EndocrinologyEndocrinology

Major endocrine glands in Major endocrine glands in the bodythe body

CHEMISTRY OF CHEMISTRY OF HORMONESHORMONES

Peptide hormonesPeptide hormones: largest, most complex, and most : largest, most complex, and most common hormones. Examples include insulin and common hormones. Examples include insulin and prolactinprolactin

Steroid hormonesSteroid hormones: lipid soluble molecules : lipid soluble molecules synthesized from cholesterol. Examples include synthesized from cholesterol. Examples include gonadal steroids (e.g testosterone and estrogen) gonadal steroids (e.g testosterone and estrogen) and adrenocortical steroids (e.g. cortisol and and adrenocortical steroids (e.g. cortisol and aldosterone).aldosterone).

AminesAmines: small molecules derived from individual : small molecules derived from individual amino acids. Include catecholamines (e.g. amino acids. Include catecholamines (e.g. epinephrine produced by the adrenal medulla), and epinephrine produced by the adrenal medulla), and thyroid hormones.thyroid hormones.

EicosanoidsEicosanoids: small molecules synthesized from fatty : small molecules synthesized from fatty acid substrates (e.g. arachidonic acid) located acid substrates (e.g. arachidonic acid) located within cell membraneswithin cell membranes

MODES OF HORMONE MODES OF HORMONE DELIVERYDELIVERY

ENDOCRINE: Most common (classical) mode, ENDOCRINE: Most common (classical) mode, hormones delivered to target cells by blood.hormones delivered to target cells by blood.

PARACRINE: Hormone released diffuses to its PARACRINE: Hormone released diffuses to its target cells through immediate extracellular target cells through immediate extracellular space. space. Blood is not directly involved in the delivery.Blood is not directly involved in the delivery.

AUTOCRINE: Hormone released feeds-back AUTOCRINE: Hormone released feeds-back on the cell of origin, again without entering on the cell of origin, again without entering blood circulation.blood circulation.

NEUROENDOCRINE: Hormone is produced NEUROENDOCRINE: Hormone is produced and released by a neuron, delivered to target and released by a neuron, delivered to target cells by blood.cells by blood.

HORMONE-TARGET CELL HORMONE-TARGET CELL SPECIFICITYSPECIFICITY

Only target cells, or cells that have Only target cells, or cells that have specific receptors, will respond to specific receptors, will respond to the hormone’s presence. the hormone’s presence. The strength of this response will The strength of this response will

depend on:depend on: Blood levels of the hormoneBlood levels of the hormone The relative numbers of receptors for that The relative numbers of receptors for that

hormone on or in the target cellshormone on or in the target cells The affinity (or strength of interactions) of The affinity (or strength of interactions) of

the hormone and the receptor.the hormone and the receptor.

HALF-LIFE, ONSET, and HALF-LIFE, ONSET, and DURATION of HORMONE DURATION of HORMONE

ACTIVITYACTIVITY

The affinity of hormones to their The affinity of hormones to their specific receptors is typically very high specific receptors is typically very high

The actual concentration of a The actual concentration of a circulating hormone in blood at any circulating hormone in blood at any time reflects: time reflects: Its rate of release.Its rate of release. The speed of its inactivation and removal The speed of its inactivation and removal

from the body. from the body.

The half-life is the time required for the The half-life is the time required for the hormone to loose half of its original hormone to loose half of its original effectiveness (or drop to half of its original effectiveness (or drop to half of its original concentration.concentration.

The time required for hormone effects to The time required for hormone effects to take place varies greatly, from almost take place varies greatly, from almost immediate responses to hours or even days.immediate responses to hours or even days.

In addition, some hormones are produced in In addition, some hormones are produced in an inactive form and must be activated in an inactive form and must be activated in the target cells before exerting cellular the target cells before exerting cellular responses. responses.

In terms of the duration of hormone action, In terms of the duration of hormone action, it ranges from about 20 minutes to several it ranges from about 20 minutes to several hours, depending on the hormone. hours, depending on the hormone.

CONTROL OF HORMONE CONTROL OF HORMONE RELEASERELEASE

The synthesis and secretion of most The synthesis and secretion of most hormones are usually regulated by hormones are usually regulated by negative feedback systemsnegative feedback systems..

