Endocrine System Overview

Module Objectives

• Identify major endocrine glands and distinguish between endocrine, autocrine, and paracrine secretions
• Compare and contrast the endocrine system and nervous system
• Describe how hormones are classified according to their chemical composition
• Explain how steroid and nonsteroid hormones affect their target cells
• Discuss how negative feedback mechanisms regulate hormone secretion
• Describe actions of pituitary hormones and explain how secretion of each is regulated
• Describe actions of hormones secreted by the thyroid and parathyroid glands and explain how secretion of each is regulated
• Describe actions of hormones secreted by adrenal glands and explain how secretion of each is regulated
• Describe actions of hormones secreted by the pancreas and explain how secretion of each is regulated
• Describe actions of hormones secreted by pineal gland, thymus, reproductive organs, digestive organs, heart, and kidneys
• Describe the general adaptation syndrome
• Describe changes associated with aging of the endocrine system

General Characteristics of Endocrine System

• Regulates body functions to maintain homeostasis and coordinates communication
• Unique system, organs are not anatomically connected but are connected functionally
• Major endocrine glands include pituitary, thyroid, parathyroid, adrenal, pancreas, pineal, thymus, and ovaries and testes (reproductive)

Hormone Action

• Hormones are released into extracellular fluid and diffuse into blood
• Transport method depends on whether the hormone is lipid-soluble or water-soluble
• Hormones are potent in small concentrations

Chemistry of Hormones

• Hormones are organic compounds
• Two general types: steroid and nonsteroid
• Steroid hormones are derived from cholesterol; examples include sex hormones and adrenal cortex hormones
• Nonsteroid hormones are derived from other substances; examples include amines, peptides, proteins, and glycoproteins

Hormone Actions (Nonsteroid Hormones)

• Cannot penetrate lipid bilayer in cell membranes
• Bind to receptors on target cell membranes
• Considered first messengers
• Often use cyclic adenosine monophosphate (cAMP) as a second messenger in a signal transduction process

Hormones of the Thyroid Gland

• Are amines but share qualities with steroid hormones
• Poor water-solubility; transported bound to plasma proteins
• Bind to receptors inside cells, usually in nucleus, causing transcription of particular genes in DNA
• Likely enter cells through specific transport methods

Parathyroid Hormone

• Released in response to low blood calcium levels
• Increases blood calcium and decreases phosphate
• Acts on bones, kidneys, and intestines to cause calcium release or conservation
• Acting on kidneys stimulates production of active Vitamin D
• Increases calcium absorption from the digestive system

Adrenal Glands

• Sit like caps atop each kidney (suprarenal glands)
• Contains two functionally distinct portions: cortex and medulla
• Cortex produces steroid hormones: aldosterone, cortisol, sex hormones etc., divided into three layers (zona)
• Medulla secretes amine hormones: epinephrine and norepinephrine

Pineal and Thymus Glands

• Pineal gland: Located in the brain, secretes melatonin, regulates circadian rhythm
• Thymus gland: Found in mediastinum, secretes thymosins, promotes development of T-lymphocytes, critical for immune function

Other Endocrine Glands (Reproductive, Digestive, Heart, Kidney)

• Ovaries and Testes/Reproductive organs: Produce sex hormones for reproduction/ovulation (female), testosterone production (male), placental hormones
• Digestive Glands: Produce digestive hormones, controlling digestion
• Heart (atrial): Produces natriuretic peptides, stimulating Na+ secretion in urine
• Kidney: Produces erythropoietin, stimulating red blood cell formation

Stress and General Adaptation Syndrome (GAS)

• Survival depends on maintaining homeostasis, factors threatening internal/external environment potentially life-threatening
• Hypothalamus activates sympathetic nervous system and adrenal hormones
• Stress is the condition produced in response to stressors
• Three stages of stress response (GAS): Alarm, Resistance, Exhaustion

Life-Span Changes in Endocrine System

• Endocrine glands decrease in size
• GH levels decline, leading to decreased muscular and skeletal strength
• ADH levels increase, due to slower elimination by liver and kidneys
• Calcitonin levels decrease, increasing risk of osteoporosis
• PTH level changes contribute to osteoporosis, especially in females
• Insulin resistance may develop
• Changes in melatonin secretion affect the body clock
• Thymosin production declines, increasing risk of infections

Diabetes Mellitus

• Type 1 diabetes: autoimmune disorder; lack of insulin production; often diagnosed in childhood
• Type 2 diabetes: body cells unable to recognize insulin; often diagnosed in adulthood
• Results in hyperglycemia (high blood glucose) disrupting carbohydrate, protein, and fat metabolism; can damage eyes, heart, kidneys, and nerves

Pancreas

• Elongated, flattened organ posterior to the stomach
• Exocrine function: secretes digestive juices into the intestines via the pancreatic duct
• Endocrine function: pancreatic islets (Islets of Langerhans); secrete hormones into body fluids; include glucagon (increases blood glucose), Insulin (decreases blood glucose), Somatostatin (inhibits glucagon and insulin secretion)

Regulation of Blood Glucose

• Pancreatic islets secrete insulin after a meal to facilitate glucose uptake and glycogen formation
• Pancreatic islets secrete glucagon between meals to break down glycogen into glucose, increasing blood glucose levels