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Anatomy & Physiology 2e: 17.1 An Overview of the Endocrine System

Anatomy & Physiology 2e
17.1 An Overview of the Endocrine System
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table of contents
  1. Cover
  2. Title Page
  3. Copyright
  4. Table Of Contents
  5. Chapter 1. An Introduction to the Human Body
    1. 1.0 Introduction
    2. 1.1 How Structure Determines Function
    3. 1.2 Structural Organization of the Human Body
    4. 1.3 Homeostasis
    5. 1.4 Anatomical Terminology
    6. 1.5 Medical Imaging
  6. Chapter 2. The Chemical Level of Organization
    1. 2.0 Introduction
    2. 2.1 Elements and Atoms: The Building Blocks of Matter
    3. 2.2 Chemical Bonds
    4. 2.3 Chemical Reactions
    5. 2.4 Inorganic Compounds Essential to Human Functioning
    6. 2.5 Organic Compounds Essential to Human Functioning
  7. Chapter 3. The Cellular Level of Organization
    1. 3.0 Introduction
    2. 3.1 The Cell Membrane
    3. 3.2 The Cytoplasm and Cellular Organelles
    4. 3.3 The Nucleus and DNA Replication
    5. 3.4 Protein Synthesis
    6. 3.5 Cell Growth and Division
    7. 3.6 Cellular Differentiation
  8. Chapter 4. The Tissue Level of Organization
    1. 4.0 Introduction
    2. 4.1 Types of Tissues
    3. 4.2 Epithelial Tissue
    4. 4.3 Connective Tissue Supports and Protects
    5. 4.4 Muscle Tissue
    6. 4.5 Nervous Tissue
    7. 4.6 Tissue Injury and Aging
  9. Chapter 5. The Integumentary System
    1. 5.0 Introduction
    2. 5.1 Layers of the Skin
    3. 5.2 Accessory Structures of the Skin
    4. 5.3 Functions of the Integumentary System
    5. 5.4 Diseases, Disorders, and Injuries of the Integumentary System
  10. Chapter 6. Bone Tissue and the Skeletal System
    1. 6.0 Introduction
    2. 6.1 The Functions of the Skeletal System
    3. 6.2 Bone Classification
    4. 6.3 Bone Structure
    5. 6.4 Bone Formation and Development
    6. 6.5 Fractures: Bone Repair
    7. 6.6 Exercise, Nutrition, Hormones, and Bone Tissue
    8. 6.7 Calcium Homeostasis: Interactions of the Skeletal System and Other Organ Systems
  11. Chapter 7. Axial Skeleton
    1. 7.0 Introduction
    2. 7.1 Divisions of the Skeletal System
    3. 7.2 Bone Markings
    4. 7.3 The Skull
    5. 7.4 The Vertebral Column
    6. 7.5 The Thoracic Cage
    7. 7.6 Embryonic Development of the Axial Skeleton
  12. Chapter 8. The Appendicular Skeleton
    1. 8.0 Introduction
    2. 8.1 The Pectoral Girdle
    3. 8.2 Bones of the Upper Limb
    4. 8.3 The Pelvic Girdle and Pelvis
    5. 8.4 Bones of the Lower Limb
    6. 8.5 Development of the Appendicular Skeleton
  13. Chapter 9. Joints
    1. 9.0 Introduction
    2. 9.1 Classification of Joints
    3. 9.2 Fibrous Joints
    4. 9.3 Cartilaginous Joints
    5. 9.4 Synovial Joints
    6. 9.5 Types of Body Movements
    7. 9.6 Anatomy of Selected Synovial Joints
    8. 9.7 Development of Joints
  14. Chapter 10. Muscle Tissue
    1. 10.0 Introduction
    2. 10.1 Overview of Muscle Tissues
    3. 10.2 Skeletal Muscle
    4. 10.3 Muscle Fiber Excitation, Contraction, and Relaxation
    5. 10.4 Nervous System Control of Muscle Tension
    6. 10.5 Types of Muscle Fibers
    7. 10.6 Exercise and Muscle Performance
    8. 10.7 Smooth Muscle Tissue