As hormone levels rise, they stimulate As hormone levels rise, they stimulate target organ responses. These in turn, target organ responses. These in turn, inhibit further hormone release. inhibit further hormone release.

The stimuli that induce endocrine glands The stimuli that induce endocrine glands to synthesize and release hormones to synthesize and release hormones belong to one of the following major types:belong to one of the following major types: HumoralHumoral NeuralNeural Hormonal Hormonal

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cAMP & IPcAMP & IP33

PituitaryPituitary GlandGland

The “Master The “Master Gland”Gland”

The pituitary has The pituitary has been called the been called the “Master” gland in “Master” gland in the body.the body.

This is because This is because most of the most of the pituitary hormones pituitary hormones control other control other endocrine glandsendocrine glands

Hormones of the anterior Hormones of the anterior pituitarypituitary

There are 6 main hormones which are secreted by There are 6 main hormones which are secreted by the adenohypophysis:the adenohypophysis:

1) Growth hormone (also known as somatotropin).1) Growth hormone (also known as somatotropin).

2) Thyroid-stimulating hormone (also known as 2) Thyroid-stimulating hormone (also known as thyrotropin).thyrotropin).

3) Adrenocorticotropic hormone (also known as 3) Adrenocorticotropic hormone (also known as corticotropin).corticotropin).

4) Prolactin.4) Prolactin.

5) Follicle-stimulating hormone.5) Follicle-stimulating hormone.

6) Luteinizing hormone.6) Luteinizing hormone.

Control of pituitary gland Control of pituitary gland secretionsecretion

Secretion of each hormone by the Secretion of each hormone by the adenohypophysis is controlled by adenohypophysis is controlled by neurohormones secreted by nerves in neurohormones secreted by nerves in the hypothalamus.the hypothalamus.

In most cases there are two In most cases there are two neurohormones controlling the neurohormones controlling the secretion of a pituitary hormone. One secretion of a pituitary hormone. One which stimulates pituitary secretion which stimulates pituitary secretion and one which inhibits pituitary and one which inhibits pituitary secretion. secretion.

Neurohormones:Neurohormones:

Are hormones secreted by nerve cells. Are hormones secreted by nerve cells. These are true hormones, since they These are true hormones, since they are secreted into the bloodstream.are secreted into the bloodstream.

All are secreted by neurosecretory All are secreted by neurosecretory neurons in the hypothalamus.neurons in the hypothalamus.

They are secreted into the hypophyseal They are secreted into the hypophyseal portal system, which then carries the portal system, which then carries the blood to the anterior pituitary.blood to the anterior pituitary.

Pituitary portal systemPituitary portal system Arterioles break into capillaries in the Arterioles break into capillaries in the

hypothalamus. hypothalamus. The axons of the neurosecretory cells form The axons of the neurosecretory cells form

plexuses with these capillaries. plexuses with these capillaries. Downstream, the capillaries combine into a vein Downstream, the capillaries combine into a vein

which carries the blood to the pars distalis. which carries the blood to the pars distalis. The vein breaks into a capillary network which The vein breaks into a capillary network which

supplies all the cells of the anterior lobe. supplies all the cells of the anterior lobe. Thus, the neurohormones are carried directly Thus, the neurohormones are carried directly

(well, sort of) from the hypothalamus to the (well, sort of) from the hypothalamus to the adenohypophysis. adenohypophysis.

Portal Portal systemsystem

Growth hormone (GH)Growth hormone (GH) Growth hormone is secreted by somatotrophs.Growth hormone is secreted by somatotrophs.

GH is a protein hormone consisting of a single GH is a protein hormone consisting of a single peptide chain of 191 amino acids. peptide chain of 191 amino acids.

GH secretion is stimulated by the secretion of GH secretion is stimulated by the secretion of GGrowth rowth HHormone ormone RReleasing eleasing HHormone (GHRH) by ormone (GHRH) by the hypothalamus.the hypothalamus.

GH secretion is inhibited by the secretion of GH secretion is inhibited by the secretion of somatostatin by the hypothalamus.somatostatin by the hypothalamus.

GH activates a tyrosine kinase receptor.GH activates a tyrosine kinase receptor.

