A&P 1 Classwork 16: The Endocrine System PDF

**[A&P 1 Class work 16]: The Endocrine System** ------------------------------------------ **The overview of the Endocrine System** ------------------------------------------ Communication is a process in which a sender transmits signals to one or more receivers to control and coordinate act...

**[A&P 1 Class work 16]: The Endocrine System** ------------------------------------------ **The overview of the Endocrine System** ------------------------------------------ Communication is a process in which a sender transmits signals to one or more receivers to control and coordinate actions. In the human body, two major organ systems participate in relatively "long distance" communication: the nervous system and the endocrine system. Together, these two systems are primarily responsible for maintaining homeostasis in the body. **Neural and Endocrine Signaling** The nervous system uses two types of intercellular communication---electrical and chemical signaling---either by the direct action of an electrical potential, or in the latter case, through the action of chemical neurotransmitters such as serotonin or norepinephrine. Neurotransmitters act locally and rapidly. In contrast, the **endocrine system** uses just one method of communication: chemical signaling. These signals are sent by the endocrine organs, which secrete chemicals---the **hormone**---into the extracellular fluid. Hormones are transported primarily via the bloodstream throughout the body, where they bind to receptors on target cells, inducing a characteristic response. As a result, endocrine signaling requires more time than neural signaling to prompt a response in target cells, though the precise amount of time varies with different hormones. In addition, endocrine signaling is typically less specific than neural signaling. **Endocrine and Nervous Systems** **Endocrine system** **Nervous system** ------------------------- ---------------------- ----------------------- Signaling mechanism(s) Chemical Chemical/electrical Primary chemical signal Hormones Neurotransmitters Distance traveled Long or short Always short Response time Fast or slow Always fast Environment targeted Internal Internal and external ### Structures of the Endocrine System ### This diagram shows the endocrine glands and cells that are located throughout the body. The endocrine system organs include the pineal gland and pituitary gland in the brain. The pituitary is located on the anterior side of the thalamus while the pineal gland is located on the posterior side of the thalamus. The thyroid gland is a butterfly-shaped gland that wraps around the trachea within the neck. Four small, disc-shaped parathyroid glands are embedded into the posterior side of the thyroid. The adrenal glands are located on top of the kidneys. The pancreas is located at the center of the abdomen. In females, the two ovaries are connected to the uterus by two long, curved, tubes in the pelvic region. In males, the two testes are located in the scrotum below the penis. ### Figure:  Endocrine System Endocrine glands and cells are located throughout the body and play an important role in homeostasis. ### Other Types of Chemical Signaling In contrast to the endocrine signaling, autocrine signaling takes place within the same cell. An **autocrine** (auto- = "self") is a chemical that elicits a response in the same cell that secreted it. Local intercellular communication is the province of the **paracrine**, also called a paracrine factor, which is a chemical that induces a response in neighboring cells. -------------- **Hormones** -------------- **Endocrine Glands and Their Major Hormones** **Endocrine gland** **Associated hormones** **Chemical class** **Effect** ----------------------- ------------------------------------------- -------------------- --------------------------------------------------------------------------------------------------- Pituitary (anterior) Growth hormone (GH) Protein Promotes growth of body tissues Pituitary (anterior) Prolactin (PRL) Peptide Promotes milk production Pituitary (anterior) Thyroid-stimulating hormone (TSH) Glycoprotein Stimulates thyroid hormone release Pituitary (anterior) Adrenocorticotropic hormone (ACTH) Peptide Stimulates hormone release by adrenal cortex Pituitary (anterior) Follicle-stimulating hormone (FSH) Glycoprotein Stimulates gamete production Pituitary (anterior) Luteinizing hormone (LH) Glycoprotein Stimulates androgen production by gonads Pituitary (posterior) Antidiuretic hormone (ADH) Peptide Stimulates water reabsorption by kidneys Pituitary (posterior) Oxytocin Peptide Stimulates uterine contractions during childbirth Thyroid Thyroxine (T~4~), triiodothyronine (T~3~) Amine Stimulate basal metabolic rate Thyroid Calcitonin Peptide Reduces blood Ca^2+^ levels Parathyroid Parathyroid hormone (PTH) Peptide Increases blood Ca^2+ ^levels Adrenal (cortex) Aldosterone Steroid Increases blood Na^+^ levels Adrenal (cortex) Cortisol, corticosterone, cortisone Steroid Increase blood glucose levels Adrenal (medulla) Epinephrine, norepinephrine Amine Stimulate fight-or-flight response Pineal Melatonin Amine Regulates sleep cycles Pancreas Insulin Protein Reduces blood glucose levels Pancreas Glucagon Protein Increases blood glucose levels Testes Testosterone Steroid Stimulates development of male secondary sex characteristics and sperm production Ovaries Estrogens and progesterone Steroid Stimulate development of female secondary sex characteristics and prepare the body for childbirth ### ### Types of Hormones The hormones of the human body can be divided into two major groups on the basis of their chemical structure. - Hormones derived from amino acids include amines, peptides, and proteins. - Those derived from lipids include steroids. These chemical groups affect a hormone's distribution, the type of receptors it binds to, and other aspects of its function.  **Figure:** **Amine, Peptide, Protein, and Steroid Hormone Structure** ### ### Pathways of Hormone Action The message a hormone sends is received by a **hormone receptor**. #### #### **[Pathways Involving Intracellular Hormone Receptors]** This illustration shows the steps involved with the binding of lipid-soluble hormones. Lipid-soluble hormones, such as steroid hormones, easily diffuse through the cell membrane. The hormone binds to its receptor in the cytosol, forming a receptor-hormone complex. The receptor-hormone complex then enters the nucleus and binds to the target gene on the cell's DNA. Transcription of the gene creates a messenger RNA that is translated into the desired protein within the cytoplasm. It is these proteins that alter the cell's activity. **Figure:** **Binding of Lipid-Soluble Hormones **A steroid hormone directly initiates the production of proteins within a target cell. Steroid hormones easily diffuse through the cell membrane. The hormone binds to its receptor in the cytosol, forming a receptor--hormone complex. The receptor--hormone complex then enters the nucleus and binds to the target gene on the DNA. Transcription of the gene creates a messenger RNA that is translated into the desired protein within the cytoplasm. #### **Pathways Involving Cell Membrane Hormone Receptors**  **Figure:** **Binding of Water-Soluble Hormones **Water-soluble hormones cannot diffuse through the cell membrane. These hormones must bind to a surface cell-membrane receptor. The receptor then initiates a cell-signaling pathway within the cell involving G proteins, adenylyl cyclase, the secondary messenger cyclic AMP (cAMP), and protein kinases. In the final step, these protein kinases phosphorylate proteins in the cytoplasm. This activates proteins in the cell that carry out the changes specified by the hormone. ### Regulation of Hormone Secretion To prevent abnormal hormone levels and a potential disease state, hormone levels must be tightly controlled. The body maintains this control by balancing hormone production and degradation. Feedback loops govern the initiation and maintenance of most hormone secretion in response to various stimuli. #### **Role of Feedback Loops** This diagram shows a negative feedback loop using the example of glucocorticoid regulation in the blood. Step 1 in the cycle is when an imbalance occurs. The hypothalamus perceives low blood concentrations of glucocorticoids in the blood. This is illustrated by there being only 5 glucocorticoids floating in a cross section of an artery. Step 2 in the cycle is hormone release, where the hypothalamus releases corticotropin-releasing hormone (CRH). Step 3 is labeled correction. Here, the CRH release starts a hormone cascade that triggers the adrenal gland to release glucocorticoid into the blood. This allows the blood concentration of glucocorticoid to increase, as illustrated by 8 glucocorticoid molecules now being present in the cross section of the artery. Step 4 is labeled negative feedback. Here, the hypothalamus perceives normal concentrations of glucocorticoids in the blood and stops releasing