    9. 10.8 Development and Regeneration of Muscle Tissue
  15. Chapter 11. The Muscular System
    1. 11.0 Introduction
    2. 11.1 Describe the roles of agonists, antagonists and synergists
    3. 11.2 Explain the organization of muscle fascicles and their role in generating force
    4. 11.3 Explain the criteria used to name skeletal muscles
    5. 11.4 Axial Muscles of the Head Neck and Back
    6. 11.5 Axial muscles of the abdominal wall and thorax
    7. 11.6 Muscles of the Pectoral Girdle and Upper Limbs
    8. 11.7 Appendicular Muscles of the Pelvic Girdle and Lower Limbs
  16. Chapter 12. The Nervous System and Nervous Tissue
    1. 12.0 Introduction
    2. 12.1 Structure and Function of the Nervous System
    3. 12.2 Nervous Tissue
    4. 12.3 The Function of Nervous Tissue
    5. 12.4 Communication Between Neurons
    6. 12.5 The Action Potential
  17. Chapter 13. The Peripheral Nervous System
    1. 13.0 Introduction
    2. 13.1 Sensory Receptors
    3. 13.2 Ganglia and Nerves
    4. 13.3 Spinal and Cranial Nerves
    5. 13.4 Relationship of the PNS to the Spinal Cord of the CNS
    6. 13.5 Ventral Horn Output and Reflexes
    7. 13.6 Testing the Spinal Nerves (Sensory and Motor Exams)
    8. 13.7 The Cranial Nerve Exam
  18. Chapter 14. The Central Nervous System
    1. 14.0 Introduction
    2. 14.1 Embryonic Development
    3. 14.2 Blood Flow the meninges and Cerebrospinal Fluid Production and Circulation
    4. 14.3 The Brain and Spinal Cord
    5. 14.4 The Spinal Cord
    6. 14.5 Sensory and Motor Pathways
  19. Chapter 15. The Special Senses
    1. 15.0 Introduction
    2. 15.1 Taste
    3. 15.2 Smell
    4. 15.3 Hearing
    5. 15.4 Equilibrium
    6. 15.5 Vision
  20. Chapter 16. The Autonomic Nervous System
    1. 16.0 Introduction
    2. 16.1 Divisions of the Autonomic Nervous System
    3. 16.2 Autonomic Reflexes and Homeostasis
    4. 16.3 Central Control
    5. 16.4 Drugs that Affect the Autonomic System
  21. Chapter 17. The Endocrine System
    1. 17.0 Introduction
    2. 17.1 An Overview of the Endocrine System
    3. 17.2 Hormones
    4. 17.3 The Pituitary Gland and Hypothalamus
    5. 17.4 The Thyroid Gland
    6. 17.5 The Parathyroid Glands
    7. 17.6 The Adrenal Glands
    8. 17.7 The Pineal Gland
    9. 17.8 Gonadal and Placental Hormones
    10. 17.9 The Pancreas
    11. 17.10 Organs with Secondary Endocrine Functions
    12. 17.11 Development and Aging of the Endocrine System
  22. Chapter 18. The Cardiovascular System: Blood
    1. 18.0 Introduction
    2. 18.1 Functions of Blood
    3. 18.2 Production of the Formed Elements
    4. 18.3 Erythrocytes
    5. 18.4 Leukocytes and Platelets
    6. 18.5 Hemostasis
    7. 18.6 Blood Typing
  23. Chapter 19. The Cardiovascular System: The Heart
    1. 19.0 Introduction
    2. 19.1 Heart Anatomy
    3. 19.2 Cardiac Muscle and Electrical Activity
    4. 19.3 Cardiac Cycle
    5. 19.4 Cardiac Physiology
    6. 19.5 Development of the Heart
  24. Chapter 20. The Cardiovascular System: Blood Vessels and Circulation
    1. 20.0 Introduction
    2. 20.1 Structure and Function of Blood Vessels
    3. 20.2 Blood Flow, Blood Pressure, and Resistance
    4. 20.3 Capillary Exchange
    5. 20.4 Homeostatic Regulation of the Vascular System
    6. 20.5 Circulatory Pathways