Functions of GH:Functions of GH: GH has effects of every cell of the body, GH has effects of every cell of the body,

either directly or indirectly. Primarily, it either directly or indirectly. Primarily, it decreases the uptake and metabolism of decreases the uptake and metabolism of glucose. (Elevates plasma glucose)glucose. (Elevates plasma glucose)

Increases the breakdown of fat. (Increases Increases the breakdown of fat. (Increases the blood levels of fatty acids)the blood levels of fatty acids)

Increases the uptake of amino acids from Increases the uptake of amino acids from the blood and increases protein synthesis the blood and increases protein synthesis in cell. (Decreases plasma amino acids)in cell. (Decreases plasma amino acids)

Actions of GH on specific cell Actions of GH on specific cell types:types:

Muscle cells:Muscle cells:

Increases amino acid uptakeIncreases amino acid uptake Increases protein synthesisIncreases protein synthesis Decreases glucose uptakeDecreases glucose uptake

Net resultNet result: : Increased Increased Lean body massLean body mass

Chondrocytes:Chondrocytes:

increases uptake of sulfurincreases uptake of sulfur increases chondroitin sulfate productionincreases chondroitin sulfate production increases DNA, RNA synthesisincreases DNA, RNA synthesis increases Protein synthesisincreases Protein synthesis increases Amino acid uptakeincreases Amino acid uptake increases Collagen synthesisincreases Collagen synthesis increases Cell size and numberincreases Cell size and number

Net resultNet result: Increased : Increased Linear growthLinear growth

Hepatocytes:Hepatocytes:

Stimulates the production of Stimulates the production of somatomedins by the liver.somatomedins by the liver.

These somatomedins directly These somatomedins directly regulate metabolic function in target regulate metabolic function in target cells. They are also called insulin-cells. They are also called insulin-like growth factors, or IGFs. like growth factors, or IGFs.

Adipocytes:Adipocytes:

Decreases glucose uptakeDecreases glucose uptake Increases lypolysisIncreases lypolysis

Net resultNet result: Decreased : Decreased AdiposityAdiposity

Other cell types in general:

Increased protein synthesis Increased DNA, RNA synthesis Increased cell size and number

Net result: Increased organ size

Increased organ function

Other considerations:Other considerations:

GH has a short half-life of about 20 GH has a short half-life of about 20 minutes. However, the IGFs are minutes. However, the IGFs are much longer lived (Tmuch longer lived (T1/21/2 of about 20 of about 20 hours).hours).

GH and Insulin actions are GH and Insulin actions are correlated:correlated:

When there is ample dietary intake of proteins and When there is ample dietary intake of proteins and carbohydrates, then amino acids can be used for carbohydrates, then amino acids can be used for protein synthesis and growth.protein synthesis and growth.

Under these conditions, both insulin and GH Under these conditions, both insulin and GH secretion are stimulated.secretion are stimulated.

Net result:Net result: Amino acids are shunted to protein synthesis Amino acids are shunted to protein synthesis and glucose is shunted to metabolism.and glucose is shunted to metabolism.

However, under conditions where only However, under conditions where only carbohydrates are ingested, insulin secretion is carbohydrates are ingested, insulin secretion is increased, but GH secretion is decreased.increased, but GH secretion is decreased.

Net result: Net result: Both glucose AND amino acids are shunted to Both glucose AND amino acids are shunted to metabolism.metabolism.

Pathophysiology of abnormal Pathophysiology of abnormal GH secretion:GH secretion:

Hyposecretion:Hyposecretion:

Pre-adolescents: Pre-adolescents: Decreased GH secretion (or sensitivity) results Decreased GH secretion (or sensitivity) results

in slow growth and delayed onset of sexual in slow growth and delayed onset of sexual maturation. These children also tend to be maturation. These children also tend to be slightly chubby. slightly chubby.

Post-adolescents:Post-adolescents: Generally, no serious problems are associated Generally, no serious problems are associated

with hyposecretion of GH in mature individuals. with hyposecretion of GH in mature individuals. However, in very severe cases there can be However, in very severe cases there can be progeria (rapid and premature aging).progeria (rapid and premature aging).

Hypersecretion:Hypersecretion: Pre-adolescents: (before closure of Pre-adolescents: (before closure of

epiphyseal plates)epiphyseal plates)

Hypersecretion results in gigantism, Hypersecretion results in gigantism, where affected individuals grow where affected individuals grow extremely rapidly and become extremely rapidly and become abnormally tall (even over 2.4 m). Body abnormally tall (even over 2.4 m). Body proportions remain relatively normal. proportions remain relatively normal. Usually, there are cardiovascular Usually, there are cardiovascular complications later in life.complications later in life.

