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Endocrine System: Hormone Mechanisms and Regulation - Prof. Allen, Lecture notes of Anatomy

University of Hong Kong Anatomy

Prof. Nora Allen

An overview of the endocrine system, focusing on how it controls and integrates body processes through hormones. It discusses the types of glands (exocrine and endocrine), chemical messengers (hormones, autocrines, paracrines), and the action of hormones on target cells. The document also covers hormone release, circulation, and the roles of various hormones, including those from the pituitary, thyroid, and parathyroid glands. It explains the mechanisms of hormone action, including water-soluble and lipid-soluble hormones, and the regulation of hormone levels through feedback systems. Structured to provide a clear understanding of the endocrine system's functions and regulatory processes, making it a valuable resource for students studying physiology and endocrinology.

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The Endocrine System

Endocrine System Overview

• Endocrine system acts with the nervous

system of body cells

o to coordinate and integrate the

activity of our body cells.

• Influences metabolic activities via hormones

transported in blood

• Responses are slower but longer lasting than

nervous system responses

• Endocrinology: study of hormones and

endocrine organs

Controls and integrates body processes

how does the endocrine system actually control

and integrate our body’s processes?

- it controls and integrates things like

reproduction, maintenance of

electrolytes, water, and nutrient balance,

growth and development, regulation of

our cellular metabolism and our energy

balance and mobilization of body

defenses

Types of Glands

Exocrine glands

• Produce non-hormonal substances

(examples: sweat, saliva, oils)

• Have ducts to carry secretions to membrane

surface

Endocrine glands

• Produce hormones

• Lack ducts

The endocrine system is going to be made up of

several glands including:

• the pituitary gland found in the brain

• the thyroid and the parathyroid glands

found in the cervical region of the body.

• The adrenal glands, which are going to be

attached to the kidneys

• pineal gland which is found in the brain.

• The hypothalamus is also considered as a

neuroendocrine gland because it has both

nervous as well as endocrine tissues, and

does send hormones to other endocrine

glands.

Some organs also have exocrine and endocrine

functions. This includes:

• the pancreas, which is going to secrete

insulin but also secretes some exocrine

secretions as well.

• gonads, which are the testis in males and

the ovaries in females

• In a pregnant woman, the placenta has

both exocrine and endocrine function as

well.

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The Endocrine System Endocrine System Overview

Endocrine system acts with the nervous system of body cells o to coordinate and integrate the activity of our body cells.
Influences metabolic activities via hormones transported in blood
Responses are slower but longer lasting than nervous system responses
Endocrinology: study of hormones and endocrine organs Controls and integrates body processes how does the endocrine system actually control and integrate our body’s processes?
it controls and integrates things like reproduction, maintenance of electrolytes, water, and nutrient balance, growth and development, regulation of our cellular metabolism and our energy balance and mobilization of body defenses Types of Glands Exocrine glands
Produce non-hormonal substances (examples: sweat, saliva, oils )
Have ducts to carry secretions to membrane surface Endocrine glands
Produce hormones
Lack ducts The endocrine system is going to be made up of several glands including:
the pituitary gland found in the brain
the thyroid and the parathyroid glands found in the cervical region of the body.
The adrenal glands, which are going to be attached to the kidneys
pineal gland which is found in the brain.
The hypothalamus is also considered as a neuroendocrine gland because it has both nervous as well as endocrine tissues, and does send hormones to other endocrine glands. Some organs also have exocrine and endocrine functions. This includes:
the pancreas, which is going to secrete insulin but also secretes some exocrine secretions as well.
gonads, which are the testis in males and the ovaries in females
In a pregnant woman, the placenta has both exocrine and endocrine function as well.

