In this section, you will explore the following questions:
- How is hormone production regulated?
- What are examples of different stimuli that control hormone levels in the body?
Connection for AP® Courses
Connection for AP® Courses
Hormone production and release are often controlled by negative feedback or positive feedback, although regulation by negative feedback is more common. In negative feedback, a stimulus elicits the release of a hormone that produces a response, and once the response reaches a certain level, it sends a signal that stops additional release of the hormone. In this way, the concentration of hormones in blood is maintained within a narrow range. For example, the anterior pituitary stimulates the thyroid gland to release thyroid hormones T3 and T4. Increased levels of these hormones result in feedback to the hypothalamus and anterior pituitary to inhibit further signaling to the thyroid gland.
Some hormones are regulated by positive feedback in which the response is amplified; an example of hormone regulation by positive feedback is the production of oxytocin during labor and childbirth as uterine contractions cause the production and release of more oxytocin. Disruptions in the mechanisms of feedback often result in deleterious consequences, such as diabetes mellitus resulting in decreased insulin production.
Information presented and the examples highlighted in the section support concepts outlined in Big Idea 2 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices.
|Big Idea 2||Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.|
|Enduring Understanding 2.C||Organisms use feedback mechanisms to regulate growth and reproduction, and to maintain dynamic homeostasis.|
|Essential Knowledge||2.C.1 Organisms use feedback mechanisms to maintain their internal environments and respond to external environmental changes.|
|Science Practice||6.1 The student can justify claims with evidence.|
|Learning Objective||2.15 The student can justify a claim made about the effect(s) on a biological system at the molecular, physiological or organism level when given a scenario in which one or more components within a negative regulatory system is altered.|
Hormone production and release are primarily controlled by negative-feedback. In negative feedback systems, a stimulus elicits the release of a substance; once the substance reaches a certain level, it sends a signal that stops further release of the substance. In this way, the concentration of hormones in blood is maintained within a narrow range. For example, the anterior pituitary signals the thyroid to release thyroid hormones. Increasing levels of these hormones in the blood then give feedback to the hypothalamus and anterior pituitary to inhibit further signaling to the thyroid gland, as illustrated in Figure 28.14. There are three mechanisms by which endocrine glands are stimulated to synthesize and release hormones: humoral stimuli, hormonal stimuli, and neural stimuli.
Hyperthyroidism is a condition in which the thyroid gland is overactive. Hypothyroidism is a condition in which the thyroid gland is underactive. Identify which of the conditions the following two patients would most likely have and explain why.
Patient A has symptoms including weight gain, cold sensitivity, low heart rate, and fatigue.
Patient B has symptoms including weight loss, profuse sweating, increased heart rate, and difficulty sleeping.
- Patient A has an underactive thyroid, because less thyroid hormones decreases metabolism. Patient B has an overactive thyroid, because more thyroid hormones increases metabolism.
- Patient A has an overactive thyroid, because less thyroid hormones decreases metabolism. Patient B has an underactive thyroid, because more thyroid hormones increases metabolism.
- Patient A has an underactive thyroid, because more thyroid hormones decreases metabolism. Patient B has an overactive thyroid, because less thyroid hormones increases metabolism.
- Patient A has an overactive thyroid, because less thyroid hormones increases metabolism. Patient B has an underactive thyroid, because more thyroid hormones decreases metabolism.
The term humoral is derived from the term humor, which refers to bodily fluids such as blood. A humoral stimulus refers to the control of hormone release in response to changes in extracellular fluids such as blood or the ion concentration in the blood. For example, a rise in blood glucose levels triggers the pancreatic release of insulin. Insulin causes blood glucose levels to drop, which signals the pancreas to stop producing insulin in a negative-feedback loop.
Hormonal stimuli refers to the release of a hormone in response to another hormone. A number of endocrine glands release hormones when stimulated by hormones released by other endocrine glands. For example, the hypothalamus produces hormones that stimulate the anterior portion of the pituitary gland. The anterior pituitary in turn releases hormones that regulate hormone production by other endocrine glands. The anterior pituitary releases the thyroid-stimulating hormone, which then stimulates the thyroid gland to produce the hormones T3 and T4. As blood concentrations of T3 and T4 rise, they inhibit both the pituitary and the hypothalamus in a negative feedback loop.
In some cases, the nervous system directly stimulates endocrine glands to release hormones, which is referred to as neural stimuli. Recall that in a short-term stress response, the hormones epinephrine and norepinephrine are important for providing the bursts of energy required for the body to respond. Here, neuronal signaling from the sympathetic nervous system directly stimulates the adrenal medulla to release the hormones epinephrine and norepinephrine in response to stress.
Science Practice Connection for AP® Courses
Create a visual representation to describe the regulation of blood sugar levels, growth spurts in teenagers, and events associated with labor and childbirth. Then explain how disruptions to these regulatory processes, for example, failure to produce insulin, can affect homeostasis in the body.