The Endocrine System (Neil Barot, Jonathan Luu, Niba Nirmal)
Chapter 45 - Unit 7 (Animal Form and Function)


In our bodies, along with many other organisms, certain changes occur: usually with the body. These changes in body forms are usually caused by one of the many biological processes controlled by hormones. What's a hormone, you may ask? A hormone is a molecule that is secreted into the extracellular fluid, circulates in the blood or hemolymph, and communicates regulatory messages throughout the body. To give a simpler image, a hormone is just something that travels through your blood and sends messages to the rest of your body. You may think then: OH this is the CIRCULATORY System.... NO YOU ARE WRONG. Although the circulatory system allows a hormone to reach all cells of the body, only its target cells have the receptors that enable a response. A hormone asks for a SPECIFIC response; For example, a change in metabolism. This change must be done by the target cells that have the receptor ONLY, not the ones that do not have this particular receptor. This chemical signaling by hormones is the function of the Endocrine System.

I. Section 45.1

Hormones and other signaling molecules bind to target receptors, triggering specific response pathways.

Hormones are a very big part of the Endocrine system, and in this section I shall be focusing on them. As previously stated: A hormone is a molecule that is secreted into the extracellular fluid, circulates in the blood or hemolymph, and communicates regulatory messages throughout the body. There are multiple types of signaling molecules other than hormones. These also bind to specific receptors to get their target cells to do things. Other cells are unresponsive to this molecule, just like some cells are unresponsive to hormones. Let's start with Hormones:

A. Hormones

- Reach the target cell via the blood stream.
- Some are found in other organ systems (e.g. digestive - stomach) also sending messages
- Some are grouped into ductless organs called Endocrine Glands
- Endocrine Glands secret hormones directily into the surrounding fluid
- Their opposites are the exocrine glands (e.g. Salivary glands) which have ducts that carry substances into the body.
- Hormones mantian homeostasis
- mediate responses to encironmental stimuli
- regulate growth
- development
- reproduction
- For example they:
- coordinate the body's response to stress
- coordinate the body's response to dehydration
- coordinate the body's response to low blood glucose
- control the appearance of characteristics
- distinguish juveniles from adults

B. Local Regulators

Another type is Local Regulators which many types of cells produce. These local regulators are secreted molecules that act over short distances and reach their target cells by diffusion.
- play roles in many other processes
- including blood pressure
- regulation
- nervous system function
- reproduction
- function in paracrine and autocrine signaling
- Paracrine Signaling
- target cells lie near the secreting cell
- Autocrine Signaling
- secreted molecules act on itself instead of other cells
- Some secreted molecules have bot paracrine and autocrine activity (itself and other cells)
Technically, Local Regulators fit inside of the definition of hormones but we split them up into different categories.

C. Neurotransmitters and Neurohormones

Some secreted molecules also have the CRITICAL role of transmitting information by neurons,
Neurosecretory cells - specialized cells typically found in the brain that secrete molecules that diffuse from never cell endings into the blood stream. These cells are in the class of hormones that care called neurohormones (e.g. ADH - hormone critical to kidney function and water balance).

D. Pheromones

These are hormones that act outside of the body
Pheromones are chemicals released into the external environment.
These serve many functions
- marking trails leading to food
- defining territories
- warning predators
- attracting potential mates

E. Chemical Classes of Hormones

Hormones are divided into three groups:
- polypeptides
- amines
- steroids
For example:

As the picture above indicates, hormones vary in their solubility in aqueous and lipid-rich environments.
Polypetides : water soluable
Most Amine Hormones: water soluable
Lipids: insoluble
- Can't pass through the plasma membranes of cells.
Steroid hormes : isoluable
- Can't pass through the plasma membranes of cells.

F. Cellular Response Pathways

One of the several differences between the response pathways for water-soluable hormones and lipid-soluable hormones is the receptor location.