Other tissues and organs that produce hormones:

Adipose cells (fat cells)
Thymic cells (walls of small intestine, stomach, the kidneys and the heart) Chemical Messengers
our bodies use in order to get signals to certain cells HORMONES AUTOCRINES PARACRINES Long-distance chemical signals that travel from one place to another place by blood or lymph Chemicals that released by a cell and exert effects on the same cells that secrete them Locally acting chemicals that affect cells nearby without affecting those that secrete
Autocrine and paracrine, because of their local chemical messaging are not considered part of the endocrine system. So when we're talking about the endocrine system, we're usually specifically talking about hormones. Hormone Chemical Structure TWO MAIN CLASSES OF HORMONES: Amino acid-based hormones Steroids
Amino acid derivatives, peptides, and proteins
Synthesized from cholesterol
make up Gonadal and adrenocortical hormones A possible third class, called eicosanoids is considered a hormone by some scientists, but most classify it as a paracrine. ACTION OF HORMONES how do hormones actually act to perform their function?
though hormones can circulate all throughout the body, only cells with receptors for that hormone are able to be affected by those hormones, these cells are referred to as our target cells, and these are tissues that have receptors for specific hormones; And hormones will then alter the activity of this target cell. HORMONE ACTION ON TARGET CELLS MAY BE TO:
how hormones act on these on these target cells

Hormonal Stimuli

Hormones stimulate other endocrine organs to release their hormones.
Example: hormones of the hypothalamus A good example of this is the hormones of the hypothalamus.
Hypothalamic hormones stimulate the release of most of our anterior pituitary hormones. o Anterior pituitary hormones stimulate target cells to secrete more hormones.
Activity of the hypothalamic pituitary target endocrine organ feedback loop are going to cause hormones from the final target organs to inhibit release of the anterior pituitary organ.
The anterior pituitary releases hormones that are going to trigger the release of things like testosterone. Consequently, high testosterone levels inhibit the release of those hormones. Nervous System Modulation ➢ The nervous system can make adjustments to hormone levels when needed. ➢ It can modify stimulation or inhibition of endocrine glands. ➢ The nervous system can override normal endocrine controls. o An example of this is when you're under severe stress. ▪ The hypothalamus and your sympathetic nervous system can override the insulin from the pancreas to allow blood glucose levels to increase and prepare your body for the flight or fight response. Target Cell Specificity ➢ Target cells must have specific receptors to which hormones bind. ➢ For example, ACTH receptors are found only on certain cells of the adrenal cortex, but thyroxin receptors are found on nearly all cells of the body. Target cell activation depends on 3 factors: ▪ BLOOD LEVELS OF HORMONE ▪ RELATIVE NUMBER OF RECEPTORS ON/IN TARGET CELL ▪ AFFINITY (STRENGTH) OF BINDING BETWEEN RECEPTOR AND HORMONE The amount of hormone can influence the number of receptors for that hormone.
Up-regulation: target cells form more receptors in response to low hormone levels o allows for the cell to be more sensitive to the available hormones
Down-regulation: target cells lose receptors in response to high hormone levels ▪ Desensitizes the target cells to prevent them from overreacting to persistently high levels of hormone

Hormone Circulation

Hormones circulate in the blood either free or bound.
Steroids and thyroid hormone are attached to plasma proteins.
All others circulate without carriers. Concentration of circulating hormone reflects:
Rate of release
Speed at which it is inactivated and removed from the body Hormones can be removed from the blood by:
DEGRADING ENZYMES
KIDNEYS
LIVER

Half - life of Hormone Activity

measure how long a hormone remains or is removed from the body Half-life is the time required for the level of hormone in blood level to decrease by half.
It varies anywhere from a fraction of a minute to a week, depending on the hormone.

Onset of Hormone Activity

a value that measure of hormonal activity
onset refers to the amount of time it takes for target cells to react to hormones. Hormones have different response times:
Some responses are immediate.
Some, especially steroids, can take hours to days.
Some are inactive until they enter target cells.

Duration of Hormone Activity

The duration of the response is usually limited.
The duration of the Response measures how long the hormonal response actually lasts.
It ranges from 10 seconds to several hours.
Effects may disappear rapidly as blood levels drop, but some may persist for hours at low blood levels. Interaction of Hormones at Target Cells
Multiple hormones may act on the same target at the same time. PERMISSIVENESS SYNERGISM ANTAGONISM
One hormone cannot exert its effects without another hormone being present. Example: Reproductive hormones need thyroid hormone to have effect.
More than one hormone produces the same effects on a target cell, causing amplification. Example: Glucagon and epinephrine both cause the liver to release glucose.
One or more hormones oppose the action of another hormone. Example: Insulin decreases blood glucose levels while glucagon increases blood glucose levels.

The Hypothalamus and Pituitary Gland

The hypothalamus is connected to the pituitary gland (hypophysis) via a stalk called the infundibulum.
The pituitary secretes at least 8 major hormones.

