Endocrine

Chapter 16: The Endocrine System

16.1 The Endocrine System is One of the Body’s Two Major Control Systems

  • The endocrine system interacts with the nervous system to coordinate and integrate the activity of most body cells.

  • Hormones: Chemical messengers transported in blood to influence metabolic activities.

  • Endocrine responses are slower but longer-lasting than nervous system responses.

  • Endocrinology: The study of hormones and endocrine organs.

Table 16.1 Comparison of Nervous and Endocrine Systems

Feature

Nervous System

Endocrine System

Initiation of responses

Rapidly

Slowly

Duration of response

Short-duration

Long-duration

Method of action

Via action potentials and neurotransmitters

Via hormones released into the blood

Specificity of action

Acts at specific locations determined by axon pathways

Acts at diffuse locations—targets can be anywhere blood reaches

Distance of action

Neurotransmitters act over very short distances

Hormones act over long distances

Signal strength

Coded by frequency of action potentials

Coded by concentration of hormone

Major Processes Controlled by the Endocrine System

  • Reproduction

  • Growth and development

  • Maintenance of electrolyte, water, and nutrient balance of blood

  • Regulation of cellular metabolism and energy balance

  • Mobilization of body defenses

Types of Glands

  • Exocrine glands:

    • Produce non-hormonal substances (e.g., sweat, saliva).

    • Have ducts to carry secretion to a (cutaneous or mucous) membrane surface.

  • Endocrine glands:

    • Produce hormones.

    • Ductless; hormones secreted directly into surrounding extracellular fluid.

    • Include: pituitary, thyroid, parathyroid, adrenal, and pineal glands.

  • Hypothalamus: A neuroendocrine organ.

  • Other organs with endocrine tissue include: pancreas, gonads, placenta.

  • Additional hormone-producing tissues include: stomach, intestine, heart, kidneys, skin, thymus, bone, and adipose.

Chemical Messengers

  • Hormones: Long-distance chemical signals of the endocrine system; travel in blood to reach target cells.

  • Autocrines: Chemicals exert effects on the same cells that secrete them.

  • Paracrines: Locally acting chemicals that affect neighboring cells.

  • Autocrines and paracrines are considered “local” chemical messengers; not part of the endocrine system.

16.2 The Chemical Structure of a Hormone Determines How It Acts

  • A hormone's chemical structure determines its solubility in water, influencing the following:

    • Transportation in blood.

    • Rate of degradation.

    • Types of receptors it will act upon.

Classes of Hormones

  • Amino Acid-based hormones:

    • Include amino acid derivatives, peptides, and proteins.

    • Generally water-soluble (except thyroxine); cannot cross plasma membrane.

  • Steroids:

    • Synthesized from cholesterol.

    • Lipid-soluble; can cross plasma membranes.

    • Includes gonadal and adrenocortical hormones.

  • Eicosanoids: A possible third class, often classified as paracrines and autocrines due to their localized effects.

16.3 Hormones Act Through Second Messengers or by Activating Specific Genes

  • Only cells with receptors for a specific hormone are affected; these are known as target cells.

  • Hormones can increase or decrease the rates of normal cellular processes in target cells.

Target Cell Changes

Hormones typically produce one or more of the following changes in target cells:

  • Alter plasma membrane permeability and/or membrane potential via ion channels.

  • Stimulate enzyme/protein synthesis.

  • Activate or deactivate enzymes.

  • Induce secretory activity.

  • Stimulate mitosis.

Mechanisms of Hormone Action

  • Depending on their chemical nature and receptor location, hormones act in one of two ways:

  1. Water-soluble hormones (all amino acid-based hormones except thyroid hormone):

    • Act on plasma membrane receptors.

    • Cannot cross plasma membrane.

    • Often coupled via G proteins to second messengers.

  2. Lipid-soluble hormones (steroid and thyroid hormones):

    • Act on intracellular receptors that directly activate genes.

    • Can diffuse across plasma membranes.

16.4 Three Types of Stimuli Cause Hormone Release

  • Hormone Blood Levels:

    • Controlled by negative feedback mechanisms.

    • The stimulus triggers hormone secretion; as levels rise, they cause specific target organ effects, subsequently reducing the stimulus for further release.

    • Levels typically vary only within a narrow range.

    • Hormone release can be triggered by:

    • Endocrine gland stimuli.

    • Nervous system modulation.

Endocrine Gland Stimuli

  • Endocrine glands can synthesize and secrete hormones in response to:

    • Humoral stimuli: Changing blood levels of ions and nutrients directly stimulate hormone release (e.g., declining blood concentration triggers the parathyroid gland to secrete PTH).

