Post-Departure Reflections

Anatomy & Physiology – The Endocrine System

The endocrine system is a system of glands that regulate the body and maintain homeostasis. A gland is a cluster of cells or an organ specialized in secreting or excreting various substances including hormones, enzymes, metabolites, and other molecules.

There are two types of glands:

  • Exocrine glands secrete into ducts;
  • Endocrine glands are ductless and secrete into the bloodstream.

The endocrine system produces more than 20 different hormones which function as chemical messengers that transfer information from one set of cells to another. Hormones help regulate the body’s metabolic activity including growth, hunger, and temperature.

Hormones are cell specific. When a hormone is secreted, it travels through the bloodstream until reaching target cells which have receptors that only attach to that particular hormone. Once the hormone locks onto the receptors, the hormone-receptor molecule transmits chemical instructions to the target cell.

Using a feedback system similar to how a thermostat keeps a house at a constant temperature, the endocrine system helps the body keep the right level of hormones in the bloodstream, which can be influenced by factors such as stress, infection, and changes in the balance of fluid and minerals in blood.


The Endocrine Glands





The hypothalamus is the primary link between the endocrine and nervous systems. It produces chemicals that either stimulate or suppress hormone secretions from the pituitary gland.

For the hormones regulated by the pituitary gland, the hypothalamus sends a signal to the pituitary gland in the form of a released hormone , which stimulates the pituitary to secrete a stimulating hormone into the circulation. The stimulating hormone then signals the target gland to secrete its hormone. As the level of this hormone rises in the bloodstream, the hypothalamus and the pituitary gland shut down secretion of the releasing hormone and the stimulating hormone, which in turn slows the secretion of the target gland. This system results in stable blood concentrations of the hormones that are regulated by the pituitary gland.

The hypothalamus also secretes a hormone called somatostatin , which causes the pituitary gland to stop the release of growth hormones.



A small gland no bigger than a pea, the pituitary is located at the base of the brain just beneath the hypothalamus.

It is often called the master gland because it produces hormones that control several other endocrine glands.

Major Pituitary Hormones



Major target organ(s)

Major Physiologic Effects


Growth hormone

Liver, adipose tissue. Promotes growth (indirectly), control of protein, lipid, and carbohydrate metabolism.

Thyroid-stimulating hormone

Thyroid gland. Stimulates secretion of thyroid hormones.

Adrenocorticotropic hormone

Adrenal gland (cortex). Stimulates secretion of glucocorticoids.


Mammary gland. Milk production.

Luteinizing hormone

Ovary and testis. Control of reproductive function.

Follicle-stimulating hormone

Ovary and testis. Control of reproductive function.


Antidiuretic hormone

Kidney. Conservation of body water.


Ovary and testis. Stimulates milk ejection and uterine contractions.

The pituitary is divided into the anterior lobe and the posterior lobe.

The anterior lobe regulates the activity of the thyroid, adrenals, and reproductive glands. It produces growth hormones which stimulates the growth of bone and other body tissues. It also plays a role in the body’s handling of nutrients and minerals.

The posterior lobe of the pituitary releases an antidiuretic hormone, which helps control the balance of water in the body and oxytocin, which triggers the contractions of the uterus when a woman is giving birth.

The pituitary also secretes endorphins, chemicals that act on the nervous system to both reduce feelings of pain and produce feelings of euphoria. In addition, the pituitary controls ovulation and the menstrual cycle in women and secretes hormones that signal the reproductive organs of men and women to make sex hormones.



Located in the neck, the thyroid produces hormones that control the rate that the body metabolizes food to produce energy. The production and release of thyroid hormones is controlled by thyrotropin , secreted by the pituitary gland. The more thyroid hormone there is in a person’s bloodstream, the faster that chemical reactions occur in the body.

Thyroid hormones help bones grow and develop, and play a role in brain and nervous system development in children.



Attached to the thyroid are four small glands known collectively as the parathyroids, which release a hormone that, along with calcitonin produced by the thyroid, regulates the blood’s calcium level.


Adrenal Glands

There are two adrenal glands, one on top of each kidney. The glands have two parts: the outer part is called the adrenal cortex. It produces hormones called corticosteroids that regulate the body’s salt and water balance, response to stress, metabolism, the immune system, and sexual development and function.

The inner part is the adrenal medulla, which produces catecholamines including epinephrine. Also known as adrenaline, epinephrine increases blood pressure and heart rate when the body becomes stressed.



Located in the middle of the brain, the pineal gland secretes melatonin, a hormone that plays a part in regulating your sleep cycle.

The Organization of the Endocrine System


The Reproductive Glands. Ovaries and Testes

The gonads, or sex organs, are the main source of sex hormones. Male gonads, located in the scrotum, secrete hormones called androgens, including testosterone. These hormones control the onset of puberty and sexual maturation. Ovaries are the female gonads. They produce eggs and secrete estrogen and progesterone. Estrogen controls the start of puberty and sexual maturation and, together with progesterone, regulates menstruation.


Pancreas and the Islets of Langerhans

The Islets of Langerhans are hormone producing cells located throughout the pancreas. There are five types of cells in an islet:

  • Alpha cells make glucagon, which raises the blood sugar level.
  • Beta cells make insulin. Along with glucagon, insulin regulates blood sugar.
  • Delta cells make somatostatin, which inhibits the release of numerous other hormones in the body.
  • PP cells contain polypeptide.
  • There are also D1 cells, but little is known about their function.


Hormonal Disorders

Type 1 Diabetes. This occurs when the pancreas fails to produce enough insulin. Symptoms include excessive thirst, hunger, frequent urination, and weight loss. In children and teens, the condition is typically an autoimmune disorder where antibodies produced by the immune system attack and destroy the beta cells that produce insulin. Long term complications from diabetes can include kidney problems, nerve damage, blindness, early onset of coronary disease, and stroke. Requires daily insulin shots.

Type 2 Diabetes. This occurs when the body is unable to respond to insulin normally. It is believed that excess body fat plays a role in the insulin resistance. Symptoms and possible complications of type 2 diabetes are essentially the same as those of type 1. The difference is that it may be controlled through diet and exercise or with medication, although some people will still need insulin injections.



The endocrine system regulates many metabolic processes, working in concert with the nervous system to keep the body working efficiently. The next lesson focuses on the circulatory system, which is responsible for supplying the body with nourishment and oxygen.

Supplemental Material

Endocrine System – Glands & Hormones

The Endocrine System

Endocrinology – Overview

Hormones & the endocrine system

Course Discussion