Endocrine System


The Endocrine System


Click here to view an animation of the endocrine system


The endocrine system is a set of hormone secreting glands within the body of an animal. The function of the endocrine system is homeostasis, communication and response to stimuli. The endocrine system regulates the internal environment of the animal for growth, survival and reproduction as well as allowing it to respond to changes in its external environment.

The endocrine system’s glands secrete chemical messages we call hormones. These signals are passed through the blood to arrive at a target organ, which has cells possessing the appropriate receptor. Exocrine glands (not part of the endocrine system) secrete products that are passed outside the body. Sweat glands, salivary glands, and digestive glands are examples of exocrine glands.

The other communication method in the body is the nervous system. Although there are differences between them, they complement each other in many responses, e.g., response to danger.

The difference between nervous and endocrine control are as follows:

1. Nervous response is faster.

2. Nervous response is shorter in duration.

3. Nervous response stops quicker.

  1. Nervous response is much more local.
  2. Nerve ‘messages’ are conducted electrically; endocrine ‘messages’ are carried chemically.


Most hormones are made of protein. They are called peptides. Peptides are short chains of amino acids; most hormones are peptides. They are secreted by the pituitary, parathyroid, heart, stomach, liver, and kidneys.

Some hormones are steroid based. Steroids are lipids derived from cholesterol. Testosterone is the male sex hormone. Estradiol, similar in structure to testosterone, is responsible for many female sex characteristics. Steroid hormones are secreted by the gonads, adrenal cortex, and placenta.

Hormones are usually slow to act but, once they act, they remain active for long periods of time and, also, their effects remain for a long time.

Endocrine Glands

There are 10 endocrine glands. As stated previously, other organs such as the stomach, intestines, kidneys, heart, brain, and placenta also make hormones.

Click here to take an online quiz on the location of the endocrine glands

The Pituitary Gland

The pituitary gland is often called the master gland. That is because the pituitary gland produces hormones that regulate other endocrine glands. Some hormones produced by the pituitary gland are:

1.                Follicle Stimulating Hormone (FSH): Will be discussed in a later Chapter of the syllabus.

2.                Luteinising Hormone (LH): Will be discussed in a later Chapter of the syllabus.

3.                Growth Hormone (GH): Causes body cells to absorb amino acids and form protein for growth. The main function is to cause the elongation of bones.

4.                Prolactin: stimulates milk formation by the breast after the birth of the baby.

5.                Oxytocin: stimulates muscle contraction of uterus during birth, stimulates muscle contraction in the milk ducts during breast-feeding.

6.                Antidiuretic Hormone (ADH): causes increased water reabsorption by kidneys.

7.                Thyroid Stimulating Hormone (TSH): Combines with iodine at the thyroid gland to produce thyroxine.

Overproduction of GH causes gigantism and underproduction causes dwarfism.

The Hypothalamus


The hypothalamus links the nervous system with the endocrine system. It produces hormones that control the pituitary gland’s responses to messages from the brain and other hormones. Some these hormones, called releasing hormones, stimulate the pituitary gland to make other hormones. Others, called release inhibiting hormones, prevent the production of pituitary hormones.

An example is growth hormone releasing factor. This causes the production of growth hormone (GH) by the pituitary gland.

The Pineal Gland

This gland is in the brain. One hormone produced there is melatonin. Synthesis and release of melatonin is stimulated by darkness and inhibited by light. But even without visual cues, the level of melatonin in the blood rises and falls on a daily (circadian) cycle with peak levels occurring in the wee hours of the morning. Melatonin is readily available in drug stores and health food stores, and it has become quite popular. Ingesting even modest doses of melatonin raises the melatonin level in the blood to as much as 100 times greater than normal. These levels appear to promote going to sleep and thus help, insomnia to hasten recovery from jet lag, and to not to have dangerous side effects.

The Thyroid Gland

          The thyroid gland produces the hormone called thyroxin. Thyroxin controls the rate of all the body’s internal reactions. In other words, thyroxin controls the rate of the body’s metabolism.

Physical conditions related to abnormal thyroid function are:

Hypothyroidism- Under Production of Thyroxine

1.                CretinismUnder production of thyroxin in young children. This results in low metabolic rates and results in retarded physical and mental development.

2.                Myxoedema- Under production of thyroxin in adults. Characteristics are tiredness, lack of energy, slow mental and physical activity, and weight gain.

3.                Goitre- Swelling of the thyroid caused by myxoedema.



In cases of low production of thyroxine tablets are available to increase the thyroxine in the body. Since thyroxine needs iodine to be produced iodine is also administered to boost thyroxine levels.

