Endocrine System - Lecture Notes

Endocrine System

The endocrine system is a network of ductless glands that secrete hormones directly into the bloodstream to regulate various physiological processes. These hormones control growth, metabolism, reproduction, and homeostasis. (e.g., pituitary, thyroid, adrenal).

Target Cells

Cell with specific receptors for a hormone

Second messengers

Intracellular signaling molecules (e.g., cAMP, Ca²⁺) used by amino acid-based hormones to exert effects

Negative Feedback Mechanism

A process where rising hormone levels inhibit further release, maintaining homeostasis

Mechanisms of Hormone Action: Amino acid-based hormones

Bind to cell surface receptors, often using second messengers like cAMP to activate protein kinases and cause cellular responses.

Mechanisms of Hormone Action: Steroid hormones

Lipid-soluble, pass through cell membranes, bind to intracellular receptors, and directly influence gene expression.

Types of Stimuli for Hormone Release: Humoral

Changes in blood levels of ions/nutrients (e.g., Ca2+ stimulates PTH release)

Types of Stimuli for Hormone Release: Neural

Nerve fibers stimulate hormone release (e.g., sympathetic stimulation of adrenal medulla)

Types of Stimuli for Hormone Release: Hormonal

Hormones stimulate release of other hormones (e.g., hypothalamic hormones stimulate anterior pituitary)

Pituitary Gland and Hypothalamus

The pituitary gland, located at the base of the brain, is divided into anterior and posterior lobes. It is closely linked to the hypothalamus, which regulates its activity

Posterior Pituitary

Releases neurohormones made in the hypothalamus

Posterior Pituitary: Oxytocin

Stimulates uterine contractions during childbirth and milk ejection

Posterior Pituitary: ADH (antidiuretic hormone)

Promotes water retention by kidneys, regulates water balance

Anterior Pituitary

• Produces and releases its own hormones, regulated by hypothalamic releasing/inhibiting hormones via the hypophyseal portal system

Anterior Pituitary: Growth hormone (GH)

Stimulates body growth, protein synthesis, fat breakdown

Anterior Pituitary: Thyroid-stimulating hormone (TSH)

Stimulates thyroid gland to release thyroid hormones

Anterior Pituitary: Adrenocorticotropic hormone (ACTH)

Stimulates adrenal cortex to release corticosteroids (especially cortisol)

Anterior Pituitary: Follicle-stimulating hormone (FSH) and Luteinizing hormone (LH)

Control gamete production and sex hormone release

Anterior Pituitary: Prolactin (PRL)

Promotes milk production; mainly controlled by inhibition (dopamine/PIH)

Growth Hormone Regulation

• Stimulated by GHRH; inhibited by GHIH (somatostatin).

• Promotes growth, especially in bones and muscles; mobilizes fats, increases blood glucose.

• Insulin-like growth factors (IGFs): Mediate many GH effects.

• Acromegaly: Excess GH in adults; enlarged hands, feet, face.

Thyroid Gland

The thyroid gland is located in the neck, below the larynx, and consists of two lobes connected by an isthmus. It produces hormones that regulate metabolism and development.

Thyroid Gland: Thyroxine (T4) and Triiodothyronine (T3)

Increase basal metabolic rate, regulate growth and development.

Thyroid Gland: Goiter

Enlarged thyroid, often due to iodine deficiency

Thyroid Gland: Grave’s disease

• Autoimmune hyperthyroidism

• Symptoms included elevated:

  • Metabolic rate

  • Sweating

  • Rapid heartbeat

  • Weight loss

  • Exophthalmos (bulging eyes)

Parathyroid Glands

Small glands located on the posterior aspect of the thyroid gland. They regulate calcium homeostasis

Parathyroid Glands: Parathyroid hormone (PTH)

Increases blood calcium by stimulating osteoclasts, increasing intestinal absorption, and kidney reabsorption of calcium

Adrenal Glands

Located on top of each kidney, the adrenal glands consist of the cortex and medulla, each producing different hormones

Adrenal Cortex: Zona glomerulosa

Produces mineralocorticoids (e.g., aldosterone) - regulate Na⁺ and K⁺ balance, blood pressure

Adrenal Cortex: Zona fasciculata

Produces glucocorticoids (e.g., cortisol) – regulate metabolism, stress response, increase blood glucose, suppress inflammation

Adrenal Cortex: Addison’s disease

Hyposecretion of adrenal cortex hormones; symptoms include fatigue, weight loss, hypotension

Adrenal Cortex: Cushing's syndrome

Hypersecretion of cortisol; symptoms include hyperglycemia, loss of muscle/bone, fat redistribution (moon face, buffalo hump)

Adrenal Medulla

• Produces catecholamines: epinephrine (80%) and norepinephrine (20%).

• Part of the sympathetic nervous system; increases heart rate, blood pressure, blood glucose, and bronchodilation.

