Anatomy and Physiology – Excretory System

Anatomy and Physiology – Excretory System

The Excretory System:

• The excretory system is the system of an organism's body that performs
• the function of excretion, the bodily process of discharging wastes.
• The Excretory system is responsible for the elimination of wastes produced
• by homeostasis.
• The excretory system is essential to one‘s health. Its responsibility is to
• remove waste from the body. The excretory system is made up of numerous
• organs that work in unison to ensure that waste is effectively removed from
• your body. Below are the details of the excretory system organs, along with

the roles they play in detoxification.

Primary Excretory System Organs

• Kidneys
• Kidneys are bean-shaped organs of a reddish brown color that are
• found in the sides of the vertebral column. Once the body has
• extracted what it needs from food and drink, it sends the wastes to the
• kidneys. The kidneys filter the wastes, including urea, salt and
• excess water, which are flushed out of the body as urine.
• Wastes in the blood come from tissue breakdown, and from food.
• After the body takes and uses what it needs, it sends the wastes to the
• kidneys. The kidneys are bean-shaped organs, about the size of a
• fist. Having atleast one kidney is mandatory for living, unless treated
• immidietally. Because of the enormous amounts of blood passing
• through the kidney, kidneys sxtract 180 liters of fluid daily. If we
• extracted all of the fluid as urine, we would lose nutrients and
• dehydrate, Kidneys are responsible for filtering the filtrate and
• returning most of the solutes and water to the blood.
• Skin
• The skin performs its excretory function via the sweat glands. These
• glands produce sweat that contains salt, excess oils, water, and other
• unnecessary substances which are then excreted out of the body
• through small pores. Sweating also helps to cool the body during
• evaporation. Your skin has a very important part in the excretory
• system. It holds moisture into every part of your body. In the
• excretory system, the skins job is to regulate one's body temperature.
• The salt in the skin helps in evaporation of the water off of the body,
• to cool off one who is hot. Sweat is excreted through sweat glands.
• Sweating helps the body maintain a cool, consistent temperature,
• which also helps one maintain homeostasis.
• Lungs
• The lungs are very important excretory organs as they expel carbon
• dioxide from the body via exhalation. The lungs use cells known as
• alveoli to remove the carbon dioxide from our blood. Otherwise, the
• carbon dioxide would accumulate and have a detrimental effect to our

body.

Accessory Excretory System Organs

• Liver
• Although considered a secondary, or accessary excretory system
• organ, the liver plays a vital part in keeping the body clean. Harmful
• poisons and chemicals that are either produced in the body or
• consumed are broken down and detoxified by the liver. For
• example, a bi-product of the metabolic process within the body is
• ammonia and the liver processes this into urea, a less harmful
• substance which continues to be filtered and excreted
• as urine. When we eat fatty foods, the liver orders the gall bladder to
• release bile into the intestines. The bile then puts the waste products
• into the intestines. Then the intestines absorb the fats into the blood
• stream. The liver is then given the blood, and removes all waste
• products from it as it passes through. The liver removes the iron,
• which involves red blood cell production. Amino acids are then
• broken down and passed into the kidney. The kidney uses these to
• carryout their function in the excretory system. The liver then stores
• the vitamins, and repeats its cycle.
• Gallbladder
• Although the gallbladder does not have a highly significant role to
• play in the excretory system, it does have a function that assists the
• overall process. Bile, a liquid produced by the liver to break down
• waste, is first stored in the gallbladder. When needed, it is
• discharged into the small intestine whose role is to break down fats,
• ethanol and other acidic wastes.
• Urinary Bladder
• The waste fluid that is created in the liver and collected in the kidney
• is transferred into the urinary bladder where it is temporarily stored
• until the individual urinates. The urinary bladder provides a short
• term solution for storing urine in the body until it is ultimately
• discharged. After the kidneys filter the fluid, the remaining wastes go
• to your bladder. The organ stores the urine, and keeps storing it until
• you can feel it. When you can feel it, it means that the bladder has
• become full, and you must urinate to release the wastes from your
• body.
• Ureters
• The ureters tubes of smooth muscle fiber transfer liquid waste
• from the kidneys into the urinary bladder. The urine is moved with
• peristaltic movements which force the urine away from the kidneys.
• The ureters also have ureterovesical valves which ensure the waste
• fluid does not travel back into the kidney.
• Urethra
• The urethra runs through the penis in males, and serves as a carrier
• of semen as well as urine for their ultimate discharge out of the
• body. The urethra tube is shorter in females and is just above the
• vaginal opening.
• Large Intestine
• Food particles are absorbed into the blood stream via the small
• intestine. The undigested substances are transferred to the large
• intestine which essentially serves as a storage organ for the
• excretory products. The descending, ascending and transverse
• colons also facilitate the absorption of leftover vitamins, water and
• salt. The distal straight section (known as the rectum) is used for the
• storage of waste products (feces) before they are excreted from the
• body via the anal canal with the help of internal and external

