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The endocrine system, like the nervous system, is a regulatory system. It is composed of glands that secrete their products, hormones, into the bloodstream. Hormones, which are chemical messengers, bind to receptors on cells and affect the behavior of the cells. A response to hormones is generally slower than a response to a nerve impulse. The major hormone-producing organs are the endocrine glands . Endocrine glands do not have ducts; they release their secretions directly into the bloodstream. Endocrine glands are scattered throughout the body and do not have direct connections to one another.
The hypothalamus controls the secretions of the pituitary gland. The activity of the hypothalamus is influenced by the levels of hormones in the blood and sensory information that enters the central nervous system. In the hypothalamus, special neurons extend their axons into the pituitary gland. When the neurons are stimulated, the vesicles at the ends of the axon terminals release their contents into the pituitary gland. The hormones will then be diffused into the capillaries.
The pituitary gland is bean-sized and dangles on a slender stalk of tissue at the base of the skull. It is divided into two parts. The front part, the anterior lobe, produces seven major peptide hormones.
The back portion of the pituitary, the posterior lobe, releases two other hormones. ADH (antidiuretic hormone) stimulates the kidneys to reabsorb more water from the collecting tubules that comprise them. Oxytocin stimulates the contraction of muscles in the uterus, which helps the mother to push a baby out during childbirth. It also causes the release of breast milk of a nursing mother. Oxytocin’s function in males is not known.
The pineal gland is a pea-sized gland located in the top of the brain. It secretes melatonin. The complete function of the pineal gland in humans is not well known. Melatonin seems to be released as a response to darkness, so the pineal gland is thought to be involved in daily biorhythms. It has also been implicated in mood disorders.
The thyroid gland is located at the base of the neck and wraps around the upper part of the trachea just below the larynx. It produces several hormones, but the most important is thyroxine. Thyroxine increases the body’s metabolic rate and promotes the normal growth of the brain, bones, and muscles. In addition to thyroxine, the thyroid gland also secretes calcitonin. Calcitonin stimulates the deposition of calcium in bone tissue and lowers the blood calcium level.
The production of hormones by the pituitary gland is initiated by the
The correct choice is A, because the hypothalamus acts as the director of the endocrine system by stimulating secretion of hormones by the endocrine glands. The adrenal medulla is a specialized part that secretes neurohormones, adrenaline (or epinephrine), and noradrenaline (norepinephrine). The thyroid gland secretes thyroxine that increases metabolic rate and promotes growth. The pineal gland secretes melatonin in response to darkness.
The parathyroid glands are attached to or embedded in the back surface of the thyroid gland. There are usually four parathyroid glands, which secrete parathyroid hormone (PTH). PTH regulates the calcium levels in the blood by increasing the reabsorption of calcium in the kidneys and the digestive system. In addition, PTH is important in promoting proper nerve and muscle function, as well as maintaining bone structure.
The adrenal glands are pyramid-shaped structures that sit on top of the kidneys, one gland on each kidney. Each adrenal gland is divided into two parts: the adrenal cortex (outer) and the adrenal medulla (inner).
The adrenal cortex makes up 80 percent of the gland. It produces more than two dozen corticosteroids, which are essential for normal body function. One example is aldosterone. Aldosterone regulates the reabsorption of sodium and the excretion of potassium by the kidneys. Cortisol helps to control the rate of metabolism of carbohydrates, fats, and proteins. It also helps people cope with stress and acts to reduce inflammation.
The adrenal medulla is a specialized part of the sympathetic nervous system. It secretes the neurohormones, adrenaline (or epinephrine), and noradrenaline (norepinephrine). These hormones prepare the body for action in emergencies. Adrenaline makes up about 80 percent of the total secretion of the adrenal medulla. Adrenaline increases heart rate, blood pressure, and blood supply to skeletal muscles. It also increases the conversion of glycogen to glucose and stimulates the body’s metabolic rate. Noradrenaline stimulates the heart muscle.
The pancreas is located just behind the stomach. The hormone-producing part of the pancreas consists of clusters of cells that resemble islands. They are called islets of Langerhans. Each islet secretes insulin and glucagons, which regulate the metabolism of blood glucose.
Insulin is a hormone that lowers blood glucose levels. It stimulates the ability of its target cells to take up and use glucose. Its major targets are the cells of the liver, skeletal muscles and fat tissue. Insulin prevents the level of glucose in blood from rising immediately after a meal. It also ensures that excess glucose will be stored for further use. A high level of glucose in the blood is an indication of diabetes mellitus, a serious disorder in which cells are unable to obtain glucose from the blood.
Glucagon stimulates the cells of the liver and skeletal muscle to break down glycogen and increase glucose levels in the blood. It also causes fat cells to break down fats so they can be used to produce carbohydrates.
The gonads (ovaries and testes) are the body’s reproductive glands. The ovaries produce the female sex hormones, estrogen, and progesterone. Estrogen is required for the development of ova and for the formation of the physical characteristics of a female. Progesterone prepares the endometrium for the arrival of a developing embryo.
The testes produce male sex hormones, androgens. Androgens, such as testosterone, are required for normal sperm production and the development of physical characteristics for a male.
Which gland produces cortisol?
The correct choice is D, because cortisol is produced and secreted by the adrenal cortex. The parathyroid secretes parathyroid hormone (PTH). The pancreas secretes insulin and glucagons. The thyroid gland produces several hormones, but most importantly thyroxine.
Diagram of Negative Feedback Mechanism
The endocrine system is regulated by a negative-feedback mechanism. This means the output of the system inversely regulates its activity. This mechanism functions to maintain homeostasis in the body. The negative-feedback mechanism is automatic and self-regulating.
The hormones produced by endocrine glands fall into two groups: polypeptides and steroids. Polypeptides, such as glucagon and thyroxine, are large proteins composed of chains of amino acids. Steroids, such as progesterone, are lipids that are produced from cholesterol. In addition, some cells and tissue produce local hormones called prostaglandins. These hormones are fatty acids.
Secretions from ductless glands are called
The correct choice is C. Endocrine glands are ductless and secrete hormones. Enzymes are proteins that act as catalysts. Excretory fluids come from the urinary system. Digestive fluids come from the digestive system.