From the Guidelines
When a person has decreased oxygen availability, lactic acid levels would increase significantly due to a shift from aerobic to anaerobic metabolism, leading to metabolic acidosis and decreased blood pH. This is because cells rely on glycolysis and lactic acid fermentation to produce ATP without adequate oxygen, as seen in the context of hypoxia and exercise 1. Several other compounds would also be present in higher concentrations, including:
- Pyruvate, which accumulates as it cannot enter the Krebs cycle efficiently, instead being converted to lactate
- Hydrogen ions, leading to metabolic acidosis with decreased blood pH
- Hypoxanthine, xanthine, and uric acid from ATP degradation
- Carbon dioxide, as retention would occur, raising blood CO2 levels and potentially leading to respiratory acidosis, as described in the BTS guideline for oxygen use in adults 1
- Ketone bodies, which might increase as alternative energy sources
- Stress hormones like epinephrine, norepinephrine, and cortisol, which would be elevated to help compensate for the hypoxic state
- Hypoxia-inducible factor (HIF) would be activated, leading to increased erythropoietin production over time to enhance oxygen-carrying capacity through increased red blood cell production. The increase in lactic acid production is a key factor, as it contributes to muscle task failure and increases pulmonary ventilation, as lactic acid buffering results in an increase in carbon dioxide production and acidosis stimulates the carotid bodies 1.
From the Research
Metabolic Shift Due to Decreased Oxygen Availability
When oxygen availability decreases, the body's metabolism shifts from aerobic to anaerobic. This shift leads to an increase in the production of certain compounds. The key compounds that would be present in higher concentrations due to this shift are:
- Lactate: As stated in 2, 3, 4, 5, 6, lactate is the end product of anaerobic metabolism of glucose. Its accumulation in the blood signals an increase in production or a decrease in utilization, or both.
- Lactic Acid: As mentioned in 3, lactic acidosis is defined as a state of metabolic acidosis due to an increase in the blood concentration of lactate.
- Pyruvate: Although not directly mentioned as increasing, the lactate/pyruvate ratio is discussed in 3, 6, implying that pyruvate levels may also be affected by the metabolic shift.
Reasons for Increased Concentrations
The reasons for the increased concentrations of these compounds include:
- Hypoperfusion: As stated in 2, hypoperfusion represents an imbalance between systemic oxygen demand and oxygen availability, leading to tissue hypoxia and increased lactate production.
- Anaerobic Metabolism: The shift to anaerobic metabolism due to decreased oxygen availability leads to increased production of lactate, as mentioned in 4, 5, 6.
- Impaired Lactate Utilization: As discussed in 2, impaired lactate utilization can also contribute to increased lactate concentrations in the blood.