In human physiology, appetite is mainly triggered by which internal signals related to hunger and energy needs?

Introduction / Context:
Appetite and hunger are common everyday experiences, but they are controlled by complex physiological mechanisms. Understanding what actually triggers appetite helps in topics such as nutrition, obesity and endocrine regulation. This question focuses on the internal signals that start the feeling of hunger rather than what happens after eating.

Given Data / Assumptions:

• Appetite is a desire to eat food.
• The brain, especially the hypothalamus, plays a major role in regulating hunger and satiety.
• Blood glucose levels and stomach status influence hunger signals.

Concept / Approach:
Hunger and appetite are regulated by a balance of hormones and nerve signals. When blood glucose levels fall or when the stomach has been empty for some time, the body releases hormones such as ghrelin, and the hunger centres in the hypothalamus are activated. This combination produces the sensation of appetite. In contrast, stretch receptors in a full stomach and hormones like leptin contribute to satiety, reducing appetite. Oxygen concentration in the blood and stored fat levels do not directly trigger the immediate feeling of hunger.

Step-by-Step Solution:
Step 1: Recall that the hypothalamus is the part of the brain that controls hunger and thirst. Step 2: Remember that low blood glucose and an empty stomach signal an energy deficit. Step 3: Link these conditions with increased hunger hormone levels, which stimulate appetite. Step 4: Recognise that a stretched stomach after a heavy meal tends to reduce appetite, not trigger it. Step 5: Confirm that high oxygen levels or stored fat are not immediate triggers of hunger signals.

Verification / Alternative check:
Physiology textbooks describe how hunger increases when people skip meals. Blood glucose falls, the stomach empties and hormones like ghrelin rise. In experiments, stimulation of the lateral hypothalamus produces feeding behaviour, further proving the central role of this brain region. When people eat, stretch receptors and hormones associated with satiety become active and the hypothalamus reduces hunger signalling. This pattern supports the idea that decreased blood glucose and hypothalamic signalling trigger appetite.

Why Other Options Are Wrong:
Option b involves stretching of the stomach after a large meal, which contributes to feeling full, not hungry.
Option c refers to high oxygen concentration in the blood, which is mainly related to respiratory function and has no direct link to appetite control.
Option d mentions large amounts of stored fat, which may influence long term metabolic regulation but does not directly trigger immediate appetite in the short term.

Common Pitfalls:
Students often confuse hunger triggers with fullness signals. They may also wrongly think that more stored fat automatically makes a person feel hungry, which is not accurate. Another pitfall is ignoring the role of the hypothalamus and focusing only on the stomach. Both hormonal and neural pathways are important, but they work together in a coordinated system.

Final Answer:
Appetite is mainly triggered by decreased blood glucose and signals from the hunger centre in the hypothalamus.