CRH. This brings blood glucocorticoid levels back to homeostasis. **Figure:** **Negative Feedback Loop **The release of adrenal glucocorticoids is stimulated by the release of hormones from the hypothalamus and pituitary gland. This signaling is inhibited when glucocorticoid levels become elevated by causing negative signals to the pituitary gland and hypothalamus. #### **Role of Endocrine Gland Stimuli** Reflexes triggered by both chemical and neural stimuli control endocrine activity. These reflexes may be simple, involving only one hormone response, or they may be more complex and involve many hormones, as is the case with the hypothalamic control of various anterior pituitary--controlled hormones. Humoral stimuli are changes in blood levels of non-hormone chemicals, such as nutrients or ions, which cause the release or inhibition of a hormone to, in turn, maintain homeostasis. ------------------------------------------ **The Pituitary Gland and Hypothalamus** ------------------------------------------ The hypothalamus--pituitary complex can be thought of as the "command center" of the endocrine system. This complex secretes several hormones that directly produce responses in target tissues, as well as hormones that regulate the synthesis and secretion of hormones of other glands. In addition, the hypothalamus--pituitary complex coordinates the messages of the endocrine and nervous systems. In many cases, a stimulus received by the nervous system must pass through the hypothalamus--pituitary complex to be translated into hormones that can initiate a response.  **Figure:** **Hypothalamus--Pituitary Complex **The hypothalamus region lies inferior and anterior to the thalamus. It connects to the pituitary gland by the stalk-like infundibulum. The pituitary gland consists of an anterior and posterior lobe, with each lobe secreting different hormones in response to signals from the hypothalamus. **Pituitary Hormones** **Pituitary lobe** **Associated hormones** **Chemical class** **Effect** -------------------- ------------------------------------ -------------------- --------------------------------------------------- Anterior Growth hormone (GH) Protein Promotes growth of body tissues Anterior Prolactin (PRL) Peptide Promotes milk production from mammary glands Anterior Thyroid-stimulating hormone (TSH) Glycoprotein Stimulates thyroid hormone release from thyroid Anterior Adrenocorticotropic hormone (ACTH) Peptide Stimulates hormone release by adrenal cortex Anterior Follicle-stimulating hormone (FSH) Glycoprotein Stimulates gamete production in gonads Anterior Luteinizing hormone (LH) Glycoprotein Stimulates androgen production by gonads Posterior Antidiuretic hormone (ADH) Peptide Stimulates water reabsorption by kidneys Posterior Oxytocin Peptide Stimulates uterine contractions during childbirth Intermediate zone Melanocyte-stimulating hormone Peptide Stimulates melanin formation in melanocytes ### Posterior Pituitary This illustration zooms in on the hypothalamus and the attached pituitary gland. The posterior pituitary is highlighted. Two nuclei in the hypothalamus contain neurosecretory cells that release different hormones. The neurosecretory cells of the paraventricular nucleus release oxytocin (OT) while the neurosecretory cells of the supraoptic nucleus release anti-diuretic hormone (ADH). The neurosecretory cells stretch down the infundibulum into the posterior pituitary. The tube-like extensions of the neurosecretory cells within the infundibulum are labeled the hypothalamophypophyseal tracts. These tracts connect with a web-like network of blood vessels in the posterior pituitary called the capillary plexus. From the capillary plexus, the posterior pituitary secretes the OT or ADH into a single vein that exits the pituitary. **Figure:** **Posterior Pituitary **Neurosecretory cells in the hypothalamus release oxytocin (OT) or ADH into the posterior lobe of the pituitary gland. These hormones are stored or released into the blood via the capillary plexus. The posterior pituitary gland does not produce hormones, but rather stores and secretes hormones produced by the hypothalamus. The paraventricular nuclei produce the hormone oxytocin, whereas the supraoptic nuclei produce ADH. These hormones travel along the axons into storage sites in the axon terminals of the posterior pituitary. In response to signals from the same hypothalamic neurons, the hormones are released from the axon terminals into the bloodstream. ### Anterior Pituitary ###  ### Figure: Anterior Pituitary The anterior pituitary manufactures seven hormones. The hypothalamus produces separate hormones that stimulate or inhibit hormone production in the anterior pituitary. Hormones from the hypothalamus reach the anterior pituitary via the hypophyseal portal system. The anterior pituitary produces seven hormones. These are the growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), beta endorphin, and prolactin. Of the hormones of the anterior pituitary, TSH, ACTH, FSH, and LH are collectively referred to as tropic hormones (trope- = "turning") because they turn on or off the function of other endocrine glands. **A summary of the pituitary hormones and their principal effects.** Diagram Description automatically generated **Figure 17.11** **Major Pituitary Hormones **Major pituitary hormones and their target organs. ----------------------- **The Thyroid Gland** ----------------------- A butterfly-shaped organ, the **thyroid gland** is located anterior to the trachea, just inferior to the larynx  Part A of this figure is a diagram of the anterior view of the thyroid gland. The thyroid gland is a butterfly-shaped gland wrapping around the trachea. It narrows at its center, just under the thyroid cartilage of the larynx. This narrow area is called the isthmus of the thyroid. Two large arteries, the common carotid arteries, run parallel to the trachea on the outer border of the thyroid. A small artery enters the superior edge of the thyroid, near the isthmus, and branches throughout the two "wings" of the thyroid. Part B of this figure is a posterior view of the thyroid. The posterior view shows that the thyroid does not completely wrap around the posterior of the trachea. The posterior sides of the thyroid wings can be seen protruding from under the cricoid cartilage of the larynx. The posterior sides of the thyroid "wings" each contain two small, disc-shaped parathyroid glands embedded in the thyroid tissue. Within each wing, one disc is located superior to the other. These are labeled the left and right parathyroid glands. Just under the inferior parathyroid glands are two arteries that bring blood to the thyroid from the left and right subclavian arteries. Part C of this figure is a micrograph of thyroid tissue. The thyroid follicle cells are cuboidal epithelial cells. These cells form a ring around irregular-shaped cavities called follicles. The follicles contain light colored colloid. A larger parafollicular cell is embedded between two of the follicular cells near the edge of a follicle.  **Figure:** **Thyroid Gland **The thyroid gland is located in the neck where it wraps around the trachea. a. Anterior view of the thyroid gland. b. Posterior view of the thyroid gland. c. The glandular tissue is composed primarily of thyroid follicles. The larger parafollicular cells often appear within the matrix of follicle cells. **[The thyroid hormones:]** 1. **Triiodothyronine** (**T~3~)**, a thyroid hormone with three iodine. 2. Tetraiodothyronine, also known as **thyroxine** (**T~4~**), a thyroid hormone with four iodine. ### ### Regulation of TH Synthesis ### Functions of Thyroid Hormones The thyroid hormones, **T~3~** and **T~4~**, are often referred to as **metabolic hormones** because their levels influence the body's basal metabolic rate, the amount of energy used by the body at rest. When T~3~ and T~4~ bind to intracellular receptors located on the mitochondria, they cause an increase in nutrient breakdown and the use of oxygen to produce ATP. In addition, T~3~ and T~4~ initiate the transcription of genes involved in glucose oxidation. Although these mechanisms prompt cells to produce more ATP, the process is inefficient, and an abnormally increased level of heat is released as a byproduct of these reactions. This so-called calorigenic effect (calor- = "heat") raises body temperature. **Calcitonin** The thyroid gland also secretes a hormone called **calcitonin** that is produced by the parafollicular cells (also called C cells) that stud the tissue between distinct follicles. Calcitonin is released in response to a rise in blood calcium levels. It appears to have a function in decreasing blood calcium concentrations by: - **Inhibiting the activity of osteoclasts**, bone cells that release calcium into the circulation by degrading bone matrix - **Increasing osteoblastic activity** - **Decreasing calcium absorption in the intestines** - **Increasing calcium loss in the urine** However, these functions are usually not significant in maintaining calcium