    • Neural stimuli: Nerve fibers stimulate hormone release (e.g., sympathetic nervous system fibers trigger adrenal medulla secretion of catecholamines).

    • Hormonal stimuli: Hormones stimulate other endocrine organs to release hormones (e.g., hypothalamic hormones regulate the anterior pituitary hormones).

Nervous System Modulation

  • The nervous system can adjust hormone levels when necessary.

  • It can modify the stimulation or inhibition of endocrine glands and may override normal endocrine controls.

    • Example: Under severe stress, the hypothalamus and sympathetic nervous system override insulin effects to increase blood glucose levels.

16.5 Cells Respond to a Hormone if They Have a Receptor for That Hormone

  • Target cells must possess specific receptors for hormone binding.

    • For instance, ACTH receptors are found only on adrenal cortex cells, while thyroxine receptors are found on nearly all body cells.

  • Factors influencing the degree of target cell activation:

    1. Blood levels of hormone.

    2. Relative number of target cell receptors.

    3. Affinity (strength) of binding between hormone and receptor.

Up-Regulation and Down-Regulation

  • Up-regulation: Target cells increase receptor numbers in response to persistently low hormone levels, making them more sensitive.

  • Down-regulation: Target cells decrease receptor numbers in response to persistently high hormone levels to prevent overstimulation.

16.6 Half-Life, Onset, and Duration of Hormone Activity

  • Hormones circulate in blood either free or bound to plasma proteins.

    • Steroids and thyroid hormones are typically bound.

    • Other hormones circulate freely.

  • Circulating hormone concentration depends on:

    1. Rate of release.

    2. Speed of inactivation and removal from the body.

  • Some hormones are rapidly degraded by enzymes; others are excreted by the liver or kidneys.

  • Half-life: Time required for the hormone level in blood to decrease by half; can vary from minutes to weeks depending on the hormone.

Response Times and Duration

  • Different hormones evoke target organ responses at varying speeds; some responses appear almost immediately, while others may take hours or days, particularly for steroid hormones.

  • Duration of hormone action varies: 10 seconds to several hours, influenced by whether the hormone is water or lipid soluble.

16.7 Interaction of Hormones at Target Cells

  • Multiple hormones may act on the same target simultaneously

    • Permissiveness: One hormone cannot exert its effects without another present (e.g., reproductive hormones require thyroid hormone).

    • Synergism: Two or more hormones produce the same effects at target cells, resulting in amplified effects (e.g., glucagon and epinephrine causing liver to release glucose).

    • Antagonism: One or more hormones oppose the action of another (e.g., insulin and glucagon).

16.8 The Hypothalamus Controls Hormone Release from the Pituitary Gland

  • The hypothalamus connects to the pituitary gland via a stalk (infundibulum).

    • The pituitary gland has two major lobes, secreting at least eight hormones.

    • The posterior pituitary is composed of neural tissue and stores two neurohormones (oxytocin and ADH), while the anterior pituitary is glandular tissue that manufactures and secretes six hormones.

Pituitary-Hypothalamic Relationships

  • Posterior Pituitary:

    • Contains pituicytes and axons of hypothalamic neurons.

    • The hypothalamic-hypophyseal tract connects hypothalamus to posterior pituitary, allowing release of oxytocin and ADH on demand.

  • Anterior Pituitary:

    • Derived from oral mucosa outpocketing and connected to hypothalamus via a hypophyseal portal system (Primary capillary plexus, hypophyseal portal veins, and secondary capillary plexus).

    • Hormones from hypothalamus (releasing and inhibiting) travel to anterior pituitary to regulate secretion.

16.9 The Posterior Pituitary and Hypothalamic Hormones
Oxytocin

  • Released during childbirth and breastfeeding.

    • Stimulant of uterine contractions (childbirth) and triggers milk ejection (breastfeeding).

    • Functions as a neurotransmitter in the brain.

    • Secretion is inhibited by stress.

Antidiuretic Hormone (ADH)

  • Triggers reabsorption of water from kidneys to reduce blood osmolarity.

  • Stimulus for release includes high blood osmolarity, low blood volume, pain, and specific drugs; inhibited by alcohol.

  • High concentrations can induce vasoconstriction, warranting the alternative name vasopressin.

16.10 Anterior Pituitary Hormones

  • The anterior pituitary secretes six hormones; all are peptide/protein hormones that act mainly with cAMP second messenger systems with exceptions of GH and PRL.

    • Growth Hormone (GH)

    • Promotes growth and has complex metabolic actions.

