The mechanisms for the absorption and carriage of Oxygen in animals are incredibly diverse. However, all the mechanisms in nature, from the spongocoel and osculum of sponges to the lungs of mammals, share three common features that enable the distribution of Oxygen within any organism. Firstly, because gases need to be dissolved in water to diffuse in and out of cells, respiratory surfaces must stay moist to be effective. Secondly, to enable the most effective rate of diffusion the cells that line the respiratory surfaces have to be very thin.
Finally, to accommodate an adequate rate of gaseous exchange respiratory systems need a large surface area (in relation to the rest of the organism) that is in contact with the environment that the organism is in (Audesirk 2002). The most basic mechanism for the absorption and carriage of Oxygen in animals is found in sponges. The bodies of sponges are arranged around a network of pores, passages and chambers. Flagellated cells called choanocytes line the internal chambers of a sponge; these choanocytes generate a water current that circulates throughout the sponge via the many pores that a sponge is built around.
Thus the cells of a sponge can independently respire from a renewable source: the water the sponge inhabits. Cnidaria, like Porifera, lack any specialised respiratory system. Instead these hydras, jellies, sea anemones and corals have a gastro vascular cavity. This cavity has one opening that acts as mouth and anus, and deals with both digestion and gaseous exchange. Cnidaria, like sponges, employ a method of pumping water to keep fresh oxygen entering the gastro vascular cavity and so enabling continuous gaseous exchange.
However, unlike sponges (which possess no true body tissues), Cnidaria have simple muscular structure. This allows them to contract and relax their bodies to create water currents for digestion and respiration. . Flatworms (Platyhelminthes) and Roundworms (Nematoda) employ a system of direct diffusion between their body cells and their environment. The diminutive size and flat shape of flatworms, and the long very thin proportions of round worms creates a sufficiently large enough respiratory area (in relation to the rest of their bodies), to allow this direct diffusion to occur.
All the previously mentioned animals are classed as organisms where circulatory and respiratory systems are absent. The Segmented Worms (annelids), like us have a closed circulatory system: this is a series of vessels with blood that possesses oxygen-carrying haemoglobin (see Diagram 1, Appendix ‘A’). However, like Flatworms and Roundworms, Segmented Worms acquire oxygen by direct diffusion through the skin and so are classed as being absent of a respiratory system (see Table 1, Appendix ‘A’). All these previously mentioned respiratory and circulatory are only effective because the animals using them have low metabolic rates.
Arthropoda, (insects, arachnids and crustaceans), are considered the lowest animal phyla on the evolutionary tree to possess both a circulatory and respiratory system. Arthropods utilise three different mechanisms to acquire oxygen. Firstly, many aquatic arthropods use gills: these vary in structure and physical location upon the body. For instance: mayfly nymphs have gills that are leaf-like in appearance, located on the first seven abdominal segments and dragonfly nymphs have gills that are folds in the rectum.
By pumping water in and out of the rectum oxygen is supplied to the gills (Borror 1989). Secondly, various arthropods employ a system of tracheae and in the most basic form just a singular trachea. The tracheal system is composed of three principle parts. Cuticular tubes that open out onto the exterior of arthropods at points known as spiracles and extend internally to very fine closed branches known as tracheoles. These tracheoles extend through and penetrate (without breaking cell membranes) the living tissues of the arthropod.
Air enters the body through the spiracles, passes through the tracheae and down into the tracheoles, where, by diffusion, oxygen enters the cells of the body. In smaller arthropods basic diffusion is sufficient to supply the tracheal system, but in larger arthropods active ventilation by using abdominal muscles, movement of internal organs or even leg or wing movements is employed to aid ventilation of the tracheal system (Borror 1989). This type of respiratory system is known as an open tracheal system. When a tracheal system has permanently closed spiracles it is known as a closed tracheal system.
In a closed tracheal system gaseous exchange takes place by diffusion through the body wall directly into the sub-dermal tracheal system. This ‘closed tracheal system’ is used by many aquatic and parasitic insects. A third mechanism of respiration that is used by arthropods is the book lungs, specialised structures used principally by arachnids. Book lungs are basically pockets where the body wall has been thinned and folded into sheets. These sheets have a permeable skin covering and blood flows through the folds.
These pockets are arranged in pairs, and depending on the species involved, there are between one and four pairs of book lungs. The flow of air into these lungs can be by simple diffusion but also muscular contraction is also utilised by some species to accelerate airflow into the lungs (Kozloff 1990). Various semi-aquatic arthropods also use fine hairs upon their bodies to trap a film of air against their bodies whilst submerged. Dissolved oxygen in the water will actually diffuse into this film of air as the oxygen levels in the film of air deplete.
In this way the insect gets several times the amount of oxygen out of the film of air than what was in the air when the insect submerged. Arthropods, unlike annelids, have a well-developed circulatory system that enables the transport of oxygen throughout their bodies. The circulatory system used by arthropods is called an open circulatory system. This is because the blood is not contained within its’ own system of vessels: instead blood is moved from the heart into spaces that are collectively known as haemocoel.
The typical arthropod heart is a tubular structure positioned along the back of the gut in the thorax. It often has a main (and usually singular) blood vessel extending from it down through the thorax and into the abdomen. Also this heart has it’s own haemocoelic space, known as the pericardial sinus, from which blood is returned to the heart via pairs of openings known as ostia. These ostia can be closed when the heart is contracting and so stop any backflow. One of the plasmas that run through the circulatory systems in arthropods contains haemocyanin.