    7. 20.6 Development of Blood Vessels and Fetal Circulation
  25. Chapter 21. The Lymphatic and Immune System
    1. 21.0 Introduction
    2. 21.1 Anatomy of the Lymphatic and Immune Systems
    3. 21.2 Barrier Defenses and the Innate Immune Response
    4. 21.3 The Adaptive Immune Response: T lymphocytes and Their Functional Types
    5. 21.4 The Adaptive Immune Response: B-lymphocytes and Antibodies
    6. 21.5 The Immune Response against Pathogens
    7. 21.6 Diseases Associated with Depressed or Overactive Immune Responses
    8. 21.7 Transplantation and Cancer Immunology
  26. Chapter 22. The Respiratory System
    1. 22.0 Introduction
    2. 22.1 Organs and Structures of the Respiratory System
    3. 22.2 The Lungs
    4. 22.3 The Process of Breathing
    5. 22.4 Gas Exchange
    6. 22.5 Transport of Gases
    7. 22.6 Modifications in Respiratory Functions
    8. 22.7 Embryonic Development of the Respiratory System
  27. Chapter 23. The Digestive System
    1. 23.0 Introduction
    2. 23.1 Overview of the Digestive System
    3. 23.2 Digestive System Processes and Regulation
    4. 23.3 The Mouth, Pharynx, and Esophagus
    5. 23.4 The Stomach
    6. 23.5 Accessory Organs in Digestion: The Liver, Pancreas, and Gallbladder
    7. 23.6 The Small and Large Intestines
    8. 23.7 Chemical Digestion and Absorption: A Closer Look
  28. Chapter 24. Metabolism and Nutrition
    1. 24.0 Introduction
    2. 24.1 Overview of Metabolic Reactions
    3. 24.2 Carbohydrate Metabolism
    4. 24.3 Lipid Metabolism
    5. 24.4 Protein Metabolism
    6. 24.5 Metabolic States of the Body
    7. 24.6 Energy and Heat Balance
    8. 24.7 Nutrition and Diet
  29. Chapter 25. The Urinary System
    1. 25.0 Introduction
    2. 25.1 Internal and External Anatomy of the Kidney
    3. 25.2 Microscopic Anatomy of the Kidney: Anatomy of the Nephron
    4. 25.3 Physiology of Urine Formation: Overview
    5. 25.4 Physiology of Urine Formation: Glomerular Filtration
    6. 25.5 Physiology of Urine Formation: Tubular Reabsorption and Secretion
    7. 25.6 Physiology of Urine Formation: Medullary Concentration Gradient
    8. 25.7 Physiology of Urine Formation: Regulation of Fluid Volume and Composition
    9. 25.8 Urine Transport and Elimination
    10. 25.9 The Urinary System and Homeostasis
  30. Chapter 26. Fluid, Electrolyte, and Acid-Base Balance
    1. 26.0 Introduction
    2. 26.1 Body Fluids and Fluid Compartments
    3. 26.2 Water Balance
    4. 26.3 Electrolyte Balance
    5. 26.4 Acid-Base Balance
    6. 26.5 Disorders of Acid-Base Balance
  31. Chapter 27. The Sexual Systems
    1. 27.0 Introduction
    2. 27.1 Anatomy of Sexual Systems
    3. 27.2 Development of Sexual Anatomy
    4. 27.3 Physiology of the Female Sexual System
    5. 27.4 Physiology of the Male Sexual System
    6. 27.5 Physiology of Arousal and Orgasm
  32. Chapter 28. Development and Inheritance
    1. 28.0 Introduction
    2. 28.1 Fertilization
    3. 28.2 Embryonic Development
    4. 28.3 Fetal Development
    5. 28.4 Maternal Changes During Pregnancy, Labor, and Birth
    6. 28.5 Adjustments of the Infant at Birth and Postnatal Stages
    7. 28.6 Lactation
    8. 28.7 Patterns of Inheritance
  33. Creative Commons License
  34. Recommended Citations
  35. Versioning

