Hypoxemia

Hypoxemia or reduced Oxygen in 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 Monoxide poisoning (poisons hemoglobin).

4. Reduced Hemoglobin content in Erythrocyte s. 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 Hematocrit e.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.

SEE ALSO

Hypoxia

Hypoxemia can also be caused by shunts in the pulmonary circulation. Shunts are areas of pulmonary blood that are not ventilated and hence have a ventilation/perfusion radio of 0. No gas exchange will occur here and as a result the pulmonary venous blood will have a lower O2 level (hence hypoxemia). Shunts can be caused by collapsed alveoli that are still perfused or a block in ventilation to an area of the lung. Standard existing shunts include the thesbian vessels which empty into the left ventricle and the bronchial circulation which supply the bronchi with oxygen.

SEE ALSO

Hobler KE, Carey LC. Effect of acute progressive hypoxemia on cardiac output and plasma excess lactate. Ann Surg. 1973 Feb;177(2):199-202.
PMID: 4572785 - indexed for MEDLINE

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Hypoxemia