Endocrine System

Learning Outcomes

  • Identify the structure and function of the endocrine system

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, which interpret these messages and act on them.

It seems like a far fetched idea that a small chemical can enter the bloodstream and cause an action at a distant location in the body. Yet this occurs in our bodies everyday of our lives. The ability to maintain homeostasis and respond to stimuli is largely due to hormones secreted within the body. Without hormones, you could not grow, maintain a constant temperature, produce offspring, or perform the basic actions and functions that are essential for life.

The endocrine system provides an electrochemical connection from the hypothalamus of the brain to all the organs that control the body metabolism, growth and development, and reproduction.

There are two types of hormones secreted in the endocrine system: Steroidal (or lipid based) and non-steroidal, (or protein based) hormones.

The endocrine system regulates its hormones through negative feedback, except in very specific cases like childbirth. Increases in hormone activity decrease the production of that hormone. The immune system and other factors contribute as control factors also, altogether maintaining constant levels of hormones.

Types of Glands

Exocrine Glands are those which release their cellular secretions through a duct which empties to the outside or into the lumen (empty internal space) of an organ. These include certain sweat glands, salivary and pancreatic glands, and mammary glands. They are not considered a part of the endocrine system.

The pineal gland and pituitary gland are located in the brain. The thyroid gland is located in the neck.The thymus is located in the chest. The adrenal gland and the pancreas are located near the kidneys in the abdomen. The ovaries and testes are located in the pelvic cavity.

Figure 1. Major endocrine glands. (Male left, female on the right.) 1. Pineal gland 2. Pituitary gland 3. Thyroid gland 4. Thymus 5. Adrenal gland 6. Pancreas 7. Ovary 8. Testis

Endocrine Glands are those glands which have no duct and release their secretions directly into the intercellular fluid or into the blood. The collection of endocrine glands makes up the endocrine system.

  1. The main endocrine glands are the pituitary (anterior and posterior lobes), thyroid, parathyroid, adrenal (cortex and medulla), pancreas and gonads.
  2. The pituitary gland is attached to the hypothalamus of the lower forebrain.
  3. The thyroid gland consists of two lateral masses, connected by a cross bridge, that are attached to the trachea. They are slightly inferior to the larynx.
  4. The parathyroid glands are four masses of tissue, two embedded posterior in each lateral mass of the thyroid gland.
  5. One adrenal gland is located on top of each kidney. The cortex is the outer layer of the adrenal gland. The medulla is the inner core.
  6. The pancreas is along the lower curvature of the stomach, close to where it meets the first region of the small intestine, the duodenum.
  7. The gonads (ovaries and testes) are found in the pelvic cavity.

Hormones and Types

A hormone is a type of chemical signal. They are a means of communication between cells.

The endocrine system produces hormones that are instrumental in maintaining homeostasis and regulating reproduction and development. A hormone is a chemical messenger produced by a cell that effects specific change in the cellular activity of other cells (target cells). Unlike exocrine glands (which produce substances such as saliva, milk, stomach acid and digestive enzymes), endocrine glands do not secrete substances into ducts (tubes). Instead, endocrine glands secrete their hormones directly into the surrounding extra cellular space. The hormones then diffuse into nearby capillaries and are transported throughout the body in the blood.

The endocrine and nervous systems often work toward the same goal. Both influence other cells with chemicals (hormones and neurotransmitters). However, they attain their goals differently. Neurotransmitters act immediately (within milliseconds) on adjacent muscle, gland, or other nervous cells, and their effect is short-lived. In contrast, hormones take longer to produce their intended effect (seconds to days), may affect any cell, nearby or distant, and produce effects that last as long as they remain in the blood, which could be up to several hours.

Table 1 shows the major hormones, their target and their function once in the target cell.

Table 1. Major Horomes
Endocrine Gland Hormone Released Chemical Class Target Tissue/Organ Major Function of Hormone
Hypothalamus Hypothalamic releasing and inhibiting hormones Peptide Anterior pituitary Regulate anterior pituitary hormone
Posterior Pituitary Antidiuretic (ADH) Peptide Kidneys Stimulates water reabsorption by kidneys
Oxytocin Peptide Uterus, mammary glands Stimulates uterine muscle contractions and release of milk by mammary glands
Anterior Pituitary Thyroid stimulating (TSH) Glycoprotein Thyroid Stimulates thyroid
Adrenocorticotropic (ACTH) Peptide Adrenal cortex Stimulates adrenal cortex
Gonadotropic (FSH, LH) Glycoprotein Gonads Egg and sperm production, sex hormone production
Prolactin (PRL) Protein Mammary glands Milk production
Growth (GH) Protein Soft tissue, bones Cell division, protein synthesis and bone growth
Thyroid Thyroxine (T4) and Triiodothyronie (T3) Iodinated amino acid All tissue Increase metabolic rate, regulates growth and development
Calcitonin Peptide Bones, kidneys and intestine Lowers blood calcium level
Parathyroids Parathyroid (PTH) Peptide Bones, kidneys and intestine Raises blood calcium level
Adrenal Cortex Glucocorticoids (cortisol) Steroid All tissue Raise blood glucose level, stimulates breakdown of protein
Mineralocorticoids (aldosterone) Steroid Kidneys Reabsorb sodium and excrete potassium
Sex Hormones Steroid Gonads, skin, muscles and bones Stimulates reproductive organs and brings on sex characteristics
Adrenal Medulla Epinephrine and norepinephrine Modified amino acid Cardiac and other muscles Released in emergency situations, raises blood glucose level, “fight or flight” response
Pancreas Insulin Protein Liver, muscles, adipose tissue Lowers blood glucose levels, promotes formation of glycogen
Glucagon Protein Liver, muscles, adipose tissue Raises blood glucose levels
Testes Androgens (testosterone) Steroid Gonads, skin, muscles and bone Stimulates male sex characteristics
Ovaries Estrogen and progesterone Steroid Gonads, skin, muscles and bones Stimulates female sex characteristics
Thymus Thymosins Peptide T lymphocytes Stimulates production and maturation of T lymphocytes
Pineal Gland Melatonin Modified amino acid Brain Controls circadian and circannual rhythms, possibly involved in maturation of sexual organs

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