Pineal Gland

Produces melatonin, which regulates sleep-wake cycles and circadian rhythms

Pancreas and other Endocrine Glands

The pancreas has both endocrine and exocrine functions. Its endocrine portion (islets of Langerhans) regulates blood glucose

Pancreas and other Endocrine Glands: Alpha cells

Secrete glucagon – raises blood glucose by promoting glycogen breakdown and gluconeogenesis

Pancreas and other Endocrine Glands: Beta cells

Secrete insulin – lowers blood glucose by promoting glucose uptake and glycogen synthesis

Pancreas and other Endocrine Glands: Diabetes mellitus

Chronic hyperglycemia due to insulin deficiency (Type 1) or resistance (Type 2)

Pancreas and other Endocrine Glands: Signs

Polyuria (excess urination), polydipsia (excess thirst), polyphagia (excess hunger)

Exocrine Gland

Producing secretions that are released into ducts

Endocrine gland

Producing hormones that are released into the blood

Type 1 Diabetes

• Insulin-dependent diabetes mellitus

• Absent production and release of insulin by beta cells

• Occur at young age

• Not associated with obesity; genetic predisposition

• Viral Infection leading to an autoimmune condition

• Treatment requires injections of insulin

Type 2 Diabetes

• Insulin-independent diabetes

• Decreased insulin released from beta cells or decreased insulin effectiveness on target cells at peripheral tissues

• Tend to occur people over the age of 30 (can be found in younger people)

• Obesity plays a major role and more young people are overweight than in the past

• Can be treated with diet, exercise, and medication to improve glucose levels

Gestational Diabetes

• Seen in pregnant people

• Latter half of pregnancy

• Untreated can pose a risk to the fetus as well as increase delivery complications

Hypoglycemia

• When blood glucose levels drop below 60 mg/dL

• Not a disease, homeostatic imbalance

• Some cause are alcohol consumption on an empty stomach, liver or kidney dysfunction, insulin overdose, prolonged and intense exercise

• Symptoms include hunger, dizziness, nervousness, confusion, anxious or weak, sweating, sleepiness

• Insufficient glucose to the brain or from activation of sympathetic nervous system

Other Hormone-Producing Organs: Leptin

Produced by adipose tissue; regulates appetite and energy balance

Other Hormone-Producing Organs: Renin

Produced by kidneys; regulates blood pressure

Other Hormone-Producing Organs: Cholecalciferol

Inactive vitamin D3, converted to calcitriol in kidneys

Other Hormone-Producing Organs: Calcitriol

Active vitamin D; enhances calcium absorption in intestines

Other Hormone-Producing Organs: Thymus

Produces thymulin, thymopoietins, and thymosins - involved in T-cell development and immune response

Hormone Functions: Regulation of metabolism

Thyroids hormones, cortisol, insulin, glucagon

Hormone Functions: Growth and development

GH, IGFs, Thyroid hormones, sex hormones

Hormone Functions: Electrolyte and water balance

Aldosterone, ADH, PTH

Hormone Functions: Reproduction

FSH, LH, estrogen, testosterone, prolactin

Hormone Functions: Stress response

Cortisol, epinephrine, norepinephrine

Second messenger (cAMP) pathway

Hormone binds receptors → activates G protein → activates adenylate cyclase → converts ATP to cAMP → activates protein kinases → cellular response

Blood Glucose Regulation

• Insulin: Glucose uptake (increase), Blood glucose (decreases)

• Glucagon: Glycogenolysis, gluconeogensis (increase), Blood glucose (increase)

Aging and the Endocrine System

Aging reduces the efficiency of endocrine system functions, and often hormonal levels decreases

• Growth Hormone (GH) decreases

• Leads to loss of weight and body mass

• Testosterone and estrogen levels decline as we age

Endocrine System Features:

• Communication method: Endocrine glands secrete hormones; transported through the blood

• Target Stimulation: Any cell in the body with the receptor for the hormone

• Response Time: Slow, seconds to minutes to hours

• Range of Effect: Widespread effects

• Duration of response: Long-lasting: minutes to weeks, may continue after stimulus is removed

Nervous System Features

• Communication method: A neurotransmitter is released from a neuron into a synaptic cleft

• Target Stimulation: Other neurons, muscle cells, and glands

• Response Time: Fast, milliseconds or seconds

• Range of Effect: Localized, specific effects

• Duration of response: Short-term: Milliseconds; terminates with removal or stimulus

What are the Major Endocrine Organs and their location within the body

• Pituitary - Hypothalamus

• Pineal - Hypothalamus

• Thyroid Glands- Thyroid

• Parathyroid - Thyroid

• Adrenal glands - Above kidneys

Water-soluble hormones

• Biogenic amines and proteins

• Readily dissolve within aqueous environment of the blood

• Hormones do not generally require transport proteins

• Generally released directly into the blood and transported to target cells

• Bound to transport proteins, function to prolong biological half-life

Lipid-soluble hormones

• Steroids, calcitriol, thyroid hormone

• Non-polar molecules that do not dissolve readily within aqueous environment of the blood

• Require transport proteins, which are water-soluble proteins synthesized by the liver

• Some are selective

• Transport protein is beneficial because it protects the hormone and prevents early destruction

Antidiuretic hormone (ADH)

• Target Organs the kidney and blood vessels

• ADH Role: Antidiuretic hormone tells your kidneys to save water and make less urine.

• Alcohol Effect: Alcohol stops the brain from releasing ADH.

• Kidney Response: Without ADH, your kidneys let more water pass out into your urine.

• Result: You lose more water and feel thirsty or dehydrated.