Hypoxia (medical)
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For other uses of the term "hypoxia", see hypoxia (disambiguation). Hypoxia is a pathological condition in which the body as a whole (
generalised hypoxia) or region of the body (
tissue hypoxia) is deprived of adequate
oxygen supply. Low oxygen content in the blood is referred to as
hypoxaemia. Hypoxia in which there is complete deprivation of oxygen supply is referred to as
anoxia.
Generalised hypoxia occurs in normal people when they ascend to
high altitude, where it causes
altitude sickness, and the potentially fatal complications of altitude sickness,
high altitude pulmonary oedema (
HAPE) and
high altitude cerebral oedema (
HACE). Hypoxia also occurs in healthy individuals when breathing mixtures of gases with a low oxygen content, for example while diving underwater, especially with closed-circuit
rebreather systems that control the amount of oxygen in the air breathed in.
Altitude training uses mild hypoxia to increase the concentration of red blood cells in the body for increased athletic performance.
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Symptoms
Symptoms of generalized hypoxia depend on its severity and speed of onset. In the case of
altitude sickness, where hypoxia develops gradually, the symptoms include
headaches, fatigue, shortness of breath, and
nausea. In severe hypoxia, or hypoxia of very rapid onset,
seizures,
coma and death occur. Severe hypoxia induces a blue discolouration of the skin, called
cyanosis (
haemoglobin is blue when it is not bound to oxygen (
deoxyhaemoglobin), as opposed to the rich red colour that it has when bound to oxygen (
oxyhaemoglobin)).
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Causes of tissue hypoxia
- Hypoxic hypoxia when there is an inadequate supply of oxygen. The term "hypoxic hypoxia" refers to the fact that hypoxia occurs as a consequence of low partial pressure of oxygen in arterial blood, in contrast to the other causes of hypoxia listed below, in which the partial pressure of oxygen in arterial blood is normal. Hypoxic hypoxia may be due to:
- Low partial pressure of atmospheric oxygen (e.g. at high altitude).[1]
- Inadequate pulmonary ventilation (e.g. in chronic obstructive pulmonary disease or respiratory arrest).
- Shunts in the pulmonary circulation or a right-to-left shunt in the heart. Shunts can be caused by collapsed alveoli that are still perfused or a block in ventilation to an area of the lung. Whatever the mechanism, blood meant for the pulmonary system is not ventilated and so no gas exchange occurs (the ventilation/perfusion ratio is zero). Normal anatomical shunt occurs in everyone, because of the thebesian vessels which empty into the left ventricle and the bronchial circulation which supplies the bronchi with oxygen.
- Anemic hypoxia in which arterial oxygen pressure is normal, but total oxygen content of the blood is reduced.[2]
- Hypemic Hypoxia when there is an inability of the blood to carry oxygen.
- Histotoxic hypoxia in which quantity of oxygen reaching the cells is normal, but the cells are unable to effectively use the oxygen.
- Ischemic, or stagnant hypoxia in which there is a local restriction in the flow of otherwise well-oxygeneated blood. The oxygen supplied to the region of the body is then insufficient for its needs. Examples are cerebral ischemia, ischemic heart disease and Intrauterine hypoxia, which is an unchallenged cause of perinatal death.
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Pathophysiology
After mixing with water vapour and expired
CO2 in the lungs, oxygen diffuses down a pressure gradient to enter
arterial blood around where its partial pressure is 100mmHg (13.3kPa).
[1] Arterial blood flow delivers oxygen to the peripheral tissues, where it again diffuses down a pressure gradient into the cells and into their
mitochondria. These bacterial like
cytoplasmic structures strip
hydrogen from fuels (
glucose,
fats and some
amino acids) to burn with oxygen to form
water. Released energy (originally from the
sun and
photosynthesis) is stored as
ATP, to be later used for energy requiring metabolism. The fuel's carbon is oxidized to CO2, which diffuses down its partial pressure gradient out of the cells into venous blood to finally be exhaled by the lungs. Experimentally, oxygen diffusion becomes rate limiting (and lethal) when arterial oxygen partial pressure falls to 40mmHg or below.
If oxygen delivery to cells is insufficient for the demand (hyoxia), hydrogen will be shifted to pyruvic acid converting it to lactic acid. This temporary measure (anaerobic metabolism) allows small amounts of energy to be produced. Lactic acid build up in tissues and blood is a sign of inadequate mitochondrial oxygenation, which may be due to hypoxemia, poor blood flow (e.g. shock) or a combination of both.[3] If severe or prolonged it could lead to cell death.
Excess lactic acid would explain the soreness and the huge number of
knots in my muscles
