Endocrine System Overview

The Endocrine System

• The endocrine system works with the nervous system to coordinate and integrate bodily functions.
• It influences metabolic processes through hormones transported in the blood.
• These responses are slower but last longer than nervous system responses.

Endocrine System Overview

• The endocrine system controls and integrates:
  • Reproduction
  • Growth and development
  • Maintaining blood electrolyte, water, and nutrient balance.
  • Regulating cellular metabolism and energy balance.
  • Mobilizing body defenses.

Exocrine Glands

• Produce non-hormonal substances (e.g., sweat, saliva).
• Have ducts to carry secretions to membrane surfaces.

Endocrine Glands

• Produce hormones.
• Lack ducts.

Endocrine Glands Examples

• Pituitary, thyroid, parathyroid, adrenal, and pineal glands
• Hypothalamus (a neuroendocrine organ)
• Some organs have both exocrine and endocrine functions (e.g., pancreas, gonads, placenta)
• Other tissues produce hormones (e.g., adipose cells, thymus, cells in the walls of small intestine, stomach, kidneys, and heart).

Hormone Chemical Structure

• Two main classes of hormones:
  • Amino acid-based hormones (amino acid derivatives, peptides, and proteins)
  • Steroid hormones (synthesized from cholesterol)
  • Eicosanoids (are sometimes also treated as hormones, but mainly considered local chemical messengers)

Hormone Action

• Hormones act on target cells with specific receptors.
• Hormone action on target cells may alter plasma membrane permeability; stimulate synthesis of enzymes or proteins; activate or deactivate enzymes; induce secretory activity; and stimulate mitosis.
• Hormones act in one of two ways depending on their chemical nature and receptor location:
  • Water-soluble hormones (all amino acid-based hormones except thyroid hormone) act on plasma membrane receptors via G protein second messengers.
  • Lipid-soluble hormones (steroid and thyroid hormones) act on intracellular receptors that directly activate genes.

Hormone Release

• Blood hormone levels are controlled by negative feedback systems.
• Hormone release is triggered by:
  • Humoral Stimuli: changing blood levels of ions and nutrients directly affecting hormone secretion.
  • Neural Stimuli: nerve fibers stimulating hormone release.
  • Hormonal Stimuli: hormones stimulating other endocrine organs to release hormones.

Target Cell Specificity

• Target cells must have specific receptors to which the hormone binds.
• Target cell activation depends on:
  • Blood levels of the hormone
  • Relative number of receptors on the cell
  • Affinity of binding between the hormone and receptor
  • 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.

Half-Life, Onset, and Duration of Hormone Activity

• Hormones circulate free or bound to plasma proteins.
• Hormones are removed from blood through degradation, by the kidneys, liver, or by other metabolic processes.
• Half-life: time required for the level of hormone in the blood to decrease by half; ranges from fractions of a minute to a week.
• Hormones have different response times, from immediate effects to effects that take hours or days to manifest.

Interaction of Hormones at Target Cells

• Synergism: more than one hormone produces the same effects on the target cell, causing amplification.
• Antagonism: one or more hormones oppose the action of another hormone.
• Permissiveness: one hormone cannot exert its effects without another hormone being present.

Hypothalamus and Pituitary Gland

• The hypothalamus is connected to the pituitary gland (hypophysis) via the infundibulum.
• The pituitary gland has two lobes:
  • Posterior pituitary: composed of neural tissue, secretes neurohormones (oxytocin and antidiuretic hormone (ADH)).
  • Anterior pituitary: composed of glandular tissue, secretes hormones in response to hormones from the hypothalamus (tropic hormones).