Human Endocrine System: Organs and Roles

• The nervous and endocrine systems are the body's communication networks
• The endocrine system's effects are slower and longer-lasting compared to the nervous system

Human Endocrine Organs

• Endocrine glands consist of cells that produce and release hormones into the bloodstream
• Hormones are biochemicals that travel in the bloodstream, altering the metabolism of one or more cells
• Key endocrine glands include the hypothalamus, pituitary gland, pineal gland, thyroid gland, parathyroid gland, adrenal glands, pancreas, ovaries (female), and testes (male)

Human Endocrine System

• Main tissue types epithelial, connective, and nervous
• Epithelial tissue forms the bulk of most glands and secretes various hormones
• Connective tissue circulates hormones throughout the body in the blood
• Nervous tissue in the brain secretes some hormones and controls the release of others; certain neurons also secrete hormones

Roles of the Human Endocrine System

• Hormones circulate throughout the entire body but only cause a response in particular target cells
• Target cells are those that express the corresponding hormone receptors

Types of Hormones based on Solubility

• Water-soluble hormones bind to receptor proteins on the outside surface of the target cell and activate other proteins inside the cell, causing a quick response
• Lipid-soluble hormones diffuse across the target cell membrane and bind to receptor proteins inside, altering gene expression and causing a slower response
• Water-soluble hormones use receptors on the cell surface and trigger second messengers
• They do not change gene expression and have a relatively quick response time
• Lipid-soluble hormones use receptors inside the cell and do change gene expression
• They have a longer lasting response time

Hypothalamus and Pituitary Gland

• The hypothalamus and pituitary gland oversee endocrine control and work together to secrete hormones that coordinate the actions of other endocrine glands
• The hypothalamus regulates hormones to maintain homeostasis
• It receives sensory input from the nervous system and secretes hormones in response
• Specialized neurons extend from the hypothalamus to secrete hormones into the pituitary gland

Hypothalamus

• It adjusts hormone production based on negative feedback systems
• These systems respond to current hormone levels to keep hormone levels steady in the blood

Posterior Pituitary Gland

• It releases two hormones produced in the hypothalamus: antidiuretic hormone (ADH, aka vasopressin) and oxytocin
• ADH stimulates cells in the kidneys to return water to the bloodstream
• Oxytocin stimulates the ejection of milk from mammary glands and induces contractions of the uterus during childbirth

Anterior Pituitary Gland

• The hypothalamus regulates the anterior pituitary gland's activity, which produces and secretes six hormones
• Growth hormone (GH) promotes growth and development of all tissues
• Prolactin stimulates milk production at mammary glands
• Thyroid-stimulating hormone (TSH) increases metabolic hormone production at the thyroid gland
• Adrenocorticotropic hormone (ACTH) stimulates the secretion of metabolic hormones from the outer portion of the adrenal glands
• Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) stimulate the testes and ovaries to release sex hormones
• Endorphins are natural pain relievers that bind to the receptors of the target cells in the brain

Metabolism

• Influenced by hormones secreted by: the thyroid gland, parathyroid gland, adrenal glands, pancreas, and pineal gland

Thyroid Gland

• Thyroxine and triiodothyronine are two thyroid hormones that increase metabolism in target cells of all tissue types
• Calcitonin decreases blood calcium levels by increasing calcium deposition in bone cells

Parathyroid Gland

• Parathyroid glands are typically four small cell groups embedded in the back of the thyroid gland
• Parathyroid hormone (PTH) increases calcium levels in blood and tissue fluid, opposing calcitonin

Adrenal Gland

• Adrenal glands are about the size of walnuts and are located atop of the kidneys
• Glands produce different hormones, playing a role in long and short term stress response
• The adrenal medulla produces epinephrine and norepinephrine, in response to exercise, trauma, fear, and excitement during the "fight or flight" response
• The adrenal cortex, regulated by ACTH from the anterior pituitary, is its outer portion
• The adrenal medulla is the inner portion that releases hormones stimulated by the sympathetic nervous system
• Adrenal cortex produces glucocorticoids that help mobilize energy, elevate blood pressure, and reduce inflammation
• Chronic stress can make responses unhealthy

Pancreas

• It is an elongated gland about the size of a hand, attached to the small intestine
• Clusters of cells in the pancreas secrete insulin and glucagon to regulate glucose usage
• Insulin lowers blood glucose by absorbing glucose from the bloodstream
• Glucagon raises blood glucose by releasing stored glucose back into the bloodstream

Diabetes

• It is caused by too much glucose in the blood, while the body's cells are starved of glucose
• Type I (insulin-dependent) usually occurs during childhood
• Type II (non-insulin dependent) usually occurs during adulthood
• Beta cells in pancreas don't produce any insulin
• Insulin is not produced by beta cells in the pancreas therefore glucose is not removed from bloodstream causing diabetes
• Type II diabetes
• Prolonged overproduction of insulin leads to desensitization of the insulin receptors, therefore glucose is not removed from the bloodstream, causing diabetes
• In type II diabetes a person's cells become insensitive to insulin released by the pancreas. This causes a blood sugar rise because cells fail to absorb sugar from bloodstream

Pineal Gland

• Produces melatonin, a hormone that regulates sleep-wake cycles
• Darkness initiates melatonin synthesis, while light inhibits it
• Higher melatonin levels signal the body towards sleep

Ovaries and Testes

• Ovaries produce: Progesterone and estrogen
• Testes produce: Testosterone
• Progesterone regulates menstrual cycle
• Estrogen regulates menstural cycle, maintains secondary sex characteristics in females
• Testosterone promotes sperm development and sex characteristics in males