Hormones participate in regulation in all physiological systems. In this section we will examine a few major examples of hormonal action that are critical in regulating the internal environment. For simplicity we will focus on humans, but these systems are similar in all mammals.
focus your learning
The thyroid gland secretes the hormone thyroxine, which has multiple effects on metabolism, and calcitonin, which reduces blood calcium levels.
Calcitriol increases blood calcium.
Clusters of endocrine cells called islets of Langerhans in the pancreas secrete the hormones insulin, glucagon, and somatostatin.
The adrenal gland consists of the adrenal medulla, which secretes epinephrine and norepinephrine, and the adrenal cortex, which secretes steroid hormones.