Bio 11.1 PDF Endocrine System

Automatic ZoomActual SizePage Width100%50%75%100%125%150%200%300%400% Chapter 11: Endocrine System 372 Lesson 11.1 **Endocrinology** Introduction The **endocrine system** participates in cellular communication and regulation of responses in the body. Unlike the nervous system (see Chapter 12),...

Automatic ZoomActual SizePage Width100%50%75%100%125%150%200%300%400% Chapter 11: Endocrine System 372 Lesson 11.1 **Endocrinology** Introduction The **endocrine system** participates in cellular communication and regulation of responses in the body. Unlike the nervous system (see Chapter 12), which provides responses on a short-term scale (eg, reflexes, sensory perception), the endocrine system typically modulates body responses over longer periods of time (eg, growth, development). The endocrine system uses chemical messengers known as **hormones**, which are carried in the bloodstream for long-distance communication among the body\'s cells. For example, cortisol, produced by cortical cells of the adrenal glands, is carried in the bloodstream to distant target cells, including liver and skeletal muscle cells, to mediate the stress response (Figure 11.1). **Figure 11.1** The endocrine system is involved in long-distance, cell-to-cell signaling. A diagram of a person\'s body Description automatically generated Chapter 11: Endocrine System 373 This lesson explores types of hormones, endocrine glands, endocrine signaling, and how the endocrine system is regulated. 11.1.01 Hormone Structure and Function **Hormones** are signaling molecules that are secreted from endocrine tissues into the circulation. Although hormones are typically present at low concentrations in the bloodstream, binding of hormones to target cell hormone receptors can cause profound responses. Hormones can be classified using several schemes. One way to classify hormones is based on structure and divides hormones into three broad categories, as shown in Figure 11.2: **Peptide hormones** are small proteins; therefore, peptide hormones consist of [amino acids](javascript:void(0)) linked by [peptide bonds](javascript:void(0)). **Steroid hormones** are lipids with a [carbon skeleton](javascript:void(0)) derived from cholesterol molecules. **Amino acid--derived hormones** are small molecules that are modified versions of the amino acids tyrosine or tryptophan. These hormones are sometimes grouped together with peptide hormones. Chapter 11: Endocrine System 374 **Figure 11.2** Classifying hormones based on structure.  Chapter 11: Endocrine System 375 Another way to classify hormones is based on their influence on other hormones. **Tropic hormones** are hormones that target endocrine tissues to influence secretion of other hormones. Through actions on other hormones, tropic hormones influence and contribute to a specific response. For example, thyroid-stimulating hormone (TSH), a tropic hormone produced by the anterior pituitary gland, causes the release of thyroid hormone from the thyroid gland, which stimulates increased metabolism in tissues. In contrast to tropic hormones, **direct hormones** act directly on target cells to elicit nonendocrine responses (ie, responses other than hormone secretion). For example, the anterior pituitary hormone prolactin, a direct hormone, causes a nonendocrine response (ie, promotion of milk production and secretion by [exocrine](javascript:void(0)) mammary glands). Some hormones can act as both a tropic hormone and a direct hormone. For example, growth hormone (GH) released by the anterior pituitary gland causes tropic effects by stimulating the liver to release other hormones that mediate growth in bone, cartilage, and soft tissue. GH also causes direct tissue effects by stimulating glucose uptake, fat utilization, and protein synthesis. Therefore, GH can be classified as both a tropic and a direct hormone (Figure 11.3). **Figure 11.3** Growth hormone has both direct and tropic effects. A diagram of a human body Description automatically generated Chapter 11: Endocrine System 376 11.1.02 Endocrine Glands Unlike **exocrine glands**, which contain ducts and secrete products (eg, sweat, tears, milk) onto [epithelial](javascript:void(0)) surfaces, **endocrine glands** lack ducts and secrete products (ie, hormones) into the bloodstream. The major endocrine glands in the body are shown in Figure 11.4. Other organs and some tissues can also produce hormones (eg, renal erythropoietin production, adipose tissue leptin production). Specific hormones produced by endocrine glands and the effects of these hormones are detailed in Lesson 11.2. **Figure 11.4** Major endocrine glands and their products.  