sphincters.

Common Diseases Affecting Excretory System Organs

• Kidney Stones
• Kidney stones are believed to form from crystals that have separated
• from urine, forming hard masses in the urinary tract, though the exact
• cause is unknown. Symptoms for kidney stones include extreme pain,
• cramping in the lower abdominals and back, nausea, and vomiting.
• Most kidney stones can be passed by increasing your intake of water
• to flush them out, although surgery may be needed in some cases.
• Urethritis
• Urethritis is a viral or bacterial infection that causes inflammation of
• the urethra. Symptoms for urethritis vary between the sexes.
• Symptoms for men include pain or swelling of the penis, blood in
• urine or semen, frequent urination and pain during ejaculation.
• Symptoms for females include pain during urination, abdominal
• pain, fever, chills, frequent urination, vaginal discharge and pelvis
• pain. Urethritis is usually treated with anti-viral medication, or
• antibiotics. Painkillers are often used to help sufferers combat the
• symptoms.
• Pyelonephritis
• Pyelonephritis is a type of urinary tract infection that travels from the
• urethra or bladder and to the kidneys. This infection occurs when
• bacteria enter the body through the urinary tract. Symptoms include
• frequent urination, burning during urination, blood in the urine, pain
• in the groin and abdominal pain. Pyelonephritis is usually treated with
• oral anti-biotics, although the anti-biotics are sometimes administered
• intravenously in cases of severe infections.
• Cystitis
• Cystitis is the medical term for inflammation of the bladder and it is
• one of the most common disease that affects excretory system organs.
• As the bladder stores urine before it is excreted from the body,
• bacteria can build up in the bladder and cause cystitis.
• Urinary Tract Infection
• Urinary tract infection (UTI) is the infection of the urethra or the
• bladder. The symptoms include abdominal pain, painful or difficult
• urination and fever. The best way to avoid UTI is by drinking loads of
• water.
• The Endocrine glands:
• Endocrine glands are glands of the endocrine system that secrete their
• products, hormones, directly into the blood rather than through a duct.
• The major glands of the endocrine system include the
• pineal gland,pituitary gland, pancreas, ovaries, testes, thyroid
• gland, parathyroidgland, hypothalamus and adrenal glands.
• When hormone levels reach a certain normal amount, the endocrine
• system helps the body to keep that level of hormone in the blood. For
• example, if the thyroid gland has secreted the right amount of thyroid
• hormones into the blood, the pituitary gland senses the normal levels
• of thyroid hormone in the bloodstream. Endocrine
• glands are glands of the endocrine system that secrete their
• products, hormones, directly into the blood rather than through a duct.
• The main endocrine glands include the pituitary gland, pancreas,
• ovaries, testes, thyroid gland, and adrenal glands.
• The main function of endocrine glands is to
• secrete hormones directly into the bloodstream. Hormones are
• chemical substances that affect the activity of another part of the body
• (target site). In essence, hormones serve as messengers, controlling
• and coordinating activities throughout the body.
• Glands are located throughout various parts of the human body. These
• glands take on the critical task of releasing hormones, and as a whole,
• they are most commonly referred to as the endocrine system.
• Major endocrine glands and other organs