Post- adolescents: (after epiphyseal Post- adolescents: (after epiphyseal closure).closure).

Hypersecretion results in tissue Hypersecretion results in tissue enlargement. This is particularly true of enlargement. This is particularly true of the bones, which get heavier and the bones, which get heavier and thicker. They cannot elongate since the thicker. They cannot elongate since the epiphyseal plates are closed. A common epiphyseal plates are closed. A common symptom is a coarsening of the facial symptom is a coarsening of the facial features and enlargement of the hands features and enlargement of the hands and feet. This condition is known as and feet. This condition is known as acromegaly.acromegaly.

Treatments of GH secretion Treatments of GH secretion disorders:disorders:

Hypersecretion is usually caused by a Hypersecretion is usually caused by a tumour in the pituitary gland. tumour in the pituitary gland. Treatment consists of surgical or Treatment consists of surgical or radiation ablation of the tumour mass.radiation ablation of the tumour mass.

Hyposecretion is usually treated in Hyposecretion is usually treated in children by hormone replacement children by hormone replacement therapy. This is generally not required therapy. This is generally not required in adults, unless GH secretion is in adults, unless GH secretion is completely abolished.completely abolished.

Prolactin (PRL)Prolactin (PRL)

Structurally, very similar to growth Structurally, very similar to growth hormone (single peptide chain of 198 hormone (single peptide chain of 198 amino acids).amino acids).

PRL is secreted by mammotrophs PRL is secreted by mammotrophs (also referred to as lactotrophs).(also referred to as lactotrophs).

Secretion of PRL is also under dual Secretion of PRL is also under dual control by the hypothalamus.control by the hypothalamus.

Primarily under inhibitory control. This Primarily under inhibitory control. This means that if there is an injury to the means that if there is an injury to the hypophyseal portal system which blocks hypophyseal portal system which blocks hypothalamic regulation of the pituitary hypothalamic regulation of the pituitary gland, PRL levels increase. All other pituitary gland, PRL levels increase. All other pituitary hormone levels decrease when this happens.hormone levels decrease when this happens.

Dopamine is secreted by neuroendocrine Dopamine is secreted by neuroendocrine cells in the hypothalamus and inhibits PRL cells in the hypothalamus and inhibits PRL release.release.

PRL release is stimulated by thyrotropin PRL release is stimulated by thyrotropin releasing hormone (TRH), vasoactive releasing hormone (TRH), vasoactive intestinal peptide (VIP) and at least one intestinal peptide (VIP) and at least one other as yet unidentified factor.other as yet unidentified factor.

PRL activates a tyrosine kinase receptor.PRL activates a tyrosine kinase receptor.

Functions of PRL:Functions of PRL: In humans, the only effects of PRL so far identified are on In humans, the only effects of PRL so far identified are on

reproduction and nursing. reproduction and nursing.

PRL is important in stimulating differentiation of breast tissue PRL is important in stimulating differentiation of breast tissue during development.during development.

Stimulates further development of mammary glands during Stimulates further development of mammary glands during pregnancy.pregnancy.

Stimulates milk production (lactation) after pregnancy.Stimulates milk production (lactation) after pregnancy.

PRL has a role in regulation of the female reproductive cycle. PRL has a role in regulation of the female reproductive cycle. However, its precise role has not be delineated yet. Excess However, its precise role has not be delineated yet. Excess PRL secretion is know to block synthesis and release of PRL secretion is know to block synthesis and release of gonadotropins, disrupting menstruation and causing gonadotropins, disrupting menstruation and causing infertility.infertility.

PRL also can regulate male fertility, but how it does so PRL also can regulate male fertility, but how it does so remains unclear.remains unclear.

Pathophysiology of PRL Pathophysiology of PRL secretion:secretion:

Hyposecretion is never seen. However, Hyposecretion is never seen. However, hyperprolactinemia (excess secretion of hyperprolactinemia (excess secretion of PRL) is a fairly common disorder. Symptoms PRL) is a fairly common disorder. Symptoms in women usually include amenorrhea in women usually include amenorrhea (cessation of menstruation), galactorrhea (cessation of menstruation), galactorrhea (abnormal lactation) and infertility. In men, (abnormal lactation) and infertility. In men, infertility and galactorrhea are the most infertility and galactorrhea are the most common symptoms.common symptoms.

