What does the endocrine system primarily regulate within the body?

The endocrine system is primarily responsible for regulating a range of bodily functions through the release of hormones. One of its key roles is to regulate growth, development, and sexual activities, which encompasses various life stages from childhood through adulthood. Hormones produced by glands such as the pituitary, thyroid, and gonads influence physical growth, maturation, and reproductive functions.

For instance, growth hormones stimulate growth in tissues and bones, while sex hormones (like estrogen and testosterone) are crucial for the development of secondary sexual characteristics and reproductive processes. This overarching control allows the body to adapt to changes throughout different life stages.

In contrast, the other options focus on functions that are predominantly governed by other systems. Respiration and heart rate are primarily regulated by the nervous system and certain hormonal responses, while blood circulation and nutrient absorption are managed mainly by the circulatory and digestive systems. Likewise, muscle contraction and movement are controlled through the muscular and nervous systems.