Major endocrine glands

Gland Function

• Hypothamlus
• (1)
• The hypothalamus is situated in the brain, at the base of the optic
• chiasm and is attached to the pituitary via a stalk-like structure. It
• acts as a collecting centre for information concerned with the
• internal well being of the body and uses much of this information to
• regulate the secretion of the hormones produced by the pituitary.
• Pituitary (1) The pituitary gland is an important gland and it is often referred to
• as the 'master gland', because it controls several of the other
• hormone glands. It is usually about the size of a pea and is situated
• in a bony hollow beneath the base of the brain and just behind the
• bridge of your nose. The gland consists of two parts (often called
• lobes) each of which has different functions. The pituitary gland is

also sometimes called the Hypophysis.

Thyroid (1) &

• parathyroid (4)
• The thyroid gland is situated in the front part of the neck, near the
• windpipe. Embedded in the rear surface of this gland are four
• parathyroid glands. The thyroid gland controls many body
• functions, including heart rate, temperature and metabolism. Both
• these glands play a role in the metabolism of calcium in the body.
• Adrenals (2) The adrenal glands (each of which weighs about 4 grams and is
• about the size of your thumb) are situated just above the kidneys
• and consist of two parts, the adrenal medulla and the adrenal cortex.
• These glands produce hormones which are essential for life and
• help us cope with stress.
• Pineal (1) The pineal gland is a tiny body located at the base of the brain. It
• produces the hormone melatonin.

Other organs in the body containing endocrine tissue

Gland Function

• Kidneys (2) The kidneys are situated near the middle of the back, just below the
• cage. These glands control the blood fluid and mineral levels within
• body by processing the blood to remove waste products and any exc
• fluid.
• Ovaries (2) The ovaries are situated either side of the uterus. In addition to containi
• the egg cells necessary for reproduction, they produce the hormon
• Oestrogen and Progesterone which are necessary for menstruation a
• producing the other female sexual characteristics.
• Testes (2) The testes are situated in a pouch that hangs outside the male body. Th
• produce the hormones necessary for the production of sperm and the oth
• male sexual characteristics.
• Pancreas (1) In addition to its digestive functions, cells in the pancreas regulate
• blood sugar that provides the body with energy.
• Hypothalamus
• The hypothalamus is a part of the brain located superior and anterior to the
• brain stem and inferior to the thalamus. It serves many different functions in
• the nervous system, and is also responsible for the direct control of the
• endocrine system through the pituitary gland. The hypothalamus contains
• special cells called neurosecretory cells—neurons that secrete hormones:
• Thyrotropin-releasing hormone (TRH)
• Growth hormone-releasing hormone (GHRH)
• Growth hormone-inhibiting hormone (GHIH)
• Gonadotropin-releasing hormone (GnRH)
• Corticotropin-releasing hormone (CRH)
• Oxytocin
• Antidiuretic hormone (ADH)
• All of the releasing and inhibiting hormones affect the function of the
• anterior pituitary gland. TRH stimulates the anterior pituitary gland to
• release thyroid-stimulating hormone. GHRH and GHIH work to regulate the
• release of growth hormone—GHRH stimulates growth hormone release,
• GHIH inhibits its release. GnRH stimulates the release of follicle stimulating
• hormone and luteinizing hormone while CRH stimulates the release of
• adrenocorticotropic hormone. The last two hormones—oxytocin and
• antidiuretic hormone—are produced by the hypothalamus and transported to

the posterior pituitary, where they are stored and later released.