G. Water Soluble Hormones

- secreted by exocytosis
- travel freely in the bloodstream
- bind to cell-surface signal receptors
- binding induces changes in cytoplasmic molecules
- binding also sometimes alters gene transcription (synthesis of messenger RNA molecules)

H. Lipid-Soluble Hormones

- Diffuse out across the membranes of endocrine cells
- Travel in the bloodstream bound to tranport proteins
- Once they diffuse to target cells, they bind to intracellular signal receptors
- This triggers changes in gene transcription (Just like in Water Soluble Hormones)

I. Water Soluble Hormones

The binding of water-soluble hormones to a signal receptor protin triggers events at the plasma membrane that result in a cellular response.
- response can vary
- can be activation of enzyme
- can be change in the uptake of specific molecules
- can be change of secretion of specific molecules
- can be rearrangement of the cytoskeleton
- can cause proteins in cytoplasm to move into the nucleus
- this can then alter transcription of SPECIFIC genes.

All these changes are called: Signal Transduction
The series of changes in cellular proteins that converts extracellular chemical signals to a specific intracellular response.
- These typically involve multiple steps
- Also typically include very specific molecular interactions.

An example of how signal transuction contributes to hotrmones is when you find yourself in a stressful situation. For example, you are running to catch your bus and since you are stressing, your adrenal glands secret epinephrine. This epinephrine then binds to a G protein-coupled receptor in the plasma membrane which triggers a "cascade of events" involving synthesis of cyclic AMP as a second messenger.
After this "cascade" the net result is that the liver releases glucose into the bloodstream, providing the fuel you need to chase the bus (LOL).

J. Lipid Soluble Hormones

Intracellular receptors usually perform the entire task of transducing a signal within a the target cell which directly triggers the cell's response. Almost ALWAYS a response to a lipid-soluble hormone is the change in gene expression.
Steroid hormones receptors are located in the cytosol prior to binding to a hormone. When it binds, a hormone-receptor complex forms which then moves into the nucleus. Then it interacts with DNA/DNA-binding protein stimulating transcription of specific genes. Most Lipid soluble hormones that are NOT steroid molecules are typically located in the nucleus. (An example is Vitamin D). Recent experiments indicate that lipid soluble hormones can sometimes trigger responses at the cell surface without first entering the nucleus.

K. Multiple Effects of Hormones

Many hormones have multiple responses in the body. This response is based on the hormone itself and also the target cell. Going back to Epinephrine, it both creaks down glycogen in the liver and increases blood flow to the major skeletal muscles while decreasing in other places. This hormone is able to do this all at the same time. These varied effects enhance the rapid reactions of the body in emergencies! In some cases, a hormone has different effects in different species as well! For example, thyroxine which is produced by the thyroid gland which regulates the metabolism in frogs humans and many other vertebrates but in frogs it also stimulates resoption of the tadpole's tail in its metamorphosis into an adult.

L. Signaling by Local Regulators

Going back to what I said earlier in case you forgot (lol), local regulators are secreted molecules that link neighboring cells or provide feedback to the secreting cell. Then they act on their target cells getting responses faster then hormones. Several types of chemical compounds function as local regulators.
Poly peptide local regulators include
- cytokines
------ play a role in immune responses
- growth factors
------ stimulate cell proliferation and differentiation
------ many cells are dependent on this to:
----------- grow
----------- divide
----------- develop normally
- nitric oxide
----- Also know as NO (chemical formula)
----- consists of nitrogen double-bonded to oxygen
----- serves in the body as neurotransmitter
----- serves in the body as a local regulator
----- activates enzyme that relaxes smooth muscle cells improving blood flow to tissues
----- enables sexual function in males by increasing blood flow....
- porstagladins
----- modified fatty acids
----- first discovered in the prostate gland secretions
----- contribute to semen
----- contribute to fever in the immune system
----- contribute to inflammation in the immune system
----- help maintain a protective lining in the stomach
---------- this is the reason why too much asprin is bad
---------- asprins effects are due to the inhibition of prostaglandin synthesis

Section 45.2

Negative feedback and antagonistic hormone pairs are common features of the Endocrine System

Previously, it was discussed how hormones and other signaling molecules had worked, and what their position and roles were in a cell. This section concentrates on how regulatory pathways which control hormone secretion are organized.

Major Endocrine glands in Humans

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A. Simple Hormone Pathways

When there is an internal or environmental (outside) stimulus, endocrine cells create and send out particular hormones, depending on the stimulus. This hormone then travels through the bloodstream to reach its target cells, where it interacts with specific receptors. When this occurs, signal transduction occurs, which brings about a physiological response. Lastly, the response leads to a reduction in the stimulus, and the pathway shuts off.

