39-1 The Endocrine System 39-1 The Endocrine System The endocrine system is made up of glands that release their products into the bloodstream. These products deliver messages throughout the body. The chemicals released by the endocrine system can affect almost every cell in the body. Hormones Hormones are chemicals released in one part of the body that travel through the bloodstream and affect the activities of cells in other parts of the body. Hormones bind to specific chemical receptors on cells. Cells that have receptors for a particular hormone are called target cells. If a cell does not have receptors or the receptors do not respond to a hormone, that hormone has no effect on it. A gland is an organ that produces and releases a secretion. There are two kinds of glands: Exocrine glands release secretions through ducts directly to the organs that use them. Endocrine glands release their secretions directly into the bloodstream. Hypothalamus The hypothalamus makes hormones that control the pituitary gland. In addition, the hypothalamus makes hormones that are stored in the pituitary gland. Pituitary gland The pituitary gland produces hormones that regulate many of the other endocrine glands. Parathyroid glands The parathyroid glands release parathyroid hormone, which regulates the level of calcium in the blood. Thymus During childhood, the thymus releases thymosin, which stimulates T cell development and proper immune response. Adrenal glands The adrenal glands release epinephrine and norepinephrine, which help the body respond to stress. Testis The testes produce testosterone, which is responsible for sperm production and the development of male secondary sex characteristics. Ovary Ovaries produce estrogen and progesterone. Estrogen is required for the development of female secondary sex characteristics and for the development of eggs. Progesterone prepares the uterus for a fertilized egg. Pancreas The pancreas produces insulin and glucagon, which regulate the level of glucose in the blood. Thyroid The thyroid produces thyroxine, which regulates metabolism throughout the body. Pineal gland The pineal gland releases melatonin, which is involved in rhythmic activities, such as daily sleep-wake cycles. Hormone Action Hormones are classified as either steroids or nonsteroids. Steroid hormones are produced from a lipid called cholesterol. Nonsteroid hormones include proteins, small peptides, and modified amino acids. Steroid Hormones Steroid hormones can cross cell membranes easily. Steroid Hormone Action Steroid hormone Receptor Nucleus Hormone-receptor complex Altered cellular function DNA Protein synthesis mRNA A steroid S teroid hormone enters a hormone cell directly across its membrane. It binds to a receptor to form a hormonereceptor complex. HRoercmepotnoer-receptor complex The hormonereceptor complex enters the nucleus, where it binds to a DNA control sequence. DNA Nucleus Binding initiates transcription of genes to mRNA. mRNA mRNA moves into the cytoplasm to direct protein synthesis. Altered cellular function Protein synthesis Hormone-receptor complexes regulate gene expression. Because steroid hormones affect gene expression directly, they can produce dramatic changes in cell and organism activity. Nonsteroid Hormones Nonsteroid hormones cannot pass through the cell membrane. A nonsteroid hormone binds to receptors on the cell membrane. This activates an enzyme on the inside of the membrane. Nonsteroid hormone (first messenger) This enzyme activates secondary messengers that carry the message of the hormone inside the cell. These messengers activate and inhibit many cell activities. ATP Enzyme activities Altered cellular function cAMP (second messenger) Prostaglandins All cells (except red blood cells) produce small amounts of hormonelike substances called prostaglandins. Prostaglandins are modified fatty acids. They affect nearby cells and tissues, and are known as “local hormones.” The endocrine system is regulated by feedback mechanisms that function to maintain homeostasis. Feedback inhibition = too much of a substance signals to stop producing the substance. Example: Controlling Metabolism Thyroxine, a hormone of the thyroid gland, affects the activity of cells throughout the body, increasing their rate of metabolism. A drop in thyroxine decreases the metabolic activity of cells. If thyroxine is low, the hypothalamus secretes thyrotropin-releasing hormone (TRH), which stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH). TSH stimulates the release of thyroxine. High levels of thyroxine in the blood inhibit secretion of TRH and TSH, which stops the release of additional