question: At fundamental level, what exactly happens so that our cells in body dies if they cannot get oxygen for a longer period?
comment: *Everything* that your cells do "costs" a chemical called ATP. ATP is the money that our cells use to do literally anything--even making more ATP. Oxygen is part of the process of making ATP. (Side note: yes, there is an anaerobic process for making ATP, but it can't go on forever). So when you run out of oxygen, you're using ATP without making more. When you run out of ATP, you can't make more ATP--and at that point, your cell is pretty much dead.
passages: Oxygen (O) must be present in every breathing gas. This is because it is essential to the human body's metabolic process, which sustains life. The human body cannot store oxygen for later use as it does with food. If the body is deprived of oxygen for more than a few minutes, unconsciousness and death result. The tissues and organs within the body (notably the heart and brain) are damaged if deprived of oxygen for much longer than four minutes.
   Section::::Background.:Typical values.
   Section::::Scope of risk.
   In conditions where the proportion of oxygen in the air is low, or when the partial pressure of oxygen has decreased, less oxygen is present in the alveoli of the lungs. The alveolar oxygen is transferred to hemoglobin, a carrier protein inside red blood cells, with an efficiency that decreases with the partial pressure of oxygen in the air. 
   In peripheral tissues, oxygen again diffuses down a pressure gradient into cells and their mitochondria, where it is used to produce energy in conjunction with the breakdown of glucose, fats, and some amino acids.
  
  Hypoxia can result from a failure at any stage in the delivery of oxygen to cells. This can include decreased partial pressures of oxygen, problems with diffusion of oxygen in the lungs, insufficient available hemoglobin, problems with blood flow to the end tissue, and problems with breathing rhythm.
   The minimum tissue and venous partial pressure of oxygen which will maintain consciousness is about . This is equivalent to approximately in the lungs. Approximately 46 ml/min oxygen is required for brain function. This equates to a minimum arterial ppO of at 868 ml/min cerebral flow.
   Section::::History.
   In medicine, oxygen saturation refers to "oxygenation", or when oxygen molecules () enter the tissues of the body. In this case blood is oxygenated in the lungs, where oxygen molecules travel from the air and into the blood. Oxygen saturation (() sats) measure the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen. Fish, invertebrates, plants, and aerobic bacteria all require oxygen for respiration.
  
  Section::::In environmental science.
   BULLET::::- central region – Per-ARNT-Sim (PAS) domain, which facilitates heterodimerization
  
  BULLET::::- C-terminus – recruits transcriptional coregulatory proteins
  
  Section::::Members.
  
  The following are members of the human HIF family:
  
  Section::::Function.
  
  HIF1α expression in Haematopoietic Stem Cells explains the quiescence nature of stem cells for being metabolically maintaining at a low rate so as to preserve the potency of stem cells for long periods in a life cycle of an organism.
   The body maintains a stable level of oxygen saturation for the most part by chemical processes of aerobic metabolism associated with breathing. Using the respiratory system, red blood cells, specifically the hemoglobin, gather oxygen in the lungs and distribute it to the rest of the body. The needs of the body's blood oxygen may fluctuate such as during exercise when more oxygen is required or when living at higher altitudes. A blood cell is said to be "saturated" when carrying a normal amount of oxygen. Both too high and too low levels can have adverse effects on the body.
  
  Section::::Measurement.
   When their hemoglobin molecules are deoxygenated, red blood cells release S-nitrosothiols, which also act to dilate blood vessels, thus directing more blood to areas of the body depleted of oxygen.
  
  Red blood cells can also synthesize nitric oxide enzymatically, using L-arginine as substrate, as do endothelial cells. Exposure of red blood cells to physiological levels of shear stress activates nitric oxide synthase and export of nitric oxide, which may contribute to the regulation of vascular tonus.
   Under normal aerobic conditions, approximately 4% of the oxygen metabolized by mitochondria is converted to superoxide ion, which can subsequently be converted to hydrogen peroxide, hydroxyl radical and eventually other reactive species including other peroxides and singlet oxygen, which can, in turn, generate free radicals capable of damaging structural proteins and DNA. Certain metal ions found in the body, such as copper and iron, may participate in the process. (In Wilson's disease, a hereditary defect that causes the body to retain copper, some of the symptoms resemble accelerated senescence.) These processes termed oxidative stress are linked to the potential benefits of dietary polyphenol antioxidants, for example in coffee, red wine and tea.
   Other common causes of low hemoglobin include loss of blood, nutritional deficiency, bone marrow problems, chemotherapy, kidney failure, or abnormal hemoglobin (such as that of sickle-cell disease).
  
  The ability of each hemoglobin molecule to carry oxygen is normally modified by altered blood pH or CO, causing an altered oxygen–hemoglobin dissociation curve. However, it can also be pathologically altered in, e.g., carbon monoxide poisoning.
   Section::::Changes in the glycolytic pathway.:Overview of glycolytic changes and consequences.
  