Thyroxine Excess (Hyperthyroidism)

Thyroxine secretion is above normal. This causes a raised level of metabolism. Symptoms of over production of thyroxin are bulging eyes, weight loss heat production, nervousness, irritability, and anxiety. This condition is called Grave’s Disease. Corrective measures for Grave’s Disease are:

1.    Drugs to suppress thyroid activity

2.    Surgically remove part of the gland

3.    Use radioactive iodine to destroy some of the gland.

The Parathyroids

parathyroids behind thyroid gland

There are 4 parathyroid glands. They are located within the thyroid gland. The hormone they produce is called parathormone. This hormone stimulates the release of calcium from the bones. That is why we must continue to include calcium in our diet even when our bones are fully grown.


The Thymus Gland

          This gland is located behind the breastbone. It produces the hormone thymosin. This hormone causes white blood cells (lymphocytes) to become mature and active. These blood cells, as previously discussed in the Blood web page, are involved in the body’s immune system.

The Adrenal Glands

Click here to view an animation of the adrenal glands


Diagram showing the location of the adrenal glands

The adrenal glands are located on top of each kidney. They secrete the hormone called adrenaline (also called epinephrine). This hormone prepares the body for stress and is released when we are frightened or feel stress. It does the following:

1.                Increases blood flow to the heart, muscles, and brain.

2.                Reduces blood flow to the kidneys. This helps reduce blood loss if we are cut. It causes us to get pale.

3.                Opens the bronchioles allowing us to get more air.

4.                Increases glucose levels in the blood.

5.                Increases heartbeat rate.

6.                Increase muscular contraction and strength.

7.                Increases mental alertness.



As discussed in the Human Nutrition web page the pancreas secretes pancreatic juice for the digestive system.

In addition, the pancreas produces the hormone called insulin. This hormone is produced in groups of cells called Islets of Langerhans.  Insulin is needed because it reduces blood glucose levels in the blood. It causes cells, especially fat and muscle cells, to absorb glucose from the blood. The glucose is needed for cellular respiration or converted into glycogen. The glycogen is stored in the liver or the muscles for future use in cellular respiration.

Diabetes is a serious condition that results from 1 of 2 causes. In type 1 diabetes, the pancreas no longer makes insulin and therefore blood glucose cannot enter the cells to be used for energy. In type 2 diabetes, either the pancreas does not make enough insulin or the body is unable to use insulin correctly. Symptoms of diabetes are high glucose levels in the blood and urine, the production of large amounts of urine, severe thirst, loss of weight, and tiredness.

Injections of insulin, which are taken daily, the control of carbohydrate intake, exercise, and weight control treat diabetes.


Anabolic Steroids

Anabolic steroids are hormone supplements that habe been used. They build up muscle, speed up recovery of muscle from injury, and help strengthen bones. There are many serious side effects such as liver and adrenal gland failure, infertility, impotence, and the development of male characteristics in females that can result if they are misused. They are also, sometimes given to animals to promote increased lean muscle (meat) production. This practice is banned in the EU.

Control of Thyroxine Level

          Control of thyroxine level as well as many other hormones is done by negative feedback. If the thyroxine level is normal the pituitary gland is inhibited from releasing thyroid stimulating hormone (TSH). As a result, no further thyroxine is produced. When thyroxine levels are low the pituitary gland produces TSH. This causes more thyroxine to be produced by the thyroid gland.

An Example of negative feedback in the role of the thyroid in maintaining body temperature at 37°C.:

  1. The hypothalamus of the brain detects a drop in blood temperature.
  2. The hypothalamus stimulates the pituitary to secrete TSH (thyroid-stimulating hormone).
  3. This hormone stimulates the thyroid to increase its secretion of thyroxine.
  4. The higher concentration of thyroxine increases metabolism and heat production increases.
  5. The blood is warmed back to normal temperature.


  1. Hypothalamus detecting raised blood temperature and reduces its stimulation of the pituitary.
  2. High thyroxine levels inhibiting the release of TSH from the pituitary.
  3. The increased level of thyroxine leads to the limitation or reduction of its secretion.
  4. Body’s metabolism slows down as a result of less thyroxin. The body’s temperature goes down.


Review Chart  of Major Hormonal Glands

Where the Hormone is Produced Hormone(s) Secreted Hormone Function
Adrenal Glands Adrenalin Causes Emergency Responses (fight/flight)
Pituitary Gland Growth hormone Affects growth and development; stimulates protein production
Pancreas Insulin Lowers blood sugar levels; stimulates metabolism of glucose, protein, and fat


Growth Hormone Releasing Factor Causes growth hormone to be made
Pineal Gland Melatonin Controls body rhythms
Parathyroid Glands Parathyroid hormone (Parathormone) Affects bone formation and excretion of calcium and phosphorus
Thyroid Thyroxine Controls Metabolism
Thymus Thymosin Matures white blood cells


Eye Dissection


Cow Eye Dissection

How do we see? The eye processes the light through photoreceptors located in the eye that send signals to the brain and tells us what we are seeing. There are two types of photoreceptors, rods and cones. These photoreceptors are sensitive to the light. Rods are the most sensitive to light and therefore provide gray vision at night. Cones are mainly active in bright light and enable you to see color. There are 100 million rods compared to the 3 million cones located in your retina. The photoreceptors help you adjust to night and day. For example, if you walk inside from the sun, you can not initially see anything. This is due to the activity of the cones and the lack of activity of the rods. The rods become activated and adapted to the dim light, resulting in gray images formed in the dark. The same thing happens when you leave a dark movie theatre during the day. The rods are mainly activated and the cones have to adjust to sunlight when you leave the theatre.