An example of this would be how secretin, a hormone, works and creates pancreatic juice


In this example, HCL, or Hydrochloric acid, is released into the duodenum in the small intestine (stimulus). Because the small intestine has a low pH, it stimulates certain endocrine cells, called S cells, to release secretin, the hormone involved in the creation of pancreatic juice. Secretin enters the bloodstream and reaches the target cells in the pancreas, which is a gland located behind the stomach. This then causes them to release bicarbonate, which raises the pH in the duodenum. This then stimulates formation of pancreatic juice.

A feedback loop connecting the response to the initial stimulus is a characteristic of control pathways. For many hormones, the response pathway involves negative feedback, which is when the response reduces the initial stimulus, rather than enhancing it. By stopping the decreasing the hormone signaling, negative feedback prevents excessive activity. Negative feedback is quite important in hormone pathways, especially those involved with maintaining homeostasis.

B. Insulin and Glucagon: Control of Blood Glucose

In humans, it is important to control your blood glucose, because metabolic balance depends on a blood glucose concentration at about 90mg. Glucose is necessary for cellular respiration, and it is also important for carbon skeletons and biosynthesis.

Insulin and glacugon, which are two hormones, regulate the concentration of glucose in the blood. Both hormones operate in a simple endocrine pathway regulated by negative feedback, which was discussed above. When blood glucose rises above the set point, insulin is created, which triggers the uptake of glucose from the blood, decreasing the blood glucose concentration. However, glucagon does the opposite. When blood glucose drops below the set point, glucagon is released which promotes the release of glucose into the blood, which increases the blood glucose concentration. Because they have opposite effects, these two hormones control the concentration of glucose in the blood quite effectively.

Both insulin and glacugon are created in the pancreas. Throughout the pancreas are clusters of endocrine cells called the islets of Langerhans .
Islets - a portion of tissue structurally distinct from surrounding tissues.
Islets have alpha cells and beta cells
Alpha cells = Glucagon
Beta cells = Insulin


Overall, only 1-2% of the pancreas is made up of hormones, while other cells in the pancreas produce things such as bicarbonate ions and other digestive enzymes.

C. Target tissues for Insulin and Glucagon

Insulin can lower blood glucose levels by making body cells take up glucose from the bloodstream, and it can also lower blood glucose levels by slowing glycogen breakdown in the liver and stopping the conversion of amino acids and glycerol to glucose.
Glucagon influences blood glucose levels through its effects on target cells in the liver. This is because the liver, as well as skeletal muscles and adipose tissues, store a large amount of energy.
- Liver and muscles store energy (sugar)
- Adipose tissue converts sugars to fats

D. Diabetes Mellitus

Diabetes Mellitus is caused by a deficiency of insulin or a slow response of insulin to target tissues. It is a disruption in glucose homeostasis, and can affect many things such as the heart, eyes, kidneys, and blood vessels. Because blood glucose rises, and there is no insulin for cells to take in the necessary amount of glucose, metabolic needs cannot be met. Because of this, fat begins being used in the process of cellular respiration. In even more severe cases, acid that is formed by the breakdown of fat can enter the bloodstream, lowering the pH of the blood, which can be a life-threatening situation.
People with diabetes have a high level of glucose in their blood, and their kidneys are unable to absorb all of the nutrients. Because of this, the glucose that remains is released through urine, which may be a test of diabetes.
There are two types of diabetes

1. Type 1 diabetes - It is also called insulin-dependent diabetes. It is a disorder in where the immune system destroys the beta cells created by the pancreas. This type of diabetes usually appears during childbirth, and this destroys the person's ability to create insulin. Treatment for this type of diabetes is injecting insulin into the body several times a day. Insulin is usually obtained through genetically engineered bacteria.
2. Type 2 diabetes - It is non-insulin dependent diabetes. It is shown by the failure of target cells to respond to insulin. Even though insulin is produced, the target cells are unable to take in the glucose from the blood, and the blood glucose level remains quite high. It can be gained through heredity, but excess weight and lack of exercise can also contribute and increase the risk. Most people (about 90%) with diabetes have type two diabetes

Section 45.3

The Endocrine and Nervous Systems Act Individually and Together in regulating animal physiology

A. Coordination of Endocrine and Nervous Systems in Invertebrates

In all animals but the simplest invertebrates, the endocrine and nervous systems are integrated in the control of repro1duction and development.