Pineal Gland

• The pineal gland is a small pinecone-shaped mass of glandular tissue found
• just posterior to the thalamus of the brain. The pineal gland produces the
• hormone melatonin that helps to regulate the human sleep-wake cycle
• known as the circadian rhythm. The activity of the pineal gland is inhibited
• by stimulation from the photoreceptors of the retina. This light sensitivity
• causes melatonin to be produced only in low light or darkness. Increased
• melatonin production causes humans to feel drowsy at nighttime when the
• pineal gland is active.
• Thymus
• The thymus is a soft, triangular-shaped organ found in the chest posterior to
• the sternum. The thymus produces hormones called thymosins that help to
• train and develop T-lymphocytes during fetal development and childhood.
• The T-lymphocytes produced in the thymus go on to protect the body from
• pathogens throughout a person‘s entire life. The thymus becomes inactive
• during puberty and is slowly replaced by adipose tissue throughout a
• person‘s life.
• Gonads
• The gonads—ovaries in females and testes in males—are responsible for
• producing the sex hormones of the body. These sex hormones determine the
• secondary sex characteristics of adult females and adult males.
• Testes: The testes are a pair of ellipsoid organs found in the scrotum of
• males that produce the androgen testosterone in males after the start of
• puberty. Testosterone has effects on many parts of the body, including the
• muscles, bones, sex organs, and hair follicles. This hormone causes growth
• and increases in strength of the bones and muscles, including the accelerated
• growth of long bones during adolescence. During puberty, testosterone
• controls the growth and development of the sex organs and body hair of
• males, including pubic, chest, and facial hair. In men who have inherited
• genes for baldness testosterone triggers the onset of androgenic alopecia,
• commonly known as male pattern baldness.
• Ovaries: The ovaries are a pair of almond-shaped glands located in the
• pelvic body cavity lateral and superior to the uterus in females. The ovaries
• produce the female sex hormones progesterone and estrogens. Progesterone
• is most active in females during ovulation and pregnancy where it maintains
• appropriate conditions in the human body to support a developing fetus.
• Estrogens are a group of related hormones that function as the primary
• female sex hormones. The release of estrogen during puberty triggers the
• development of female secondary sex characteristics such as uterine
• development, breast development, and the growth of pubic hair. Estrogen
• also triggers the increased growth of bones during adolescence that lead to
• adult height and proportions.
• The main glands and organs of the endocrine system include:
• Pituitary gland – is inside the brain. It oversees the other glands and keeps
• hormone levels in check. It can bring about a change in hormone production
• somewhere else in the system by releasing its own ‗stimulating‘ hormones.
• The pituitary gland is also connected to the nervous system through part of
• the brain called the hypothalamus. The hormones released
• gland are gonadotropins (LH and FSH), growth hormone (GH), thyroid
• stimulating hormone (TSH), adrenocorticotropic hormone (ACTH),
• prolactin, antidiuretic hormone and oxytocin.
• The pituitary gland, also known as the hypophysis, is a small pea-sized
• lump of tissue connected to the inferior portion of the hypothalamus of the
• brain. Many blood vesselssurround the pituitary gland to carry the hormones
• it releases throughout the body. Situated in a small depression in
• the sphenoid bone called the sella turcica, the pituitary gland is actually
• made of 2 completely separate structures: the posterior and anterior pituitary
• glands.
• Posterior Pituitary: The posterior pituitary gland is actually not glandular
• tissue at all, but nervous tissue instead. The posterior pituitary is a small
• extension of the hypothalamus through which the axons of some of the
• neurosecretory cells of the hypothalamus extend. These neurosecretory cells
• create 2 hormones in the hypothalamus that are stored and released by the
• posterior pituitary:
• 2.
• Oxytocin triggers uterine contractions during childbirth and the
• release of milk during breastfeeding.
• Antidiuretic hormone (ADH) prevents water loss in the body by
• increasing the re-uptake of water in the kidneys and reducing
• blood flow to sweat glands.
• Anterior Pituitary: The anterior pituitary gland is the true glandular
• part of the pituitary gland. The function of the anterior pituitary gland
• is controlled by the releasing and inhibiting hormones of the
• hypothalamus. The anterior pituitary produces 6 important hormones:
• Thyroid stimulating hormone (TSH), as its name suggests, is a
• tropic hormone responsible for the stimulation of the thyroid
• gland.
• Adrenocorticotropic hormone (ACTH) stimulates the adrenal
• cortex, the outer part of the adrenal gland, to produce its
• hormones.
• Follicle stimulating hormone (FSH) stimulates the follicle cells
• of the gonads to produce gametes—ova in females and sperm in
• males.
• Luteinizing hormone (LH) stimulates the gonads to produce the
• sex hormones—estrogens in females and testosterone in males.
• Human growth hormone (HGH) affects many target cells
• throughout the body by stimulating their growth, repair, and
• reproduction.
• Prolactin (PRL) has many effects on the body, chief of which is
• that it stimulates the mammary glands of the breast to produce
• milk.
• Thyroid gland – sits in the neck at the front of the windpipe. It releases
• thyroid hormone (T4 and T3) which is required for metabolism and body
• homeostasis. It is controlled by TSH which is produced
• gland through a feed-back loop. The thyroid gland is a butterfly-shaped
• gland located at the base of the neck and wrapped around the lateral sides of
• the trachea. The thyroid gland produces 3 major hormones:
• Calcitonin

