Endocrine glands: Endocrine organs, called glands, secrete hormones into the bloodstream. Hormones affect the activity of target sites that are often located far from the site of release. Exocrine organs direct the function of their target sites by releasing their active.

Human endocrine system: The major endocrine organs include the hypothalamus and the hypophysis, or pituitary gland.   Other endocrine glands within the body include: thyroid, parathyroids, adrenals, pancreas, ovaries, and testes.

• The hypothalamus: The hypothalamus is located in the forebrain, directly above the pituitary gland. The hypothalamus receives input from other parts of the brain and from peripheral nerves. This input affects neurosecretory cells within the hypothalamus.
• The pituitary gland: The anterior pituitary synthesizes its own hormones. Capillaries within the anterior pituitary receive signals from the hypothalamus that tell the anterior pituitary whether or not to release certain hormones.
• The thyroid gland: The thyroid gland is a bilobed structure found at the trachea. It synthesizes and secretes three hormones:
  1. thyroxine (T4),
  2. triiodothyronine (T3), and
  3. calcitonin.

  The parathyroids are four small glands embedded in the thyroid. They produce and secrete parathyroid hormone (PTH).

• The adrenal gland: The adrenal glands are located on top of the kidneys. Each gland is subdivided into an outer adrenal cortex and an inner adrenal medulla.
• The pancreas: The pancreas is both an endocrine organ and an exocrine organ. The exocrine portion of the pancreas secretes digestive enzymes into the pancreatic duct. The endocrine portion of the pancreas secretes hormones, including insulin and glucagon.
• The testes: The testes are responsible for the synthesis and secretion of androgens, such as testosterone. Interstitial cells, located between the seminiferous tubules of the testes, produce androgens.
• The ovaries: The ovaries produce and secrete steroid hormones known as estrogens and progesterone.

The Menstrual Cycle and Pregnancy

From puberty to menopause, hormonal interplay between the hypothalamus, pituitary, and ovaries results in the monthly menstrual cycle, which has four distinct phases: follicular phase and the luteal phase.

Fertilization on or around day 14 of the menstrual cycle is followed by implantation of the zygote and development of the placenta.

Mechanisms of hormone action

Hormones may be grouped into three major categories, depending on their chemical structures:

1. Peptide hormones include simple peptides like ADH,
2. Steroid hormones include fat derived compounds like estrogen and
3. Amino acid-derived hormones include epinephrine and T4.

This tutorial discusses the endocrine system and the glands that produce the major hormones.

Specific Tutorial Features:

• Graphical illustrations of the anatomical location of the hypothalamus and pituitary glands are presented.
  
• The production, release and the regulation of the major hormones is discussed with illustrations.

Series Features:

• Concept map showing inter-connections of new concepts in this tutorial and those previously introduced.
  
• Definition slides introduce terms as they are needed.
  
• Visual representation of concepts
  
• Examples given throughout to illustrate how the concepts apply.
  
• A concise summary is given at the conclusion of the tutorial.

Endocrine glands

Endocrine vs. exocrine organs
Human endocrine system

• The hypothalamus
• The pituitary gland
• The thyroid gland
• The adrenal gland
• The pancreas
• The testes
• The ovaries

The Menstrual Cycle and Pregnancy

Mechanisms of hormone action