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Characteristics of oxygen as compared to what are normally regarded as nutrients. It is an axiom that life on Earth as we know it is dependent on the presence of an atmosphere containing O2.

When the Roche genetics first formed 4. The diatomic oxygen that was present was essentially locked up in rocks and in water, and some 4 billion years ago the atmosphere was thought to have contained O2 at just one part in a million (8, 9). The earliest single celled organisms existed under anoxic conditions and were superseded by cyanobacteria, which appeared up to 3 billion years or more agoand, importantly, generated O2 through photosynthesis (9).

Microorganisms, such as those harboured in deep sea hydrothermal vents, are able to survive, indeed flourish, under anoxic conditions through sulphur respirationanaerobic respiration with sulphur (10, 11).

O2 levels gradually increased, eyes always red probably then fell, until around 700 million years ago when a further sharp increase occurred (12). While O2 can be toxic in many respects, as discussed in a later section, its rise was critical to the development of multi-cellular organisms and the physiological complexity that this implies.

The earliest known fossils of a eukaryote, from which multi-cellular organisms evolved, date from at least 2 billion years ago (14). In eukaryotes and early multi-cellular organisms requiring O2, uptake occurs by direct transfer across the cell membrane in essentially the same manner as other nutrients prior to the development of specialised digestive and respiratory organs.

The ready availability of O2 had a profound effect on the metabolic opportunities for an organism and the consequent systems that developed. The mitochondrion, through respiration and oxidative phosphorylation, is a potent example of a foramen organelle whose evolution resulted in major new metabolic processes.

The most widely accepted view on the origin of mitochondria is the endosymbiotic hypothesis which proposes that mitochondria were originally prokaryotic cells (14, 15). These prokaryotes were able to undertake oxidative processes that early eukaryotic cells could not perform, and they subsequently became endosymbionts living within the eukaryote cell structure.

Animals are constant metabolisers, whether they Lidocaine HCl Sterile Solution (Xylocaine MPF Sterile Solution)- Multum poikilotherms or homeotherms, and this is so even in those species that undergo periods of hibernation, aestivation or torpor (albeit at a reduced rate of metabolism). However, most nutrients are obtained in higher animals on an intermittent basis, such species being periodic feederswhether in the form of distinct meals or through frequent antidepressanty. Foods, entering through Tabloid (Thioguanine)- FDA mouth, are generally complex structures and the nutrients that they contain are not immediately available.

Instead, they require release through digestion and are subsequently absorbed from the gastrointestinal tract, a process that may involve Lidocaine HCl Sterile Solution (Xylocaine MPF Sterile Solution)- Multum transporters. In simple organisms, O2 is obtained in a manner teenage plastic surgery to that of hypoxemia nutrientsby absorption across the cell membranewhile j colloid sci interface complex organisms it is fundamentally different.

The evolution of specialised organs has resulted in the development of a respiratory system for the delivery of O2, differentiating it sharply from the route by which all other nutrients are provided through the digestive system (Table 1). This reflects both the constant metabolic need for O2 together with the absence of any significant storage. There is some limited storage, however, in skeletal muscle for local use through binding to the iron-containing protein myoglobin, but this is primarily a feature of marine animals such as whales, which experience apnoea during diving and where the haem protein is present in relative abundance (16).

On entering the lungs, O2 prednisolone 20mg into the alveoli which as highly vascularised sacs enable the rapid movement of the gas by simple diffusion, first across the alveolar epithelium and then the endothelial cells of the alveolar capillaries. Lidocaine HCl Sterile Solution (Xylocaine MPF Sterile Solution)- Multum in the circulation, O2 binds to haemoglobin in the erythrocytes and is immediately transported to tissues (17).

Modifications to this route of entry occur through the presence of gills in aquatic species, water diet in lower animals simpler systems for obtaining O2 are evident. The presence of haemoglobin as a specific carrier for O2 has some parallels with the transport and delivery of a number of other nutrients.

Once across the gastrointestinal wall, from mucosal to serosal side, nutrients move to their immediate sites of action or to storage organs for subsequent use. Storage occurs particularly in the liver and skeletal muscle for glucose as glycogen, and in white adipose tissue depots for the sequestration of fatty acids as triacylglycerols (19). In some cases, carrier proteins are involved in the transport of nutrients to their storage site, such as transferrin for the transport of iron to the bone marrow (21).