The signals that direct molting and metamorphosis in insects originate in the brain. There, neurosecretory cells produce prolhoracicolropic hormone (PTTH), a peptide neurohormone. In response to PTTH, the prothoracic glands, a pair of endocrine glands just behind the brain, release ecdysone. Ecdysone promotes each successive molt, as well as the metamorphosis of the caterpillar into a butterfly during the final molt.
There are small endocrine glands found just behind the brain. They are called the corpora allata. These glands secrete a signaling molecule know as juvenile hormone. One of the many functions of juvenile hormones is to maintain larval (juvenile) characteristics.

Juvenile hormone influences development indirectly by modulating the activity of ecdysone. In the presence of high levels of juvenile hormone, ecdysone stimulates molting that results in a larger larva. At the end of the larval stage, the level of juvenile hormone wanes. When the juvenile hormone level is low, ecdysone-induced molting produces the cocoon, or pupal form, within which metamorphosis occurs.

B. Coordination of Endocrine and Nervous Systems in Vertebrates

In Vertebrates, the hypothalamus plays a central role in integrating the endocrine and nervous systems. It receives info from nerves through the body and other pats of the brain. In many vertebrates, nerve signals from the brain pass sensory information to the hypothalamus bout seasonal change and the availability of a mate.

Signals from the hypothalamus travel to the pituitary gland, a gland located at its base. Roughly the size and shape of a lima bean, the pituitary has discrete posterior and anterior parts (lobes), which are actually two glands, the posterior pituitary and the anterior pituitary.The posterior pituitary and the anterior pituitary.


These glands initially develop in separate regions of the embryo. Al- though they fuse together later in development, their functions are distinct.

The posterior pituitary, or neurohypophysis, is an extension of the hypothalamus that grows downward toward the mouth during embryonic development. The posterior pituitary stores and secretes two hormones made by the hypothalamus.

The anterior pituitary, or adenohypopllysis, develops from a fold of tissue at the roof of the embryonic mouth; this tissue grows upward toward the brain and eventually loses its connection to the mouth. Hormones released by the hypothalamus regulate secretion of hormones by the anterior pituitary.

Under the control of the hypothalamus, the anterior pituitary and posterior pituitary produce a set of hormones central to endocrine signaling throughout the body,

C. Posterior Pituitary Hormones

The posterior pituitary releases 2 neurohormones
- Oxytocin
----- functions is to regulate milk release during nursing
----- A stimulus received by a sensory neuron stimulates a neurosecretory cell that causes this release.
- Antidiuretic hormone
----- also known as vasopressin
----- Helps regulate blood osmolarity
----- One of the several hormones that regulate kidney function
----- Increases water retention in the kidney
----- decrease urine volume


D. Anterior Pituitary Hormones

These hormones create, as well as regulate many types of hormones from the hypothalamus.
Each hypothalamus hormone can be one of two things:

- A releasing hormone – releases hormones
- An inhibiting hormone – inhibits/stops the release of hormones


An example of this would be TRH, which is thyrotropin-releasing hormones, which is a product of the hypothalamus that stimulates the anterior pituitary to secrete thyrotropin.

The hypothalamic hormones are secreted into capillaries, near the base of the hypothalamus. These capillaries then empty into short blood vessels, which then subdivide into a second capillary bed.

E. Hormone Cascade Pathways

Hormones from the hypothalamus, anterior pituitary, as well as the target glands are organized into the hormone cascade pathway. Signals from the brain stimulate the hypothalamus to secrete a hormone, which then stimulates/inhibits the release of hormones from the anterior pituitary. The anterior pituitary hormone then stimulates the secretion of yet another hormone (target cell), which then leads to things such as metabolic or developmental effects.


An example of this pathway would be with a child exposed to the cold. When a young child is exposed to cold, their body temperature drops which caused the hypothalamus to release TRH. TRH then targets the anterior pituitary, which then creates TSH. TSH then acts on the thyroid hormone, which then leads to things such as increased metabolic rate, which raises temperature.

Hormone cascade pathways usually have negative feedback, just like simple hormone pathways.