 Triiodothyronine (T3)

• Thyroxine (T4)
• Calcitonin is released when calcium ion levels in the blood rise above a
• certain set point. Calcitonin functions to reduce the concentration of calcium
• ions in the blood by aiding the absorption of calcium into the matrix of
• bones. The hormones T3 and T4 work together to regulate the body‘s
• metabolic rate. Increased levels of T3 and T4 lead to increased cellular
• activity and energy usage in the body
• Parathyroid gland – there are usually four parathyroid glands which lie
• alongside the thyroid gland. The parathyroid gland is involved in calcium,
• phosphate and vitamin D regulation. The parathyroid glands are 4 small
• masses of glandular tissue found on the posterior side of the thyroid gland.
• The parathyroid glands produce the hormone parathyroid hormone (PTH),
• which is involved in calcium ion homeostasis. PTH is released from the
• parathyroid glands when calcium ion levels in the blood drop below a set
• point. PTH stimulates the osteoclasts to break down the calcium containing
• bone matrix to release free calcium ions into the bloodstream. PTH also
• triggers the kidneys to return calcium ions filtered out of the blood back to
• the bloodstream so that it is conserved.
• Adrenal glands – there are two adrenal glands which sit on top of each
• kidney. They make a number of different hormones. The outside part of the
• gland (adrenal cortex) makes cortisol, aldosterone and sex hormones. The
• centre of the adrenal gland (adrenal medulla) makes adrenaline. Adrenaline
• is an example of a hormone that is under the control of the nervous system.
• The adrenal glands are a pair of roughly triangular glands found
• immediately superior to the kidneys. The adrenal glands are each made of 2
• distinct layers, each with their own unique functions: the outer adrenal
• cortex and inner adrenal medulla.
• Adrenal cortex: The adrenal cortex produces many cortical hormones in 3
• classes: glucocorticoids, mineralocorticoids, and androgens.
• Glucocorticoids have many diverse functions, including the breakdown of
• proteins and lipids to produce glucose. Glucocorticoids also function to
• reduce inflammation and immune response.
• Mineralocorticoids, as their name suggests, are a group of hormones that
• help to regulate the concentration of mineral ions in the body.
• Androgens, such as testosterone, are produced at low levels in the adrenal
• cortex to regulate the growth and activity of cells that are receptive to male
• hormones. In adult males, the amount of androgens produced by the testes is
• many times greater than the amount produced by the adrenal cortex, leading
• to the appearance of male secondary sex characteristics.
• Adrenal medulla: The adrenal medulla produces the hormones epinephrine
• and norepinephrine under stimulation by the sympathetic division of the
• autonomic nervous system. Both of these hormones help to increase the flow
• of blood to the brain and muscles to improve the ―fight-or-flight‖ response
• to stress. These hormones also work to increase heart rate, breathing rate,
• and blood pressure while decreasing the flow of blood to and function of
• organs that are not involved in responding to emergencies.
• Pancreas – an organ of digestion which is inside the abdomen. It makes
• insulin, which controls the amount of sugar in the bloodstream. It also makes
• other hormones such as glucagon and somatostatin. The pancreas is a large
• gland located in the abdominal cavity just inferior and posterior to
• the stomach. The pancreas is considered to be a heterocrine gland as it
• contains both endocrine and exocrine tissue. The endocrine cells of the
• pancreas make up just about 1% of the total mass of the pancreas and are
• found in small groups throughout the pancreas called islets of Langerhans.
• Within these islets are 2 types of cells—alpha and beta cells. The alpha cells
• produce the hormone glucagon, which is responsible for raising blood
• glucose levels. Glucagon triggers muscle and liver cells to break down the
• polysaccharide glycogen to release glucose into the bloodstream. The beta
• cells produce the hormone insulin, which is responsible for lowering blood
• glucose levels after a meal. Insulin triggers the absorption of glucose from
• the blood into cells, where it is added to glycogen molecules for storage.
• Ovaries – are inside the female pelvis. They make female sex hormones like
• oestrogen.
• Testes – they hang in the male scrotal sack. They make male sex hormones
• like testosterone.
• Other lesser known endocrine organs include:
• Adipose tissue (fat tissue) – is recognised to be metabolically important. It
• releases hormones such as leptin, which affect appetite, and is also a site of
• oestrogen production. Insulin also acts on adipose tissue.
• Kidneys – produce erythropoietin (EPO) which stimulates red blood cell
• production, produce renin which is needed for blood pressure regulation and
• produce the active form of Vitamin D (1–25 dihydroxy vitamin D3)
• Gut – an increasing number of hormones in the gut are being researched and

