Endocrine System Overview

Endocrine System Overview

• The endocrine system is a messenger system comprised of hormones released by internal glands directly into the circulatory system.
• These hormones target and regulate distant organs.
• The hypothalamus is the neural control center for endocrine systems in vertebrates.
• The endocrine and nervous systems work together to coordinate body functions and are primarily responsible for homeostasis.
• The study of the endocrine system and its disorders is called endocrinology.

Endocrine Glands

• The major endocrine glands in humans are the thyroid, parathyroid, pituitary, pineal, adrenal glands, and the testicles (males) and ovaries (females).
• Other organs such as the hypothalamus, pancreas, and thymus also have endocrine functions.
• The hypothalamus and pituitary glands are part of the neuroendocrine system—linking the endocrine system to the nervous system.
• The pituitary gland is about the size of a pea.
• Pituitary hormones include: growth hormone (bones and tissue growth), prolactin (breastfeeding), oxytocin (uterine contractions and milk ejection), and antidiuretic hormone (water retention).
• Growth hormone is vital for skeletal muscle and bone growth impacting final body size.
• Hyposecretion during childhood leads to dwarfism; hypersecretion during childhood results in gigantism.
• The pineal gland produces melatonin regulating circadian rhythm.

Thyroid and Parathyroid Glands

• The thyroid gland is butterfly-shaped, located anterior to the trachea and inferior to the larynx.
• It produces thyroid hormones (T3 and T4), increasing metabolism by controlling glucose oxidation and conversion to energy (ATP).
• Hypothyroidism (low thyroid hormone) causes low metabolic rate, weight gain, lethargy, and cold sensitivity.
• Hyperthyroidism (high thyroid hormone) causes high metabolic rate, weight loss, hyperactivity, and heat intolerance.
• Parafollicular cells of the thyroid gland release calcitonin to decrease blood calcium levels.
• Parathyroid glands are located on the posterior surface of the thyroid gland and produce parathyroid hormone.

Adrenal Glands

• The adrenal glands are located above the kidneys.
• They secrete epinephrine and norepinephrine (fight-or-flight responses).
• Cortisol—the primary stress hormone—is also produced by the adrenal glands.

Pancreas

• The pancreas releases insulin from beta cells to decrease blood glucose levels.
• It releases glucagon from alpha cells to increase blood glucose levels.
• The pancreas has both exocrine and endocrine functions. Acinar cells secrete digestive enzymes, while endocrine function involves islets of Langerhans producing insulin and glucagon for glucose regulation.

Gonads

• The testes secrete testosterone.
• The ovaries secrete estrogen and progesterone.
• These hormones control reproductive processes.

Stress Response

• Stress is any potential or actual threat to homeostasis.
• Stress involves physiological changes triggered by stressors.
• The stress response is a combination of sympathetic nervous system (immediate response) and the hypothalamic-pituitary-adrenal (HPA) axis response (longer-term response), keeping the body's internal environment stable.

Hormones

• Hormones act as mediators that regulate cellular activity throughout the body.
• Hormones reach their target cells via the blood stream.
• Cells with specific receptors for a given hormone are called target cells.

Local Hormones

• Autocrine hormones act on the cells that produced them.
• Paracrine hormones act on neighboring cells.

Circulating Hormones

• Circulating hormones travel through the circulatory system to act on distant target cells.

Neural vs. Endocrine Signaling

• Endocrine signaling uses chemical signals (hormones), can be fast or slow, and has widespread effects.
• Nervous system signaling uses electrical and chemical signals (neurotransmitters), is always fast, and typically produces localized effects.

Hypothalamic-Pituitary-Adrenal (HPA) Axis

• The hypothalamus plays a crucial role in connecting the nervous and endocrine systems.
• Secretion of corticotrophin-releasing hormone (CRH) activates the release of adrenocorticotropic hormone (ACTH) from the pituitary gland.
• ACTH triggers the adrenal gland to produce cortisol, the major stress hormone.
• Cortisol affects nearly every organ system in the body.

Cortisol Effects (short-term)

• Increased heart rate and blood pressure
• Increased memory and attention
• Increased glucose levels
• Increased muscle use
• Increased fat use

Cortisol Effects (long-term)

• Cardiovascular disease, hypertension
• Cognitive impairment, depression
• Diabetes
• Osteoporosis
• Muscle loss
• Cancer
• Obesity

Negative Feedback Control of Cortisol

• High cortisol levels trigger negative feedback mechanisms in the brain to suppress further cortisol production.
• Chronic stress can impair this negative feedback mechanism leading to elevated cortisol levels.

This quiz covers the key aspects of the endocrine system, including its glands, hormones, and functions. Understand how the endocrine system interacts with the nervous system to maintain homeostasis and regulate bodily functions. Test your knowledge on major endocrine glands and their roles in human health.