UNIT 4: Homeostasis Chapter 10: The Endocrine System pg. 466 10.2: The Endocrine Glands pg. 473 482 Hormones are produced by the endocrine system. These chemicals maintain homeostasis within the body. The endocrine system is made up of many glands located throughout the body. The glands produce hormones and secrete them into the circulatory system to be delivered to their target cells. Some hormones are secrete from organs, such as; the heart, liver and the kidneys. Figure 1: The major endocrine glands and cells in humans.
The Vertebrate Endocrine System Hypothalamus is the region of the brain that releases hormones to control the pituitary gland, which, in turn, controls other endocrine glands. Neurohormone is a hormone produced by neurons, such as in the hypothalamus that controls the production of other hormones in the pituitary gland. Pituitary gland is a two lobed gland within the cranial cavity that produces hormones that control the other endocrine glands. The hypothalamus gland is found at the base of the brain and is part of the nervous systems. The hypothalamus receives nerve impulses and produces special type of hormones, called a neurohormone. The hypothalamus coordinates actions between the brain and the endocrine system. Neurohormone leaves the hypothalamus and travels to the pituitary gland. The hypothalamus is also in direct contact with some sensory neurons to monitor changes in body chemistry and temperature. Pituitary gland is located in the cranium, below the brain, and has two lobes, anterior and posterior. Neurohormones from the hypothalamus travel to the pituitary gland via the portal vein. Two types of neurohormones secreted from the hypothalamus are releasing and inhibiting hormones. These hormones will affect the anterior portion of the pituitary gland, either stimulating or inhibiting hormones secretion. The hormones secreted from the pituitary gland control many other endocrine glands, therefore the pituitary is known as the master gland. See Table 1: Hypothalamic and Pituitary Hormones, pg. 474
Figure 2: The hypothalamus and the pituitary gland link the endocrine system and the nervous system. Neurohormones produced by the hypothalamus control the anterior and posterior pituitary hormones, which control the endocrine glands. Anterior Pituitary Gland Anterior Pituitary Gland produces six major hormones: prolactin, growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-stimulating hormone, and luteinizing hormone. Prolactin (PRL) is responsible for reproductive activities and parental care in vertebrates. PRL stimulates secretory cells in the mammary glands during late pregnancies and milk synthesis.
Growth Hormone (GH) stimulates cell division, protein synthesis and bone growth in children and adolescents. GH binds to muscles and other target tissues, causing them to release insulin-like growth factor (IGF), which directly stimulates growth. GH also controls the conversion of glycogen to glucose and fats to fatty acids to maintain levels in blood. GH also regulates the intake fatty acids and amino acids by cells and limiting the uptake of glucose by cells. Lack of GH can lead to dwarfism or the over secretion can lead to gigantism. (2.7 m) Thyroid-stimulating hormone (TSH) is responsible for stimulating the thyroid gland to increase in size and release thyroid hormones. Adrenocorticotropic hormone (ACTH) initiates the release of hormones from the adrenal cortex. Follicle stimulating hormone (FSH) affects egg development and female hormones in females and sperm production in males. Luteinizing hormone (LH) regulates part of the menstrual cycle in females and sex hormones in males. FSH and LH are known as Gonadotropins because they regulate the activities of the gonads (ovaries and testes) Melanocyte-stimulating hormone (MSH) and endorphins are secreted from the anterior pituitary gland. MSH is responsible for the secretion of melanin that makes your skin go darker in the sun. Endorphins (natural painkillers) act as neurotransmitters in the pathways that control pain, inhibiting pain perception.
Posterior Pituitary Gland The posterior pituitary gland stores and releases two important hormones produced in the hypothalamus; antidiuretic hormone and oxytocin into the circulatory system. Antidiuretic Hormone (ADH) is responsible for the stimulating of the kidney cells to absorb water from the urine, to increase blood volume. Sensory receptors in the hypothalamus are stimulates by the high concentration of Na+. The kidneys are able to maintain a homeostatic balance of extracellular fluid in the body. Approximately 85% of the water that passes through the nephron is reabsorbed from the proximal convoluted tubule the rest is absorbed from the distal convoluted tubule only if ADH is present. Oxytocin stimulates the production of milk from the mammary glands and it also plays a key role in the stimulation of contractions during child birth. Thyroid and Parathyroid Hormones Thyroid gland is an endocrine gland located in the throat that is regulated by the hypothalamus-pituitary system. Parathyroid hormone is a hormone secreted by the parathyroid glands hat controls calcium and phosphate levels in the blood. The thyroid is located in the front of the throat, and it secretes hormones that have a wide range of effects. Thyroxine (T4) contains four iodine atoms, is responsible for increases the metabolic rate of cells and is essential for body growth. The thyroid is vital for growth, development, maturation, and metabolism is all mammals. A goiter is a condition created when the thyroid is over stimulated, the neck begins to swell. Iodine supplements are added to food to prevent goiters in many countries.
Figure 6: the negative feedback control of parathyroid hormone (PTH) and calcitonin by the Ca 2+ level in the blood. Adrenal Hormones The adrenal medulla secretes two adrenal hormones, epinephrine and norepinephrine that act as neurotransmitters. Epinephrine prepares your body for handling stress. The adrenal cortex secretes several hormones. Aldosterone is involved in osmoregulation and control of blood pressure. Pineal Gland Pineal gland is an endocrine gland located in or on the brain of vertebrates that secretes the hormone melatonin to regulate biological rhythms.