being understood to effect metabolism and appetite. Included are glucagonlike peptide 1 (GLP–1), ghrelin which stimulates appetite, and somatostatin.

• Problems of the endocrine system
• Numerous problems can occur in the endocrine system. These can be
• considered as excessive or deficient hormone production. Endocrine organs
• are also prone to tumours (adenomas) which can over produce hormones.
• Some problems of the endocrine system include:
• Diabetes – too much sugar in the blood caused by problems with insulin
• production. This includes type 1 diabetes (deficiency of insulin) and type 2
• diabetes (initially excessive, then deficiency, of insulin).
• Menstruation abnormalities – irregular menstruation or lack of
• menstruation. Some causes of this include polycystic ovarian syndrome
• (PCOS), pituitary adenoma or primary ovarian failure (POF).
• Thyroid problems – when the gland is overactive (hyperthyroidism) or
• underactive (hypothyroidism). Thyroid nodules are common but thyroid
• cancers are rare.
• Parathyroid problems – an enlargement or one of more of the parathyroid
• glands can lead to high calcium levels in the blood (hypercalcemia).
• Pituitary adenomas – these are tumours of the pituitary gland that can
• make too much of a certain hormone or cause deficiencies of hormones.
• These tumours can be small (microadenomas) or large (macroadenomas).
• Neuro-endocrine tumours – these are rare to tumours of certain endocrine
• glands (usually the adrenal gland, pancreas or small bowel). These can
• include too much adrenaline released by the adrenal gland
• (pheochromocytoma), or too much hormone 5–HIAA from a carcinoid
• tumour which causes diarrhoea and flushing.
• Functions of the endocrine system
• Some of the roles of the endocrine system include:
• Growth
• Repair
• Sexual reproduction
• Digestion
• Homeostasis (constant internal balance).
• How hormones work
• A hormone will only act on a part of the body if it ‗fits‘. A hormone can be
• thought of as a key, and its target site (such as an organ) has specially
• shaped locks on the cell walls. If the hormone fits the cell wall, then it will
• work.
• The hormones can set off a cascade of other signaling pathways in the cell to
• cause an immediate effect (for instance, insulin signaling leads to a rapid
• uptake of glucose into muscle cells) or a more delayed effect
• (glucocorticoids bind to DNA elements in a cell to switch on the production
• of certain proteins, which takes a while to produce).
• The endocrine system is a tightly regulated system that keeps the hormones
• and their effects at just the right level. One way this is achieved is through
• ‗feedback loops‘. The release of hormones is regulated by other hormones,
• proteins or neuronal signals.
• The released hormone then has its effect on other organs. This effect on the
• organ feeds back to the original signal to control any further hormone