F. Tropic Hormones

TSH is an example of a tropic hormone, which is a hormone that regulates the function of endocrine glands and cells. There are also other tropic hormones as well, such as FSH (Follicle-stimulating hormone), LH (Luteinizing hormone), and ACTH (Adrenocorticotropic hormone).

FSH and LH take part in the reproductive system in the gonads, so they are also known as gonadotropins.

ACTH stimulates the production and secretion of steroid hormones, which are released by the adrenal cortex, which is discussed in section four.

G. Nontropic Hormones

Nontropic hormones are hormones that regulate the function of non-endocrine glands and cells. Examples of these would be prolactin (PRL) and melanocyte-stimulating hormone (MSH).

Prolactin has many different effects throughout different species. In mammals, in stimulates mammary gland growth and milk synthesis. It regulates fat metabolism and reproduction in birds, and it also delays metamorphosis in amphibians. It also regulates salt and water balance in freshwater fishes.

Melanocyte-stimulating hormone regulates the activity of pigment-containing cells in amphibians. However, in mammals, it acts on the neurons in the brain, which inhibits hunger.

H. Growth Hormone

Growth hormones are secreted by the anterior pituitary, and it stimulates growth through both tropic and nontropic effects. The liver is a major target, and it responds to GH by releasing insulin-like growth factors, or IGF. IGF then circulates in blood, and it directly stimulates bone growth as well as cartilage growth. Growth hormones also have an effect on metabolism as well, which tends to raise blood glucose levels, which opposes the effect of insulin.

Abnormal production of these growth hormones in humans can cause problems and disorders, but it depends on if there are too many growth hormones excreted, or too little. Hyperexcretion, which is when too many are excreted, can lead to gigantism, which is when the person grows abnormally tall. Hypoexcretion, which is when too little are excrete, can lead to pituitary dwarfism.

Section 45.4

Endocrine Glands respond to diverse stimuli in regulating metabolism, homeostasis, development, and behavior

Endocrine signaling helps regulate animal physiology, such as metabolism, homeostasis, reproduction, and development. There are neurosecretory cells that produce prothoracicotropic hormone, a peptide neurohormone.

A. Thyroid Hormones

The thyroid hormone is secreted by the thyroid gland, and it regulates both homeostasis and development in vertebrates. However, in humans and other mammals, it regulates other things such as bioenergetics, and it also helps maintain blood pressure, heart rate, muscle tone, etc. The thyroid gland consists of two lobes on the ventral surface of the trachea. Thyroid hormones are actually two very similar hormones derived from tyrosine.
The two hormones are:
- Triiodothyronine (T3) - it contains three iodine atoms
- Thyroxine (T4) - it contains four iodine atoms
The thyroid usually secretes T4, but most target cells convert it back to T3 by removing an iodine atom.
Too much thyroid hormone, or too little of it can be problematic. If there is too much, it can cause problems such as weight loss, a lot of sweating, high body temperature, high blood pressure, and irritability. This condition is called hyperthyroidism. The most common hyperthyroidism is Graves disease,
If there is too little thyroid hormone, it can cause problems such as weight gain, lethargy, and intolerance to cold. This condition is called hypothyroidism.


Proper thyroid function needs a good amount of iodine in a diet. Iodine can be obtained by things such as iodized salt or seafood, but many people throughout the world, especially in third world countries, are unable to obtain the necessary amount of iodine in their diet. Because of this, their bodies are unable to produce T3 and T4, eventually leading to goiter, which is a swelling of the neck.

Thyroid hormones have a variety of functions throughout vertebrates
- Controls the metamorphosis of a tadpole into a frog
- In vertebrates, it helps the normal functioning of bone-forming cells and the branching of nerve cells during embryonic development of the brain.

Diseases such as congenital hypothyroidism are inherited deficiencies, and can cause things such as slowed skeletal growth and poor mental development. These defects can be prevented or at least helped, by being treated with thyroid hormones early in life. Iodine deficiencies also cause the same problems as congenital hypothyroidism, but can be prevented if iodized salt is used.

Parathyroid Hormone and Vitamin D: Control of Blood Calcium

Calcium is necessary for the normal function of all cells, therefore the control of blood calcium is quite important. If calcium in blood falls dramatically, skeletal muscles begin to contract, which is a fatal condition called tetany. However, if calcium rises above normal, calcium phosphate can precipitate throughout the body, leading to the damage of many organs.

