Module 8: Endocrine System

Question 1

Endocrine System - what is involved?

Answer 1

Question 2

Endocrine System Functions

Answer 2

Endocrine system functions
 Role in reproductive and CNS development in fetus
 Stimulating growth and development during childhood and
adolescence
 Sexual reproduction
 Maintaining homeostasis
 Responding to emergency demands

Question 3

Endocrine System - how it functions

Answer 3

Exert their effects by recognizing their target tissues and attaching to receptor sites in a “lock-and-key” type of mechanism
 Control endocrine activity by stimulating or inhibiting hormone synthesis/ secretion
 Positive and negative feedback

Question 4

Lipid Soluble vs Water Soluble Hormones

Answer 4

Lipid soluble
* Bind to plasma proteins for transport
* Cross cell membrane by simple diffusion
* Thyroid hormones and steroids

 Water soluble
* Circulate freely in blood to target tissues
* Insulin, growth hormone, prolactin

Question 5

Hormonal Systems: Negative and Positive Feedback

Answer 5

Ensure hormones are released at the right amounts at the right times

Question 6

Negative Feedback

Answer 6

Negative feedback mechanisms are the most common way the body regulates hormone levels. This system operates similarly to a thermostat in a heating system:

Set Point: Just as a thermostat has a set temperature it aims to maintain, the body has set levels (set points) for various hormones that it considers optimal for physiological function.

Detection of Deviation: If the hormone level deviates from this set point (either too high or too low), sensors in the body detect this change, akin to a thermostat sensing the room temperature has moved away from the set temperature.

Response to Restore Balance: In response to a deviation, the body will either increase or decrease the production and secretion of the hormone to bring its level back to the set point. For example, if the blood glucose level rises, insulin secretion increases to lower the glucose level back to its normal range. Once the desired hormone level (or physiological effect) is achieved, the secretion of the regulating hormone is dialed back.

Result: The end result is a stable concentration of hormones, which allows the body to maintain homeostasis.

Question 7

Positive Feedback

Answer 7

Positive feedback mechanisms, on the other hand, are less common and work in a way that amplifies the initial action until a specific event or function is completed. This system encourages a rise in hormone levels until an external action stops their release:

Initial Trigger: The process begins with an initial trigger, which causes an increase in the secretion of a hormone.

Amplification: Instead of correcting the deviation, the body responds by increasing the hormone level even further. This amplification continues the action initiated by the hormone.

Completion of a Specific Event: The positive feedback loop is typically designed to continue until a specific physiological event is completed. For example, during childbirth, the hormone oxytocin is released to stimulate uterine contractions. The presence of contractions triggers the release of more oxytocin, which intensifies the contractions further. This loop continues until delivery is completed, at which point the stimulus for oxytocin release is removed.

Termination: Once the desired outcome is achieved (e.g., childbirth), the stimulus for further hormone release is removed, stopping the cycle.

Question 8

Role of the Nervous System in Hormone Secretion

Answer 8

Initial Stimulus: The body encounters a stimulus such as pain, fear, sexual excitement, or a stressor. This stimulus is detected by sensory receptors and relayed to the central nervous system (CNS) for processing.

CNS Response: The CNS, upon recognizing the stimulus, evaluates it and determines the appropriate response. This involves integrating sensory information and coordinating an immediate reaction through the autonomic nervous system (ANS), particularly the sympathetic nervous system (SNS).

SNS Activation: The SNS is part of the ANS that is responsible for the body’s ‘fight or flight’ response. When the CNS signals a stressor, the SNS gets activated.

Hormonal Secretion by Endocrine Glands: The activation of the SNS leads to the stimulation of certain endocrine glands. For instance, the adrenal glands, located on top of the kidneys, are prompted to secrete catecholamines, including adrenaline (epinephrine) and noradrenaline (norepinephrine).

Question 9

Hormonal Effects on the Body

Answer 9

Catecholamines: These hormones are released into the bloodstream and exert several effects aimed at preparing the body for a rapid response to the stressor. Key effects include:

Increased Heart Rate and Blood Pressure: To ensure that muscles and vital organs receive more oxygen and nutrients, the heart rate and blood pressure increase.

Enhanced Lung Function: The bronchioles dilate, improving oxygen intake, which is crucial for increased physical activity during the ‘fight or flight’ response.