release. The pituitary gland is well known for its feedback loops

Other Hormone Producing Organs

• In addition to the glands of the endocrine system, many other non-glandular
• organs and tissues in the body produce hormones as well.
• Heart: The cardiac muscle tissue of the heart is capable of producing the
• hormone atrial natriuretic peptide (ANP) in response to high blood
• pressure levels. ANP works to reduce blood pressure by triggering
• vasodilation to provide more space for the blood to travel through. ANP also
• reduces blood volume and pressure by causing water and salt to be excreted
• out of the blood by the kidneys.
• Kidneys: The kidneys produce the hormone erythropoietin (EPO) in
• response to low levels of oxygen in the blood. EPO released
• travels to the red bone marrow where it stimulates an increased production
• of red blood cells. The number of red blood cells increases the oxygen
• carrying capacity of the blood, eventually ending the production of EPO.
• Digestive System: The hormones cholecystokinin (CCK), secretin, and
• gastrin are all produced by the organs of the gastrointestinal tract. CCK,
• secretin, and gastrin all help to regulate the secretion of pancreatic juice,
• bile, and gastric juice in response to the presence of food in the stomach.
• CCK is also instrumental in the sensation of satiety or ―fullness‖ after eating
• a meal.
• Adipose: Adipose tissue produces the hormone leptin that is involved in the
• management of appetite and energy usage by the body. Leptin is produced at
• levels relative to the amount of adipose tissue in the body, allowing the brain
• to monitor the body‘s energy storage condition. When the body contains a
• sufficient level of adipose for energy storage, the level of leptin in the blood
• tells the brain that the body is not starving and may work normally. If the
• level of adipose or leptin decreases below a certain threshold, the body
• enters starvation mode and attempts to conserve energy through increased
• hunger and food intake and decreased energy usage. Adipose tissue also
• produces very low levels of estrogens in both men and women. In obese
• people the large volume of adipose tissue may lead to abnormal estrogen
• levels.
• Placenta: In pregnant women, the placenta produces several hormones that
• help to maintain pregnancy. Progesterone is produced to relax the uterus,
• protect the fetus from the mother‘s immune system, and prevent premature
• delivery of the fetus. Human chorionic gonadotropin (HCG) assists
• progesterone by signaling the ovaries to maintain the production of estrogen
• and progesterone throughout pregnancy.
• Local Hormones: Prostaglandins and leukotrienes are produced by every
• tissue in the body (except for blood tissue) in response to damaging stimuli.
• These two hormones mainly affect the cells that are local to the source of
• damage, leaving the rest of the body free to function normally.
• Prostaglandins cause swelling, inflammation, increased pain sensitivity, and
• increased local body temperature to help block damaged regions of the body
• from infection or further damage. They act as the body‘s natural bandages to
• keep pathogens out and swell around damaged joints like a natural cast to
• limit movement.
• Leukotrienes help the body heal after prostaglandins have taken effect by
• reducing inflammation while helping white blood cells to move into the
• region to clean up pathogens and damaged tissues.
• Endocrine System vs. Nervous System Function
• The endocrine system works alongside of the nervous system to form the
• control systems of the body. The nervous system provides a very fast and
• narrowly targeted system to turn on specific glands and muscles throughout
• the body. The endocrine system, on the other hand, is much slower acting,
• but has very widespread, long lasting, and powerful effects. Hormones are
• distributed by glands through the bloodstream to the entire body, affecting
• any cell with a receptor for a particular hormone. Most hormones affect cells
• in several organs or throughout the entire body, leading to many diverse and

powerful responses.

