Final answer:
The liver and kidneys are crucial for maintaining glucose concentrations when dietary intake is low, as they both can produce glucose through gluconeogenesis. The liver also releases glucose from its glycogen stores.
Step-by-step explanation:
The brain requires a constant input of glucose for energy, particularly because it uses about 20-25% of the body's glucose even though it only makes up about 2% of the body's weight. When dietary glucose is low, the metabolic activity of the liver and the kidneys is necessary to maintain the required concentration of glucose for the brain.
The liver has a central role in regulating blood glucose levels. It stores glycogen and releases glucose through glycogenolysis. Additionally, the liver can produce glucose from non-carbohydrate sources in a process called gluconeogenesis. The kidneys also contribute to gluconeogenesis and can produce glucose during prolonged fasting.
Skeletal muscle can contribute to glucose availability by providing amino acids and lactate for gluconeogenesis in the liver, yet on its own, it does not release glucose into the bloodstream. Adipose tissue also plays a role, but mainly in providing fatty acids for energy, sparing glucose for the brain, yet it does not directly regulate glucose concentration.