El sistema endocrino, estudiado en endocrinología, está compuesto por células que secretan hormonas y glándulas endocrinas, regulando la homeostasis del cuerpo junto con el sistema nervioso. Las hormonas actúan como mensajeros químicos que afectan células específicas, siendo liberadas en pequeñas cantidades y regulando procesos como el crecimiento, reproducción y balance de energía. Las secreciones hormonales son controladas por mecanismos de retroalimentación, y el hipotálamo juega un papel crucial en la regulación hormonal del cuerpo.

1. Chapter -9
The Endocrine System
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2. Introduction
 Endocrinology is the study of the endocrine system, which is
composed of hormone-secreting cells (e.g. brain, heart
,intestine) and endocrine glands.
 Glands are group of cells that produces chemical messagers
 Hormones are chemical messengers that are secreted into the
bloodstream and stimulate the physiology of cells in another
tissue or organ, often a considerable distance away.
 Endocrine system together with nervous system regulate
homeostasis of the body
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Introduction….
1. The nervous system: Regulates homeostasis by releasing
chemical messengers called neurotransmitters
2. The endocrine system:
Regulates homeostasis by releasing chemical messengers
called hormones
 Difference b/n hormones and NTs
 Speed of action
 Duration of action
 Distribution
 Common aspects:
 small quantities
 regulate other cells & tissues
 act through receptors
 functional overlap between some hormones &
neurotransmitters

4. Endocrine glands
 The body contains two kinds of glands:
 exocrine glands
 endocrine glands.
 Exocrine glands (exo- outside) secrete their products into ducts
that carry the secretions into different parts of the body
 includes sudoriferous (sweat), sebaceous (oil), mucous, and
digestive glands.
 Endocrine glands (endo- within) secrete their products (hormones)
into the interstitial fluid then into blood capillaries and blood carries
them to target cells throughout the body.
 include the pituitary, thyroid, parathyroid, adrenal, and pineal
glands, etc
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1. Pituitary gland (anterior & posterior)
2. Thyroid gland.
3. Parathyroid gland.
4. Adrenal gland (cortex & medulla)
5. Islets of langerhans ( in the pancreas)
6. Male gonads ( testes) & Female gonads
7.Placenta. (Human Chorionic Gonadotropin (hCG), Human
Chorionic Somammotropin (hCS), Relaxin , Estrogen
(E),Progesterone (P))
The Principle Endocrine Glands

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Major Endocrine Glands

7. Endocrine glands…
 Several organs and tissues are not exclusively classified as
endocrine glands but contain cells that secrete hormones.
 Include the:
 hypothalamus, thymus
 kidneys, stomach, liver
 small intestine, skin
 heart, adipose tissue
 Thus, all endocrine glands and hormone-secreting cells
constitute the endocrine system
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Principal functions of hormones
1. Regulation of reproduction: gametogenesis, sexual desire,
coitus, fertilization.
2. Regulation of body growth and development.
3. Production, utilization and storage of energy
4. Homeostasis: maintenance of the internal environment in
the body (maintaining the optimum biochemical
environment).
 Water-electrolyte balance (stability of body fluid
composition)
 Regulation of acid-base balance
 Regulation of arterial blood pressure, heart rate
 Control of body temprature, emotion
 Change in mass of bone, muscle and fat

9. Hormonal activity
 A given hormone affects only specific target cells
 A target cell has a specific receptor that bind and recognize
that hormone
 Receptors are constantly being synthesized and broken down
 A target cell has 2000 to 100,000 receptors for a particular
hormone
 If a hormone is present in excess, the number of target-cell
receptors may decrease an effect called down-regulation
 makes a target cell less sensitive to a hormone
 when a hormone is deficient, the number of receptors may
increase, which is called up-regulation
 makes a target cell more sensitive to a hormone
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Hormonal activity….
Most hormones circulate in blood, coming into contact with
essentially all cells. However, a given hormone usually affects
only a limited number of cells, which are called target cells. A
target cell responds to a hormone because it bears receptors for
the hormone.

11. Circulating and local hormones
 Hormones are:
 Circulating hormones(endocrine): they pass from the
secretory cells into the blood
 Paracrine hormones: that act on neighboring cells
eg. histamine
 Autocrine hormones: that act on the same cell that secreted
them
eg. testosterone
 Local hormones usually are inactivated quickly
 Circulating hormones may linger in the blood and exert their
effects for a few minutes or occasionally for a few hours.
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Intercellular signaling

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Chemical nature of hormones
1. Amines (derived from tyrosine)
 Hydroxylation - catecholamines
 Iodination - thyroid hormones
2. Peptides/polypeptides or proteins
3. Steroids (derived from cholesterol)
 Adrenocorticoids
 Sex hormones
 Active metabolites of vitamin D3
4. Eicosanoids (derived from fatty acids)
 Prostaglandins
 Thromboxanes

14. Mechanism of Hormone Acton
 The physiological processes regulated by hormones result from
interaction of the hormone with receptors
 Nature of hormone receptors
 Receptors are larger proteins or glycoprotein
Location
On or within the cell membrane : Peptide/protein and
catecholamine
 Within the cytoplasm or nucleus: Steroid and thyroid
hormones.