Treatment usually consists of administration Treatment usually consists of administration of a dopaminergic agonist, such as of a dopaminergic agonist, such as bromocriptine.bromocriptine.

Thyroid Stimulating Thyroid Stimulating hormone (TSH)hormone (TSH)

TSH is a glycoprotein hormone composed TSH is a glycoprotein hormone composed of 2 peptide chains a and b.of 2 peptide chains a and b.

The a subunit is called “unspecific” The a subunit is called “unspecific” because it is also incorporated into two because it is also incorporated into two other unrelated pituitary hormones (LH other unrelated pituitary hormones (LH and FSH).and FSH).

The b subunit contains the biologically The b subunit contains the biologically active sites. However, it must be active sites. However, it must be combined with the a subunit in order for combined with the a subunit in order for the hormone to be active.the hormone to be active.

TSH secretion is controlled very TSH secretion is controlled very tightly by the hypothalamus.tightly by the hypothalamus.

TSH secretion is stimulated by TSH secretion is stimulated by Thyrotropin-releasing hormone Thyrotropin-releasing hormone (TRH). TRH is a tripeptide, meaning (TRH). TRH is a tripeptide, meaning it is composed of three amino acids.it is composed of three amino acids.

TRH secretion is stimulated by TRH secretion is stimulated by thermal and caloric signals in the thermal and caloric signals in the brain.brain.

Control of TSH secretionControl of TSH secretion Negative control of TSH secretion Negative control of TSH secretion

occurs in two ways:occurs in two ways: Triiodothyronien or T3 (which will be Triiodothyronien or T3 (which will be

discussed later) feeds back on the discussed later) feeds back on the hypothalamus to stimulate secretion of hypothalamus to stimulate secretion of dopamine and somatostatin. These two dopamine and somatostatin. These two factors both function as TSH-release factors both function as TSH-release inhibiting factors.inhibiting factors.

T3 can feed back directly onto the T3 can feed back directly onto the thyrotrophs to directly inhibit TSH thyrotrophs to directly inhibit TSH secretion.secretion.

Function of TSH:Function of TSH:

TSH stimulates the follicular cells of the TSH stimulates the follicular cells of the thyroid to induce a number of responses:thyroid to induce a number of responses:

TSH activates both the cAMP and PIP TSH activates both the cAMP and PIP pathways: pathways: Increased cAMPIncreased cAMP Increased [CaIncreased [Ca2+2+]]ii

TSH can stimulate both cell growth (of TSH can stimulate both cell growth (of follicular cells) and secretion of Tfollicular cells) and secretion of T33 and and thyroxine ( Tthyroxine ( T44 ). ).

Adrenocorticotropic hormone Adrenocorticotropic hormone (ACTH)(ACTH)

ACTH is a single peptide chain which is relatively ACTH is a single peptide chain which is relatively small (30 amino acids).small (30 amino acids).

ACTH secretion is primarily under stimulatory ACTH secretion is primarily under stimulatory control (i.e. there isn’t an ACTH-release inhibitory control (i.e. there isn’t an ACTH-release inhibitory factor).factor).

ACTH secretion is stimulated by corticotropin ACTH secretion is stimulated by corticotropin releasing hormone (CRH).releasing hormone (CRH).

CRH secretion can be stimulated by a large CRH secretion can be stimulated by a large number of factors, most of which would be number of factors, most of which would be considered stress factors.considered stress factors.

Examples; infection, trauma, sleep cycle, anxiety, Examples; infection, trauma, sleep cycle, anxiety, depression and others. (Just remember stress).depression and others. (Just remember stress).

Functions of ACTH:Functions of ACTH:

ACTH stimulates the adrenal gland to secrete ACTH stimulates the adrenal gland to secrete cortisol.cortisol.

ACTH levels are associated with the sleep cycle.ACTH levels are associated with the sleep cycle.

ACTH stimulates the cAMP pathway in ACTH stimulates the cAMP pathway in adrenocorticol cells.adrenocorticol cells.