Haemocyanin uses copper as an oxygen binding component which often gives the blood a blue colour, compared to haemoglobin (used by virtually all vertebrates as well as some arthropods) which uses iron as its’ oxygen binding component and is red in colour (Campbell ; Reece 2002). Although the blood in arthropods contains these oxygen-binding materials, very little of the oxygen used by arthropods is gained from the blood. The previously mentioned respiratory system of arthropods is responsible for the direct delivery of oxygen to nearly every cell in an arthropods body.
Like arthropods, Mollusca, (snails, squid and clams), also have an open circulatory system. In molluscs this system utilises a heart that typically has one ventricle and one or two atria, and like arthropods, haemocoels are responsible for the majority of blood flow. The one exception to this is the class cephalapoda, the octopuses and squids: these have a closed circulatory system that ends in capillary networks. Also like the arthropods, molluscs utilise both haemocyanin and haemoglobin as oxygen-binding components. The molluscs utilise a cavity under their shells to acquire oxygen for respiration: this cavity is known as the mantle cavity.
As well as the anus and excretory pores this cavity also houses the breathing mechanism. In air breathing molluscs the wall of this cavity has been adapted to act as a lung. For other molluscs a variety of gills are used, including a gill-type unique to molluscs, ctenidia (Kozloff 1990). These are typically arranged in pairs within the mantle cavity and look like leaves. The design of the ctenidia within the cavity is often good enough to allow separate inhaling and exhaling currents to carry water in and out of the cavity.
The previously mentioned molluscs and arthropods are protosomes: that is whilst in embryonic development a space forms (known as a coelom) between the digestive tract and the body wall. Echinodermata, the phylum of the starfish and sea urchins, are deuterstome: they get a coelom that is developed from the digestive tract. Of the nine main phyla of animals only echinoderms and chordates have a deuterstome development, although echinoderms have a unique system for the absorption and carriage of oxygen. Echinoderms have a water vascular system: this is an intricate multi-purpose system of vessels.
Water is delivered to this system via an opening in the body known as the madrepoint or sieve plate. From the sieve plate water enters a centrally placed ring canal; from this canal radial vessels branch off towards the extremities of the echinoderm. In this way the oxygen in the seawater is transported throughout the echinoderm’s body. The flow of this water is crucial to the movement mechanisms employed by echinoderms, and so the constant hydraulic action used for locomotion also serves for circulation.
As well as being deuterstome, all chordates have (even if only whilst embryonic) what is known as Pharyngeal gill slits. These gill slits are the structures that allow chordates like fish to use their gills so effectively. The fish opens its’ mouth, water flows in; the water passes through the slits, situated in the back of the pharynx, and flows out over the gills and back out of the body past the operculum (an external flap supported by bone that protects the delicate gill filaments). A diagram of fish gills can be found in Appendix ‘A’ (diagram two).
This type of gills are a very efficient oxygen acquiring mechanism, they can retrieve over eighty per cent of the oxygen in the water that is passed over the gills (Campbell ; Reece 2002). All other vertebrates (and even some fish) use lungs, with the exception of some amphibians and the reptilian Testudines, or turtles. Some amphibians obtain oxygen by diffusing it straight through their skin, and turtles have sheets of moist, tightly packed cells (called epithelial surfaces) through which they gain oxygen.
These surfaces are located in the mouths and anus of turtles and they supplement the lungs. For animals that are marine or amphibious maintaining a moist surface area that promotes efficient gas exchange across cell membranes is relatively easy. However, water only contains about four to eight millilitres of dissolved oxygen per litre, compared to 210 millilitres of dissolved oxygen per litre in air. The higher concentration of oxygen in air, amongst other things, makes the actual process of breathing with lungs a lot more energy efficient than gills.
The lungs provide air to the capillaries of the circulatory system, where the oxygen diffuses across the alveolar and capillary walls and into the plasma of the bloodstream. The circulatory system of air-breathing vertebrates differs very little: oxygen rich blood from the lung capillaries is delivered to the left atria. In amphibians and most reptiles this blood then empties into a singular ventricle that also receives deoxygenated blood from the body via a right atria.
Although some mixing does occur, most of the oxygenated blood remains to the left of the ventricle and from there is pumped around the rest of the body until it ends up back on the right side of the ventricle, where it is pumped back to the lungs. The high metabolic demand of mammals and birds necessitates an even greater efficiency in oxygen delivery. So, instead of the two chambers of a fish heart, or the three chambers of amphibian and reptilian hearts, the hearts of birds and mammals contain four chambers. This permits two separate circulations to take place simultaneously.
Firstly a pulmonary circuit, dedicated to the flow of blood from and to the lungs; secondly a systemic circuit, dedicated to the flow of blood to and from the body. Oxygen rich blood flows into the left atria from the lungs, it then is pumped down into the left ventricle and then pumped up out of the left ventricle and around the body. After gaseous exchange in the body capillaries the deoxygenated blood is returned to the heart via the right atria. From here it travels through the right ventricle and so back to the capillaries in the lungs.
The absorption of oxygen in the body capillaries is of course greatly enhanced by the presence of the protein haemoglobin: this can carry up to four oxygen molecules at a time and so greatly improves the carrying capacity of the blood (97% of the oxygen in blood is transported this way (Audesirk 2002)). So, from the pores of sponges to the capillary network of mammalian lungs, oxygen absorption and carriage methods differ greatly, but the results are similar: the successful continuance of life by ingenious means.