Endocrine System Basics

Endocrine System

• The endocrine system is the second major system that exercises control over the body, characterized by rapid responses to various stimuli.
• It is made up of glands that produce and secrete hormones, which regulate the activity of cells or organs, controlling growth, metabolism, and sexual development and function.

Major Endocrine Glands

• The major endocrine glands are arranged in two groups:
  • The first group: anterior pituitary gland, adrenal cortex, thyroid gland, and sex glands (gonads).
  • The second group: posterior pituitary gland, adrenal medulla, parathyroid glands, and pancreas.

Anterior Pituitary Gland

• Controls the other members of the first group by secreting trophic hormones (e.g., ACTH or corticotrophin).
• Under the control of the hypothalamus, which secretes releasing and inhibiting hormones (e.g., corticotrophin-releasing factor).

Hormones

• Chemical messengers created by the body to transfer information from one set of cells to another, coordinating the functions of different parts of the body.
• Regulated by feedback, similar to a thermostat regulating temperature in a room.

Pituitary Gland

• Size of a grape, hanging by a stalk from the hypothalamus.
• Protected by the sphenoid bone, with two functional lobes: anterior (glandular tissue) and posterior (nervous tissue).

Hormones of the Anterior Pituitary

• There are six anterior pituitary hormones, with two affecting non-endocrine targets and four stimulating other endocrine glands (tropic hormones).
• Characteristics of all anterior pituitary hormones: proteins (or peptides), acting through second-messenger systems, and regulated by hormonal stimuli, mostly negative feedback.

Growth Hormone (GH)

• General metabolic hormone, with major effects directed to growth of skeletal muscles and long bones.
• Causes amino acids to be built into proteins and fats to be broken down for energy.

Functions of Other Anterior Pituitary Hormones

• Prolactin (PRL): stimulates and maintains milk production following childbirth; function in males is unknown.
• Adrenocorticotropic hormone (ACTH): regulates endocrine activity of the adrenal cortex.
• Thyroid-stimulating hormone (TSH): influences growth and activity of the thyroid gland.
• Gonadotropic hormones (FSH and LH): regulate hormonal activity of the gonads, stimulating follicle development in ovaries and sperm development in testes.

Pituitary-Hypothalamus Relationship

• Release of hormones is controlled by releasing and inhibiting hormones produced by the hypothalamus.
• Hypothalamus produces two hormones transported to neurosecretory cells of the posterior pituitary.

Hormones of the Posterior Pituitary

• Oxytocin: stimulates contractions of the uterus during labor and causes milk ejection.
• Antidiuretic hormone (ADH): can inhibit urine production and, in large amounts, causes vasoconstriction, leading to increased blood pressure (vasopressin).

The Thyroid Gland

• Located in the lower part of the neck, consisting of two lobes on either side of the trachea, joined together by the isthmus.
• Secrete two types of hormones: thyroid hormones (T3 and T4) and calcitonin (which regulates calcium metabolism).

Parathyroid Gland

• Small, oval-shaped glands located on the posterior surface of the thyroid gland, with two pairs: superior and inferior.
• Release parathyroid hormone (PTH), which raises serum calcium levels.

The Adrenal Glands

• Two small, flattened yellowish bodies situated on top of each kidney, with an outer cortex and an inner medulla.
• The adrenal medulla secretes catecholamines (epinephrine and norepinephrine), which elicit a fight-or-flight response.
• The adrenal cortex secretes over 30 different steroid hormones (corticosteroids), including mineralocorticoids (e.g., aldosterone), glucocorticoids (e.g., cortisol), and gonadocorticoids (e.g., testosterone, estrogen, progesterone).

Pancreas

• Located slightly behind the stomach, secreting insulin (which reduces blood glucose) and glucagon (which increases blood glucose).

Pineal Gland

• A small, reddish-gray structure situated in the midline of the brain, producing melatonin, which establishes the body's day-night cycle.

Gonads

• Testes: produce testosterone, which promotes sex characteristics, muscle development, and maturity.
• Ovaries: produce estrogen, which promotes sex characteristics, maturity, and coordination.

Learn about the endocrine system, its major glands, and how they regulate the body's activities. Understand the role of hormones in growth, metabolism, and development.