B. Parathyroid glands and hormones

Mammals have a parathyroid gland, which is a small four structured object embedded in the posterior surface of the thyroid. When blood calcium falls below a set point, these glands release parathyroid hormones (PTH).

PTH helps raise the calcium level in blood in direct and indirect ways. PTH can cause the mineral matrix in bones to decompose, which releases calcium into the blood.

In kidneys, it stimulates the reabsorption of calcium through the renal tubes. PTH also has an indirect effect in kidneys, by converting the vitamin D in the kidneys into an active hormone. Vitamin D can be obtained through food or sunlight, and its activation begins at the liver, and completes at the kidneys. The vitamin D acts directly on the intestines, stimulating the uptake of calcium from food, which stops the effect of PTH (Negative feedback).


C. Calcitonin

The thyroid gland can also contribute to homeostasis in calcium. If calcium rises above the normal set point, the thyroid gland releases calcitonin, which is a hormone that inhibits bone resorption and enhances calcium release by the kidneys.
In things such as fishes and rodents, calcitonin is required in calcium homeostasis. However, in humans, it is only needed only during the extensive bone growth during childhood.

D. Adrenal Hormones

The adrenal glands of vertebrates are associated with the kidneys. In mammals, it is made up of two glands with different cell type functions: the adrenal cortex (outside portion) and the adrenal medulla (inside portion).
Each adrenal gland is a fused endocrine and neuroendocrine gland.

E. Catecholamines from the Adrenal Medulla

When you are in danger, like being chased by a dangerous animal such as a bear, your body releases hormones. Your heart beats faster, your breathing quickens, your thoughts speed up, and your muscles tense up. These changes are triggered by two hormones in the adrenal medulla, which are epinephrine (adrenaline) and norepinephrine (noradrenaline). Both of these hormones are catecholamines, which are a class of amine hormones created by tyrosine.

The adrenal medulla secretes adrenaline and noradrenaline as a response to stress. A major activity of these two hormones is to increase the amount of chemical energy available for immediate use. Both hormones increase the rate of glycogen breakdown in the liver and skeletal muscles, promote glucose release by liver cells, and stimulate the release of fatty adds from fat cells. The glucose released by this and fatty acids can be used by body cells as fuel.

They also have effects on the cardiovascular and respiratory systems. The two hormones can increase heart rate, stroke volume,and dilate the bronchioles in the lungs.
Epinephrine has a stronger effect on heart and metabolic rates, while norepinephrine helps modulate blood pressure.

F. Steroid Hormones from the Adrenal Cortex

Hormones from the adrenal cortex also function in the body's response to stress. However, it responds to endocrine signals, rather than nervous input. When you are stressed, the hypothalamus secretes a hormone, which stimulates the anterior pituitary to release the tropic hormone, ACTH. When ACTH reaches the adrenal cortex, it stimulates the endocrine cells to create a group of steroids called corticosteroids.

There are two types of corticosteroids:

1. Glucocorticoids - These have a big part in glucose metabolism. Glucocorticoids augment the effects of glucagon, which promotes the creation of glucose from non-carbohydrate sources, making glucose more available as fuel. An example of this would be cortisol. Cortisol acts on skeletal muscle, and it causes the breakdown of muscle proteins. They are then sent to the liver and kidneys, and are converted into glucose and released into the blood.
When the level of glucocorticoids tips over the set point, they can suppress parts of the immune system. Because of this, glucocorticoids are usually used to treat diseases such as arthritis. However, the long-term use of glucocorticoids has bad side effects, so drugs such as asprin and ibuprofen are usually only used for chronic inflammatory conditions.

2. Mineralocorticoids - They take part in mineral metabolism. They act mostly on maintaining salt and water balance.
An example of this would be aldosterone, which function in ions and water homeostasis in blood. Low blood pressure leads to the production of angiotenin II, which then stimulates the secretion of aldosterone. Aldesterone than stimulates cells in the kidneys to reabsorb sodium ions and water from the filtrate, which raises blood pressure and volume. Aldosterone also functions in the body’s response to severe stress, because the ACTH levels increase, which increases the rate at which the adrenal cortex releases aldosterone.


All steroid hormones are synthesized by cholesterol, and their structures only differ in small ways. However, these small differences still cause a large difference in effects.

