Final answer:
The hypothalamus is the part of the endocrine system that secretes releasing factors, initiating a cascade of hormonal activity by controlling the pituitary gland, which in turn regulates other glands such as the thyroid and adrenal glands.
Step-by-step explanation:
The structure of the endocrine system responsible for secreting releasing factors is the hypothalamus. The hypothalamus plays a crucial role in the endocrine system by producing hormones known as 'releasing factors,' which are then sent through the hypophyseal-hypothalamic portal vein to the pituitary gland. The pituitary is often referred to as the 'master gland' because it produces hormones that regulate other endocrine glands. Therefore, the hypothalamus indirectly controls various physiological processes by regulating the release of hormones from the pituitary gland. The endocrine system includes several other glands, such as the thyroid, parathyroid, adrenal glands, and gonads, but it is the hypothalamus that originates the signalling molecules that orchestrate the hormonal symphony.