The endocrine system consists of glands and tissues that secrete hormones. This page will give many examples of the close association between the endocrine and nervous systems. Like the nervous system, the endocrine system is intimately involved in homeostasis. Hormones are chemical signals that affect the behavior of other glands or tissues.
Hormones influence the metabolism of cells, the growth and development of body parts, and homeostasis. Endocrine glands are ductless; they secrete their hormones into tissue fluid. From there, they diffuse into the bloodstream for distribution throughout the body. Endocrine glands can be contrasted with exocrine glands, which have ducts and secrete their products into these ducts. For example, the salivary glands send saliva into the mouth by way of the salivary ducts.
Figure 10.1 depicts the locations of the major endocrine glands in the body, and Table 10.1 lists the hormones they release. Each type of hormone has a different composition. Even so, hormones can be categorized as either peptides (which include proteins, glycoproteins, and modified amino acids) or steroids. Protein hormones, such as insulin, must be administered by injection. If these hormones were taken orally, they would be acted on by digestive enzymes. Steroid hormones, such as those in birth control pills, can be taken orally because they can pass through the plasma membrane without prior digestion.
Hormones and Homeostasis
The effect of hormones is usually controlled in two ways: (1) Negative feedback opposes their release, and (2) antagonistic hormones oppose each other’s actions. Notice in Table 10.1 that several hormones directly affect the blood glucose, calcium, and sodium levels. Other hormones are involved in the function of various organs, including the reproductive organs.
Some hormones or their effects are controlled by a negative feedback system. The result is that the activity of the hormone is maintained within normal limits. The negative feedback system can be sensitive to either a resulting condition or to the blood level of a hormone. For example, when the blood glucose level rises, the pancreas secretes insulin, which causes the liver to store glucose and the cells to take it up. When blood glucose lowers, the secretion of insulin is inhibited, and the pancreas stops producing insulin. On the other hand, when the blood level of thyroid hormones rises, the anterior pituitary stops secreting thyroid-stimulating hormones. These examples illustrate regulation by negative feedback.
The actions of a hormone can also be controlled by the presence of an antagonistic hormone. The effect of insulin, for example, is offset by the production of glucagon by the pancreas. Insulin lowers the blood glucose level, while glucagon raises it. Also, the thyroid lowers the blood calcium level, but the parathyroids raise the blood calcium level. In subsequent sections of this chapter, we will point out other instances in which hormones work opposite to one another, and thereby bring about the regulation of a substance in the blood.
Figure 10.1 The endocrine system. Anatomical location of major endocrine glands in the body. The hypothalamus and pituitary gland are in the brain, the thyroid and parathyroids are in the neck, and the adrenal glands and pancreas are in the pelvic cavity. The gonads include the ovaries in females, located in the pelvic cavity, and the testes in males, located outside this cavity in the scrotum. Also shown are the pineal gland, located in the brain, and the thymus gland, which lies within the thoracic cavity.