G. Gonadal Sex Hormones

Sex hormones affect many things, including growth, development, reproductive cycles, and sexual behavior. Adrenal glands may secrete a small amount of these hormones, but the testes and ovaries are the major sources of these hormones.

These gonads produce three types of steroid hormones:
1. Androgen
2. Estrogens
3. Progestins

All three of these hormones are found in both males and females, but they are found at different levels and amounts.

The testes mainly create androgens, the main one being testosterone. Testosterone first functions before birth, which was proved by a French researcher named Alfred Jost, who concluded that for mammals, female development is the default process in embryos.

H. Puberty and Anabolic Steroids

Androgens also have a major role in puberty. High concentrations of androgen lead to a low voice, as well as male patterns of hair growth. It also leads to an increase in bone and muscle mass. Because of this muscle building (anabolic) action of testosterone, some athletes decide to take in these anabolic steroids as supplements, even though it is prohibited in sports.

Anabolic steroids, even though it increases muscle mass, have many side effects as well, including severe acne breakouts as well as liver damage. Because anabolic steroids have a negative feedback effect on testosterone production, it also leads to low sperm count and testicular size.

I. Other steroid hormones

Estrogens, the most important one being estradiol, has control of the maintenance for the female reproductive system, as well as female secondary sex characteristics.
Mammals have progestins, which include progesterone, and they are involved in preparing and maintaining tissues of the uterus, which is required for the growth and development of the embryo.

Androgens, estrogens, and progestins are all components of hormone cascade pathways. The synthesis of these hormones is controlled by gonadatropins from the anterior pituitary gland.

J. Melatonin and Biorhythms

The pineal gland, which is a small mass of tissue near the center of the brain, synthesizes the hormone melatonin, which is a modified amino acid. Depending on the species, the pineal gland contains light·sensitive cells or has nervous connec- tions from the eyes that control its secretory activity.

Melatonin regulates functions related to light, and it can affect skin pigmentation in vertebrates. However, its main functions relate to biological rhythms associated with reproduction.

Melatonin is secreted at night, and the amount that is secreted depends on how long the night is.