Hormone Properties

• Once hormones have been produced by glands, they are distributed through
• the body via the bloodstream. As hormones travel through the body, they
• pass through cells or along the plasma membranes of cells until they
• encounter a receptor for that particular hormone. Hormones can only affect
• target cells that have the appropriate receptors. This property of hormones is
• known as specificity. Hormone specificity explains how each hormone can
• have specific effects in widespread parts of the body.
• Many hormones produced by the endocrine system are classified as tropic
• hormones. A tropic hormone is a hormone that is able to trigger the release
• of another hormone in another gland. Tropic hormones provide a pathway of
• control for hormone production as well as a way for glands to be controlled
• in distant regions of the body. Many of the hormones produced by the

pituitary gland, such as TSH, ACTH, and FSH are tropic hormones.

Hormonal Regulation

• The levels of hormones in the body can be regulated by several factors. The
• nervous system can control hormone levels through the action of the
• hypothalamus and its releasing and inhibiting hormones. For example, TRH
• produced by the hypothalamus stimulates the anterior pituitary to produce
• TSH. Tropic hormones provide another level of control for the release of
• hormones. For example, TSH is a tropic hormone that stimulates the thyroid
• gland to produce T3 and T4. Nutrition can also control the levels of
• hormones in the body. For example, the thyroid hormones T3 and T4 require
• 3 or 4 iodine atoms, respectively, to be produced. In people lacking iodine in
• their diet, they will fail to produce sufficient levels of thyroid hormones to
• maintain a healthy metabolic rate. Finally, the number of receptors present in
• cells can be varied by cells in response to hormones. Cells that are exposed
• to high levels of hormones for extended periods of time can begin to reduce
• the number of receptors that they produce, leading to reduced hormonal
• control of the cell.
• Classes of Hormones
• Hormones are classified into 2 categories depending on their chemical
• make-up and solubility: water-soluble and lipid-soluble hormones. Each of
• these classes of hormones has specific mechanisms for their function that
• dictate how they affect their target cells.
• Water-soluble hormones: Water-soluble hormones include the peptide and
• amino-acid hormones such as insulin, epinephrine, HGH, and oxytocin. As
• their name indicates, these hormones are soluble in water. Water-soluble
• hormones are unable to pass through the phospholipid bilayer of the plasma
• membrane and are therefore dependent upon receptor molecules on the
• surface of cells. When a water-soluble hormone binds to a receptor molecule
• on the surface of a cell, it triggers a reaction inside of the cell. This reaction
• may change a factor inside of the cell such as the permeability of the
• membrane or the activation of another molecule. A common reaction is to
• cause molecules of cyclic adenosine monophosphate (cAMP) to be
• synthesized from adenosine triphosphate (ATP) present in the cell. cAMP
• acts as a second messenger within the cell where it binds to a second
• receptor to change the function of the cell‘s physiology.
• Lipid-soluble hormones: Lipid-soluble hormones include the steroid
• hormones such as testosterone, estrogens, glucocorticoids, and
• mineralocorticoids. Because they are soluble in lipids, these hormones are

able to pass directly through the phospholipid bilayer of the plasma membrane and bind directly to receptors inside the cell nucleus. Lipidsoluble hormones are able to directly control the function of a cell from these receptors, often triggering the transcription of particular genes in the DNA to produce "messenger RNAs (mRNAs)" that are used to make proteins that affect the cell‘s growth and function.