thyroxine. Controlling Metabolism The hypothalamus is also sensitive to temperature. If body temperature drops, it produces extra TRH. TSH is released, which causes the release of more thyroxine. Thyroxine increases oxygen consumption and cellular metabolism. Increased metabolic activity maintains a core temperature. Complementary Hormone Action Sometimes two hormones with opposite effects act to regulate part of the body’s internal environment. Such a complementary system regulates the level of calcium ions in the bloodstream. Two hormones that regulate calcium concentration are calcitonin and parathyroid hormone (PTH). Calcitonin decreases the level of calcium in the blood, while PTH increases it. If calcium levels are too high, the thyroid secretes calcitonin. Calcitonin signals the kidneys to reabsorb less calcium. Calcitonin also reduces the amount of calcium absorbed in the intestines and stimulates calcium deposition in the bones. If calcium levels drop too low, PTH is released by the parathyroids. PTH, with vitamin D, stimulates the intestine to absorb more calcium from food. PTH also causes the kidneys to retain calcium, and it stimulates bone cells to release calcium stored in bone tissue into the bloodstream. 39-2 Human Endocrine Glands 39-2 Human Endocrine Glands The endocrine glands are scattered throughout the body. The human endocrine system regulates a variety of activities. Any improper functioning of an endocrine gland may result in a disease or a disorder. The major glands of the endocrine system include: • • • • • • • the pituitary gland the hypothalamus the thyroid gland the parathyroid glands the adrenal glands the pancreas the reproductive glands The pituitary gland secretes nine hormones that directly regulate many body functions and controls the actions of several other endocrine glands. • The pituitary gland is a structure at the base of the skull. •The gland is divided into two parts: the anterior pituitary and the posterior pituitary. The Pituitary Gland Hypothalamus Anterior pituitary Posterior pituitary Pituitary gland The hypothalamus is the part of the brain attached to the posterior pituitary. The hypothalamus controls the secretions of the pituitary gland. Hypothalamus Pituitary gland The hypothalamus is influenced by hormone levels in the blood and by sensory information. Interactions between the nervous system and the endocrine system take place at the hypothalamus. The close connection between the hypothalamus and the pituitary gland means that the nervous and endocrine systems act together to coordinate body activities. The thyroid gland is located at the base of the neck and wraps around the upper part of the trachea. The thyroid gland has the major role in regulating the body's metabolism. The thyroid produces thyroxine. Thyroxine regulates the metabolic rate of cells. Thyroid Gland Larynx Thyroid gland Esophagus Trachea Thyroid Disorders Hyperthyroidism: the body produces too much thyroxine. It is characterized by elevated temperature and metabolic rate, increased blood pressure, and weight loss. Hypothyroidism: the body produces too little thyroxine. It is characterized by lower temperature and metabolic rate, lack of energy, and weight gain. Goiter: enlargement of thyroid gland. Caused by an iodine deficiency. The four parathyroid glands are found on the back surface of the thyroid gland. Hormones from the parathyroid glands act to maintain homeostasis of calcium levels in the blood. Parathyroid glands secrete parathyroid hormone (PTH). PTH regulates calcium levels in the blood by increasing reabsorption of calcium in the kidneys and by increasing uptake of calcium from the digestive system. PTH affects other organ systems, promoting proper nerve and muscle function and bone structure. Parathyroid Glands Thyroid gland Parathyroid glands The adrenal glands are two pyramidshaped structures that sit on top of the kidneys, one gland on each kidney. The adrenal glands release hormones that help the body prepare for and deal with stress. Adrenal Glands Adrenal gland Kidney An adrenal gland has an outer part called the adrenal cortex and an inner part called the adrenal medulla. Adrenal cortex Adrenal medulla Adrenal Cortex •The adrenal cortex produces over 24 steroid hormones. •The hormone aldosterone regulates reabsorption of sodium ions and the excretion of potassium ions by the kidneys. •The hormone cortisol controls the rate of metabolism of carbohydrates, fats, and proteins. Adrenal Medulla •The release of hormones from the adrenal medulla prepares the body for energy-intense activities. •The two hormones released by the adrenal medulla are epinephrine