Biology Lecture 4: The Endocrine System

Study Notes

The endocrine system is a control system of ductless glands that secrete hormones within specific organs.
Hormones act as "messengers" and are carried by the bloodstream to different cells in the body.

Exocrine glands release their cellular secretions through a duct, including sweat glands, salivary and pancreatic glands, and mammary glands.
Endocrine glands have no duct and release their secretions directly into the intercellular fluid or into the blood.

The main endocrine glands are: pituitary, thyroid, parathyroid, adrenal, pancreas, and gonads.
Pituitary gland is attached to the hypothalamus of the lower forebrain.
Thyroid gland consists of two lateral masses connected by a cross bridge, attached to the trachea.
Parathyroid glands are four masses of tissue embedded in the thyroid gland.
Adrenal glands are located on top of each kidney, with the cortex being the outer layer and the medulla being the inner core.
Pancreas is located along the lower curvature of the stomach, near the duodenum.
Gonads (ovaries and testes) are found in the pelvic cavity.

Hormones are chemical messengers produced by cells that effect specific changes in target cells.
Hormones can be chemically classified into four groups: amino acid-derived, polypeptide and proteins, steroids, and eicosanoids.
Hormones activate target cells by one of two methods, depending on their chemical nature: lipid-soluble hormones or water-soluble hormones.

Lipid-soluble hormones (steroid hormones and thyroid hormones) diffuse through cell membranes and bind to a receptor protein, activating specific genes.
Water-soluble hormones (polypeptide, protein, and amino acid hormones) bind to a receptor protein on the plasma membrane, stimulating the production of second messengers such as cyclic AMP (cAMP).