ‘respiratory organ’) it provides very little in the way of protection when compared to mammalian skin for example (which has evolved as a protective barrier). This restricts amphibians to generally aquatic/damp habitats where less stress is posed on the skin.
As mentioned above, amphibians also have a pair of saclike lungs, although they have a limited surface area and are not nearly as efficient as mammalian lungs. Without ribs or a diaphragm (and the lack of involvement of the chest muscles in ventilation) amphibians must use a force pump mechanism to ventilate the lungs.
The force pump mechanism of breathing is very energy expensive, the mechanism is as follows. With the mouth and nostrils closed, the amphibian will lower the floor of the oral cavity (resulting in the distinctive expansion of the throat) resulting in a reduction of pressure. The nostrils are then opened allowing air to rush into the mouth (as pressure equalises). The air in the mouth is then forced into the lungs by further contraction of the oral cavity floor.
Fish live in predominantly aquatic environments however there are exceptions, such as the lung fish which are able to utilise lungs to obtain oxygen. Aquatic fish however use a set of respiratory organs known as gills.
Gills are highly vascularised with a large surface area, short diffusion distance and an always moist surface. Their structure consists of:
- Four gill arches (either side of the head) or two gill arches in cartilaginous fish
- Each gill arch then has two rows of gill filaments. There are many gill filaments per column.
- On the upper and lower sides of each gill filament there are many lamellae. The blood flow within the lamellae is counter-current meaning the deoxygenated blood flow direction is opposite to the oxygenated water flow direction. This mechanism maintains a concentration gradient thus increasing the efficiency of the respiration process. Cartilaginous fish do not have a counter current flow system as they lack bones which are needed to have the opened out gill that bony fish have.
In order to pass the oxygenated water over the gills fish have two main methods, either ram ventilate (swim through the water with an open mouth forcing water to pass over the gills, stopping would cause the fish to drown) or use a similar method to amphibians – a double force pump. Both of these methods are unfortunately energy expensive.
The mechanism of the double force pump method is as follows. The fish utilises the expansion and contraction of two cavities, the buccal cavity (mouth) and the opercular cavity (gills):
- Water is sucked into the buccal cavity by expanding both the buccal and opercular cavities. The operculum (the bony flap on the outside of the gills which is used to protect them) remains closed to prevent water escaping.
- By contracting the buccal cavity water is now forced over the gills into the opercular cavity. The mouth and operculum remain closed to keep the cavities sealed.
- With the mouth still closed, the operculum opens and the opercular cavity now contracts forcing the deoxygenated water out.
The exposure of the gills to water poses certain problems concerning salt movement. In freshwater salt is able to easily diffuse out of the fish into the hypotonic water. To maintain a healthy salt concentration within their body, they:
- Do not drink freshwater (which would dilute the body salt levels)
- Excrete a mucus over their body surface to minimise water and salt movement
- Excrete large volumes of diluted urine
- Obtain more salt from their diet and absorb as much as possible through the gills
Marine fish on the other hand, have the completely opposite problem. Water is continuously lost from the fish and salt is continually being taken up. To maintain healthy levels of water and salt marine fish:
- Excrete a small volume of isotonic urine (The kidneys lack loops of Henle meaning they are unable to concentrate their urine)
- Chloride is actively pumped out of the fish by specialised chloride cells in the epithelium of the gill, whilst sodium diffuses out between epithelial cells along the electrical gradient
- Marine, cartilaginous fish have a specialised salt excreting gland in their rectum
- They are also able to increase the osmolarity of their tissue fluids enough so that they actually extract water from seawater by osmosis
- To increase their water levels, they must drink the seawater and excrete the salt via the specialised rectum glands – this is energy expensive however.
Compared to other species, mammalian respiration is highly efficient; there is a very large surface area within the lungs which is maximised by the bubble like structure of the alveoli. The lungs also benefit from very thin membranes between the moist layer within the alveoli and the blood. The blood supply to the lungs is very great.
Mammals have a sealed thoracic cavity, which is sealed by the diaphragm. In conjunction with the ribs, the two sets of muscles are able to control breathing. Normally, the diaphragm’s relaxed position recoils (decreasing the thoracic volume) whereas in the contracted position it is pulled downwards (increasing the thoracic volume). This process works in conjunction with the intercostal muscles connected to the rib cage. Contraction of these muscles lifts the rib cage, thus aiding in increasing the thoracic volume. Relaxation of the diaphragm compresses the lungs, effectively decreasing their volume while increasing the pressure inside them. The intercostal muscles simultaneously relax, further decreasing the volume of the lungs. This increased pressure forces air out of the lungs. Conversely, contraction of the diaphragm increases the volume of the (partially empty) lungs, decreasing the pressure inside, which creates a partial vacuum. Environmental air then follows its pressure gradient down to fill the lungs. The ventilation system of mammals is basically a suction pump.
Just like mammals, birds have ribs, although they lack a diaphragm to seal the thoracic cavity. The thoracic and abdominal cavities are thus not separated and this single large body cavity is known as the coelom.
The lungs of the bird are connected to the wall of the coelom by connective tissue and are unable to enlarge themselves like mammalian lungs. Instead air is moved in and out of the bird by expanding the coelom; this enlarges air sacs connected to the coelom causing air to pass through the lungs and into the air sacs.
A complete avian respiratory cycle involves two inspirations and two expirations (unlike mammals which involve only one). Whereas mammals have lungs in which air is inspired and then exhaled, you can think of birds as having 3 sets of lungs. They have anterior (cranial) air sacs, posterior (caudal) air sacs and the lungs themselves. Air is shifted around these 3 sets of respiratory organs during inhalation and exhalation, the exact mechanism is below:
- During the first inspiration the air stream is divided into two as the trachea divides. The air does not go directly to the lung, but instead travels to the caudal (posterior) air sacs. A small amount of air will pass through the caudal air sacs to the lung.
- During the first expiration, the air is moved from the posterior air sacs into the lungs. Blood capillaries flow through the lungs and this is where the oxygen and carbon dioxide are exchanged.
- When the bird inspires the second time, the air moves to the cranial air sacs. (Whilst more air is inhaled into the caudal air sacs)
- On the second expiration, the air moves out of the cranial air sacs, into the trachea and out of the nostrils. (Whilst the air inspired during the second inspiration moves through the lungs).
Notice how efficient this is, before the initial breath has even been exhaled, more air has been brought into the respiratory system.
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