Energy Mobilization: Glucose and fatty acids are released into the bloodstream, providing immediate energy for muscles.

Question 10

Circadian Rhythms

Answer 10

Circadian rhythms are 24-hour cycles that are part of the body’s internal clock, running in the background to carry out essential functions and processes. One of the most well-known circadian rhythms is the sleep-wake cycle, which is influenced by external light and darkness cues.

Impact on Hormonal Secretion: Many hormones exhibit circadian fluctuations in their secretion. For example, cortisol, often referred to as the “stress hormone,” has a circadian rhythm that typically peaks in the early morning, shortly after waking, and gradually decreases throughout the day, reaching its lowest levels at night. This pattern helps regulate various body functions, including metabolism, immune response, and the ability to respond to stress.

Dark-Light Cycle: The exposure to light and darkness directly influences the production of melatonin, the hormone responsible for regulating sleep. Melatonin levels increase in response to darkness, promoting sleep, and decrease with light exposure, promoting wakefulness.

Question 11

Ultradian Rhythms

Answer 11

Ultradian rhythms are cycles shorter than 24 hours, which can be observed in the fluctuation of hormones and other biological processes.

Reproductive Cycles: A key example of an ultradian rhythm is the pulsatile secretion of gonadotropin-releasing hormone (GnRH), which regulates the release of two other hormones crucial for reproduction: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These hormones have ultradian rhythms in their secretion, which are crucial for the menstrual cycle in females and spermatogenesis in males.

Impact on Fertility and Reproduction: The precise regulation and timing of these hormonal releases are essential for normal reproductive function, including ovulation in women and the production of sperm in men.

Question 12

Hypothalamus with Pituitary Gland

Answer 12

**acting as a key link between the central nervous system (CNS) and the endocrine system, particularly influencing the function of the pituitary gland

**located next to the pituitary gland at the base of the brain

Influence on Pituitary Gland:

Direct Control: The hypothalamus exerts control over the pituitary gland through both direct and indirect means. Directly, it produces and releases hormones that travel to the pituitary gland and either stimulate or inhibit the release of pituitary hormones. For instance, the hypothalamus releases thyrotropin-releasing hormone (TRH) to stimulate the pituitary to release thyroid-stimulating hormone (TSH), which in turn stimulates the thyroid gland.

Indirect Control via the Pituitary Gland: Indirectly, the hypothalamus influences various bodily functions by controlling the secretion of a range of pituitary hormones that act on other endocrine glands, such as the adrenal glands, thyroid gland, and gonads (testes and ovaries), regulating stress responses, metabolism, growth, and reproduction.

Question 13

Hypothalamus with Integration with the Central Nervous System

Answer 13

Receiving CNS Input: The hypothalamus receives a vast array of signals from the CNS, including sensory, emotional, and higher cognitive inputs. This allows the hypothalamus to respond to changes in the internal and external environment by adjusting hormone levels accordingly.

Neural Circuit Coordination: Neurons within the hypothalamus create circuits that integrate the endocrine system with the autonomic nervous system (ANS). This integration enables the hypothalamus to initiate endocrine responses to a variety of physiological states and environmental stimuli, ensuring a coordinated response that maintains homeostasis.

Question 14

Hypothalamus Key Functions and Hormonal Regulation

Answer 14

Homeostasis: Through its influence on the pituitary gland and its ability to integrate neural and endocrine functions, the hypothalamus plays a critical role in maintaining homeostasis. It regulates vital processes such as temperature control, hunger and satiety, water balance, sleep cycles, and emotional responses.

Endocrine Regulation: The hypothalamus produces several hormones that directly affect pituitary gland function. These include corticotropin-releasing hormone (CRH), gonadotropin-releasing hormone (GnRH), growth hormone-releasing hormone (GHRH), somatostatin (inhibits growth hormone release), and dopamine (inhibits prolactin release).

Question 15

Pituitary - Structure/Location

Answer 15

The pituitary gland, also known as the hypophysis, is a small but crucial part of the human endocrine system. Located in the sella turcica, a saddle-shaped depression in the sphenoid bone at the base of the brain, it sits directly under the hypothalamus. The two parts of the brain are connected by a structure called the infundibular (hypophyseal) stalk, which allows for both neural and vascular communication between the hypothalamus and the pituitary gland. The pituitary gland is often referred to as the “master gland” because it controls the functions of many other endocrine glands.

