Metabolic Pathways and Energy Substrates in Human Tissues

Heart Muscle

The heart is the ultimate aerobic tissue. Its activity depends heavily on glucose oxidation. It utilizes the following fuels and metabolic pathways:

• Glucose: Oxidation
• Fatty Acids: Beta-oxidation
• Ketone Bodies: Oxidation of acetoacetate

The heart also plays a role in the clearance of lactic acid produced by muscles.

Nervous System

The nervous system primarily utilizes:

• Glucose: Oxidation
• Ketone Bodies: Oxidation of acetoacetate

Adipose Tissue

Adipocytes serve as the body's energy reserves, storing free fatty acids as triacylglycerols. Because these cells lack the enzyme to catalyze the conversion of glycerol to glycerol-3-phosphate, they must utilize glucose.

Glucose concentration is a determinant factor in the esterification and release of fatty acids:

• Low glucose levels: Decrease esterification and increase fatty acid release.
• High glucose levels: Increase esterification and decrease fatty acid release.

Skeletal Muscle

Skeletal muscle utilizes the following fuels and catabolic pathways:

• ATP and Phosphocreatine: Immediate energy reserves.
• Glucose: Derived from blood glucose or stored glycogen (via glycolysis and glycogenolysis).
• Fatty Acids: Beta-oxidation.
• Ketone Bodies: Degradation of acetoacetate.

In a resting state, muscle tissue uses either fatty acids or ketone bodies synthesized by the liver, depending on the body's nutritional status.

Kidney

The cells of the nephron can oxidize blood glucose, ketone bodies, fatty acids, and amino acids. The kidney acts as a metabolic buffer, assisting in the purification of lactic acid, particularly during post-exercise recovery.

Liver

The liver collects structural blocks absorbed by the small intestine and distributes them to other tissues. Hepatocytes use the degradation of amino acids and glucose as an energy source, while also utilizing glucose for carbohydrate biosynthesis.

Once hexoses are absorbed, the liver can release glucose into the blood, playing an essential role in maintaining constant blood glucose levels.