The Endocrine System

Overview of the Endocrine System

The endocrine system works in conjunction with the nervous system to coordinate and integrate the activities of body cells. It primarily influences metabolic activities through the release of hormones into the bloodstream. Compared to the nervous system, endocrine responses are slower but tend to last longer.

• Endocrinology: The scientific study of hormones and endocrine organs.

• Exocrine glands: Produce nonhormonal substances (e.g., sweat, saliva) and have ducts that carry secretions to membrane surfaces.

• Endocrine glands: Ductless glands that produce hormones, which are long-distance chemical signals transported in blood or lymph.

• Major endocrine glands: Pituitary, thyroid, parathyroid, adrenal, and pineal glands.

• Neuroendocrine organ: The hypothalamus functions as both a neural and endocrine organ.

• Organs with both exocrine and endocrine functions: Pancreas, gonads, placenta.

• Other hormone-producing tissues: Adipose cells, thymus, and cells in the walls of the small intestine, stomach, kidneys, and heart.

Hormone Chemical Structure

Classification of Hormones

Hormones are classified based on their chemical structure, which determines their mechanism of action and solubility.

• Amino acid–based hormones: Includes amino acid derivatives, peptides, and proteins. Most hormones belong to this class.

• Steroid hormones: Synthesized from cholesterol. Includes gonadal and adrenocortical hormones.

• Eicosanoids: Includes prostaglandins and prostacyclins. These are involved in inflammation, fever, blood pressure regulation, and blood clotting. Most scientists classify them as paracrines rather than true hormones.

Action of Hormones

Mechanisms of Hormone Action

Hormones affect only specific target cells that possess receptors for them. Although hormones circulate throughout the body, only target cells respond.

• Hormones can:

  • Alter plasma membrane permeability and/or membrane potential by opening or closing ion channels.

  • Stimulate synthesis of enzymes or other proteins.

  • Activate or deactivate enzymes.

  • Induce secretory activity.

  • Stimulate mitosis (cell division).

• Hormones act via two main mechanisms, depending on their chemical nature:

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

     • Act on plasma membrane receptors.

     • Use G protein–mediated second messengers (e.g., cyclic AMP, PIP2).

     • Cannot enter the cell; effects are indirect.

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

     • Act on intracellular receptors that directly activate genes.

     • Can enter the cell; effects are direct.

Hormone Release

Regulation of Hormone Secretion

Blood levels of hormones are primarily regulated by negative feedback mechanisms, ensuring that hormone concentrations remain within a narrow, optimal range.

• Increased hormone effects on target organs can inhibit further hormone release.

• Endocrine glands are stimulated to synthesize and release hormones in response to three types of stimuli:

Target Cell Specificity

Factors Affecting Target Cell Activation

For a hormone to affect a cell, the cell must have specific receptors for that hormone. The degree of target cell activation depends on:

• Blood levels of the hormone.

• Relative number of receptors on or in the target cell.

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

The number of receptors can change in response to hormone levels:

• Up-regulation: Target cells form more receptors in response to low hormone levels.

• Down-regulation: Target cells lose receptors in response to high hormone levels, preventing overreaction.

Hormone Interactions at Target Cells

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

• Synergism: More than one hormone produces the same effect, amplifying the response (e.g., glucagon and epinephrine both cause the liver to release glucose).

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

The Hypothalamus and Pituitary Gland

Hypothalamic–Pituitary Relationships

The hypothalamus is connected to the pituitary gland (hypophysis) via the infundibulum. The pituitary gland consists of two major lobes:

• Posterior pituitary (neurohypophysis): Maintains a neural connection to the hypothalamus via the hypothalamic-hypophyseal tract. Stores and releases two neurohormones (oxytocin and antidiuretic hormone [ADH]) produced by the hypothalamus.

• Anterior pituitary (adenohypophysis): Composed of glandular tissue and connected to the hypothalamus via the hypophyseal portal system (primary capillary plexus, portal veins, secondary capillary plexus). Produces and releases seven hormones in response to hypothalamic releasing and inhibiting hormones.

Posterior Pituitary Hormones

• Oxytocin: Stimulates strong uterine contractions during childbirth and triggers milk ejection during lactation. Both actions are regulated by positive feedback mechanisms.

• Antidiuretic hormone (ADH): Released in response to high solute concentration in the blood, pain, low blood pressure, or certain drugs. Targets kidney tubules to reabsorb more water, reducing urine formation. Inhibited by alcohol and diuretics. At high concentrations, causes vasoconstriction (also called vasopressin).

Clinical Note: Diabetes insipidus is caused by ADH deficiency due to damage to the hypothalamus or posterior pituitary, resulting in excessive urination and thirst. Patients must remain well hydrated.

Anterior Pituitary Hormones

• All seven hormones are peptide hormones.

• Four are tropic hormones (tropins) that regulate the secretion of other endocrine glands.

Growth Hormone (GH)

• Direct actions: Glucose-sparing (anti-insulin) effects, increases blood fatty acids, encourages protein synthesis.

• Indirect actions: Stimulates liver, skeletal muscle, and bone to produce insulin-like growth factors (IGFs), which promote cell division, collagen formation, and bone matrix deposition.

• Major targets: Bone and skeletal muscle.

Clinical Note:

• Hypersecretion of GH: Causes gigantism in children (excessive growth, height up to 8 feet) and acromegaly in adults (overgrowth of hands, feet, face).

• Hyposecretion of GH: Causes pituitary dwarfism in children (short stature, ~4 feet); usually no problems in adults.

Thyroid-Stimulating Hormone (TSH)

• Stimulates normal development and secretory activity of the thyroid gland.

• Release is triggered by thyrotropin-releasing hormone (TRH) from the hypothalamus.

• Inhibited by rising blood levels of thyroid hormones (negative feedback on pituitary and hypothalamus).

Adrenocorticotropic Hormone (ACTH)

• Stimulates the adrenal cortex to release corticosteroids.

• Release is triggered by corticotropin-releasing hormone (CRH) from the hypothalamus, with highest levels in the morning.

• Release is increased by stressors such as fever, hypoglycemia, and other internal/external factors.

Gonadotropins (FSH and LH)

• FSH: Stimulates production of gametes (eggs or sperm).

• LH: Promotes production of gonadal hormones (estrogen, progesterone, testosterone). In females, triggers ovulation and hormone release; in males, stimulates testosterone production.

• Absent from blood in prepubertal children; release is triggered by gonadotropin-releasing hormone (GnRH) during and after puberty.

Prolactin (PRL)

• Stimulates milk production in females.

• Blood levels rise toward the end of pregnancy; suckling stimulates PRL release and maintains milk production.

Additional info: Melanocyte Stimulating Hormone (MSH) is involved in regulating skin pigmentation, though its role in humans is less prominent than in other animals.