Endocrine

Endocrine system

The body system that works with the nervous system to maintain homeostasis by releasing hormones that regulate growth, metabolism, reproduction, and stress responses.

Function of the endocrine system in development

Controls the differentiation of the reproductive and central nervous systems in the developing fetus.

Function of the endocrine system in growth

Coordinates overall body growth during childhood and adolescence.

Function of the endocrine system in reproduction

Regulates the activities of the male and female reproductive systems.

Function of the endocrine system in homeostasis

Helps maintain internal stability of the body’s environment.

Adaptive response

Initiates corrective and adaptive hormonal responses to emergencies or stress.

Type of signal in the nervous system

Electrical impulses.

Type of signal in the endocrine system

Chemical signals called hormones.

Speed of nervous system signals

Very fast, usually in milliseconds.

Speed of endocrine system signals

Slower, usually taking minutes to hours.

Duration of nervous system response

Short-term and immediate.

Duration of endocrine system response

Longer-lasting and sustained.

Principal endocrine glands

Pituitary, thyroid, adrenal, hypothalamus, parathyroid, pancreas, ovaries, and testes.

Other hormone-producing tissues

Pineal gland, thymus, kidneys, heart, and digestive viscera.

Hormone

A chemical messenger secreted into the blood that binds to specific target cells to cause a response.

Target cell

A cell that has specific receptors for a particular hormone.

Factors affecting hormone action

Hormone concentration, number of receptors, and receptor-hormone affinity.

Effects of hormones on target cells

Change membrane permeability or potential, stimulate or inhibit enzyme activity, induce secretion, activate protein synthesis, and stimulate mitosis.

Peptide hormone

A hormone made of several amino acids linked together.

Amino acid derivative hormone

A hormone derived from a single modified amino acid (usually tyrosine).

Steroid hormone

A hormone made from cholesterol that is lipid-soluble and acts through gene activation.

Amino acid-based hormones

Water-soluble hormones that act through membrane receptors and second messenger systems.

Steroid hormones mechanism

Bind to cytoplasmic or nuclear receptors to change gene expression and produce new proteins.

Examples of amino acid-based hormones

Growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-stimulating hormone, luteinizing hormone, prolactin, oxytocin, antidiuretic hormone, thyroid hormone, parathyroid hormone, calcitonin, epinephrine, and norepinephrine.

Examples of steroid hormones

Aldosterone, cortisol, testosterone, progesterone, and estrogen.

Negative feedback

The main mechanism that regulates hormone secretion by reducing hormone release once balance is achieved.

Neural stimulus

A nerve impulse that directly triggers a gland to release a hormone.

Humoral stimulus

A change in the blood’s chemical composition that triggers hormone release.

Hormonal stimulus

Hormones from one gland stimulate or inhibit another gland (e.g., hypothalamus → pituitary → target gland).

Pituitary gland location

Lies in the sella turcica of the sphenoid bone and is connected to the hypothalamus by the infundibulum.

Releasing hormone

A hormone that stimulates another gland to secrete its hormone.

Inhibiting hormone

A hormone that suppresses the secretion of another hormone.

Tropic hormone

A hormone that controls the activity or growth of another endocrine gland.

Antidiuretic Hormone (ADH)

Conserves body water by reducing urine output and causes blood vessel constriction at high levels.

Oxytocin

Stimulates uterine contractions during labor and triggers milk ejection during breastfeeding.

Follicle-Stimulating Hormone (FSH)

In females, stimulates follicle and estrogen production; in males, stimulates sperm production.

Luteinizing Hormone (LH)

In females, triggers ovulation and stimulates estrogen and progesterone production; in males, stimulates testosterone secretion.

Adrenocorticotropic Hormone (ACTH)

Maintains adrenal cortex cells and stimulates them to release glucocorticoids (such as cortisol).

Thyroid-Stimulating Hormone (TSH)

Regulates the release of thyroxine (T4) and triiodothyronine (T3) from the thyroid gland.

Prolactin (PRL)

Promotes breast development during pregnancy and stimulates milk production after birth.

Growth Hormone (GH)

Stimulates skeletal and tissue growth, increases protein synthesis, decreases glucose uptake, and promotes fat use for energy.

Thyroxine (T4) and Triiodothyronine (T3)

Increase metabolic rate, oxygen use, Na⁺/K⁺ pump activity, and mitochondria number; essential for normal CNS, skeletal, and reproductive development.

Calcitonin

Lowers blood calcium levels by inhibiting osteoclast activity and stimulating osteoblast activity.

Parathyroid Hormone (PTH)

Raises blood calcium levels by stimulating osteoclasts, enhancing calcium retention in the kidneys, and promoting intestinal calcium absorption.

Aldosterone (Mineralocorticoid)

Regulates electrolytes by controlling sodium and potassium elimination by the kidneys.

Cortisol (Glucocorticoid)

Increases blood glucose as a response to stress by promoting energy release.

Sex Steroids (Gonadocorticoids)

Influence reproductive functions and secondary sex characteristics.

Epinephrine and Norepinephrine (Catecholamines)

Mimic sympathetic stimulation (‘fight or flight’), increasing heart rate, blood flow, and alertness.

Insulin

Lowers blood glucose by increasing glucose uptake by cells and preventing glycogen breakdown.

Glucagon

Raises blood glucose by promoting glycogen breakdown and glucose production in the liver.

Testosterone

Produced by the testes and adrenal cortex; responsible for male reproductive functions and traits.

Estrogen

Produced by the ovaries and adrenal cortex; regulates female reproductive system functions.

Pituitary Gigantism

Excess growth hormone before growth plates close, causing extreme height.

Pituitary Dwarfism

Growth hormone deficiency during development, causing short stature.

Acromegaly

Excess growth hormone in adults causing enlarged hands, face, and tongue.

Hypothyroidism (Myxedema)

Decreased metabolism, low body temperature, and reduced heart rate and blood pressure.

Hyperthyroidism

Increased metabolism, high body temperature, rapid heart rate, and high blood pressure.