Key Terms of Chapter 45

1. Hormone - a molecule that is secreted into the extracellular fluid, circulates in the blood or hemolymph, and communicates regulatory messages throughout the body
2. Endocrine system - one of the two basic systems for communication and regulation through- out the body
3. Nervous system - a network of specialized cells-neurons-that transmit signals along dedicated pathways
4. Endocrine glands - other endocrine cells are grouped in ductless organs
5. Local regulators - secreted molecules that act over short distances and reach their target cells solely by diffusion
6. Paracrine – signaling where target cells lie near the secreting cell
7. Autocrine - signaling where the target cells are on the cell that secreted it
8. Neurotransmitters - secreted molecules by neurons that diffuse a very short distance to bind receptors on the target cells
9. Neurohormones - molecules that are secreted by neurosecretory cells by the brain
10. Pheromones - chemicals that are released into the external environment
11. Signal transduction - the series of changes in cellular proteins that converts the extracellular chemical signal to a specific intracellular response
12. Epinephrine - hormone secreted by adrenal glands, also called adrenaline
13. Cytokines - A polypeptide local regulator which play a role in most immune responses
14. Growth factors - which stimulate cell proliferation and differentiation
15. Nitric oxide - consists of nitrogen double-bonded to oxygen and serves in the body as both a neurotransmitter and a local regulator
16. Prostaglandins - group of local regulators that are modified fatty acids
17. Pancreas - a gland located behind the stomach
18. Negative feedback - a loop in which the response reduces the initial stimulus
19. Insulin - riggers uptake of glucose from the blood, decreasing the blood glucose concentration
20. Glucagon - promotes the release of glucose into the blood, in- creasing the blood glucose concentration
21. Islets of Langerhans - clusters of endocrine cells scattered throughout the pancreas
22. Diabetes Mellitus - disorder caused by a deficiency of insulin or a decreased response to insulin in target tissues.
23. Ecdysone - it promotes each successive molt in insects, as well as the metamorphosis of the caterpillar into a butterfly during the final molt
24. Juvenile hormone - a signaling molecule; one of the many functions of juvenile hormone is to maintain larval characteristics
25. Hypothalamus - it plays a central role in integrating the endocrine and nervous systems. It receives information from nerves throughout the body and from other parts of the brains and it initiates endocrine signaling appropriate to environmental conditions
26. Pituitary gland - gland located at the base of the hypothalamus
27. Posterior pituitary - an extension of the hypothalamus that grows downward toward the mouth during embryonic development. The posterior pituitary stores and secretes two hormones made by the hypothalamus
28. Anterior pituitary - this tissue grows upward toward the brain and eventually loses its con- nection to the mouth. Hormones released by the hypothala- mus regulate secretion of hormones by the anterior pituitary
29. Oxytocin - regulate milk release during nursing; this function is mediated by a simple neurohormone pathway
30. Positive feedback - reinforces a stimulus, leading to an even greater response
31. Antidiuretic hormone - it helps regulate blood osmolarity
32. Tropic Hormone - a hormone that regulates the function of endocrine cells or glands
33. Prolactin - remarkable for the diversity ofits effects among vertebrate species. For example, prolactin stimulates mammary gland growth and milk synthesis in mammals, reg- ulates fat metabolism and reproduction in birds, delays meta- morphosis in amphibians, and regulates salt and water balance in freshwater fishes
34. Melanocyte-stimulating hormone - regulates the activity of pigment-containing cells in the skin of some amphibians (as well as fishes and reptiles). In mammals, MSH appears to act on neurons in the brain, inhibiting hunger
35. Growth Hormone - secreted by the anterior pituitary; stimulates growth through tropic and nontropic effects.
36. Thyroid Gland - consists of two lobes on the ventral surface ofthe trachea
37. Triiodothyronine - contains three iodine atoms, derived from tyrosine
38. Thryoxine - contains four iodine atoms, derived from tyrosine
39. Parathyroid Glands - a set of four small structures embedded in the posterior surface ofthe thyroid
40. Parathyroid Hormone - hormone released by parathyroid glands; raises blood calcium
41. Calcitonin - a hormone that inhibits bone resorption and enhances calcium release by the kidney
42. Adrenal glands - each adrenal gland is actually made up of two glands with different cell types, functions, and embryonic origins: the adrenal cortex, the outer portion, and the adrenal medulla, the central portion
43. Catecholamines - a class of amine hormones synthesized from the amino acid tyrosine
44. Corticosteroids - when ACTH reaches the adrenal cortex via the bloodstream, it stimulates the endocrine cells to synthesize corticosteroids. The two main types of corticosteroids in humans are glucocorticoids and mineralocorticoids.
45. Glucocorticoids - have a primary effect on glucose metabolism. Augmenting the fuel-mobilizing effects of glucagon from the pancreas, glucocorticoids promote glucose synthesis from noncarbohydrate sources, such as proteins, making more glucose available as fuel.
46. Mineralocorticoids - named for their effects on mineral metabolism, they act principally in maintaining salt and water balance.
47. Androgens - testes primarily synthesize this, the main one being testosterone
48. Testosterone - Has a major role in puberty, where high levels of this and other androgen lower voice, as well as development of male sex characteristics
49. Estrogens - responsible for the maintenance ofthe female reproductive system and the development of female secondary sex characteristics
50. Progestins - primarily involved in preparing and maintaining tissues of the uterus required to support the growth and development of an embryo.


In conclusion, the endocrine system has many functions overall. There are many organs throughout the body that secrete hormones, which travel throughout the bloodstream, and then attach to their target cells and activate them. There are many different types of hormones, as well as local regulators, and they have many different ways of traveling, such as diffusion, moving through lipids, etc. Hormones have many functions as well, such as maintaining homeostasis in calcium, as well as glucose, using hormones such as insulin and glucagon. Without hormones such as insulin, people can get diseases and disorders such as diabetes. Hormones also maintain metabolism, development, and behavior as well. Secreting things such as adrenaline, hormones also change things such as heart rate, beat, blood pressure, in order to help people get out of dangerous situations. Hormones such as testosterone also influence the body quite a bit, such as being activated in stages of life such as puberty. Even though hormones such as testosterone are secreted naturally, people take advantage of this and decide to take hormones through supplements, such as anabolic steroids, even though they are not allowed. Doing so results in many bad side effects, including bad acne breakouts as well as liver damage. Overall, the endocrine system maintains your body’s homeostasis, and helps keep you alive.