Final answer:
The four main components of the Harris hypothesis are releasing hormones, inhibiting hormones, portal system, and target cells, which all contribute to the regulatory functions of the endocrine system.
Step-by-step explanation:
The Harris hypothesis relates to the coordinated effort of structures and chemicals within the endocrine system to regulate physiological processes via hormones. The four main components of the Harris hypothesis are releasing hormones, inhibiting hormones, the portal system, and target cells. Releasing and inhibiting hormones originate in the hypothalamus and travel through the portal system to the pituitary gland, which in turn secretes hormones that affect target cells in various other glands or tissues.