ACTH can directly inhibit CRH secretion ACTH can directly inhibit CRH secretion (negative feedback).(negative feedback).

Follicular-Stimulating Follicular-Stimulating hormone (FSH)hormone (FSH)

Luteinizing Hormone (LH) Luteinizing Hormone (LH) These are generally grouped together and called These are generally grouped together and called

gonadotropines.gonadotropines.

Gonadotropins are secreted by the gonadotrophs, Gonadotropins are secreted by the gonadotrophs, which synthesize and secrete which synthesize and secrete bothboth LH and FSH. LH and FSH.

Both LH and FSH are peptide hormones.Both LH and FSH are peptide hormones.

Secretion of gonadotropins is mainly under positive Secretion of gonadotropins is mainly under positive control.control.

Hypothalamus secretes gonadotropin-releasing Hypothalamus secretes gonadotropin-releasing hormone (GnRH) which stimulates gonadotrophs to hormone (GnRH) which stimulates gonadotrophs to secrete both LH and FSH.secrete both LH and FSH.

Functions of LH and Functions of LH and FSH:FSH:

LH and FSH stimulate secretion of the sex steroids LH and FSH stimulate secretion of the sex steroids by the gonads. Mainly estrogen in women and by the gonads. Mainly estrogen in women and testosterone in men.testosterone in men.

FSH also stimulates gonadal release of inhibin, FSH also stimulates gonadal release of inhibin, which serves as a negative feedback factor to block which serves as a negative feedback factor to block release of FSH by pituitary.release of FSH by pituitary.

LH and FSH stimulate the gonadal release of LH and FSH stimulate the gonadal release of activin, which can have positive feedback on activin, which can have positive feedback on gonadotropin secretion by the pituitary.gonadotropin secretion by the pituitary.

Gonadal secretion of estrogen and testosterone can Gonadal secretion of estrogen and testosterone can negatively feedback on both the hypothalamus, to negatively feedback on both the hypothalamus, to reduce GnRH secretion, and the gonadotrophs reduce GnRH secretion, and the gonadotrophs directly, to reduce gonadotropin secretions.directly, to reduce gonadotropin secretions.

Hormones of the posterior Hormones of the posterior pituitary:pituitary:

Remember that the neurohypophysis serves as a Remember that the neurohypophysis serves as a storage organ for hormones produced by storage organ for hormones produced by neurosecretory cells in the hypothalamus.neurosecretory cells in the hypothalamus.

There are two hormones secreted by the There are two hormones secreted by the neurohypophysis:neurohypophysis: 1) antidiuretic hormone (ADH)1) antidiuretic hormone (ADH) 2) oxytocin2) oxytocin

Both hormones are peptide hormones containing Both hormones are peptide hormones containing 9 amino acid residues.9 amino acid residues.

They differ in only 2 amino acids, but have very They differ in only 2 amino acids, but have very different functions.different functions.

Both activate the PIP pathway in the target cells.Both activate the PIP pathway in the target cells.

ADHADH Term: diuresis ö means production of urine.Term: diuresis ö means production of urine.

ADH inhibits urine production, i.e. conserves water ADH inhibits urine production, i.e. conserves water in the body.in the body.

Main target for ADH are the cells in the kidney Main target for ADH are the cells in the kidney which reabsorb water (will be covered in detail in which reabsorb water (will be covered in detail in the section on renal physiology).the section on renal physiology).

ADH secretion is stimulated by either an increase ADH secretion is stimulated by either an increase in the osmotic concentration of the blood, or by a in the osmotic concentration of the blood, or by a decrease in blood volumedecrease in blood volume usually sensed by a decrease in blood pressure.usually sensed by a decrease in blood pressure.

Secretion of ADH causes retention of water, Secretion of ADH causes retention of water, which will tend to counteract both an which will tend to counteract both an increase in blood concentration and/or increase in blood concentration and/or decrease in blood volume.decrease in blood volume.

cannot overcome serious blood loss.cannot overcome serious blood loss.

Conversely, excess consumption of water Conversely, excess consumption of water will have two effects:will have two effects: increase blood volume (and pressure).increase blood volume (and pressure). decrease blood concentration.decrease blood concentration.

Under these conditions ADH secretion is Under these conditions ADH secretion is inhibited.inhibited.

This results in formation of This results in formation of more more urine, which is urine, which is usually fairly dilute.usually fairly dilute.

