INTRODUCTION TO
THE ENDOCRINE SYSTEM

The body uses two systems to communicate with the body: the endocrine system and the nervous system. The endocrine system is a communication system that utilizes signaling molecules (hormones and neurohormones) to send information from one part of the body to another.
- Hormones are chemical signals that are synthesized and secreted by endocrine cells/glands.
- Neurohormones are chemical signals that are synthesized and secreted by neurons. Hormones/neurohormones travel via diffusion or via the blood stream to their target cells.
Our bodies can only function within a relatively very narrow range of values for all of its physiological functions. The ability of the body to maintain these physiological parameters within the necessary ranges needed to sustain life is called HOMEOSTASIS.
What is Homeostasis?

Homeostasis is the ability of the body's systems to maintain a stable, relatively constant internal environment. Homeostasis is the tendency to resist change in order to maintain a stable, relatively constant internal environment. Homeostasis refers to the maintenance of relatively constant internal conditions. For example, your body shivers to maintain a relatively constant body temperature when the external environment gets colder.
How is Homeostasis Maintained?
The function of maintaining homeostasis is provided by the endocrine system. The endocrine system exerts its control through various positive feedback loops. Maintaining homeostasis requires that the body continuously monitors its internal conditions. Each of the body's physiological parameters has a SET POINT. The human body must maintain homeostasis within just a few points of the body's set point value. Without this, the body can quickly become out of balance and death can occur.
Your cells, tissues, organs, and organ systems all function to maintain homeostasis. Homeostasis is a state of balance that keeps certain physiological parameters within a fairly narrow range of values. Without homeostasis, the body quickly dies.
The function of maintaining homeostasis is provided by the endocrine system. The endocrine system exerts its control through various positive feedback loops. Maintaining homeostasis requires that the body continuously monitors its internal conditions. Each of the body's physiological parameters has a SET POINT. The human body must maintain homeostasis within just a few points of the body's set point value. Without this, the body can quickly become out of balance and death can occur.
Your cells, tissues, organs, and organ systems all function to maintain homeostasis. Homeostasis is a state of balance that keeps certain physiological parameters within a fairly narrow range of values. Without homeostasis, the body quickly dies.

The body uses two systems to communicate with the body:
- the endocrine system
- the nervous system.
- Hormones are chemical signals that are synthesized and secreted by endocrine cells/glands
- Receptors on the cell surface will use second messenger systems to cause a rapid, non-genomic effect.
- Receptors in the cytoplasm and nucleus will influence transcription/translation causing aslower genomic effect.
- If a cell does not contain the receptor that is specific for that hormone, then it will not respond to the hormone .
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.
- Endocrine Glands are those glands which have no duct and release their secretions directly into the intracellular fluid or into the blood. The collection of endocrine glands makes up the endocrine system.
HORMONE CLASSIFICATIONS The classification of a hormone can help us understand how they affect the target cell.
Hormones can be classified in three different classes:
Hormones can be classified in three different classes:
- Steroid Hormones
- Non-Steroidal Hormones
- Peptide Hormones
- Amino Acid Derived Hormones

Non-Steroidal Hormones
Non-steroidal hormones include
1) peptide hormones
2) amino acid-derived hormones PROPERTIES OF NON-STEROIDAL HORMONES
1) Non-Steroidal hormones are hydrophillic so they are soluble in bodily fluids
2) Non-Steroidal hormone receptors are located on the cell's surface so they cause a rapid non-genomic effect
3) Non-Steroidal hormones have a short half-life.
Peptide hormones
Peptide hormones are synthesized through transcription and translation and are stored by the cell in storage vesicles until the cell receives a signal to secrete the hormone. Most hormones are considered peptide hormones.
Amino acid-derived hormones
Amino acid-derived hormones are formed from amino acid precursors. For example, the amino acid tryptopohan is used by the body to make a dopamine, norepinephrine, epinephrine, melatonin and serotonin. The amino acid tyrosine is used to make the thyroid hormones triiodothyronine (T3) and thyroxine (T4) which regulate the metabolism and energy levels of the body. The amino acid glutamic acid is used to make histamines which function as part of the body's immune system.
These experiments relate to the body's endocrine pathways and demonstrate how hormones help our body maintain homeostasis.
Non-steroidal hormones include
1) peptide hormones
2) amino acid-derived hormones PROPERTIES OF NON-STEROIDAL HORMONES
1) Non-Steroidal hormones are hydrophillic so they are soluble in bodily fluids
2) Non-Steroidal hormone receptors are located on the cell's surface so they cause a rapid non-genomic effect
3) Non-Steroidal hormones have a short half-life.
Peptide hormones
Peptide hormones are synthesized through transcription and translation and are stored by the cell in storage vesicles until the cell receives a signal to secrete the hormone. Most hormones are considered peptide hormones.
Amino acid-derived hormones
Amino acid-derived hormones are formed from amino acid precursors. For example, the amino acid tryptopohan is used by the body to make a dopamine, norepinephrine, epinephrine, melatonin and serotonin. The amino acid tyrosine is used to make the thyroid hormones triiodothyronine (T3) and thyroxine (T4) which regulate the metabolism and energy levels of the body. The amino acid glutamic acid is used to make histamines which function as part of the body's immune system.
These experiments relate to the body's endocrine pathways and demonstrate how hormones help our body maintain homeostasis.
Feedback Loops
Homeostasis is maintained through feedback loops. There are 2 types of feedback loops; Negative and Positive.
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Control centers in the brain play roles in regulating physiological parameters and keeping them within the normal range. As the body works to maintain homeostasis, any significant deviation from the normal range will be resisted and homeostasis restored through a process called a feedback loop.
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A feedback loop has three basic components:
- Sensors / Receptors - Monitors the internal physiological parameter and reports changes to the Control Center
- The Control Center - Receives sensory input and compares the reported physiological parameter's value to the set point.
- Effectors - If the Control Center finds that the value of the physiological parameter is too far away from the set point, the control center will send a command to effectors. The effector will function to bring the physiological parameter closer to the set point. is the component in a feedback system that causes a change to reverse the situation and return the value to the normal range. Effectors are muscles and glands.