and norepinephrine. Epinephrine and norepinephrine: • increase heart rate, blood pressure, and blood flow to the muscles. •cause air passageways to open wider, allowing for an increased intake of oxygen. •stimulate the release of extra glucose into the blood to help produce a sudden The pancreas has both exocrine and endocrine functions. •It is a digestive gland whose secretions break down food. •It produces insulin and glucagon. QuickTimeª and a decompressor are needed to see this picture. Insulin and glucagon (produced by pancreas) help to keep the level of glucose in the blood stable. Insulin stimulates cells in the liver and muscles to remove sugar from the blood and store it as glycogen or fat. Glucagon stimulates the liver to break down glycogen and release glucose back into the blood. Maintaining Blood Sugar Levels •When glucose levels rise, the pancreas releases insulin. •Insulin stimulates cells to take glucose out of the bloodstream. Glucose taken out of circulation is stored as glycogen in the liver and skeletal muscles. In fat tissue, glucose is converted to lipids. When blood glucose level drops, glucagon is released from the pancreas. Glucagon stimulates liver cells and skeletal muscles to break down glycogen and increase glucose levels. It causes fat cells to break down fats for production of carbohydrates. This makes more chemical energy available and helps raise the blood glucose level back to normal. Diabetes Mellitus •When the pancreas fails to produce or properly use insulin, diabetes mellitus occurs. Blood sugar video The gonads are the body’s reproductive glands. The gonads serve two important functions: the production of gametes, and the secretion of sex hormones. The female gonads—the ovaries— produce eggs. The male gonads—the testes— produce sperm. The gonads also produce sex hormones. The ovaries produce the female sex hormones estrogen and progesterone. Progesterone prepares the uterus for the arrival of a developing embryo. Estrogen is needed for the development of eggs and for the formation of physical characteristics of the female body. The testes produce testosterone, which is needed for normal sperm production and development of male physical characteristics. 39-3 The Reproductive System Sexual Development •In humans, the reproductive system produces, stores, and releases specialized sex cells known as gametes. •Sperm + egg = zygote, the single cell from which all cells of the human body develop. • Puberty is a period of rapid growth and sexual maturation during which the reproductive system becomes fully functional. •When puberty ends, reproductive organs are fully developed. •Puberty usually begins between the ages of 9 and 15, and usually starts one year earlier in females than in males. Puberty begins when the hypothalamus signals the pituitary to produce increased levels of two hormones that affect the gonads. These hormones are follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The Male Reproductive System • Release of FSH and LH stimulates cells in the testes to produce testosterone. •FSH and testosterone stimulate the development of sperm. The main function of the male reproductive system is to produce and deliver sperm. A sperm cell consists of: • a head, which Head Nucleus contains the nucleus • a midpiece, which contains energy-releasing mitochondria • a tail, which propels the cell forward Midpiece Mitochondria Tail Male Reproductive System Urinary bladder Vas deferens Pubic bone Seminal vesicle Urethra Rectum Penis Prostate gland Epididymis Testis Scrotum Bulbourethral gland The testes are located in an external sac called the scrotum. The scrotum is located outside the body cavity, where it is between 1 and 3 degrees cooler than normal body temperature. The lower temperature helps sperm development. Within each testis are clusters of hundreds of tiny tubules called seminiferous tubules. The seminiferous tubules are tightly coiled and twisted together. Sperm are produced in the seminiferous tubules. Sperm produced in the seminiferous tubules move into the epididymis, where they mature and are stored. From there, sperm move into a tube called the vas deferens, which extends up from the scrotum into the abdominal cavity. The vas deferens merges with the urethra, the tube that leads to the outside of the body through the penis. Glands lining the reproductive tract produce seminal fluid. Seminal fluid nourishes sperm and protects them from the acidity of the female reproductive tract. The combination of sperm and seminal fluid is called semen. Male reproductive diagram Seminiferous tubules- In testes, where sperm are made (meiosis) Epididymis- where