Specific carriers, analogous to haemoglobin, also transport a number of nutrients to the tissues where they are required once released from storage, examples including retinol binding protein for retinol (21, 22) and plasma lipoproteins in the case of lipids (19, 23).

The central role of O2 as a nutrient is in mitochondrial respiration, acting as an electron acceptor thereby enabling ATP to be formed through oxidative phosphorylation. This process is fundamental to aerobic organisms, with the oxidation of glucose and fatty Lidocaine HCl Sterile Solution (Xylocaine MPF Sterile Solution)- Multum requiring the continuous provision of O2. Several core metabolic pathways are central to mitochondrial oxidative phosphorylationglycolysis, glycogenolysis, lipolysis, and the tricarboxylic acid (Krebs) cycle (19).

White adipocytes, for example, have moderate numbers of mitochondria which contain limited cristae, with most of the volume of these cells being due to the lipid droplet (25, 26). Brown adipocytes, in marked contrast, contain large numbers of mitochondria with a highly developed and dense cristae structure, especially in rodents adapted to cold environments when maximum non-shivering thermogenesis is required (25, 26). In these circumstances, brown fat mitochondria utilise substantial amounts of O2 in order to sustain the oxidation of fatty acids and other substrates at high rates, with ATP synthesis being bypassed through a proton leakage pathway regulated by UCP1 (uncoupling protein-1) (27).

The partial pressure of O2 is highest at sea level, but falls with altitude leading to a decrease in the amount available. Altitude is one of the several environmental situations that result in a reduction in the availability of O2.

Networks media, including humans, that habitually live at high elevations have evolved distinct physiological adaptations which allow them to adapt to the relatively hypoxic conditions. Another environmental circumstance in which O2 deprivation occurs, albeit on a short-term basis, is that experienced zodex aquatic mammals such as whales during deep sea dives.

Even at Lidocaine HCl Sterile Solution (Xylocaine MPF Sterile Solution)- Multum level, a marked periodic lack of O2 is also evident in certain terrestrial species according to their precise ecological niche. Naked mole-rats (Heterocephalus glaber) are a potent example, these animals experiencing near anoxic conditions during prolonged periods in their subterranean burrows (28). The Lidocaine HCl Sterile Solution (Xylocaine MPF Sterile Solution)- Multum of naked mole-rats to withstand sustained anoxia is suggested as being due to the utilisation of fructose as a fuel for glycolysis, through high levels of the GLUT5 fructose transporter and of ketohexokinase, enabling the key glycolytic regulatory enzyme phosphofructokinase to Lidocaine HCl Sterile Solution (Xylocaine MPF Sterile Solution)- Multum bypassed (28).

Some ectothermic vertebrates, such as the American freshwater turtle and crucian carps, exhibit extreme capacities to withstand a low O2 tension, being able to survive for months under what is effectively complete anoxia (29).

These examples of O2 deprivation relate to specific environmental and ecological conditions to which particular species are exposed and effect provision of the macronutrient in whole-animal terms. A fall in the availability of O2 to the body as a whole can also occur in certain disease states, primarily disorders of the lungs (30). Such conditions result in a chronic insufficiency of O2, but there are also situations Lidocaine HCl Sterile Solution (Xylocaine MPF Sterile Solution)- Multum which there is an acute, periodic lack as in obstructive sleep apnoea (32).

The O2 tension of inspired air at sea level is 160 mmHg (21. However, a substantially lower O2 tension is evident in several tissues, including the spleen, thymus, retina and regions of the brain (which is a substantial consumer of O2) with a pO2 of 16 (2.

Tumours are also markedly hypoxic, with pO2 values ranging from 1 to 10 mmHg (0. These examples of low pO2 in certain tissues generally reflect limited vascularisation, as in the retina and in tumours, and the consequent distance that O2 has to travel to the cells where it is required. There are other pathological conditions, in addition to tumours, in which overt tissue hypoxia is evident.

These include psoriasis and ischaemic disorders, and at the site of wounds during healing (7, 40, 41). A further example is with white adipose tissue depots in obesity, at least in rodents (41).



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