All Multicellular Organisms Must Coordinate Their Functions The many cells, tissues, and organs in the animal body need to communicate so that the whole organism can develop and function effectively The endocrine system consists of different types of secretory cells that release a variety of signaling molecules called hormones

The Endocrine System Animal hormones are produced by specialized cells that are often organized into discrete organs called endocrine glands Endocrine glands release hormones into body fluids, which then carry these chemical messengers throughout the body Some endocrine cells are embedded as single cells or clusters of cells within other specialized tissues and organs

The Endocrine System The hypothalamus is responsible for coordinating the endocrine system and integrating it with the nervous system The hypothalamus contains both neurons that interact with the brain and also endocrine cells that produce hormones

The Endocrine System The hypothalamus makes releasing and inhibiting hormones that direct the pituitary to release another set of hormones These hormones from the pituitary will travel through the circulatory system to regulate the various endocrine glands.

The Endocrine System Hormones are often distributed through the body by the circulatory system and therefore move only as fast as blood is circulated Hormones are effective in small amounts because they bind to target cells with great specificity and tenacity

Hormones and their Target Cell A hormone released by one cell causes one or more specific responses in the target cells, which may lie in more than one type of tissue Hormones can act on target cells by binding to plasma membrane receptors or intracellular receptors

Hormones and their Target Cell A single hormone can produce a diversity of effects in a variety of potential target cells

Hormonal Signals to the Cell Hormonal signals are amplified inside the target cell and alter key cellular processes There are four main categories of cellular responses produces by a hormonal signal: Changes in protein production Changes in metabolism Changes in the activity of the cytoskeleton Changes in plasma membrane transport

Hormonal Signals to the Cell Animals usually produce hormones in tiny amounts measured in micrograms When hormone molecules bind to receptors in the target cell, they set in motion a chain of events that may activate thousands of protein molecules in that cell Through signal amplification, just a few hormone molecules can have a substantial impact on the whole body

Hormonal Signals to the Cell Signal transduction is the process in which a signal received by a membrane receptor is relayed within the cytoplasm to produce responses from the target cell Notice the amplification of the singnal

Hormonal Signals to the Cell Some hormones bind to receptors on the plasma membrane Others, like steroids, are able to cross the plasma membrane and deliver the hormonal message within the cell

Specific Hormonal Processes we will Cover Glucose homeostasis The fight-or-flight response Growth Menstruation

Glucose Homeostasis Hormones that regulate homeostasis must act quickly to maintain a constant internal environment Glucose homeostasis is controlled by the pancreas The pancreas, function as endocrine glands and also as exocrine (ducted) glands The pancreas contains clusters of endocrine cells called islet cells that produce and release insulin and glucagon

Glucose Homeostasis Insulin and glucagon are hormones that act in opposite ways to maintain homeostasis in blood glucose levels Insulin acts on target cells throughout the body, but especially in the liver, fat tissue, and skeletal muscles, signaling these cells to increase their uptake of glucose from the blood Insulin= into cell out of blood Causes decrease blood sugar Glucagon= gone from cell into blood Causes increased blood sugar

Figure 29.4 Balancing Levels of Glucose in the Blood Two hormones—insulin and glucagon—work in opposite ways to regulate blood levels of glucose. The width of the red circuit indicates the relative amounts of glucose in the bloodstream at different times.

Disruption in Glucose Homeostasis: Diabetes Too much glucose in the blood results in diabetes Most cases of type 1 diabetes result from: autoimmune damage to the islet cells preventing insulin production or production of a defective form of insulin by the pancreas Type 2 diabetes occurs when: too little insulin is produced or the receptors on target cells respond poorly to insulin

Short Term Control: Fight-or-Flight Response The adrenals regulate the fight-or-flight response The adrenal glands are a pair of endocrine glands that sit on top of the kidneys and release epinephrine and norepinephrine, which coordinate our response to sudden stress

Short Term Control: Fight-or-Flight Response Epinephrine stimulates glycogen breakdown in liver and skeletal muscle cells, which raises blood glucose levels and in turn increases the speed and force with which the heart contracts

Regulating Long-Term Processes: Growth The pituitary gland produces growth hormone (GH), which promotes the growth of bones and stimulates increases in muscle mass Too much or too little GH can have lifelong consequences on an individual, resulting in a variety of conditions

Regulating Long-Term Processes: Reproduction Animals rely on hormones to regulate nearly all aspects of reproduction In humans, hormones influence nearly all aspects of sexual development and reproduction, including sperm production in males and the menstrual cycle of women

Sex Hormones Play a Role in Sexual Development before Birth It is through the action of the hormones that a fetus develops into a male or a female By the seventh week of the fetus’s development, the sex glands, or gonads, are producing the sex hormones These sex hormones signal genes in their target cells to begin the process of sexual development The gonads of both sexes produce estrogens, progestogens, and androgens in varying amounts

Sex Hormones Play a Role in Sexual Development before Birth Estrogen produces female characteristics, while progestogens, including progesterone, create a suitable environment for a developing fetus Androgens, such as testosterone, stimulate cells to develop the characteristics of maleness

Sex Hormones Coordinate Sexual Maturation at Puberty GnRH During puberty, the hypothalamus makes Gonadotropin Releasing Hormone (GnRH) that activates the production of gonadatropins made in the pituitary gland These gonadatropins, Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) coordinate the development of sperm in males and play a role in regulating the menstrual cycle in females

Sex Hormones Coordinate the Menstrual Cycle In humans females, individual eggs mature and are released in a hormone-driven sequence of events known as the menstrual cycle The menstrual cycle is marked by a succession of hormones that stimulate the release of an egg and prepare the uterine lining to grow and thicken in preparation for a potential pregnancy Between 40 and 50 years of age, a drop in the levels of estrogen and progesterone results in menopause, in which the menstrual cycle ceases permanently

Sex Hormones Coordinate the Menstrual Cycle The menstrual cycle is marked by a succession of hormones that stimulate the release of an egg and prepare the uterine lining to grow and thicken in preparation for a potential pregnancy

Sex Hormones Coordinate the Menstrual Cycle Figure 29.9 The Human Menstrual Cycle Depends on the Sequential Release of Several Hormones Follicle-stimulating hormone (FSH) launches a new menstrual cycle by stimulating the growth of ovarian follicles. Developing follicles produce estrogen. When estrogen levels reach a certain threshold, they trigger release of luteinizing hormone (LH) from the pituitary, which triggers ovulation. An estrogen, especially the large amounts of progesterone secreted by the corpus luteum, stimulates a buildup of the uterine lining in preparation for a possible pregnancy. If fertilization fails to occur, the corpus luteum dies after about 14 days, hormone concentrations crash, and the uterine lining is sloughed off.