It is divided into three distinct lobes, each with its own unique functions and hormone productions: the anterior lobe (adenohypophysis), the posterior lobe (neurohypophysis), and the intermediate lobe.

Question 16

Anterior Lobe (Adenohypophysis)

Answer 16

The anterior lobe makes up about three-quarters of the total weight of the pituitary gland. It functions primarily through signals it receives from the hypothalamus via releasing or inhibiting hormones that travel through the hypophyseal portal system. The adenohypophysis secretes several key hormones, including:

Growth Hormone (GH): Stimulates growth of bones and tissues.
Thyroid-Stimulating Hormone (TSH): Stimulates the thyroid gland to produce thyroid hormones.
Adrenocorticotropic Hormone (ACTH): Stimulates the adrenal cortex to release cortisol and other glucocorticoids.
Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH): Control reproductive processes including ovulation and sperm production.
Prolactin (PRL): Stimulates milk production in breastfeeding women.

Question 17

Posterior Lobe (Neurohypophysis)

Answer 17

The posterior lobe is an extension of the hypothalamus and stores and releases hormones produced by the hypothalamus. It does not produce hormones itself but releases them into the bloodstream when signaled by the hypothalamus. The main hormones released by the neurohypophysis include:

Antidiuretic Hormone (ADH, also known as vasopressin): Regulates water balance by controlling the amount of water reabsorbed by the kidneys.

Oxytocin: Stimulates uterine contractions during childbirth and milk ejection during breastfeeding.

Question 18

Pineal Gland

Answer 18

 Located in brain
 Composed of photoreceptive cells
 Primary function is secretion of melatonin

The pineal gland, located in the brain, does not directly have photoreceptive cells in the way that the eyes do. Instead, it receives information about environmental light conditions indirectly through a complex pathway from the eyes. The eyes contain photoreceptor cells (rods and cones) that detect light, and this information is relayed to the brain and ultimately to the pineal gland through neural pathways

The primary function of the pineal gland is to secrete melatonin, a hormone that regulates sleep-wake cycles, also known as circadian rhythms. The production and release of melatonin are influenced by the perception of light and darkness, which is detected by the eyes.

The pineal gland’s secretion of melatonin is inversely related to the amount of light received by the retina. Melatonin levels are low during the day and increase in the evening and throughout the night, peaking in the early hours before dawn.

Question 19

Thyroid Gland

Answer 19

The thyroid gland is a vital endocrine organ located in the front part of the neck, below the Adam’s apple. It plays a crucial role in metabolism, growth, and development of the human body through the production and release of hormones. The primary hormones produced by the thyroid gland are Thyroxine (T4), Triiodothyronine (T3), and Calcitonin.

Question 20

Thyroxine (T4)

Answer 20

Production: T4 is the primary hormone produced by the thyroid gland, constituting about 90% of the thyroid hormone output.

Function: Despite being less active than T3, T4 acts as a precursor to T3, meaning that it is converted into T3 in target tissues by the removal of one iodine atom. T4 plays a vital role in regulating metabolism, influencing growth, heart rate, body temperature, and metabolism.

Regulation: The production of T4 is regulated by the thyroid-stimulating hormone (TSH), which is secreted by the pituitary gland. The hypothalamus and pituitary gland monitor the level of thyroid hormones in the blood and adjust TSH secretion accordingly to maintain hormonal balance.

Question 21

Triiodothyronine (T3)

Answer 21

Production: T3 is produced in smaller quantities by the thyroid gland, with about 20% of the circulating T3 being directly released by the thyroid. The majority of T3 in the bloodstream is derived from the conversion of T4 into T3 in peripheral tissues.

Potency and Effects: T3 is more potent than T4 and has greater metabolic effects. It is the active form of thyroid hormone that interacts with nuclear receptors inside cells to regulate gene expression, thereby influencing metabolism, energy production, and the regulation of various other physiological processes.

Regulation: Like T4, the production and release of T3 are regulated by TSH from the pituitary gland, with feedback mechanisms ensuring that hormone levels remain within a healthy range.