sperm matures & waits Vas deferens- tube from testes Urethra- where sperm & urine exit The Female Reproductive System • The primary reproductive organs in the female are the ovaries. •The ovaries are located in the abdominal cavity. The main function of the female reproductive system is to produce eggs. In addition, the female reproductive system prepares the female's body to nourish a developing embryo. Female Reproductive System Fallopian tube Ovary Uterus Urinary bladder Cervix Rectum Pubic bone Vagina Urethra Puberty in females starts when the hypothalamus signals the pituitary gland to release FSH and LH. FSH stimulates cells within the ovaries to produce estrogen. QuickTimeª and a decompressor are needed to see this picture. Egg Development • Each ovary contains about 400,000 primary follicles, which are clusters of cells surrounding a single egg. •The follicle helps an egg mature for release into the reproductive tract, where it can be fertilized. • Eggs develop within their follicles. QuickTimeª and a decompressor are needed to see this picture. Egg Release •When a follicle has matured, its egg is released from the ovary in a process called ovulation. •The follicle breaks open, and the egg is swept of the two Fallopian from the ovary into one tubes. QuickTimeª and a decompressor are needed to see this picture. • While in the Fallopian tube, an egg can be fertilized. •After a few days, the egg passes from the Fallopian tube into the uterus. •If the egg is not fertilized it passes through the cervix, and finally out of the vagina. •The vagina leads to the outside of the body. The Menstrual Cycle •The menstrual cycle is controlled by internal feedback mechanisms between the reproductive system and the endocrine system. •The menstrual cycle takes an average of 28 days. During the menstrual cycle, an egg develops and is released from an ovary. The uterus is prepared to receive a fertilized egg. If the egg is fertilized, it is implanted in the uterus and embryonic development begins. If the egg is not fertilized, it is discharged. The menstrual cycle has four phases: • follicular phase • ovulation • luteal phase • menstruation The Menstrual Cycle Menstrual Cycle Ovulation • The follicular phase begins when estrogen levels in the blood are low. • The anterior pituitary secretes FSH and LH, which cause a follicle to develop to maturity. • As the follicle develops, cells surrounding the egg enlarge and produce more estrogen. • Estrogen causes the lining of the uterus to thicken. Ovulation •This phase occurs midway through the cycle and lasts 3–4 days. •The pituitary gland produces more FSH and LH. •The release of these hormones causes the follicle to rupture, and a mature egg is released into one of the Fallopian tubes. QuickTimeª and a decompressor are needed to see this picture. Luteal Phase •The luteal phase begins after the egg is released. •As the egg moves in the Fallopian tube, the follicle turns yellow and is called the corpus luteum. •The corpus luteum continues to release estrogen but also begins to release progesterone. Progesterone stimulates growth and development of the blood supply and surrounding tissue. Within a few days of implantation, the uterus and the growing embryo will release hormones that keep the corpus luteum functioning for several weeks. This allows the lining of the uterus to nourish and protect the developing embryo. Menstruation •If fertilization does not occur, the corpus luteum will begin to disintegrate. •The follicle breaks down and releases less hormones, which makes the uterine lining detach. •This tissue, blood, and the unfertilized egg are discharged through the vagina. •This phase is menstruation, and it lasts Sexually Transmitted Diseases • Diseases that spread from one person to another during sexual contact are called sexually transmitted diseases (STDs). •STDs are a serious problem in the U.S., infecting millions of people each year and accounting for thousands of deaths. STDs caused by bacteria include chlamydia, syphilis, and gonorrhea. STDs caused by viruses include hepatitis B, genital herpes, genital warts, and HIV/AIDS. Female reproductive diagram Ovary- egg develops & is released Fallopian tube- Fertilization usually happens here Uterus- Where baby develops Vagina- birth canal QuickTimeª and a decompressor are needed to see this picture. 