Antidiuretic Hormone (ADH) The hypothalamus contains osmoreceptors that monitors a solute concentration. - If the concentration of a solute is too high, the posterior pituitary is triggered to secrete antidiuretic hormone. o This hormone targets the kidney tubules and causes the reabsorption of more water to inhibit or prevent urine formation. ▪ this is where this hormone gets its name as diuresis means your urine formation and this is Anti diuresis. - release is also triggered by pain, low blood pressure and certain drugs. - Antidiuretic hormone can also be inhibited by alcohol and certain drugs called diuretics. - High concentrations of ADH can also cause vasoconstriction and so sometimes ADH is referred to as Vasopressin B. Anterior Pituitary-hypothalamic Relationships The hypothalamus secretes releasing and inhibiting hormones to the anterior pituitary to regulate hormone secretion. Anterior Pituitary Hormones ALL 6 HORMONES ARE PEPTIDE HORMONES.
- Luteinizing hormone (LH)
  - Follicle-stimulating hormone (FSH)
  - Thyroid-stimulating hormone (TSH)
  - Prolactin (PRL)
  - Growth hormone (GH)
  - Adrenocorticotropic hormone (ACTH) All but 2 are tropic hormones(tropins)that regulate the secretion of other hormones.

Except the PRL AND GH

Growth Hormone (GH)

GH is also called somatotropin as it is produced by somatotropic cells.
Tropic hormone, also called somatotropin
Origin: somatotropic cells
Function: direct actions on metabolism and indirect grow-promoting actions DIRECT ACTIONS ON METABOLISM ➢ Glucose-sparing actions decrease rate of cellular glucose uptake and metabolism (anti-insulin effects) ➢ Triggers liver to break down glycogen into glucose ➢ Increases blood levels of fatty acids for use as fuel and encourages cellular protein synthesis INDIRECT ACTIONS ON GROWTH ➢ GH triggers liver, skeletal muscle, and bone to produce insulin-like growth factors (IGFs) ➢ IGFs stimulate cellular uptake of nutrients to: ▪ Synthesize DNA and proteins needed for cell division ▪ Formation of collagen ▪ Deposition of bone matrix ➢ GH stimulates most cells to enlarge and divide o (major targets: bone and skeletal muscle) Growth Hormone (GH) Regulation of Secretion
GH release or inhibition is chiefly regulated by hypothalamic hormones on somatotropic cells

Negative Feedback Loop for TSH Thyroid stimulating hormone - This is a tropic hormone that is also sometimes referred to as thyrotropin - Produced by thyrotropic cells that stimulate the normal development and secretory activity of the thyroid. - Its release is going to be triggered by thyrotropin releasing hormone in the hypothalamus - It is inhibited by rising blood levels of thyroid hormone that then act on the pituitary and the hypothalamus - It is also inhibited by the growth of hormone-inhibiting hormone Negative feedback loop for the thyroid stimulating hormone - Hypothalamus is going to release thyrotropin releasing hormone which goes to the anterior pituitary o This stimulates the release of thyroid stimulating hormone, which then act on the thyroid gland causing the thyroid to release thyroid hormones, which will then go to our target cells - Increasing blood levels of thyroid hormone o Act as a negative feedback loop and it inhibits the release of thyrotropin releasing hormone from the hypothalamus as well as thyroid stimulating hormone from the anterior pituitary 3. Adrenocorticotropic Hormone (ACTH) - Also called corticotropin Origin: (secreted by) corticotropic cells Precursor: pro-opiomelanocortin Function: stimulates adrenal cortex to release corticosteroids(e.g. cortisol and androsterone) - Regulation of ACTH release is triggered by the hypothalamic corticotropin releasing hormone in daily rhythms with the highest levels of this hormone being released in the morning Internal and external factors that alter release of CRH (corticotropin-releasing hormone) include: ➢ Fever ➢ hypoglycemia (low blood sugar), ➢ stressors - For example, stress triggers the release of corticotropin-releasing hormone, which then leads to the production of cortisol, which we sometimes refer to as stress hormone 4. Gonadotropins (FSH and LH) - these are follicle stimulating hormone and luteinizing hormone - secreted by the gonadotropic cells of the anterior pituitary

parafollicularcells o located outside the follicles

Thyroid Hormone (TH)