Question 22

Calcitonin

Answer 22

Production: Calcitonin is produced by the parafollicular cells (C cells) of the thyroid gland, which are located between the follicular cells that produce T3 and T4.

Function: Calcitonin plays a role in calcium and phosphorus metabolism. It helps to regulate blood calcium levels by inhibiting bone resorption by osteoclasts (cells that break down bone) and increasing the excretion of calcium and phosphorus in the urine. This leads to a decrease in blood calcium levels.

Regulation: The secretion of calcitonin is regulated primarily by the calcium levels in the blood; an increase in blood calcium levels stimulates calcitonin secretion.

Lowers serum calcium levels by
 Inhibiting transfer of calcium from bone to the blood
 Increasing calcium storage in bone
 Increasing renal excretion of calcium and phosphorus

Question 23

Role of Iodine

Answer 23

Iodine is an essential component in the synthesis of T3 and T4 hormones. Thyroid cells absorb iodine from the bloodstream and incorporate it into amino acids to form T3 and T4. The difference between these two hormones lies in the number of iodine atoms; T4 contains four iodine atoms, while T3 contains three. Without sufficient iodine, the thyroid cannot produce adequate amounts of these hormones, leading to various metabolic disorders.

Question 24

Effects of T3 and T4

Answer 24

Metabolic Rate and Caloric Requirements: These hormones regulate the body’s metabolic rate—the rate at which the body uses energy. An increase in T3 and T4 levels accelerates the metabolic rate, leading to higher caloric requirements.

Oxygen Consumption: T3 and T4 stimulate oxygen consumption by most tissues in the body, contributing to the basal metabolic rate (BMR). This is crucial for maintaining body temperature and supporting various metabolic functions.

Carbohydrate and Lipid Metabolism: They play significant roles in the metabolism of carbohydrates and lipids. T3 and T4 stimulate the breakdown of carbohydrates for energy and influence lipid metabolism by promoting the mobilization of fats from adipose tissue and affecting cholesterol levels.

Growth and Development: Thyroid hormones are essential for normal growth and development, particularly in the central nervous system. In children, adequate levels of T3 and T4 are crucial for brain development and growth.

Brain Function and Nervous System Activities: Beyond development, thyroid hormones continue to affect brain function and are involved in maintaining the activities of the nervous system, influencing mood, cognition, and overall neurological function.

Question 25

Parathyroid Gland

Answer 25

Parathyroid glands
 Parathyroid hormone (PTH)
* Regulates serum calcium levels
* Stimulates renal conversion of vitamin D to its most active form
* Regulated by a negative feedback system

Question 26

Adrenal Glands

Answer 26

Small, but critically important
Located on top of each kidney

Each gland composed of adrenal cortex + adrenal medulla

The adrenal medulla, the inner part of the gland, plays a key role in the body’s response to stress through the secretion of catecholamines, including epinephrine (adrenaline), norepinephrine (noradrenaline), and, to a lesser extent, dopamine.

These hormones are both neurotransmitters and hormones, serving as essential components of the sympathetic nervous system’s (SNS) “fight-or-flight” response.

Question 27

Catecholamines and Their Functions

Answer 27

Epinephrine (Adrenaline): This is the primary hormone produced by the adrenal medulla, making up about 80% of its catecholamine output. Epinephrine prepares the body for quick action by increasing heart rate, expanding air passages of the lungs, increasing blood flow to muscles, and stimulating the body to release stored energy (glucose and fatty acids). It also diverts blood away from less critical areas like the gastrointestinal tract.

Norepinephrine (Noradrenaline): This hormone acts as both a hormone and a neurotransmitter. As a hormone, it complements the actions of epinephrine but has a greater influence on peripheral vasoconstriction, which increases blood pressure. As a neurotransmitter, it is involved in alertness, arousal, and the readiness for action.

Dopamine: In the context of the adrenal medulla, dopamine acts as a precursor to the production of norepinephrine and epinephrine. It is released in smaller amounts compared to the other catecholamines and plays a role in the regulation of blood flow in certain areas of the body.

Question 28

The “Fight-or-Flight” Response

Answer 28

The “fight-or-flight” response is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival. It prepares the body to either fight the threat or flee to safety. The adrenal medulla’s secretion of catecholamines is pivotal in triggering this response, which involves:

Increased Heart Rate and Blood Pressure: To ensure that muscles receive more oxygen and nutrients for potential physical exertion.

