What mechanism allows organisms to maintain homeostasis when the level of one substance influences another or organ activity?

The mechanism that allows organisms to maintain homeostasis when the level of one substance influences another or organ activity is known as "feedback loops". There are two main types of feedback loops: negative feedback and positive feedback.

1. Negative Feedback: This is the most common mechanism for maintaining homeostasis. In a negative feedback loop, a change in a given direction causes a series of events that results in an effect that reverses that change. For example, the control of blood sugar levels in humans. When blood sugar rises after a meal, the pancreas senses the change and releases insulin. Insulin helps cells to take in glucose, reducing blood sugar levels and thereby reducing the stimulus (high blood sugar) that triggered insulin release. Once blood sugar levels are back to normal, insulin release slows down. This negates the shift from the set-point and is why it's called 'negative' feedback.

2. Positive Feedback: This is less common and typically occurs when a change from the normal conditions needs to be amplified. In positive feedback, a deviation from a normal value results in the amplification of a change, pushing the system farther away from the initial state. An example of this is the onset of labor in childbirth. When contractions begin, the hormone oxytocin is released, which increases the intensity of contractions, resulting in the birth of a child. After birth, the stimulus (stretching of the cervix) is removed, the signal stops, and the system returns to its pre-labor state.

Feedback loops are essential to the functioning of homeostatic systems and involve multiple components, like receptors to detect changes, control centers to process this information, and effectors to exert the necessary response to counteract the change.