39–4 Fertilization and Development When an egg is fertilized, human development begins. In this process, a single cell undergoes a series of cell divisions that results in the formation of a new human being. Fertilization • During sexual intercourse, sperm are released when semen is ejaculated through the penis into the vagina. •Sperm swim through the uterus into the Fallopian tubes. •if an egg is present in one of the Fallopian tubes, its chances of being fertilized are good. The egg is surrounded by a protective layer that contains binding sites to which sperm can attach. When a sperm attaches to a binding site, its head releases enzymes that break down the protective layer of the egg. The sperm nucleus enters the egg, and chromosomes from the sperm and egg are brought together. The process of a sperm joining an egg is called fertilization. Fertilization After the two haploid (N) nuclei fuse, a single diploid (2N) nucleus is formed. A diploid cell has a set of chromosomes from each parent cell. The fertilized egg is called a zygote. Early Development • While still in the Fallopian tube, the zygote begins to undergo mitosis. •Four days after fertilization, the embryo is a solid ball of about 64 cells called a morula. The stages of early development include implantation, gastrulation, and neurulation. Implantation •As the morula grows, it becomes a hollow structure with an inner cavity called a blastocyst. 6–7 days after fertilization, the • blastocyst attaches to the uterine wall. •The embryo secretes enzymes that digest a path into it. • This process is known as implantation. Fertilization Fertilization and Implantation Blastocyst cells specialize due to the activation of genes. This process, called differentiation, is responsible for the development of the various types of tissue in the body. A cluster of cells, known as the inner cell mass, develops within the inner cavity of the blastocyst. The embryo will develop from these cells, while the other cells will differentiate into tissues that surround the embryo. Gastrulation •The inner cell mass of the blastocyst gradually sorts itself into two layers, which then give rise to a third layer. Early Development The third layer is produced by a process of cell migration known as gastrulation. Amniotic cavity Primitive streak Mesoderm Ectoderm Endoderm The result of gastrulation is the formation of three cell layers—the ectoderm, the mesoderm, and the endoderm. Amniotic cavity Primitive streak Mesoderm Ectoderm Endoderm The ectoderm develops into the skin and nervous system. The endoderm forms the digestive lining and organs. Mesoderm cells differentiate into internal tissues and organs. Neurulation • Gastrulation is followed by neurulation. •Neurulation is the development of the nervous system. Shortly after gastrulation is complete, a block of mesodermal tissue begins to differentiate into the notochord. • As the notochord develops, the neural groove changes shape, producing neural Neural crest folds. Neural fold Notochord • Gradually, these folds move together to create a neural tube from which the spinal cord and the nervous system Neural crest Neural tube develop. Ectoderm Notochord Extraembryonic Membranes •As the embryo develops, membranes form to protect and nourish the embryo. •Two of these membranes are the amnion and the chorion. • The amnion Amniotic sac Placenta develops into a fluid-filled amniotic sac. Umbilical cord •The amniotic sac cushions and protects Uterus the Amnion developing embryo. Fetus Fingerlike projections called chorionic villi form on the outer surface of the chorion and extend into the uterine lining. Fetal portion of placenta Chorionic villus Amnion Umbilical cord Maternal portion of placenta Mom & baby blood do NOT mix!! Maternal artery Umbilical arteries Umbilical vein Maternal vein The chorionic villi and uterine lining form the placenta. The placenta • Lies between the uterus and the developing embryo • provides nutrients to the fetus. The placenta is the embryo's organ of respiration, nourishment, and excretion. The placenta acts as a barrier to some harmful or disease-causing agents. Some disease causing agents, such as German measles and HIV can cross the placenta. Some drugs, including alcohol and medications also can penetrate the placenta and affect development. After eight weeks, the embryo is called a fetus. After three months, most major organs and tissues are formed. During this time, the umbilical cord also forms. The umbilical cord connects the fetus to the placenta. Control of Development • The fates of many cells in the early embryo are not fixed. •The inner cell mass contains embryonic stem cells, unspecialized cells that can differentiate into nearly any specialized cell type. •Researchers are still learning the mechanisms that control stem cell differentiation. Later Development • 4–6 months after fertilization: •The heart can be heard with a stethoscope. Bone replaces cartilage