Increased Blood Glucose Levels: To provide immediate energy supplies to the body’s cells.

Increased Blood Flow to Muscles: By constricting blood vessels in less essential areas and dilating those in muscles and the heart.

Dilation of Air Passages: Allowing for greater oxygen intake to meet the increased oxygen demand.

Question 29

Adrenal Cortex

Answer 29

The adrenal cortex is the outer portion of the adrenal glands, which are located atop each kidney. It plays a vital role in the body’s stress response, metabolism, immune response, and regulation of fluid and electrolyte balance through the production of several types of steroid hormones. These hormones include glucocorticoids (such as cortisol), mineralocorticoids (such as aldosterone), and adrenal androgens.

Question 30

Glucocorticoids (Cortisol)

Answer 30

Anti-inflammatory Effects: Cortisol exerts potent anti-inflammatory and immunosuppressive actions. It inhibits the functions of white blood cells and other components of the immune system to reduce inflammation.

Effects on Glucose Metabolism: Cortisol plays a crucial role in glucose metabolism. It stimulates gluconeogenesis, the process of producing glucose from non-carbohydrate sources, which raises blood glucose levels. This is especially important during periods of stress when the body requires additional energy. Cortisol also affects the metabolism of fats, proteins, and carbohydrates to maintain blood glucose levels and support the body’s stress response.

Stress Response: Cortisol is known as the “stress hormone” because its levels increase in response to stress. It helps the body to cope with stress by providing it with the energy it needs to respond to a stressful situation.

Regulates blood glucose concentration by stimulating hepatic glucose formation
* Decreases inflammatory response by stabilizing membranes of cellular lysosomes and preventing increased capillary permeability
* Stress, burns, infection, fever, acute anxiety and hypoglycemia increase levels

Question 31

Mineralocorticoids (Aldosterone)

Answer 31

Fluid and Electrolyte Balance: Aldosterone’s primary role is to regulate the body’s balance of sodium and potassium, and consequently, maintain blood pressure and fluid balance. It promotes sodium reabsorption and potassium excretion by the kidneys. This reabsorption of sodium leads to water retention, which increases blood volume and blood pressure.

Regulation of Blood Pressure: Through its actions on sodium and water retention, aldosterone plays a critical role in regulating blood pressure and ensuring adequate blood flow to organs.

Maintains extracellular fluid volume
* Acts on renal tubule to promote renal reabsorption of sodium and excretion of potassium and hydrogen ions

Question 32

Adrenal Androgens

Answer 32

Sex Hormone Precursors: The adrenal cortex produces androgens, male sex hormones, in both men and women. Although these hormones are produced in much larger quantities by the testes in males, adrenal androgens can still influence traits such as muscle mass and libido. In females, they are precursors to estrogens and are involved in the development of secondary sexual characteristics.

Development and Puberty: Adrenal androgens contribute to the onset of puberty and influence the growth of pubic and axillary hair. They also have effects on skin, contributing to the oiliness of skin and the development of acne.

Question 33

Pancreas

Answer 33

The pancreas is a unique organ that serves both exocrine and endocrine functions, playing a crucial role in digestion and the regulation of blood glucose levels. The exocrine part of the pancreas produces digestive enzymes that are secreted into the small intestine to aid in the digestion of fats, proteins, and carbohydrates.

The endocrine component, however, is attributed to the Islets of Langerhans, which contain four main types of hormone-secreting cells: alpha (α) cells, beta (β) cells, delta (δ) cells, and F (or PP) cells. Each type of cell produces different hormones that are vital for maintaining homeostasis within the body.

The integrated functions of these cells within the Islets of Langerhans ensure the body’s blood glucose levels are tightly regulated, which is essential for energy production and overall metabolic balance. Dysfunction in any of these cells can lead to metabolic disorders, most notably diabetes mellitus, highlighting the importance of the pancreas in endocrine health.