released by Thyroid Gland
Body’s major metabolic hormone, which is found in the 2 forms : T4 (THYROXINE) T3 (TRIIODOTHYRONINE) Both are iodine-containing amine hormones.
T4 or Thyroxine o Major form consisting of 2 tyrosine molecules and 4 bound iodine atoms
T3 form or triiodothyronine o Form that has 2 tyrosine and only 3 bound iodine atoms o Can be created from T4 by enzymes found at the tissue level Thyroid hormone
Unique hormone
Affects virtually every cell in the body o There are receptors on pretty much every cell in our body
It enters the target cells and binds to intracellular receptors within the nucleus of the target cell
It then triggers transcription of various metabolic genes The effects of thyroid hormone o Increase of the basal metabolic rate o heat production, which we refer to as calorie genic effect Body’s major metabolic hormone (Thyroid hormone), which is found in 2 form : Regulates tissue growth and development Maintains blood pressure Critical for normal skeletal and nervous system development and reproductive capabilities Increases adrenergic receptors in blood vessels in order to maintain blood pressure

Calcitonin

Produced by parafollicular(C) cells in response to high Ca 2+ (blood calcium) levels
Antagonist to parathyroid hormone (PTH)

There is no known physiological role in humans at normal physiological levels for calcitonin
However, at higher than normal doses, it's going to work to inhibit osteoclast activity in the bone and prevent the release of calcium from the bone matrix. o It also is going to stimulate calcium uptake and incorporation of that calcium into the bone matrix.

Parathyroid Gland

4 - 8 tiny yellow-brown glands embedded in the posterior surface of the thyroid. Contain two types of cells:
oxyphil cells o function not clear
parathyroid cells o which secrete parathyroid hormone (PTH) or parathormone Parathyroid Hormone (PTH)
PTH is the most important hormone in Ca2+ (calcium) homeostasis ▪ Secreted in response to low blood levels of Ca2+ (calcium) and triggers osteoclast activity to break down bone and release calcium into the blood ▪ Inhibited by negative feedback loop with rising levels of Ca2+ Target organs: skeleton, kidneys, and intestine Functions of Parathyroid Hormone (PTH)
Parathyroid hormone is going to stimulate osteoclasts to digest bone matrix and release calcium into the blood.
It also works by enhancing reabsorption of calcium and secretion of phosphate by the kidneys.
Lastly, parathyroid hormones can promote activation of vitamin D by the kidneys. o Vitamin D leads to an increased absorption of calcium by our intestinal mucosa

Adrenal Gland

Structurally and functionally

Adrenal gland is actually 2 glands in one o Adrenal cortex ▪ Outer portion of adrenal gland o Adrenal medulla ▪ Inner portion of adrenal gland

Adrenal Cortex: Glucocorticoids Adrenal Cortex- Actions of Cortisol Gluconeogenesis

Prime metabolic effect
It encourages ourselves to use fatty acids for fuel so that we can save our glucose that we need for the brain. o Brain can also use glucose for energy
Enhances vasoconstriction o vasoconstriction causes a rise in blood pressure which allows for a quick distribution of nutrients to cells in that area. Excessive levels of glucocorticoids
Depress cartilage and bone formation
Inhibit inflammation by decreasing release of inflammatory chemicals
which is why sometimes people who are in pain get steroid shots
Depress immune system
cortisol has the ability to Disrupt normal cardiovascular, neural, and gastrointestinal functions Glucocorticoid drugs can control symptoms of many inflammatory diseases (arthritis, allergies) but can also cause undesirable effects. Adrenal Cortex: Gonadocorticoids
secretes weak androgens (male sex hormones) are converted to testosterone in tissue cells, some to estrogens. May contribute to:
Onset of puberty and appearance of secondary sex characteristics
Sex drive in women
Source of estrogens in postmenopausal women B. Adrenal Medulla
Deep to the adrenal cortex
Contains Medullary chromaffin cells that synthesize catecholamines: epinephrine (80%) and norepinephrine (20%) Effects of catecholamines:
Vasoconstriction
Increased heart rate
Increased blood glucose levels
Blood diverted to brain, heart, and skeletal muscle Both hormones have basically same effects, but: Epinephrine is more a stimulator of metabolic activities (Example: bronchial dilation, and blood flow to skeletal muscles and heart). Norepinephrine has more of an influence on peripheral vasoconstriction and blood pressure.