that forms the early skeleton. A layer of soft hair grows over the fetus’s skin. The fetus grows and the mother can feel it moving. During the last three months, the organ systems mature. • The fetus doubles in mass. •It can now regulate its body temperature. •The central nervous system and lungs completely develop. Childbirth • About nine months after fertilization, the fetus is ready for birth. •A complex set of factors affects the onset of childbirth. The mother’s posterior pituitary gland releases the hormone oxytocin, which affects involuntary muscles in the uterine wall. These muscles begin rhythmic contractions known as labor. The contractions become more frequent and more powerful. The opening of the cervix expands until it is large enough for the head of the baby to pass through it. At some point, the amniotic sac breaks, and the fluid it contains rushes out of the vagina. Contractions force the baby out through the vagina. The baby now begins an independent existence. Its systems quickly adapt to life outside the uterus, supplying its own oxygen, excreting waste on its own, and maintaining its own body temperature. Multiple Births Multiple Births •If two eggs are released during the same cycle and fertilized by two different sperm, fraternal twins result. •A single zygote may split apart to produce two embryos, which are called identical twins. Early Years •The first two years of life are called infancy. It is a period of rapid growth and development. •Childhood lasts from infancy until puberty. •Adolescence begins with puberty and ends with adulthood. •Puberty produces a growth spurt that will conclude in mid-adolescence. Adulthood • Development continues during adulthood. •Adults reach their highest levels of physical strength and development between the ages of 25 and 35. •Most people begin to show signs of aging in their 30s. •Around age 65, most body systems become less efficient, making homeostasis more difficult to maintain. 39-1 Cells that have receptors for a particular hormone are called a. nerve cells. b. target cells. c. exocrine cells. d. endocrine cells. 39-1 Chemicals that travel through the bloodstream and affect the activities of other cells are known as a. hormones. b. receptors. c. enzymes. d. messengers. 39-1 Which group of hormones act on target cells by binding directly to DNA in the nucleus? a. steroids b. nonsteroids c. proteins d. second messengers 39-2 Diabetes mellitus is a disease that results when the pancreas fails to produce or properly use a. glucose. b. insulin. c. glucagon. d. carbohydrate. 39-2 Metabolism is regulated by a. thyroxine. b. parathyroid hormone. c. epinephrine. d. estrogen. 39-2 The gonads are the body's a. target cells. b. exocrine glands. c. reproductive glands. d. reproductive cells. 39-2 The endocrine glands responsible for maintaining homeostasis of calcium in the blood are the a. thyroid and parathyroid glands. b. adrenal and pituitary glands. c. hypothalamus and thyroid glands. d. gonads. 39-2 Epinephrine is a hormone produced by the adrenal medulla and is responsible for a. the “fight or flight” response to stress. b. controlling the level of insulin in the blood. c. maintaining proper levels of sodium and potassium in the blood. d. regulating the water content of the body. 39-3 The process in which a mature egg is released from the follicle of an ovary is known as a. fertilization. b. ovulation. c. menstruation. d. meiosis. 39-3 An egg passes from a Fallopian tube into the cavity of the a. ovary. b. vagina. c. uterus. d. cervix. Which statement best describes male sperm cells? a. They are motile, produced in small numbers, and larger than most body cells. b. They are motile, produced in large numbers, and smaller than most body cells. c. They are nonmotile, produced in small numbers, and larger than most body cells. d. They are nonmotile, produced in large numbers, and smaller than most body cells. 39–4 Fertilization takes place in the a. ovary. b. Fallopian tube. c. cavity of the uterus. d. cervix. 39–4 The process in which a blastocyst attaches to the wall of the uterus is called a. fertilization. b. implantation. c. gastrulation. d. neurulation. 39–4 The central nervous system develops during which phase of early development? a. gastrulation b. neurulation c. implantation d. fertilization 39–4 The placenta is a structure that a. belongs entirely to the mother. b. belongs entirely to the fetus. c. brings blood from the mother and fetus close together. d. provides an impermeable barrier between the mother and the fetus. 39–4 Which of the following is not a primary germ layer? a. neural tube b. endoderm c. ectoderm d. mesoderm