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Hypoxemia

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Hypoxemia or reduced oxygenin the blood, can be caused by:

1. Low partial pressure of atmospheric oxygen (e.g., high altitudes). Partial pressure is obtained by multiplying atomospheric pressure (e.g. 760 mmHg minus 47mmHg water vapor) by the gas's fraction in air (e.g., 713 x .21=150mmHg). After mixing with expired CO2 in the lungs, oxygen diffuses down this pressure gradient to enter arterial blood around 100mmHg. 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 aminoacids) to burn with oxygen to form water. Released energy (originally from the sun and photosynthesis)is stored as ATP (adenosine triphosphate, 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.

2. Inadequate pulmonary ventilation (e.g., chronic obstructive pulmonary disease, respiratory arrest.)

3. Carbon monoxidepoisoning (poisons hemoglobin)

4. Reduced hemoglobincontent in erythrocytes. Since hemoglobin carries oxygen and carbon dioxide (and/or carbon monoxide), the quantity (volume) of oxygen carried is affected by how much hemoglobin in available and healthy. For example iron deficiency anemia lower hemoglobin levels in red blood cells and therefore hinders this carrying capacity.

5. Decreased hematocrite.g. from blood loss (blood loss anemia).

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. If severe or prolonged it could lead to cell death.

Hypoxemia

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