Negative Feedback Loop Schematic
Negative feedback loops are the body’s most common mechanisms used to maintain homeostasis. Negative feedback works like driving car down the road. Constant adjustment to the right or left is required to keep the car on the road. A negative feedback system is one that tries to keep the body constant. Negative feedback is a mechanism in which the effect of the response to the stimulus is to shut off the original stimulus or reduce its intensity. In a negative feedback loop, when a mismatch is sensed by the control center, this deviation from the set point is resisted through a physiological process that returns the body to homeostasis.
TEMPERATURE REGULATION (Thermoregulation)
The body regulates its temperature through a process called thermoregulation. Thermoregulation is the ability of the body to maintain its temperature between ~36.5–37.5 °C (or 97.7–99.5 °F). Thermoregulation is an example of negative feedback. The hypothalamus in the brain is the master switch that works as a thermostat to regulate the body’s core temperature.
The body regulates its temperature through a process called thermoregulation. Thermoregulation is the ability of the body to maintain its temperature between ~36.5–37.5 °C (or 97.7–99.5 °F). Thermoregulation is an example of negative feedback. The hypothalamus in the brain is the master switch that works as a thermostat to regulate the body’s core temperature.
Temperature regulation of the body is an example of a negative feedback loop. The negative feedback loop that regulates temperature can function to ultimately turn the internal temperature up or down as needed to get closer to the body's necessary set point.

Humans and other mammals must maintain an internal body temperature close to 98.6 degrees Fahrenheit or 37.0 degrees Celsius, despite how cold our external environment is. Our body's internal temperature must stay within a very narrow range of temperatures (between ~F95∘F/ C35∘C and F107∘F/ C41.7∘C) to avoid illness or even death.
If the temperature goes above the set core temperature:
The hypothalamus can initiate several processes to lower the body temperature. The following 3 effects occur in efforts to decrease the internal body temperature:
- Blood Flow Redirection - Blood vessels in the skin begin to dilate allowing more blood from the body core to flow to the extremities and the surface of the skin. This allows for heat loos through the skin which cools the internal temperature.
- Sweating - Sweat glands are activated to increase their secretion of sweat. Sweat pools at the skin's surface and is evaporated as it touches the air. As the sweat evaporates from the skin surface into the surrounding air, it dissipates heat and cools the skin.
- Breathing Changes - The depth of respiration increases, and a person may breathe through an open mouth instead of through the nasal passageways. This further increases heat loss from the lungs.
If the temperature falls below the set core temperature:
The hypothalamus can initiate several processes to raise the body temperature. The following 3 effects occur in efforts to increase the internal body temperature:
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Blood Glucose Levels
Between meals, blood glucose levels drop. This drop is glucose triggers the release of glucagon from the pancreas. Glucagon stimulates the liver to breakdown glycogen into glucose. This increases the blood glucose levels.
After a meal, blood glucose levels increase. The increase blood glucose triggers the release of insulin from the pancreas. Insulin activates the conversion of glucose to glycogen in the liver. This functions to decrease the blood glucose levels.
- Glucose: Insulin and Glucagon - The receptors of the pancreas are responsible for monitoring glucose levels in the blood. Pancreatic Hormones regulate blood sugar level before and after meals. - Islets; clusters of cells in pancreas
- GLUCAGON – increases sugar
- INSULIN – decreases sugar
Between meals, blood glucose levels drop. This drop is glucose triggers the release of glucagon from the pancreas. Glucagon stimulates the liver to breakdown glycogen into glucose. This increases the blood glucose levels.
After a meal, blood glucose levels increase. The increase blood glucose triggers the release of insulin from the pancreas. Insulin activates the conversion of glucose to glycogen in the liver. This functions to decrease the blood glucose levels.
Positive Feedback Loops
Positive feedback intensifies a change in the body’s physiological condition rather than reversing it. A deviation from the normal range results in more change, and the system moves farther away from the normal range. Positive feedback in the body is normal only when there is a definite end point.
Positive feedback intensifies a change in the body’s physiological condition rather than reversing it. A deviation from the normal range results in more change, and the system moves farther away from the normal range. Positive feedback in the body is normal only when there is a definite end point. Childbirth and the body’s response to blood loss are two examples of positive feedback loops that are normal but are activated only when needed.
Positive feedback intensifies a change in the body’s physiological condition rather than reversing it. A deviation from the normal range results in more change, and the system moves farther away from the normal range. Positive feedback in the body is normal only when there is a definite end point.
Positive feedback intensifies a change in the body’s physiological condition rather than reversing it. A deviation from the normal range results in more change, and the system moves farther away from the normal range. Positive feedback in the body is normal only when there is a definite end point. Childbirth and the body’s response to blood loss are two examples of positive feedback loops that are normal but are activated only when needed.