Question 34

α (Alpha) Cells

Answer 34

Hormone Secreted: Glucagon

Function: Glucagon plays a critical role in glucose metabolism. When blood glucose levels are low, alpha cells release glucagon into the bloodstream. Glucagon then signals the liver to convert stored glycogen into glucose (a process known as glycogenolysis) and release it into the bloodstream, thereby increasing blood glucose levels. Glucagon also promotes gluconeogenesis, which is the production of glucose from non-carbohydrate sources.

Question 35

β (Beta) Cells

Answer 35

Hormone Secreted: Insulin

Function: Insulin is the key regulator of blood glucose levels. When blood glucose levels are high, such as after eating, beta cells secrete insulin. Insulin facilitates the uptake of glucose by cells throughout the body, where it is used for energy, and promotes the storage of glucose in the liver and muscle cells as glycogen. Insulin also inhibits gluconeogenesis, helping to lower blood glucose levels to a normal range.

Insulin Role:
Main regulator of metabolism and storage of ingested
carbohydrates, fats, and proteins
* Facilitates glucose transport into cells, transport of amino acids across muscle membranes, and synthesis of amino acids into protein in peripheral tissues

Question 36

δ (Delta) Cells

Answer 36

Hormone Secreted: Somatostatin

Function: Somatostatin has a variety of functions, including the regulation of insulin and glucagon secretion. It acts as an inhibitor, preventing the over-secretion of these hormones. Additionally, somatostatin slows down the absorption of nutrients from the gastrointestinal tract, helping to ensure that blood glucose levels do not spike too rapidly after a meal.

Question 37

F (or PP) Cells

Answer 37

Hormone Secreted: Pancreatic Polypeptide

Function: Pancreatic polypeptide is involved in the regulation of both the endocrine and exocrine pancreatic functions. It influences the regulation of gastrointestinal motility and the secretion of digestive enzymes from the pancreas. Pancreatic polypeptide also has effects on liver glycogen levels and may play a role in controlling appetite.

Question 38

Glucagon in More Depth

Answer 38

Glucagon is a critical hormone in glucose metabolism and energy regulation, produced and secreted by alpha (α) cells of the pancreatic Islets of Langerhans. Its primary role is to raise blood glucose levels, ensuring a constant energy supply for the body, especially during fasting states or periods of low glucose availability. Glucagon’s action contrasts with that of insulin, which lowers blood glucose levels.

Glycogenolysis
Process: Glycogenolysis is the breakdown of glycogen, a stored form of glucose found in the liver and muscle tissues, into glucose molecules.
Purpose: The primary purpose of glycogenolysis is to quickly release glucose into the bloodstream, increasing blood glucose levels to meet the body’s energy demands, especially between meals or during physical activity.

Gluconeogenesis
Process: Gluconeogenesis is the production of glucose from non-carbohydrate sources, such as amino acids (from protein degradation) and glycerol (from fat metabolism).
Purpose: This process is crucial during prolonged fasting or starvation, when glycogen stores are depleted. Gluconeogenesis ensures a continuous supply of glucose for tissues that are dependent on it, such as the brain and red blood cells.

Ketogenesis
Process: Ketogenesis is the production of ketone bodies in the liver from fatty acids and ketogenic amino acids.
Purpose: Ketone bodies serve as an alternative energy source when glucose levels are low, particularly for the brain. Although not directly involved in increasing blood glucose levels, ketogenesis is important for maintaining energy balance during prolonged periods of low glucose availability, such as fasting or low carbohydrate diets. Glucagon promotes ketogenesis by stimulating the breakdown of fats (lipolysis), which releases fatty acids that are then converted to ketone bodies.

Question 39

Gerontologic Considerations
Effects of Aging on Endocrine System

Answer 39

 Decreased production and secretion
 Altered metabolism and biologic activity
 Decreased responsiveness
 Changes in circadian rhythms
 Assessment may be difficult
 Co-morbid conditions and medications that change
body’s usual response

Question 40

Endocrine Issues - too much or too little of a hormone

Answer 40

• Onset of symptoms is often gradual
• Vague symptoms can be misinterpreted
• Patients may present with acute symptoms may need
  immediate intervention for life-threatening conditions

Question 41

Thyroid Studies

Answer 41

Thyroid studies
 TSH is most sensitive and accurate thyroid test
 Additional tests include total T4, free T4, and total T3

Parathyroid laboratory studies
 Abnormal PTH levels are reflected in calcium and phosphate levels