By dissecting the eye of a cow, which is similar to the eyes of all mammals including humans, you will gain an understanding of the structure and function of the parts of the eye.

Cow eye, dissecting pan, dissecting kit, safety glasses, lab apron, and gloves

Procedure (External Structure):

  1. Obtain a cow eye, place it in your dissecting pan, & rinse the eye with water.
  2. Rotate the eye until the larger bulge or tear gland is on the top of the eye. The eye is now in the position it would be in a body as you face the body.
  3. On the outside of the eye, locate the following parts:
  • fat– surrounds the eye & cushions it from shock
  • tear or lacrimal gland – forms a bulge on the top outer area of the eye & produces tears to wash the surface of the eye
  • tear ducts – tubes to carry the tears from the gland to the eye
  • optic nerve – a white cord on the back of the eye about 3mm thick just toward the nasal side; carries messages between the eye & brain
  • muscles – reddish, flat muscles found around the eye to raise, lower, & turn (right & left) the eye
  1. Turn the eye so that it is facing you & examine these structures on the front surface of the eye:
  • eyelids – two moveable covers that protect the eye from dust, bright light, and impact
  • sclera – this is the tough, white outer coat of the eye that extends completely around the back & sides of the eye
  • cornea – a clear covering over the front of the eye that allows light to come into the eye (preservative often makes this appear cloudy)
  • iris – round black tissue through the cornea that controls the amount of light that enters the inner part of the eye (may be colored in humans)
  • pupil – the round opening in the center of the eye that allows light to enter and whose size is controlled by the iris

Click here for labeled eye model

Procedure (Internal Structure):

  1. Place the eye in the dissecting pan so it is again facing you. Using your scalpel, pierce the white part of the eye or sclera just behind the edge of the cornea. Make a hole large enough for your scissors.
  2. Using your scissors, carefully cut around the eye using the edge of the cornea as a guide. Lift the eye & turn it as needed to make the cut and be careful not to squeeze the liquid out of the eye.
  3. After completing the cut, carefully remove the front of the eye and lay it in your dissecting pan.
  4. Place the back part of the eye in the pan with the inner part facing upward.
  5. Locate the following internal structures of the eye:
  • cornea – observe the tough tissue of the removed cornea; cut across the cornea with your scalpel to note its thickness
  • aqueous humor – fluid in front the eye that runs out when the eye is cut
  • iris – black tissue of the eye that contains curved muscle fibers
  • ciliary body – located on the back of the iris that has muscle fibers to change the shape of the lens
  • lens – can be seen through the pupil; use your scalpel & dissecting needle to carefully lift & work around the edges of the lens to remove it
  • vitreous humor – fluid inside the back cavity of the eye behind the lens
  • retina – tissue in the back of the eye where light is focused; connects to the optic nerve; use forceps to separate the retina from the back of the eye & see the dark layer below it

10. Answer the worksheet questions on the cow eye dissection.

Click here for eye dissection questions

  1. Dispose of the eye as your teacher advises and rinse and return all equipment to the supply cart. Wash your hands thoroughly.

Introduction to Animals Study Guide


Introduction to Animals Study Guide

How are most animals classified?
What are the main characteristics of chordates?
How are vertebrates classified?
What are heterotrophs & give some examples.
In what ways do animals differ from plants?
What are tissues?
What determines an animal’s body plan?
In what habitat do you find most species of animals?
What is bilateral symmetry?
What does bipedal mean?
Where are the dorsal & ventral surfaces on a bipedal organism?
What is radial symmetry?
Name invertebrates that are asymmetrical, radial symmetry, & bilateral symmetry.
What does cephalization mean?
What invertebrate group was first to show cephalization?
Describe the “surfaces” of animals with radial symmetry.
Why is cephalization an advantage for animals?
What is a postanal tail & give examples of adult chordates with this characteristic?
Describe the “skeletal” support found in roundworms.
What is segmentation, & what animals exhibit this characteristic?
What is the function of kidneys, and what organisms have these organs?
How do closed & open circulatory systems differ?
How are terrestrial animals protected against water loss?
What structures show segmentation in vertebrates?
What is the advantage of having a long intestinal tract?
How are nutrients moved through a cnidarian’s body?
Describe how spiral cleavage occurs.
describe the embryo at the start of gastrulation.
What forms from endoderm in cnidarians.