Hormones and the Endocrine System
Overview: The Body’s Long-Distance Regulators
An animal hormone is a chemical signal that is secreted into the circulatory system that communicates regulatory messages within the body.
A hormone may reach all parts of the body, but only specific target cells respond to specific hormones.
A given hormone traveling in the bloodstream elicits specific responses from its target cells, while other cell types ignore that particular hormone.
Concept 45.1 The endocrine system and the nervous system act individually and together in regulating an animal’s physiology
Animals have two systems of internal communication and regulation, the nervous system and the endocrine system. …show more content…
Hormone-secreting organs called endocrine glands secrete hormones directly into the extracellular fluid, where they diffuse into the blood.
The nervous and endocrine systems overlap to some extent.
Certain specialized nerve cells known as neurosecretory cells release hormones into the blood. The hormones produced by these cells are sometimes called neurohormones.
Chemicals such as epinephrine serve as both hormones of the endocrine system and neurotransmitters in the nervous system.
The nervous system plays a role in certain sustained responses—controlling day/night cycles and reproductive cycles in many animals, for example—often by increasing or decreasing secretions from endocrine glands.
The fundamental concepts of biological control systems are important in regulation by hormones. A receptor, or sensor, detects a stimulus and sends information to a control center.
After comparing the incoming information to a set point, the control center sends out a signal that directs an effector to respond. th Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Education, …show more content…
PTH raises the level of blood Ca2+ by direct and indirect effects.
In bone, PTH induces specialized cells called osteoclasts to decompose the mineralized matrix of bone and release Ca2+ into the blood.
In the kidneys, it promotes the conversion of vitamin D to its active hormonal form.
An inactive form of vitamin D is obtained from food or synthesized in the skin.
The active form of vitamin D acts directly on the intestines, stimulating the uptake of
Ca2+ from food.
A rise in blood Ca2+ above the set point promotes release of calcitonin from the thyroid gland. Calcitonin exerts effects on bone and kidneys opposite those of PTH and thus lowers blood Ca2+ levels.
The regulation of blood Ca2+ levels illustrates how two hormones with opposite effects (PTH and calcitonin) balance each other, exerting tight regulation and maintaining homeostasis.
Each hormone functions in a simple endocrine pathway in which the hormone-secreting cells themselves monitor the variable being regulated.
In classic feedback, the response to one hormone triggers release of the