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Types of receptor
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 Cytosolic or Nuclear receptor
 Lipophilic ligand enters
cell
 Often activates gene
 Slower response
 Cell membrane receptor
 Lipophobic ligand can't
enter cell
 Fast response

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Effect of Hormones on the cells
 Membrane Permeability changes.
 Alteration of Enzyme Activity
 ATP  Cyclic AMP  Protein Kinase 
- Protein Synthesis
- Gene activation
- Initiate secretion
- Causing muscle contraction or relaxation

17. Hormone transport and metabolism
 Transport: hormones are transported in blood in two forms:
1. In the free form
2. In combination with plasma proteins (albumin & globulin). It has
3 advantages
a. prolong the biological half life of hormones
b. serve as a reservoir of hormones
c. increasing their solubility in blood
 Metabolism: metabolized in the liver or by target cells
 Excretion: urine, feces, sweat
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18. Hormone Interaction
 The responsiveness of a target cell to a hormone depends on:
 the hormone’s concentration
 the abundance of the target cell’s hormone receptors
 influences exerted by other hormones
 The actions of some hormones on target cells require a simultaneous
or recent exposure to a second hormone.
 In such cases, the second hormone is said to have a permissive
effect.
 For example, epinephrine and thyroid hormones (T3 and T4)
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19. Hormone interaction….
 When the effect of two hormones acting together is greater or
more extensive than the effect of each hormone acting alone,
the two hormones are said to have a synergistic effect.
 For example, normal development of oocytes in the ovaries
requires both follicle-stimulating hormone and estrogens.
 When one hormone opposes the actions of another hormone,
the two hormones are said to have antagonistic effects.
 insulin and glucagon
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Regulation of hormonal secretion
Hormonal secretion is regulated by at least 3 mechanisms.
1. Feedback regulatory mechanisms
 Negative feedback mechanism (most hormones)
 Positive feedback mechanism (LH- surge)
2. Nervous control of hormonal secretion
 Secretion of OT, ADH, AD, NA
3. Circadian rhythm (periodic variations)
 Cortisol, sex hormones

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Feedback control of hormone production
Feedback loops are used extensively
to regulate secretion of hormones in
the hypothalamic-pituitary axis.
An important example of a negative
feedback loop is seen in control of
thyroid hormone secretion

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Negative feedback effects

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Feedback control of insulin by glucose
concentrations

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LH surge: the positive feedback mechanism
HT
Pituitary
Ovary
GnRH
LH
Estrogen
>200 µg/ml
activates

25. Hypothalamus and Pituitary
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Hypothalamus
 HT is part of the diencephalons, which forms the floor and the
lateral wall of the 3rd ventricle.
 Located near the bottom of the brain (below thalamus); blood
brain barrier in this area is weaker than in rest of the brain
 HT represents less than 1% of the brain mass, about 5 gm
 Regardless of its size, it plays most important role in
controlling homeostasis.
 It is the main brain structure involved in regulating hormonal
levels in the body

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Function of Hypothalamus
1. Controls the ANS
2. Endocrine function
– Controls adenohypophyseal hormones
– Controls neurohypophyseal hormones
– Controls adrenal medulla
3. Regulation of body temperature
4. Contributes to the regulation of sleep, wakefulness, emotions,
sexual arousal, anger, fear, pain, and pleasure.
5. Controls food intake (hunger sensation):
6. Control of water-electrolyte balance
7. Control of sexual behavior: libido, sexual activities are
controlled by cerebral cortex, limbic system and HT.
8. Regulates MR by stimulating calorigenic hormones such as
T3/T4, AD, NA, glucocorticoids.
9. Controls milk letdown and uterine contraction.

28. Hypothalamic releasing/inhibiting hormones
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Hormones of the Hypothalamus Primary Action On Anterior Pitutary
Thyrotropin-releasing hormone Stimulates secretion of TSH by thyrotropes
Gonadotropin-releasing hormone Stimulates secretion of FSH and LH by
gonadotropes
Corticotropin-releasing hormone Stimulates secretion of ACTH by
corticotropes
Growth hormone releasing hormone Stimulates secretion of GH by somatotropes
Growth hormone inhibiting Hormone
(Somatostatin)
Inhibits secretion of GH by somatotropes
Prolactin-inhibiting hormone (Dopamine) Inhibits secretion of PRL by lactotropes
Prolactin releasing hormone (PRH) Stimulates PRL secretion lactotropes

29. The pituitary gland (hypophysis)
 Located directly below the hypothalamus
 The glands consists of two parts, each distinct from each other
 Anterior pituitary (adenohypophysis): derived from
glandular tissue
 Posterior pituitary (neurohypophysis): derived from neural
tissue
 These glands are not part of the brain, but receive direct input
from the hypothalamus via the pituitary stalk
 Bundle of both axons and blood vessels
 Axons go to posterior, blood vessels go to anterior
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30. Posterior pituitary (Neurohypophysis)
Neuroendocrine cells from
two nuclei in hypothalamus
synapse directly on to blood
vessels in posterior pituitary
• Supraoptic nucleus
• Paraventricular nucleus
When an action potential
arrives down axon, it releases
hormone directly into
bloodstream
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31. Posterior pituitary…..
 Does not synthesize hormones,
 It does store and release two hormones.
 Oxytocin
 Antidiuretic hormone (ADH) or vasoperssion
 The paraventricular nucleus synthesizes the hormone oxytocin
 Induces uterus contraction (Labor)
 Induces myoepithelial contraction (lactation)
 The supraoptic nucleus produces antidiuretic hormone (ADH)
 Promotes reabsorption of water in the renal tubules
 Vasoconstriction of arteries and arterioles
 Regulates osmolality and volume of ECF
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32. Major pituitary glands
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33. End
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