Responses to stressors by adrenal medulla are usually brief, unlike adrenal cortical hormones.

Pineal Gland

The pineal gland is a small gland that is hanging from the roof of the third ventricle.
Contains Pinealocytes that secrete melatonin, which is derived from serotonin. Melatonin may affect: Timing of sexual maturation and puberty Day/night cycles o Referred to as circadian rhythm Physiological processes that show rhythmic variations (body temperature, sleep, appetite) Production of antioxidant and detoxification molecules in cells Other Endocrine Organs Pancreas
The pancreas is a triangular gland located partially behind the stomach.
It has both exocrine and endocrine cells.
Pound in pancreatic islets

Pancreatic Islets: Glucagon

Released by alpha cells of the pancreas
Extremely potent hyperglycemic agent
Triggered by decreased blood glucose levels, rising amino acid levels, or sympathetic nervous system
The brain in particular has an absolute requirement for glucose in order to perform its metabolic functions, so this hormone is important because it helps to maintain our blood glucose homeostasis, especially during periods of fasting and starvation. Raises blood glucose levels by targeting liver to:
Break down glycogen into glucose (glycogenolysis)
Synthesize glucose from lactic acid and other noncarbohydrates (gluconeogenesis)
Release glucose into the blood

Pancreatic Islets: Insulin

Produced by the beta cells of the pancreas
Secreted when blood glucose levels increase
Synthesized as proinsulin that is then modified

Endocrine Function throughout Life Most endocrine organs operate well until old age, however:

growth hormone levels begin to decline with age and accounts for things like muscular atrophy as we get older
the thyroid hormones also decline with age and contribute to our lowering of our basal metabolic rate or how our metabolism slows as we age.
The parathyroid hormone levels are going to remain fairly constant with age, however, in older women because of the lack of estrogen, they can become more vulnerable to bone demineralization. o Effect of parathyroid hormone
Glucose tolerance can also deteriorate with age and is usually associated with insulin and Glucagon.
as we age, the ovaries undergo significant changes and become unresponsive to gonadotropins. o This problem is associated with an estrogen deficiency
In males, testosterone also begins to diminish with age. o effects are usually not seen until very old age. In a Nutshell ✓ The endocrine system acts with the nervous system. It coordinates and integrates the activity of body cells. ✓ Hormones act by binding to specific target cells and altering cell activity. ✓ There are several endocrine glands throughout the body, which secrete specific hormones and target specific cells. ✓ The pituitary gland secretes 8 different hormones: 2 posterior pituitary hormones and 6 anterior pituitary hormones.
many of these hormones control other endocrine glands. ✓ The thyroid gland secretes thyroid hormone (metabolic regulator). ✓ The parathyroid secretes parathyroid hormone (Ca2+ levels). ✓ The adrenal gland secretes hormones responsible for several different body activities (regulation of blood pressure and stress response). ✓ The pineal gland secretes the hormone melatonin (circadian rhythm). ✓ Other organs in the body with endocrine function include the pancreas, gonads, and placenta TRUE OR FALSE _____ 1. Endocrine glands are going to produce non- hormonal substances like sweat, saliva or oils. _____ 2. Paracrines are locally-acting chemicals that affect cells that are nearby. _____ 3. Thyroid stimulating hormone is referred to as a somatotropin because it is produced by somatotropic cells. It has direct actions on metabolism and indirect actions on growth promotion. _____ 4. The concentration of circulating hormones usually reflects its rate of release as well as the speed of which the hormone is inactivated or removed from the body. _____ 5. The endocrine system can make adjustments to hormone levels when needed. This is referred to as nervous system modulation. _____ 6. Prolactin is going to be secreted by the prolactin cells of the posterior pituitary. This directly stimulates milk production in females. _____ 7. Follicle-stimulating hormone is going to be responsible for the production of the female gametes only which is the ovum.

____ 8. The thyroid hormone is a unique hormone and that it affects virtually every cell in the body. _____ 9. Glucagon is going to raise our blood glucose levels by targeting the liver to break down glycogen into glucose also known as gluconeogenesis. _____ 10. The pituitary gland secretes 8 different hormones, 2 from the posterior pituitary and 6 from the anterior pituitary