17.1 An Overview of the Endocrine System

Learning Objectives

By the end of this section, you will be able to:

  • Distinguish similarities and differences between neural and hormonal communication
  • Identify the major organs of the endocrine system and their location in the body

Communication within the human body involves the transmission of signals to control and coordinate actions in an effort to maintain homeostasis.  There are two major organ systems responsible for providing these communication pathways: the nervous system and the endocrine system.

The nervous system is primarily responsible for rapid communication throughout the body.  As discussed in previous chapters, the nervous system utilizes two types of signals – electrical and chemical (Table 17.1).  Electrical signals are sent via the generation and propagation of action potentials which move along the membrane of a cell.  Once the action potential reaches the synaptic terminal, the electrical signal is converted to a chemical signal as neurotransmitters are released into the synaptic cleft.  When the neurotransmitters binds with receptors on the receiving (post-synaptic) cell, a new electrical signal is generated and quickly continues on to its destination.  In this way, neural communication enables body functions that involve quick, brief actions, such as movement, sensation, and cognition.

In contrast, the endocrine system relies on only a single method of communication: chemical signaling (Table 1).  Hormones are the chemicals released by endocrine cells that regulate other cells in the body.   Hormones are transported primarily via the bloodstream throughout the body, where they bind to receptors on target cells, triggering a response.  Because of this dependence on the cardiovascular system for transport, this type of communication is much slower than that observed for neural signaling.  As such, hormonal communication is usually associated with activities that go on for relatively long periods of time.

External Website

QR Code representing a URL

Visit this link to watch an animation of the events that occur when a hormone binds to a cell membrane receptor. What is the secondary messenger made by adenylyl cyclase during the activation of liver cells by epinephrine?

In general, the nervous system involves quick responses to rapid changes in the external environment, and the endocrine system is usually slower acting—taking care of the internal environment of the body, maintaining homeostasis, and controlling reproduction. This does not mean, however, that the two systems are completely independent of one another.  Take for example the release of adrenaline from the adrenal medulla as part of the ‘fight-or-flight’ response.  Although adrenaline uses blood for transportation throughout the body, the effects are evident within seconds after the event has occurred;  how does the response happen so quickly if hormones are usually slower acting?   It occurs so rapidly because the nervous and endocrine system are both involved in the process: it is the fast action of the nervous system responding to the danger in the environment that stimulates the adrenal glands to quickly secrete their hormones.  In such a situation, the nervous system causes a rapid endocrine response to deal with sudden changes in both the external and internal environments when necessary.

Endocrine and Nervous Systems (Table 17.1)
Endocrine systemNervous system
Signaling mechanism(s)ChemicalChemical/electrical
Primary chemical signalHormonesNeurotransmitters
Distance traveledLong or shortAlways short
Response timeFast or slowAlways fast
Environment targetedInternalInternal and external

Endocrine Organs

Hormones are released by secretory cells that are derived from epithelial tissue.  Often, these cells are clustered together, forming endocrine glands.  Unlike exocrine glands, which have a duct for conveying secretions to the outside of the body (e.g., sweat gland), endocrine glands secrete substances directly into the surrounding interstitial fluid.  From there, hormones then enter the bloodstream for distribution throughout the body.

The major endocrine glands found in the human body include the pituitary gland, thyroid gland, parathyroid glands, thymus gland, adrenal glands, pineal gland, testes, and ovaries (Figure 17.1.1). While some of the glands are pure endocrine (e.g., thyroid gland), others serve both endocrine and exocrine function. For example, the pancreas contains cells that secrete digestive enzymes and juices into the small intestine (exocrine function) and cells that secrete the hormones insulin and glucagon, which regulate blood glucose levels.

In addition to the endocrine glands, major organs of the body show endocrine function including the hypothalamus, heart, kidneys, stomach, small intestine, and liver.  Moreover, adipose tissue has long been known to produce hormones, and recent research has revealed a role for bone tissue in hormone production and secretion.

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 17.1.1 – Endocrine System: Endocrine glands and cells are located throughout the body and play an important role in homeostasis.

Other Types of Chemical Signals

In the classical definition of the endocrine system, hormones are secreted into the interstitial fluid and then diffuse into the blood or lymph for circulation throughout the body to reach target tissues.  However, in certain instances, target cells are local and do not require hormones to enter the blood.  If a chemical signal is released into the interstitial fluid and targets neighboring cells, then the activity is referred to as paracrine.  Neurotransmitter communication between a pre- and post-synaptic neuron is a good example of paracrine activity.  Alternatively, chemicals released by a cell elicit a response in the same cell that secreted it, demonstrating autocrine activity.  An example of this is type of activity is Interleukin-1, signaling molecule released in an inflammatory response that binds to receptors located on the surface of the cell releasing the molecule.

Career Connections – Endocrinologist

Endocrinology is a specialty in the field of medicine that focuses on the treatment of endocrine system disorders. Endocrinologists, the medical doctors who specialize in this field, are experts in treating diseases associated with hormonal systems, ranging from thyroid disease to diabetes mellitus.