Chapter 11: Endocrine System 377 11.1.03 Hormone Transport Hormones travel to target cells via the [blood](javascript:void(0)), which is composed primarily of water. Therefore, the solubility of a hormone in water affects how a particular hormone is transported in the blood and how that hormone exerts a physiological effect on target cells. **Peptide hormones** are [produced](javascript:void(0)) in the rough [endoplasmic reticulum (ER)](javascript:void(0)), and like most proteins in the body, they typically possess an overall charge at physiological pH. The net charges on peptide hormones make them water soluble (ie, hydrophilic) and allow peptide hormones to circulate largely in a free form (ie, unbound to proteins) in the bloodstream. However, peptide hormones cannot cross the hydrophobic [lipid bilayer](javascript:void(0)) of the target cell plasma membrane and therefore rely on intracellular molecules to transmit a signal to the interior of a cell, as depicted in Figure 11.5. **Figure 11.5** Peptide hormone transport. After production in the rough ER and modification in the [Golgi apparatus](javascript:void(0)), peptide hormones are stored within vesicles in the secreting cell until needed. When the appropriate signal is received, the vesicles travel to the plasma membrane and undergo [exocytosis](javascript:void(0)). Released peptide hormones are then carried in the bloodstream to target cells (ie, cells possessing specific peptide hormone receptors) upon which peptide hormones exert their effects, most of which involve changes to the activity of existing proteins. Due to this mechanism of production and action, peptide hormones are relatively fast acting with relatively short-lived results. A diagram of a cell Description automatically generated Chapter 11: Endocrine System 378 **Steroid hormones** are produced in the smooth ER, and most cells that secrete steroid hormones have greater amounts of smooth ER than cells that do not secrete steroid hormones. Because steroid hormones are derived from cholesterol, the resulting hormones are lipophilic (ie, hydrophobic) and can diffuse readily through the secreting cell\'s membrane. Unlike peptide hormones, steroid hormones are not stored in vesicles for later use; rather, steroid hormones diffuse through the secreting cell\'s membrane and enter into the circulation once produced. Because they are lipophilic, steroid hormones are predominantly bound to protein carriers to increase solubility during circulation in the aqueous bloodstream. Steroid hormones are typically inactive when bound to a protein carrier; therefore, carrier dissociation is typically required for hormone activation. Protein carrier--hormone pairs serve as a ready reserve of steroid hormones in the blood. The hydrophobicity of steroid hormones facilitates diffusion through target cell plasma membranes, after which the hormones bind receptors in the cytoplasm or nucleus to regulate [gene expression](javascript:void(0)) (Figure 11.6). **Figure 11.6** Steroid hormone transport. Because steroid hormones act upon DNA to influence [transcription](javascript:void(0)) and gene expression (ie, synthesis of new proteins), these hormones are slower acting than peptide hormones, the effects of which typically involve protein modification (eg, phosphorylation) rather than protein synthesis. However, the effects of steroid hormones are typically longer lasting than those of peptide hormones. **Amino acid--derived hormones** can function similarly to peptide hormones and steroid hormones based on their structure and regulation (Figure 11.7). For example, the [catecholamines](javascript:void(0)) epinephrine and norepinephrine behave like peptide hormones because they circulate unbound in the bloodstream and bind to surface receptors on target cells. Thyroid hormones are similar to steroid hormones because they are lipophilic, circulate in protein-bound form, and activate intracellular hormone receptors, but thyroid hormones are also similar to peptide hormones because they do not simply *diffuse* through the target cell membrane.  Chapter 11: Endocrine System 379 **Figure 11.7** Amino acid--derived hormone transport. The effects in the target cell once a hormone binds to its receptor (ie, endocrine signaling) are discussed in detail in Concept 11.1.04. 