Which two tissues are considered to have high energy demand?

Skeletal muscle and brain tissue are known to have high energy demands due to their specific functions and metabolic requirements.

Skeletal muscle requires significant amounts of ATP, particularly during physical activity. Muscle contraction, which relies on actin and myosin interactions, consumes energy rapidly, especially during intense exercise. The energy requirements can vary, with different types of muscle fibers (fast-twitch versus slow-twitch) utilizing different metabolic pathways for ATP production, such as anaerobic glycolysis or oxidative phosphorylation.

The brain, on the other hand, is continuously active and has a constant requirement for energy to maintain essential functions, including neuronal signaling, neurotransmitter synthesis, and maintaining the electrochemical gradients across neuronal membranes. The brain primarily uses glucose as its energy source under normal circumstances and relies heavily on oxidative metabolism for efficient ATP production.

While other tissues like the liver and retina also have significant energy demands, they do not have the same level of continuous, high-energy needs as skeletal muscle and the brain, especially under conditions of normal activity. Thus, the pairing of skeletal muscle and brain highlights two of the most energetically demanding tissues in the body.