Childbirth at full term is an example of a situation in which the maintenance of the existing body state is not desired. Enormous changes in the mother’s body are required to expel the baby at the end of pregnancy. And the events of childbirth, once begun, must progress rapidly to a conclusion or the life of the mother and the baby are at risk. The extreme muscular work of labor and delivery are the result of a positive feedback system (Figure 1.11).
Normal childbirth is driven by a positive feedback loop. A positive feedback loop results in a change in the body’s status, rather than a return to homeostasis.
The first contractions of labor (the stimulus) push the baby toward the cervix (the lowest part of the uterus). The cervix contains stretch-sensitive nerve cells that monitor the degree of stretching (the sensors). These nerve cells send messages to the brain, which in turn causes the pituitary gland at the base of the brain to release the hormone oxytocin into the bloodstream. Oxytocin causes stronger contractions of the smooth muscles in of the uterus (the effectors), pushing the baby further down the birth canal. This causes even greater stretching of the cervix. The cycle of stretching, oxytocin release, and increasingly more forceful contractions stops only when the baby is born. At this point, the stretching of the cervix halts, stopping the release of oxytocin.
There are two types of hormones secreted in the endocrine system:
- Steroidal (or lipid based)
- non-steroidal hormones
- peptide hormones
- Amino-Acid Derived Hormones
- peptide hormones
Peptide hormones
Peptide hormones are synthesized through transcription and translation and are stored by the cell in storage vesicles until the cell receives a signal to secrete the hormone. Most hormones are considered peptide hormones. PROPERTIES OF PEPTIDE HORMONES
1) Peptide hormones are hydrophilic and are soluble in bodily fluids 2) Peptide hormones have receptors that are located on the cell's surface causing a rapid non-genomic effect 3) Peptide hormones have a short half-life |
Steroid hormones
Steroid hormones include corticosteroids and sex hormones. Steroid hormones are derived from cholesterol.
PROPERTIES OF STEROID HORMONES
1) Steroid hormones are hydrophobic so they are able to cross the cell membrane
2) Steroid hormones require a carrier protein to travel through the blood stream
3) Steroid hormones have receptors that are located in the cell's cytoplasm or nucleas causing SLOW ACTIONS.
4) Steroid hormones have a long half-life.
Steroid hormones include corticosteroids and sex hormones. Steroid hormones are derived from cholesterol.
PROPERTIES OF STEROID HORMONES
1) Steroid hormones are hydrophobic so they are able to cross the cell membrane
2) Steroid hormones require a carrier protein to travel through the blood stream
3) Steroid hormones have receptors that are located in the cell's cytoplasm or nucleas causing SLOW ACTIONS.
4) Steroid hormones have a long half-life.
Amino acid-derived hormones
Amino acid-derived hormones are formed from amino acid precursors. For example, the amino acid tryptopohan is used by the body to make a dopamine, norepinephrine, epinephrine, melatonin and serotonin. The amino acid tyrosine is used to make the thyroid hormones triiodothyronine (T3) and thyroxine (T4) which regulate the metabolism and energy levels of the body. The amino acid glutamic acid is used to make histamines which function as part of the body's immune system.
PROPERTIES OF AMINO ACID-DERIVED HORMONES
1)Amino acid-derived hormones are hydrophilic so they are soluble in bodily fluids
2) Amino acid-derived hormone receptors are located on the cell's surface so they cause a rapid non-genomic effect
3) Amino acid-derived hormones have a short half-life.
Amino acid-derived hormones are formed from amino acid precursors. For example, the amino acid tryptopohan is used by the body to make a dopamine, norepinephrine, epinephrine, melatonin and serotonin. The amino acid tyrosine is used to make the thyroid hormones triiodothyronine (T3) and thyroxine (T4) which regulate the metabolism and energy levels of the body. The amino acid glutamic acid is used to make histamines which function as part of the body's immune system.
PROPERTIES OF AMINO ACID-DERIVED HORMONES
1)Amino acid-derived hormones are hydrophilic so they are soluble in bodily fluids
2) Amino acid-derived hormone receptors are located on the cell's surface so they cause a rapid non-genomic effect
3) Amino acid-derived hormones have a short half-life.
The hypothalamic–pituitary–thyroid axis (HPT axis)
The hypothalamic–pituitary–thyroid axis (HPT axis), a.k.a. thyroid homeostasis or thyrotropic feedback control) is part of the neuroendocrine system responsible for the regulation of metabolism.
- The hypothalamus senses low circulating levels of thyroid hormones T3 (Triiodothyronine) and T4 (Thyroxine).
- The hypothalamus responds to low levels of T3 and T4 by releasing TRH (thyrotropin-releasing hormone)
- TRH stimulates the anterior pituitary to produce TSH (thyroid-stimulating hormone)
- TSH stimulates the thyroid to produce thyroid hormone s (T3 and T4) until levels in the blood return to normal.
- The ultimate effect of the release of thyroid hormones (T3 and T4) is to increase metabolic function and thereby return metabolic function back to the set-point or back to 'normal' levels
Thyroid hormone exerts negative feedback control over the hypothalamus as well as anterior pituitary, thus controlling the release of both TRH from hypothalamus and TSH from anterior pituitary gland.
- Thyroid hormone exerts negative feedback control over the hypothalamus as well as anterior pituitary, thus controlling the release of both TRH from hypothalamus and TSH from anterior pituitary gland.
Typical Feedback System
Fight or Flight - Positive Feedback Loop