homeostasis, so the importance of calcitonin is not entirely understood. Pharmaceutical preparations of calcitonin are sometimes prescribed to reduce osteoclast activity in people with osteoporosis and to reduce the degradation of cartilage in people with osteoarthritis. The hormones secreted by thyroid are summarized in. **Thyroid Hormones** **Associated hormones** **Chemical class** **Effect** ------------------------------------------- -------------------- -------------------------------- Thyroxine (T~4~), triiodothyronine (T~3~) Amine Stimulate basal metabolic rate Calcitonin Peptide Reduces blood Ca^2+^ levels ---------------------------- **The Parathyroid Glands** ---------------------------- The **parathyroid glands** are tiny, round structures usually found embedded in the posterior surface of the thyroid gland. A thick connective tissue capsule separates the glands from the thyroid tissue. Most people have four parathyroid glands, but occasionally there are more in tissues of the neck or chest. The function of one type of parathyroid cells, the oxyphil cells, is not clear. The primary functional cells of the parathyroid glands are the chief cells. These epithelial cells produce and secrete the **parathyroid hormone (PTH)**, the major hormone involved in the regulation of blood calcium levels. Part A of this diagram shows the four, small, disc-shaped parathyroid glands embedded in the posterior surface of the thyroid gland. Part B shows a micrograph of parathyroid tissue. The tissue is largely composed of cube-shaped chief cells encircling a central blood vessel. A few larger and darker-staining oxyphil cells are embedded within the many chief cells. **Figure:** **Parathyroid Glands **The small parathyroid glands are embedded in the posterior surface of the thyroid gland.  **Figure:** **Parathyroid Hormone in Maintaining Blood Calcium Homeostasis **Parathyroid hormone increases blood calcium levels when they drop too low. Conversely, calcitonin, which is released from the thyroid gland, decreases blood calcium levels when they become too high. These two mechanisms constantly maintain blood calcium concentration at homeostasis. ------------------------ **The Adrenal Glands** ------------------------ The **adrenal glands** are wedges of glandular and neuroendocrine tissue adhering to the top of the kidneys by a fibrous capsule. The adrenal glands have a rich blood supply and experience one of the highest rates of blood flow in the body. They are served by several arteries branching off the aorta, including the suprarenal and renal arteries. Blood flows to each adrenal gland at the adrenal cortex and then drains into the adrenal medulla. Adrenal hormones are released into the circulation via the left and right suprarenal veins. This diagram shows the left adrenal gland located atop the left kidney. The gland is composed of an outer cortex and an inner medulla all surrounded by a connective tissue capsule. The cortex can be subdivided into additional zones, all of which produce different types of hormones. The outermost layer is the zona glomerulosa, which releases mineralcorticoids, such as aldosterone, that regulate mineral balance. Underneath this layer is the zona fasciculate, which releases glucocorticoids, such as cortisol, corticosterone and cortisone, that regulate glucose metabolism. Underneath this layer is the zona reticularis, which releases androgens, such as dehydroepiandrosterone, that stimulate masculinization. Below this layer is the adrenal medulla, which releases stress hormones, such as epinephrine and norepinephrine, that stimulate the sympathetic ANS. **Figure:** **Adrenal Glands **Both adrenal glands sit atop the kidneys and are composed of an outer cortex and an inner medulla, all surrounded by a connective tissue capsule. The cortex can be subdivided into additional zones, all of which produce different types of hormones. **Hormones of the Adrenal Glands** **Adrenal gland** **Associated hormones** **Chemical class** **Effect** ------------------- ------------------------------------- -------------------- ------------------------------------ Adrenal cortex Aldosterone Steroid Increases blood Na^+^ levels Adrenal cortex Cortisol, corticosterone, cortisone Steroid Increase blood glucose levels Adrenal medulla Epinephrine, norepinephrine Amine Stimulate fight-or-flight response ---------------------- **The Pineal Gland** ---------------------- Recall that the hypothalamus, part of the diencephalon of the brain, sits inferior and somewhat