Final answer:
The regulating hormones from the hypothalamus first enter the anterior pituitary gland via the hypophyseal portal system, which does not enter the systemic circulation.
Step-by-step explanation:
The regulating hormones from the hypothalamus first enter the anterior pituitary gland without entering the systemic circulation. This is made possible by a specialized network known as the hypophyseal portal system. The process begins with the superior hypophyseal artery, which branches from the carotid arteries and transports blood to the hypothalamus. The superior hypophyseal artery forms the capillary plexus to the portal veins that carry the releasing and inhibiting hormones to the anterior pituitary. Once the anterior pituitary receives these signals, it either releases or suppresses its own hormones into the secondary capillary plexus, which then enters the general circulation.
The hypothalamus, located below the thalamus, is a critical component of the brain that regulates the endocrine system by sending signals to the pituitary gland. The pituitary gland, in turn, releases various hormones that affect other glands and cells. This interaction ensures the hypothalamus monitors and regulates vital behaviors affected by these hormones, such as maintaining homeostasis for food and water intake, energy expenditure, body temperature, and circadian rhythms.