  Increased expression of almost every glycolytic enzyme is seen in hypoxic tumor conditions. The over-expression of these proteins is mediated by HIF-1 and completely alters normal cellular metabolism. With decreases in the rate of mitochondrial oxidation, lactate and protons begin to accumulate. High levels of glycolysis and the production of lactate, as shown in hypoxic tumor cells, is hallmark of cancer cells even in the presence of oxygen.
   BULLET::::- Hypoxic disease-associated - for example in cyanotic heart disease where blood oxygen levels are reduced significantly, may also occur as a result of hypoxic lung disease such as COPD and as a result of chronic obstructive sleep apnea.
  
  BULLET::::- Iatrogenic - Secondary polycythemia can be induced directly by phlebotomy (blood letting) to withdraw some blood, concentrate the erythrocytes, and return them to the body.
   There are physiological and psychological disorders related to prolonged exposure to hypoxic conditions. For example, sleep apnea is a condition where there is partial, or complete, blockage of breathing during sleep. In addition, the respiratory system linked to central nervous system via base of brain. Thus, prolonged exposure to low oxygen concentration has detrimental effects on the brain.
  
  Section::::Relationship with other physiological systems.:Sensorimotor system.
   The Arc system uses a two component regulatory system. The sensor, ArcB, is an unusual histidine kinase in that it contains three signaling domains. ArcB senses the redox state of the cell, and becomes phosphorylated. The phosphate is shuttled onto various signalling domains until it winds up on ArcA, the response regulator. The phosphorylated ArcA is then able to act as either an activator or repressor for various metabolic genes. So, when oxygen is low, then gene products that utilize oxygen will be repressed, while genes that do not require oxygen will be upregulated.
   The spleen acts as a reservoir of red blood cells, but this effect is somewhat limited in humans. In some other mammals such as dogs and horses, the spleen sequesters large numbers of red blood cells which are dumped into the blood during times of exertion stress, yielding a higher oxygen transport capacity.
  
  Section::::Structure.:Human.
   The next item of interest is oxygen deficiency, questioning how oxygen deficiency impacts the brain is an important part of comprehending why ASC occurs when there is oxygen deprivation in an environment.
  
  Section::::Induction methods.:Pathologies/other.:Infections.
   Section::::Cause of aging.
   Regardless of the partial pressure of oxygen in the blood, the amount of oxygen that can be carried, depends on the hemoglobin content. The partial pressure of oxygen may be sufficient for example in anemia, but the hemoglobin content will be insufficient and subsequently as will be the oxygen content. Given enough supply of iron, vitamin B12 and folic acid, EPO can stimulate RBC production, and hemoglobin and oxygen content restored to normal.
  
  Section::::Controls of variables.:Arterial blood pressure.
  
  The brain can regulate blood flow over a range of blood pressure values by vasoconstriction and vasodilation of the arteries.
   In mammals, HVR invokes several physiological mechanisms. It is a direct result of the decrease in partial pressure of oxygen in arterial blood, and leads to increased ventilation. The body has different ways of coping with acute hypoxia. Mammals that rely on pulmonary ventilation will increase their ventilation to account for the lack of oxygen reaching the tissues. Mammals will also experience decreases in aerobic metabolism and oxygen demand, along with increases in ATP production.
   In addition to its involvement in generating respiratory rhythm, the pre-Bötzinger complex is also capable of integrating sensory information from changes in the biochemical environment, particularly oxygen. The capability to detect focal hypoxia causes an excitatory response in the motor output responsible for respiration, which causes alterations in the firing pattern of neurons within the pre-Bötzinger complex. Among these changes are the transition of a fully integrated network involving complex networks and autonomous mechanisms, to a system dependent on the activity of pacemaker neurons through sodium current activation. Hypoxia results in gasping due to the increased dependence on the sodium current and the overlap in networks between the generation of respiratory rhythm and intrinsic oxygen sensitization.
   The partial pressure of oxygen in the air in the lungs controls the oxygen loading of blood. A critical pO of in the lungs will sustain consciousness when breathing is resumed after a breath-hold dive. This is about 4% oxygen in the lungs and 45% oxygen saturation of the arterial blood. At 30 msw (4 bar), 2% by volume oxygen in the lung gas gives a pO of . At 10 msw (2 bar), for the same 2% oxygen, the pO would be , i.e. marginal. At the surface the same 2% oxygen drops to , ignoring metabolic use.
  
  Section::::Consequences.
   Hypoxemia is usually defined in terms of reduced partial pressure of oxygen (mm Hg) in arterial blood, but also in terms of reduced content of oxygen (ml oxygen per dl blood) or percentage saturation of hemoglobin (the oxygen-binding protein within red blood cells) with oxygen, which is either found singly or in combination.
  