Oxygen<span> is the third most abundant element in the universe after </span>hydrogen<span> and </span>helium<span> and the most abundant element by mass in the Earth's crust. Diatomic oxygen gas constitutes 20. 9% of the volume of air. All major classes of structural molecules in living organisms, such as proteins, carbohydrates, and fats, contain </span>oxygen<span>, as do the major inorganic compounds that comprise animal shells, teeth, and bone. </span>Oxygen<span> in the form of O2 is produced from </span>water<span> by cyanobacteria, algae and plants during photosynthesis and is used in cellular respiration for all living organisms. Green algae and cyanobacteria in marine environments provide about 70% of the free </span>oxygen<span> produced on earth and the rest is produced by terrestrial plants. </span>Oxygen<span> is used in mitochondria to help generate </span>adenosine triphosphate<span> (</span>ATP<span>) during oxidative phosphorylation. For animals, a constant supply of </span>oxygen<span> is indispensable for cardiac viability and function. To meet this demand, an adult human, at rest, inhales 1. 8 to 2. 4 grams of </span>oxygen<span> per minute. This amounts to more than 6 billion tonnes of </span>oxygen<span> inhaled by humanity per year. At a resting pulse rate, the heart consumes approximately 8-15 ml O2/min/100 g tissue. This is significantly more than that consumed by the brain (approximately 3 ml O2/min/100 g tissue) and can increase to more than 70 ml O2/min/100 g myocardial tissue during vigorous exercise. As a general rule, mammalian heart muscle cannot produce enough energy under anaerobic conditions to maintain essential cellular processes; thus, a constant supply of </span>oxygen<span> is indispensable to sustain cardiac function and viability. However, the role of </span>oxygen<span> and </span>oxygen<span>-associated processes in living systems is complex, and they and can be either beneficial or contribute to cardiac dysfunction and death (through reactive </span>oxygen<span> species). Reactive </span>oxygen<span> species (ROS) are a family of </span>oxygen<span>-derived free radicals that are produced in mammalian cells under normal and pathologic conditions. Many ROS, such as the </span>superoxide<span> anion (O2-)and </span>hydrogen peroxide<span> (H2O2), act within blood vessels, altering mechanisms mediating mechanical signal transduction and autoregulation of cerebral blood flow. Reactive </span>oxygen<span> species are believed to be involved in cellular signaling in blood vessels in both normal and pathologic states. The major pathway for the production of ROS is by way of the one-electron reduction of </span>molecular oxygen<span> to form an </span>oxygen<span> radical, the </span>superoxide<span> anion (O2-). Within the vasculature there are several enzymatic sources of O2-, including </span>xanthine<span> oxidase, the mitochondrial electron transport chain, and </span>nitric oxide<span> (NO) synthases. Studies in recent years, however, suggest that the major contributor to O2- levels in vascular cells is the membrane-bound enzyme </span>NADPH<span>-oxidase. Produced O2- can react with other radicals, such as NO, or spontaneously dismutate to produce </span>hydrogen peroxide<span> (H2O2). In cells, the latter reaction is an important pathway for normal O2- breakdown and is usually catalyzed by the enzyme </span>superoxide<span> dismutase (SOD). Once formed, H2O2 can undergo various reactions, both enzymatic and nonenzymatic. The antioxidant enzymes catalase and </span>glutathione<span> peroxidase act to limit ROS accumulation within cells by breaking down H2O2 to H2O. Metabolism of H2O2 can also produce other, more damaging ROS. For example, the endogenous enzyme myeloperoxidase uses H2O2 as a substrate to form the highly reactive compound </span>hypochlorous acid<span>. Alternatively, H2O2 can undergo Fenton or Haber-Weiss chemistry, reacting with Fe2+/Fe3+ ions to form toxic </span>hydroxyl<span> radicals (-. OH). </span>
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