Hypoxia has many different causes, but the bottom line is that metabolism in the cells of the body receive too little oxygen to function normally. Various basic causes of tissue hypoxia are shown by the list below:
A discussion of each of these different causes of hypoxia is too extensive and ambitious for this website. So the discussion on the website is limited to the effects of hypoxic hypoxia, i.e. hypoxia due to insufficient oxygen in the the blood. Hypoxic hypoxia also reveals the effects of all other forms of hypoxia upon the functioning of the body, and is also the cause of the multitude forms of hypoxia experienced by most people. So let us begin with this fascinating subject.
Hypoxia profoundly affects the functioning of the body, and some organs are more sensitive to the effects of hypoxia than are others. Of all the body organs, the brain and the eyes are most sensitive to hypoxia. However, some parts of the brain and the eyes need more oxygen to function normally than do other parts, and are accordingly more sensitive to hypoxia. So increasingly severe hypoxia first causes malfunction, and failure, of those parts of the brain and eyes consuming more oxygen, before causing malfunction and failure of those parts of the brain and eyes consuming less oxygen. This is illustrated by the different survival times of different parts of the brain during anoxia (page 278 in Liere & Stickney, "Hypoxia", 1963).
The differences in the sensitivity to hypoxia of different parts of the brain has profound consequences. Consider the example of the common experience of fainting.
The brainstem and thalamus generate the wondrous phenomenon called consciousness, and the table above shows that these parts of the brain are more resistant to the effects of hypoxia than are many other parts of the brain. Moreover, the oxygen consumption of the retina per gram retinal tissue is higher than the oxygen consumption of the brain per gram brain tissue. The experience of fainting clearly demonstrates this fact. Fainting is due to a sudden temporary severe reduction of blood flow to the head, and causes rapid onset of hypoxia of all organs and tissues in the head, resulting ultimately in loss of consciousness. Many people reporting their experience of fainting say that everything "went dark", "went black", or "went gray" before they lost consciousness. This experience clearly illustrates that the functioning of the retina failed due to hypoxia before they lost consciousness, because they consciously saw only grey, darkness, or blackness before losing consciousness.
Changes in the functioning of the brain and the eyes caused by increasingly severe hypoxia are the same for each person, because each person has the same basic body structure and function, regardless of race, sex, culture, religion, or psyche. Furthermore, each degree of degree of hypoxia affects more than one part of the brain, as well as more than one part of the eyes, and worsening degrees of hypoxia cause increasingly more parts of the brain and the eyes to malfunction. So at each level of hypoxia, mental and perceptual function is a product of the functioning of the abnormally functioning parts of the brain and the eyes, together with the functioning of the remaining normally functioning parts of the brain and the eyes. This means each level of hypoxia generates a specific cluster of symptoms and manifestations. Studies of hypoxia show it is possible to divide the symptoms and manifestations of hypoxia into four distinct levels: mild, moderate, severe, and extreme (see figure below).
These effects of hypoxia are related to the degree of saturation of hemoglobin with oxygen for people with normal hemoglobin concentration and cardiac function. A more detailed study of the effects of these four degrees of hypoxia is very revealing, of the experiences of mountaineers, of aviators, of people who have undergone hypoxic episodes, as well as revealing many aspects of the experience of dying. Readers of this website should note that all the above, as well as what follows in this website deals exclusively with the effects of acute hypoxia, and does not discuss the effects of chronic adaptation to hypoxia. Acute hypoxia is sudden onset of hypoxia lasting up to several hours.
Mild hypoxia, is normal to lower a lower level of tissue oxygenation down to a SpO2 = 80%. Mild hypoxia does not affect the functioning of the brain or the senses. Nonetheless, some people are more sensitive to the effects of hypoxia than others and may develop changes in the functioning of the brain, and the senses typical of moderate hypoxia even at mild degrees of hypoxia.
Moderate hypoxia at levels of SpO2 = 80-60% does not cause loss of consciousness or affect breathing, but does significantly affect the functioning of the brain and the senses. These changes are fascinating, wondrous, and reveal much about the experiences of hypoxia, the experience of dying, as well as the experiences reported by mountaineers climbing without oxygen, and people who fly too high in unpressurized airplanes. The partial pressures of oxygen at different altitudes with the corresponding hemoglobin saturations (SpO2) are shown in the table below (NONIN datasheet)(N.B. The SpO2 levels are AVERAGES - not absolutes. They will vary according to constitution and health of the individual. So some people will have a lower or higher SpO2 values at these percentage inhaled oxygen, altitudes, etc).
(% pressure at sea level)
Moderate hypoxia occurs when:
The manifestations of moderate hypoxia are wide-ranging and include any one or more of a multitude of hypoxia induced changes in skin color, as well as visual, motor, somatosensory, and mental function.
Cyanosis, the blue coloration of the skin and tongue due to desaturation of hemoglobin becomes evident to most observers when the SpO2 drops below 80% (Comroe 1947).
Moderate hypoxia causes narrowing of the visual fields (Klemp 2007, pages 336-337 in Liere & Stickney, "Hypoxia", 1963) resulting in tunnel vision (Lempert 1994a, page 514 in Rossen 1943), as well as blurring of vision (page 514 in Rossen 1943). This is the "tunnel experience" reported by those reporting near death experiences. The physiology of the retinal circulation explains narrowing of visual fields very well, and has been extensively discussed by Click here to read more on this subject.
