It was originally thought that the nervous system with its network of neurons spreading throughout the body was the main way the body's functions were regulated. In the 1900s researchers studying the triggers or pancreatic juices blocked the action of neurons in the intestines and round that the pancreas was still able to respond. They assumed that the gland itself was secreting an active substance (creatively called secretin) that worked on food. This substance was later called a hormone from the Greek hormon, meaning to set in motion. Correspondingly, the discovery of a separate system from the nervous system led to its being called the endocrine system from the Greek endon meaning within, and krinein meaning separate (Starr & McMillan, 2016). Key Components and Mechanics The key players in the endocrine system are the hormones and the Itad image ductless glands that produce them (which are seen in Figure 4.4).
The major glands (with examples of the hormones they secrete) are the pituitary gland (oxytocin), pineal (melatonin), thyroid (thyroxine), parathyroid (parathyroid hormone), thymus (thymosins), pancreas (insulin), adrenal (Cortisol and catecholamines), ovaries (estrogen), and testes (androgens). The ovaries and testes, collectively referred to as the gonads, are also our primary reproductive organs.
The major functions of each of these hormones are also summarized in Figure 4.4. Hormones often interact and have different functions (Starr & McMillan, 2016). The effect of one hormone can counteract the effect of another (e.g., glucagon and insulin). Sometimes, two hormones are needed to interact to cause an effect (e.g., lactation from prolactin, oxytocin, and estrogen). There is even a warm-up act function: One hormone serves to start a process that is finished by another hormone (e.g., implantation of a fertilized egg after the uterus is exposed to estrogen then progesterone). The hormones are divided into two major categories: steroids such as estrogens, testosterone, progesterone, aldosterone, and Cortisol; and nonsteroids such as the amines (e.g., norepinephrine and epinephrine), peptides (e.g., oxytocin), proteins (e.g., insulin and prolactin), and glycoproteins (e.g., follicle stimulating hormone). The steroid hormones are synthesized from cholesterol whereas the nonsteroids are derived from amino acids. The hypothalamus and the pituitary gland (described in the nervous system section) arc the major controllers of the different endocrine glands. You will see a specific detailed case of these players in action in Chapter on stress. In general, the hypothalamus and the pituitary gland interact to control secretion of hormones, nicely illustrating the partnership between the nervous and endocrine systems. In fact, the two systems are often referred to as one neuroendocrine system. The glands of the endocrine system secrete hormones directly into the bloodstream by which they are circulated to different parts of the body. Just like the lock-and-key mechanisms of our brain chemicals, neurotransmitters, hormones are also specialized to connect to unique receptors on target cells. In addition to the key role played by the endocrine system in the stress response, the functions or hormones are also directly tied in to the etiology of diabetes, a common chronic illness. When we eat, the pancreas secretes insulin that stimulates the uptake of glucose by muscles and fat cells. Insulin lowers the glucose level in the blood. Between meals our body's cells use up glucose and as the blood glucose decreases, the pancreas secretes glucagon, which breaks down amino acids and glycogen to increase the glucose level in the blood. When the body cannot produce enough insulin or when the insulin target cells cannot react to it, diabetes mellitus (or diabetes for short) develops.
The major glands (with examples of the hormones they secrete) are the pituitary gland (oxytocin), pineal (melatonin), thyroid (thyroxine), parathyroid (parathyroid hormone), thymus (thymosins), pancreas (insulin), adrenal (Cortisol and catecholamines), ovaries (estrogen), and testes (androgens). The ovaries and testes, collectively referred to as the gonads, are also our primary reproductive organs.
The major functions of each of these hormones are also summarized in Figure 4.4. Hormones often interact and have different functions (Starr & McMillan, 2016). The effect of one hormone can counteract the effect of another (e.g., glucagon and insulin). Sometimes, two hormones are needed to interact to cause an effect (e.g., lactation from prolactin, oxytocin, and estrogen). There is even a warm-up act function: One hormone serves to start a process that is finished by another hormone (e.g., implantation of a fertilized egg after the uterus is exposed to estrogen then progesterone). The hormones are divided into two major categories: steroids such as estrogens, testosterone, progesterone, aldosterone, and Cortisol; and nonsteroids such as the amines (e.g., norepinephrine and epinephrine), peptides (e.g., oxytocin), proteins (e.g., insulin and prolactin), and glycoproteins (e.g., follicle stimulating hormone). The steroid hormones are synthesized from cholesterol whereas the nonsteroids are derived from amino acids. The hypothalamus and the pituitary gland (described in the nervous system section) arc the major controllers of the different endocrine glands. You will see a specific detailed case of these players in action in Chapter on stress. In general, the hypothalamus and the pituitary gland interact to control secretion of hormones, nicely illustrating the partnership between the nervous and endocrine systems. In fact, the two systems are often referred to as one neuroendocrine system. The glands of the endocrine system secrete hormones directly into the bloodstream by which they are circulated to different parts of the body. Just like the lock-and-key mechanisms of our brain chemicals, neurotransmitters, hormones are also specialized to connect to unique receptors on target cells. In addition to the key role played by the endocrine system in the stress response, the functions or hormones are also directly tied in to the etiology of diabetes, a common chronic illness. When we eat, the pancreas secretes insulin that stimulates the uptake of glucose by muscles and fat cells. Insulin lowers the glucose level in the blood. Between meals our body's cells use up glucose and as the blood glucose decreases, the pancreas secretes glucagon, which breaks down amino acids and glycogen to increase the glucose level in the blood. When the body cannot produce enough insulin or when the insulin target cells cannot react to it, diabetes mellitus (or diabetes for short) develops.
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