    • Direct actions: Increases blood glucose levels, decreases cellular glucose uptake.

    • Indirect actions: GH triggers liver and muscles to produce IGFs promoting cellular nutrient uptake and division.

    • Regulation: Release is stimulated by GHRH and inhibited by GHIH (somatostatin).

  • Thyroid-stimulating Hormone (TSH)

    • Stimulates normal development and secretory activity of the thyroid.

    • Release influenced by TRH and inhibited by rising thyroid hormone levels.

  • Adrenocorticotropic Hormone (ACTH)

    • Stimulates release of glucocorticoids from adrenal cortex.

    • Triggered by stress and inhibited by rising glucocorticoids.

  • Follicle-stimulating Hormone (FSH)

    • Stimulates production of gametes in gonads.

    • Regulated by GnRH and inhibited by feedback from gonadal hormones.

  • Luteinizing Hormone (LH)

    • Stimulates gonadal hormone production.

    • Release regulated by GnRH and inhibited by elevated gonadal hormones.

  • Prolactin (PRL)

    • Stimulates milk production in females.

    • Levels regulated by PIH (dopamine); increased during pregnancy and suckling.

16.11 Disorders of Pituitary Hormones
Growth Hormone Disorders

  • Hypersecretion:

    • In children leads to gigantism; in adults results in acromegaly.

  • Hyposecretion:

    • Results in pituitary dwarfism.

Clinical Homeostatic Imbalances

  • Diabetes Insipidus: ADH deficiency, resulting in excessive urination and thirst.

  • Syndrome of Inappropriate ADH Secretion (SIADH): Overproduction of ADH leads to water retention, causing hyponatremia.

Thyroid Gland

  • Thyroid Hormones (TH):

    • Major metabolic hormones affecting nearly all cells.

    • Produced in T4 and T3 forms, are lipid soluble.

    • Regulate basal metabolic rate, and tissue growth, and are essential for normal development.

  • Calcitonin: Produced by parafollicular cells; helps regulate calcium levels, although with unclear physiological roles at normal levels.

  • Hyperthyroidism: Common in Graves’ disease; results from autoimmune stimulation of the thyroid, leading to increased TH levels and symptoms like weight loss and anxiety.

  • Hypothyroidism: Can lead to myxedema in adults, and cretinism in children; characterized by low metabolic rates and various adverse health effects.

Parathyroid Glands

  • Secrete parathyroid hormone (PTH): regulates calcium levels.

    • Stimulated by low calcium levels and inhibits when levels rise.

  • Hyperparathyroidism can lead to osteitis fibrosa cystica; hypoparathyroidism may result in increased neuronal excitability and tetany.

Adrenal Glands

  • Produce hormones involved in electrolyte balance and stress response.

    • Adrenal Cortex: Synthesizes corticosteroids (mineralocorticoids like aldosterone, glucocorticoids like cortisol, and gonadocorticoids).

    • Adrenal Medulla: Secretes catecholamines (epinephrine and norepinephrine) that are involved in the fight-or-flight response.

Pancreas

  • Has both endocrine (insulin and glucagon) and exocrine functions.

  • Insulin reduces blood glucose levels, while glucagon increases them.

Clinical Homeostatic Imbalance

  • Diabetes Mellitus (DM): Results from insulin hyposecretion (Type 1) or hypoactivity (Type 2), characterized by polyuria, polydipsia, and polyphagia, with associated metabolic derangements.

16.12 Hormone Functions Outside Major Glands

  • Several tissues/hormones have endocrine functions beyond major glands include:

    • Adipose tissue: Produces leptin, resistin, and adiponectin.

    • Gastrointestinal tract: Secretes hormones involved in digestion.

    • Heart: Releases atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) in response to blood volume changes.

    • Kidneys: Produce erythropoietin and renin.

    • Skeleton: Osteocalcin affects insulin sensitivity.

    • Skin: Synthesizes cholecalciferol, a precursor to active vitamin D.

    • Thymus: Involved in T lymphocyte development.

Developmental Aspects

  • Hormone-producing glands arise from all three primary germ layers.

  • Environmental pollutants can disrupt hormone function, particularly vulnerable hormones include sex hormones, thyroid hormone, and glucocorticoids.

  • Endocrine function generally remains intact until old age, with levels of some hormones such as estrogen declining dramatically around menopause.

  • Hormonal sensitivity can decrease with age, possibly leading to diabetes.

Summary

  • The endocrine system is essential for the control of metabolic processes, development, and overall homeostasis within the body. Understanding its mechanisms, hormone functions, and interactions is crucial for grasping human physiology.