anterior to the thalamus. Inferior but somewhat posterior to the thalamus is the **pineal gland**, a tiny endocrine gland whose functions are not entirely clear. The **pinealocyte** cells that make up the pineal gland are known to produce and secrete the amine hormone **melatonin**, which is derived from serotonin. ------------------------------------ **Gonadal and Placental Hormones** ------------------------------------ **Reproductive Hormones** **Gonad** **Associated hormones** **Chemical class** **Effect** ----------- ------------------------------ -------------------- --------------------------------------------------------------------------------------------------- Testes Testosterone Steroid Stimulates development of male secondary sex characteristics and sperm production Testes Inhibin Protein Inhibits FSH release from pituitary Ovaries Estrogens and progesterone Steroid Stimulate development of female secondary sex characteristics and prepare the body for childbirth Placenta Human chorionic gonadotropin Protein Promotes progesterone synthesis during pregnancy and inhibits immune response against fetus ---------------------------- **The Endocrine Pancreas** ---------------------------- The **pancreas** is a long, slender organ, most of which is located posterior to the bottom half of the stomach. Although it is primarily an exocrine gland, secreting a variety of digestive enzymes, the pancreas has an endocrine function. Its **pancreatic islets**---clusters of cells formerly known as the islets of Langerhans---secrete the hormones glucagon, insulin, somatostatin, and pancreatic polypeptide (PP).  **Figure:** **Pancreas **The pancreatic exocrine function involves the acinar cells secreting digestive enzymes that are transported into the small intestine by the pancreatic duct. Its endocrine function involves the secretion of insulin (produced by beta cells) and glucagon (produced by alpha cells) within the pancreatic islets. These two hormones regulate the rate of glucose metabolism in the body. The micrograph reveals pancreatic islets. **Hormones of the Pancreas** **Associated hormones** **Chemical class** **Effect** ----------------------------------- -------------------- --------------------------------------- Insulin (beta cells) Protein Reduces blood glucose levels Glucagon (alpha cells) Protein Increases blood glucose levels Somatostatin (delta cells) Protein Inhibits insulin and glucagon release Pancreatic polypeptide (PP cells) Protein Role in appetite **Figure:** **Homeostatic Regulation of Blood Glucose Levels **Blood glucose concentration is tightly maintained between 70 mg/dL and 110 mg/dL. If blood glucose concentration rises above this range, insulin is released, which stimulates body cells to remove glucose from the blood. If blood glucose concentration drops below this range, glucagon is released, which stimulates body cells to release glucose into the blood. ----------------------------------------------- **Organs with Secondary Endocrine Functions** ----------------------------------------------- **Organs with Secondary Endocrine Functions and Their Major Hormones** **Organ** **Major hormones** **Effects** --------------------------- ------------------------------------------------------------------------------------ ----------------------------------------------------------------------------------- Heart Atrial natriuretic peptide (ANP) Reduces blood volume, blood pressure, and Na^+^ concentration Gastrointestinal tract Gastrin, secretin, and cholecystokinin Aid digestion of food and buffering of stomach acids Gastrointestinal tract Glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide 1 (GLP-1) Stimulate beta cells of the pancreas to release insulin Kidneys Renin Stimulates release of aldosterone Kidneys Calcitriol Aids in the absorption of Ca^2+^ Kidneys Erythropoietin Triggers the formation of red blood cells in the bone marrow Skeleton FGF23 Inhibits production of calcitriol and increases phosphate excretion Skeleton Osteocalcin Increases insulin production Adipose tissue Leptin Promotes satiety signals in the brain Adipose tissue Adiponectin Reduces insulin resistance Skin Cholecalciferol Modified to form vitamin D Thymus (and other organs) Thymosins Among other things, aids in the development of T lymphocytes of the immune system Liver Insulin-like growth factor-1 Stimulates bodily growth Liver Angiotensinogen Raises blood pressure Liver Thrombopoietin Causes increase in platelets Liver Hepcidin Blocks release of iron into body fluids