Question

Glucose can be secreted into the blood by a. the liver. b. the muscles. c. the liver and muscles. d. the liver, muscles, and brain.

Short answer

The liver is the only organ capable of secreting glucose into the blood, as it has hepatocytes that secrete glucose to maintain blood glucose levels. Muscles use and store glucose but cannot secrete it back into the bloodstream due to the absence of necessary enzymes. The brain consumes glucose but does not produce or secrete it. Therefore, the correct answer is a. the liver.

Step-by-step solution

Understand the Function of the Liver

The liver plays a crucial role in maintaining blood glucose levels in the body. This organ has cells called hepatocytes that secrete glucose into the bloodstream. This process is essential to maintaining an adequate supply of glucose for other organs like the heart and brain.

Understand the Function of the Muscles

Muscles use glucose as their primary source of energy. They extract glucose from the blood and store it in the form of glycogen which they convert back into glucose when needed. However, unlike liver cells, muscle cells lack the necessary enzymes to convert their glycogen stores back into glucose and secrete it into the bloodstream. Therefore, the glucose taken by the muscles isn't added back to the bloodstream.

Understand the Function of the Brain

The brain is the primary consumer of glucose in the body to fuel its actions. It does not produce or secrete glucose. Instead, it takes glucose from the bloodstream to fulfil its energy requirements.

Answering the question

From the understanding acquired in steps 1, 2 and 3, it can be inferred that the only organ capable of secreting glucose into the blood is the liver. Thus, the correct response to the exercise is a. the liver.

Key concepts

Liver Function in Glucose Regulation

The liver is an essential organ for regulating glucose levels within the bloodstream. Acting much like a vigilant gatekeeper, it performs a balancing act to keep blood glucose within a narrow range.

One of the liver's key functions is gluconeogenesis, a process whereby it produces new glucose from non-carbohydrate sources when blood sugar levels dip too low. This newly synthesized glucose is then secreted into the blood to ensure organs and tissues, particularly the brain, receive a constant fuel supply.

Another critical role of the liver is to convert excess glucose into glycogen, a storage form of glucose, during times of plenty. This function prevents hyperglycemia, which can be harmful if persistent. When energy is needed, the liver activates glycogenolysis, where glycogen is broken down into glucose and released into the bloodstream, precisely metering out the supply to meet the body's demands.

• Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources.
• Glycogenolysis: Conversion of glycogen back into glucose when energy is required.

This dual capacity to both store and create glucose makes the liver unique and indispensable in glucose metabolism.

Muscle Glucose Metabolism

Muscles are powerhouses of the body, and glucose is the primary source of their energy during both light activities and high-intensity workouts. When glucose enters muscle tissue, it's either used immediately for energy or it's stored for future needs as glycogen. This storage is particularly vital during exercise when the demand for energy rises sharply.

However, muscle tissue has a significant limitation—it's not equipped with the enzymes to convert glycogen back into glucose to release into the bloodstream. This one-way conversion means that the glucose muscles store is earmarked solely for their own use and cannot contribute to systemic blood glucose levels.

Muscles also increase their glucose uptake from blood in response to insulin, particularly after meals, which helps in reducing high blood sugar levels. At rest, muscles can burn fatty acids for energy, conserving stored glucose primarily for physical activity where quick energy is essential.

Fueling Muscular Activity

During intense exercise:

• Muscles use glycogen reserves for immediate energy production.
• They increase glucose uptake to sustain prolonged activity.

As a result, although muscles are voracious consumers of glucose, they do not contribute to glucose secretion into the bloodstream.

Brain Glucose Consumption

The brain is the most energy-intensive organ in the body and uniquely relies almost entirely on glucose for its metabolic needs. While other organs and tissues can switch to alternative fuels like fatty acids, the brain's preference remains glucose. It consumes approximately 20% of the glucose used by the entire body, even though it only represents about 2% of the body's mass.

Because the brain cannot store glucose, it requires a continuous supply from the bloodstream. This dependency means that any fluctuations in blood glucose levels can have rapid effects on brain function, impacting concentration, alertness, and overall cognitive performance.

Hyperglycemic (high blood sugar) and hypoglycemic (low blood sugar) conditions can be particularly detrimental to brain function, with possible symptoms ranging from confusion and irritability to loss of consciousness or seizures. Insulin does not influence the brain's glucose uptake directly; neurons uptake glucose in response to their metabolic needs.

• The brain’s demand for glucose is unwavering, and it is non-negotiable.
• Optimal brain function is tied to consistent and adequate glucose delivery.

The brain’s dependency on a steady glucose supply underscores the importance of maintaining stable blood sugar levels, which is largely coordinated through the liver's glucose regulation.