In emergencies, adrenaline (epinephrine) is released by the body to override the homeostatic control of glucose. This is done to promote the breakdown of glycogen into glucose to be used in the emergency. These emergencies are often known as 'fight or flight reactions'.
Adrenaline is secreted by the adrenal glands. The secretion of it leads to increased metabolism, breathing and heart rate. Once the emergency is over, and adrenaline levels drop, the homeostatic controls are once again back in place.
Regulation of Blood Glucose

Insulin is a hormone released by the body in response to high blood glucose levels. When insulin is introduced into the bloodstream, blood glucose levels fall as a response.
When glucose is ingested, blood glucose levels increase. This increase triggers the release of insulin from eyelet cells of the pancreas. When this insulin reaches the bloodstream, blood glucose levels decrease back to normal.
Diabetes Mellitus (Type 1 or Type 2) Diabetes Mellitus is a condition in which a person's body cannot regulate blood plasma glucose levels.
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- Diabetes mellitus can result when plasma glucose levels are not regulated properly and the body cannot maintain homeostasis. Diabetes mellitus type 1 results when the beta cells of the pancreas fail to produce enough insulin therefore resulting in high plasma glucose levels.
- Diabetes mellitus type 2 results when the cells of the body do not respond to insulin properly (insulin resistance) therefore resulting in high plasma glucose levels.
Effects of Insulin

Following a meal, the body's blood glucose levels are raised. The beta cells of the pancreas sense the increased blood glucose levels and in response they secrete insulin (a protein hormone).
Insulin affects...
- the cells of the body by increasing the number of glucose transporters on the cell membrane thus increasing facilitated diffusion of glucose into the cell
- increasing the rate of glycogenesis (synthesis of glycogen) in the liver,
- increasing the uptake of amino acids and increasing the rate of protein synthesis,
- increasing lipogenesis (synthesis of fatty acids)
- slowing the rate of glycogenolysis (breakdown of glycogen into glucose)
- slowing the rate of gluconeogenesis (forming glucose from lactic acid and amino acids).
Blood Glucose Levels Following a Period of Fasting
After several hours of fasting, the body's blood glucose levels decrease below the setpoint.
- The alpha cells of the pancreas sense the decreased blood glucose levels and they secrete glucagon (a protein hormone). Glucagon affects hepatocytes (cells of the liver) and causes the following to occur:
- increase in the rate glycogenolysis (breakdown of glycogen into glucose)
- increase in the rate of gluconeogenesis (synthesizing glucose from lactic acid, glycerol and amino acids)
- slowing the rate of glycogenesis (synthesis of glycogen) in the liver, and 4) slowing the rate of lipogenesis (synthesis of fatty acids). The secretion of glucagon ultimately results in an increase of plasma glucose levels.