Blood loses water and thus volume.Blood loses water and thus volume.

OxytocinOxytocin Release of oxytocin is under neural control (like with ADH).Release of oxytocin is under neural control (like with ADH).

However, unlike ADH, the release of oxytocin is largely However, unlike ADH, the release of oxytocin is largely controlled by emotional state.controlled by emotional state.

Oxytocin is required for nursing.Oxytocin is required for nursing. Principally know as the “milk letdown factor”.Principally know as the “milk letdown factor”.

It is secreted within seconds of the onset of suckling.It is secreted within seconds of the onset of suckling.

Sensory receptors in the nipples generate afferent impulses that Sensory receptors in the nipples generate afferent impulses that stimulate the hypothalamus, triggering oxytocin secretion.stimulate the hypothalamus, triggering oxytocin secretion.

Can actually be secreted in response to auditory input, Can actually be secreted in response to auditory input, i.ei.e. in nursing . in nursing mothers in response to hearing their babies cry.mothers in response to hearing their babies cry.

Oxytocin specifically stimulates certain smooth muscles to Oxytocin specifically stimulates certain smooth muscles to contract.contract.

Primarily those of the reproductive tract and mammary glands.Primarily those of the reproductive tract and mammary glands.

Effects of OxytocinEffects of Oxytocin Oxytocin stimulation at low doses causes rhythmic Oxytocin stimulation at low doses causes rhythmic

contractions of the uterus.contractions of the uterus.

ö Oxytocin stimulation at high dose causes ö Oxytocin stimulation at high dose causes sustained tetanic uterine contractions.sustained tetanic uterine contractions.

ö Oxytocin is often used to induce labour.ö Oxytocin is often used to induce labour.

ö It is now generally believed that oxytocin ö It is now generally believed that oxytocin believed that oxytocin produced by the fetus plays believed that oxytocin produced by the fetus plays a critical role in labour.a critical role in labour.

ö Oxytocin is also used to stop post-partum ö Oxytocin is also used to stop post-partum bleeding.bleeding.

The number of oxytocin receptors in uterine smooth The number of oxytocin receptors in uterine smooth muscles increases towards the end of pregnancy.muscles increases towards the end of pregnancy.

Oxytocin affects smooth muscle cells in uterus and Oxytocin affects smooth muscle cells in uterus and vagina of non-pregnant women.vagina of non-pregnant women.

There is clear evidence that oxytocin is involved in There is clear evidence that oxytocin is involved in sexual arousal and orgasm in both men and women.sexual arousal and orgasm in both men and women.

What role it plays in men is unknown. However, it may What role it plays in men is unknown. However, it may play a strong role in reinforcing the pair-bond.play a strong role in reinforcing the pair-bond.

The role in women is only slightly better known.The role in women is only slightly better known.

Oxytocin is secreted in response to vaginal distention Oxytocin is secreted in response to vaginal distention during intercourse.during intercourse.

Oxytocin is also secreted in response to stimulation of the Oxytocin is also secreted in response to stimulation of the nipples.nipples.

Emotional considerationsEmotional considerations

Oxytocin secretion during sexual intercourse probably serves Oxytocin secretion during sexual intercourse probably serves to reinforce the male-female pair-bond.to reinforce the male-female pair-bond.

Often referred to as the “the cuddle hormone” or “the love Often referred to as the “the cuddle hormone” or “the love hormone” in the popular press.hormone” in the popular press.

Secretion of oxytocin during and after labour may play an Secretion of oxytocin during and after labour may play an important role in the formation of the mother-child pair-important role in the formation of the mother-child pair-bond.bond.

Oxytocin secreted during suckling may serve to reinforce this Oxytocin secreted during suckling may serve to reinforce this pair-bond.pair-bond.

Recent studies with knock out mice has shown that oxytocin Recent studies with knock out mice has shown that oxytocin is critical in initiating and maintaining maternal care.is critical in initiating and maintaining maternal care.

Oxytocin secreted in response to suckling can cause uterine Oxytocin secreted in response to suckling can cause uterine contractions which may play a role in the recovery of uterine contractions which may play a role in the recovery of uterine muscle tone after pregnancy and may serve to shrink the uterus muscle tone after pregnancy and may serve to shrink the uterus back to normal.back to normal.