Patients who are referred to endocrinologists may have signs and symptoms or blood test results that suggest excessive or impaired functioning of an endocrine gland or endocrine cells. The endocrinologist may order additional blood tests to determine whether the patient’s hormonal levels are abnormal, or they may stimulate or suppress the function of the suspect endocrine gland and then have blood taken for analysis. Treatment varies according to the diagnosis. Some endocrine disorders, such as type 2 diabetes, may respond to lifestyle changes such as modest weight loss, adoption of a healthy diet, and regular physical activity. Other disorders may require medication, such as hormone replacement, and routine monitoring by the endocrinologist. These include disorders of the pituitary gland that can affect growth and disorders of the thyroid gland that can result in a variety of metabolic problems.

Some patients experience health problems as a result of the normal decline in hormones that can accompany aging. These patients can consult with an endocrinologist to weigh the risks and benefits of hormone replacement therapy intended to boost their natural levels of reproductive hormones.

In addition to treating patients, endocrinologists may be involved in research to improve the understanding of endocrine system disorders and develop new treatments for these diseases.

Chapter Review

The body coordinates its functions through two major types of communication: neural and endocrine. Neural communication includes both electrical and chemical signaling between neurons and target cells. Endocrine communication involves chemical signaling via the release of hormones which travel through the bloodstream, where they elicit a response in target cells. Endocrine glands are ductless glands that secrete hormones. Many organs of the body with other primary functions—such as the heart, stomach, and kidneys—also have endocrine activity.

Interactive Link Questions

Visit this link to watch an animation of the events that occur when a hormone binds to a cell membrane receptor. What is the secondary messenger made by adenylyl cyclase during the activation of liver cells by epinephrine?

cAMP

Review Questions

An interactive H5P element has been excluded from this version of the text. You can view it online here:
https://open.oregonstate.education/aandp/?p=746#h5p-322

An interactive H5P element has been excluded from this version of the text. You can view it online here:
https://open.oregonstate.education/aandp/?p=746#h5p-323

Critical Thinking Questions

1. Describe several main differences in the communication methods used by the endocrine system and the nervous system.

2. Compare and contrast endocrine and exocrine glands.

3. True or false: Neurotransmitters are a special class of paracrines. Explain your answer.

Glossary

autocrine
chemical signal that elicits a response in the same cell that secreted it
endocrine gland
tissue or organ that secretes hormones into the blood and lymph without ducts such that they may be transported to organs distant from the site of secretion
endocrine system
cells, tissues, and organs that secrete hormones as a primary or secondary function and play an integral role in normal bodily processes
exocrine system
cells, tissues, and organs that secrete substances directly to target tissues via glandular ducts
hormone
secretion of an endocrine organ that travels via the bloodstream or lymphatics to induce a response in target cells or tissues in another part of the body
paracrine
chemical signal that elicits a response in neighboring cells; also called paracrine factor

Solutions

Answers for Critical Thinking Questions

  1. The endocrine system uses chemical signals called hormones to convey information from one part of the body to a distant part of the body. Hormones are released from the endocrine cell into the extracellular environment, but then travel in the bloodstream to target tissues. This communication and response can take seconds to days. In contrast, neurons transmit electrical signals along their axons. At the axon terminal, the electrical signal prompts the release of a chemical signal called a neurotransmitter that carries the message across the synaptic cleft to elicit a response in the neighboring cell. This method of communication is nearly instantaneous, of very brief duration, and is highly specific.
  2. Endocrine glands are ductless. They release their secretion into the surrounding fluid, from which it enters the bloodstream or lymph to travel to distant cells. Moreover, the secretions of endocrine glands are hormones. Exocrine glands release their secretions through a duct that delivers the secretion to the target location. Moreover, the secretions of exocrine glands are not hormones, but compounds that have an immediate physiologic function. For example, pancreatic juice contains enzymes that help digest food.
  3. True. Neurotransmitters can be classified as paracrines because, upon their release from a neuron’s axon terminals, they travel across a microscopically small cleft to exert their effect on a nearby neuron or muscle cell.

Annotate

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17.2 Hormones
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Anatomy and Physiology
Copyright © 2019 by Lindsay M. Biga, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Devon Quick & Jon Runyeon

Anatomy & Physiology by Lindsay M. Biga, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Devon Quick & Jon Runyeon is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.

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