11.1.04 Endocrine Signaling **Cell signaling** occurs when a signal acts upon a target cell and causes a response. There are three broad ways in which cell signaling occurs (Figure 11.8). In **autocrine signaling**, the signal acts upon the same cell that releases the signal. During **paracrine signaling**, the signal (ie, paracrine factor) diffuses through the interstitial fluid to act on a nearby cell. Finally, in **endocrine signaling**, signals (ie, hormones) released by secreting cells travel through the bloodstream to act on a more distant target cell. **Figure 11.8** Types of cell signaling. During endocrine signaling, hormones affect the function of many diverse cells and tissues throughout the body. This pattern occurs because hormone receptors can be expressed in a variety of cell types, and A diagram of a person\'s body Description automatically generated  Chapter 11: Endocrine System 380 any cell that expresses the specific receptor for a particular hormone can respond to the receptor\'s ligand (ie, hormone). The reception of a signal by a target cell is the first step in endocrine signaling. Receptors for peptide hormones are located in the plasma membrane because peptide hormones are charged and cannot readily cross the phospholipid bilayer. The receptor is often coupled with a [G](javascript:void(0)) [protein](javascript:void(0)), the function of which is detailed in Concept 5.2.02. The peptide hormone is described as the **first messenger**, which binds its receptor and triggers a **second messenger** (eg, cyclic adenosine monophosphate), another molecule inside the target cell that transmits the signal, often through phosphorylation of products. This sequence is known as a **signaling cascade** (Figure 11.9). **Figure 11.9** Example of a signaling cascade. The responses elicited by peptide hormones can occur quickly, but the effects are relatively short lived. However, a small concentration of peptide hormones can have a great effect due to the potential for signal amplification along the signaling cascade. **Signal amplification** occurs when a small number of molecules produce many more activated products. For example, a single enzyme activated by a membrane receptor can catalyze the production of hundreds of second messengers, greatly amplifying the eventual response in the cell. Because steroid hormones are lipophilic, these hormones are able to diffuse directly across the plasma membrane and bind [intracellular receptors](javascript:void(0)) in the cytosol or nucleus, and they typically do not make use of second messengers. Rather, the steroid hormone--receptor complex typically acts as a [transcription](javascript:void(0)) [factor](javascript:void(0)) and binds DNA, resulting in [increased or decreased transcription](javascript:void(0)) of target genes. Often, two steroid hormone--receptor complexes bind together (ie, dimerize) prior to DNA binding, as illustrated in Figure 11.10 for glucocorticoids (ie, steroid hormones produced by the adrenal cortex). A diagram of a cell cycle Description automatically generated Chapter 11: Endocrine System 381 **Figure 11.10** Steroid hormone signaling. Because steroid hormones influence gene expression, the cellular effects of steroid hormones take longer to produce but last longer than the cellular effects of peptide hormones. As noted in Concept 11.1.03, amino acid--derived hormones can exhibit characteristics of peptide and steroid hormones depending on the specific hormone and context. For example, like peptide hormones, binding of the amino acid--derived hormone epinephrine to its receptor activates a cascade of events in which second messengers are rapidly activated to influence glycogen metabolism as well as transcription of certain genes. In addition, like steroid hormones, the amino acid--derived thyroid hormones influence transcription of target genes upon binding thyroid hormone receptors that are members of the nuclear hormone receptor family. 11.1.05 Regulation of Endocrine System As a regulator for the rest of the body, the endocrine system *itself* must be regulated. Regulation of the endocrine system allows a return to [homeostasis](javascript:void(0)) when the body\'s normal balance is disturbed. Hormones are constantly regulated in response to the concentrations of molecules governed by those hormones. When the concentration of a particular product molecule increases, less of the hormone that caused the increased concentration is released. This process is known as **negative feedback**, as illustrated in Figure 11.11.  Chapter 11: Endocrine System 382 **Figure 11.11** Negative feedback regulates the endocrine system. In addition to negative feedback regulation, the nervous system influences much of the endocrine system\'s actions. This influence is accomplished via the [hypothalamus](javascript:void(0)), a brain structure that serves as a link between the nervous and endocrine systems. The hypothalamus regulates other endocrine structures (notably, the anterior pituitary gland) by secreting releasing and inhibiting factors. Lesson 11.2 goes into further detail about the hypothalamic-pituitary connection and how specific hormones are regulated