The Purkinje cells of the cerebellum are large cells extremely sensitive to the effects of hypoxia. Malfunction of these and other parts of the cerebellum result in cerebellar ataxia manifesting during moderate hypoxia (Peacock 1998, Johnson-2005). The result is that people with moderate hypoxia have muscle tremors, ataxia, in-coordination with diminished hand-eye coordination (pages 304 & 316-318 in Liere & Stickney, "Hypoxia", 1963). The deterioration of handwriting of people at altitudes above 16500 feet clearly illustrate these points, as does their inability to copy simple graphic figures (Hornbein 1989).
More severe degrees of moderate hypoxia also affect voluntary motor function. Initiation of movements and actual movement requires enormous mental effort (page 317 in Liere & Stickney, "Hypoxia", 1963).
Moderate hypoxia may induce a wide variety of changes in mental function varying from an attitude of serene unconcern, of calm and tranquil indifference to everything, including pain, or even hilarity, euphoria, to a sense of power with ultimate knowledge. Two descriptions of these changes are listed below.
A person exposed to a low-oxygen tension often passes through an initial stage of euphoria, accompanied by a feeling of self-satisfaction and a sense of power. The oxygen want stimulates the central nervous system so that the subject may become hilarious and sing or shout, and manifest other emotional disturbances.(page 300 in Liere & Stickney, "Hypoxia", 1963)
Hypoxia quickly affects the higher centers, causing a blunting of the finer sensibilities and a loss of sense of judgment and of self criticism. The subject feels, however, that his mind is not only quite clear but unusually keen. He develops a fixity of purpose and continues to do what he was doing when hypoxia first began to affect him, in spite of the fact that it may lead to disaster. This fixity of purpose is highly dangerous, especially when such an individual is responsible for the lives or others, such as is true of an airplane pilot. (page 300 in Liere & Stickney, "Hypoxia", 1963)
Other changes in mental function induced by moderate hypoxia are listed below.
Moderate hypoxia lasting for several hours to days can induce changes in mental function such as difficulty with concentration, planning, calculation, and memory that may take from three to twelve months to resolve (Hornbein 1989, West 1986).
Brain malfunction induced by hypoxia may even be one of the reasons why holy people seek high, and lonely places to meditate and undergo religious experiences. After climbing high in the mountains, hypoxia alters the functioning of their brains. They become unconcerned and indifferent to cold, weariness, and hunger. Wonderful feelings of calm exultation arise, and their mental processes seem unusually keen and sharp. Isolation and fixity of purpose ensure that their feelings of mystical exultation and rapt concentration continue without thought of rest or food for a considerable time. After several weeks, their bodies adapt to the low oxygen pressure in the air they breathe, and their bodies become less hypoxic. Their feelings of religious exultation fade along with their indifference to hunger and discomfort, and they decide to return to the lowlands. It takes three months to one year before the changes in mental function return to normal after several days exposure to moderate hypoxia at high altitudes. So these holy people develop long-lasting hypoxia-induced changes in mental function during their sojourn in the mountains. Upon their return to the lowlands, their followers observe the changes caused by their stay in the mountains. They interpret the indifference caused by hypoxic brain malfunction as a renewed inner calm and transcendence of the world about them. And all are happy, because meditation high in the mountains has indeed wrought wondrous changes.
Severe hypoxia is an extension of the effects of moderate hypoxia to involve even more aspects of eye and brain function, and occurs when:
Tunnel vision may still be present, but worsening of the level of hypoxia eventually causes failure of the entire retina, and total loss of vision (Rossen 1943, Andina 1937). The fact that people say they see nothing, only blackness, at this level of hypoxia means that they are still conscious.
Indeed, because people say they have tunnel vision, or only see blackness at this level of hypoxia means that people usually retain consciousness at higher levels of severe hypoxia. This has some unusual effects - effects that can be termed "apparent unconsciousness", because people are conscious but paralyzed. In this situation people appear unconscious, simply because they do not, and cannot move.
Severe hypoxia causes generalized malfunction of the cerebral cortex. This has a number of fascinating consequences resulting in most unusual experiences for those experiencing severe hypoxia.
These effects, and the state of 'apparent unconsciousness' explains the experiences reported by those undergoing near death experiences (click here for a detailed discussion of the physiology of near death experiences). For example, the experience of a woman who underwent a period of severe hypoxia caused by nearly lethal lung disease is a good example of this. She reported:
I saw them resuscitating me. It was really strange. I wasn't very high; it was almost like I was on a pedestal, but not above them to any great extent, just maybe looking over them. I tried talking to them but nobody could hear me, nobody would listen to me.(Margot Gray, "Return From Death: An Exploration of the Near Death Experience")
No-one heard this woman speak, even though she tried to speak. Failure of her primary motor cortex functions caused by severe hypoxia paralyzed her, so she could not speak, no matter how hard she tried to speak. This report is unusual, because severe hypoxia usually causes failure of the supplementary motor cortex, as well as failure of the adjacent primary motor cortex. The supplementary motor cortex is a region of the brain surface next to the primary motor cortex, and is responsible for planning and initiating voluntary movements. People affected by failure of their supplementary motor cortex functions no longer even think of speaking or moving, because thoughts of speaking or moving simply do not arise in their minds.
The effects described above are those of severe hypoxia. Progression of the severity of hypoxia finally results in extreme hypoxia. Extreme hypoxia eventually causes failure of brain stem functions. The brainstem is a part of the brain most resistant to the effects of hypoxia, and only malfunctions and fails at degrees of hypoxia causing failure of the rest of the brain. Extreme hypoxia causes failure of all cerebral cortex functions, as well as brainstem malfunction so inducing loss of consciousness, together with abnormal breathing. Even more extreme hypoxia causes failure of all brainstem functions, causing loss of consciousness and cessation of breathing, resulting in anoxia, subsequent irreversible brainstem damage and death.
These are the effects of the four degrees of hypoxia, and are summarized below