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.