The Endocrine System

This document provides information about the endocrine system, including the control of secretion, hormonal and neural control, cortisol and insulin receptors, pituitary gland, and adrenal glands. It discusses the functions and actions of various hormones and their receptors, as well as the role of the hypothalamus in regulating the pituitary gland. The document also explains the synthesis and secretion of corticosteroids and catecholamines in the adrenal glands.

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THE ENDOCRINE SYSTEM
Student’s Name
Course Name
Professor’s Name
University Name
City, State
Date

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TASK
Question 1
Control of secretion Hormone Principal Actions
Hormonal.
Hormonal control of secretion
means the release of a
hormone in response to
another hormone acting as the
stimuli triggering the release.
This happens in the case of
hypothalamus releasing
secretory or inhibitory
hormones that stimulate the
anterior pituitary to either
increase or lower the
secretion of various
hormones.
1.Thyroxine. Thyroxine is
secreted by the thyroid gland
when stimulated by thyroid
stimulating hormone from the
anterior pituitary gland.
2.Growth Hormone. Secreted
by the anterior pituitary after
stimulation by the growth
hormone releasing hormone
from the hypothalamus.
1.Increases basal metabolic
rate through increased oxygen
consumption in the tissues.
Tissues not affected by this
action include brain,
gonads,spleen,lungs and
retina. Thyroxine induces the
synthesis and activity of Na+-
K+ ATPase.
2. Has diabetogenic effect
causing insulin resistance and
by decreasing glucose uptake
and its utilization by the
target tissues such as the
muscle.
Neural.
In neural control of hormonal
1.Epinephrine. Released from
the adrenal medulla
1. Increases the systolic blood
pressure through increasing

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secretion the nervous system
is directly involved as the
stimuli that causes the
endocrine glands to release
the hormones. The
hypothalamus stimulates the
posterior pituitary through
neural mechanisms to release
its stored hormones. Adrenal
medulla is stimulated by
sympathetic nervous system
to produce catecholamines.
chromaffin cells upon
stimulation by the
sympathetic nervous system.
2. Antidiuretic hormone.
Synthesized by the
hypothalamus and stored in
posterior lobe of the pituitary
gland.
the force of contraction of the
heart and the cardiac output.
Reduces the total peripheral
resistance hence a decrease in
diastolic pressure.This effects
are due to interaction with
theadrenergic receptors found
in this tissues.
2.Acts on the kidneys to
cause water retention by
increasing reabsorption of
water in the distal convoluted
tubule and the collecting duct.
Question 2
Cortisol which is a steroid hormone released by the adrenal cortex has intracellular receptors
which are located inside the target cells, in the nucleus or in the cytoplasm and acts to alter the
gene expression of the target cell. These steroid receptors are a large group of molecules that
function as transcription factors and signal transducers (Agely et al., 2019). Cortisol enters a
target cell by simple diffusion across the cell membrane because it’s a lipid. Once inside the
target cell it binds to its receptor that is either in the cytoplasm or in the nucleus causing
conformational changes in the receptor making it viable for binding DNA termed as receptor

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activation. The hormone-receptor complex functions as a ligand-dependent transcription factor
that binds to the promoter region of responsive genes to stimulate or inhibit the transcription of
these genes.
Insulin binds to its receptor on the cell membrane which is a tetramer composed of two alpha
subunits and two beta subunits. The alpha subunit lies in the extracellular domain while the beta
subunit spans across the cell membrane. Insulin first binds to the alpha subunit causing a
conformational change in the receptor activating tyrosine kinase in the beta subunit which
phosphorylase themselves in presence of ATP (Cara and Rosenfield,2009). The activated
tyrosine kinase then phosphorylates other proteins involved in physiologic actions of insulin. The
insulin receptor complex is then taken into the target cell by endocytosis. The insulin receptor is
then degraded by intracellular proteases, stored or recycled to the cell membrane for re-use.
Target cells have receptors specific to a given hormone and activated by either lipid soluble
which are permeable to plasma membrane or water soluble which bind to cell surface receptor.
Insulin which is a water soluble hormone means it is lipophobic and has to bind to a receptor on
or within the plasma membrane to initiate intracellular signaling cascade. Cortisol which is
lopophilic passes through the plasma membrane and binds to an intracellular receptor and
changes gene expression (Fink, Pfaff & Levine , 2012).
TASK 2
QUESTION 1
The pituitary gland also called hypophysis lies in the Sella turcica at the base of the brain and it
is connected to the hypothalamus by the pituitary or hypophysial stalk and has two lobes,
anterior and posterior lobes. Anterior pituitary also called adenohypophysis acts as the engine of
the endocrine system under the influence of the hypothalamus. Hypothalamus controls the

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anterior pituitary secretions by secreting releasing and inhibitory hormones that are transported
through the hypothalamo-hypophyseal portal vessels to the anterior pituitary lobe.
Examples of hormones released by the hypothalamus that are secretory;
(i)Growth hormone releasing peptide that stimulates the release of growth hormone releasing
hormone and growth hormone
(ii)Corticotropin releasing hormone that stimulates the release of adrenocorticotropin
(iii)Thyrotrophic releasing hormone (TRH) that stimulates the release of thyroid stimulating
hormone (TSH)
(iv) Gonadotropin releasing hormone (GnRH) causing the release of luteinizing hormone (LH)
and follicular stimulating hormone (FSH)
Examples of hormones released by the hypothalamus that are inhibitory;
(i)Growth hormone inhibitory hormone or somatostatin inhibiting the growth hormone release
(ii)Prolactin inhibitory hormone inhibiting prolactin secretion
Examples of hormones released by the anterior pituitaryand their cellular origin
(i)Growth hormone(GH) from somatotropes that promotes growth through its effects in protein
formation, cell differentiation and by cell multiplication.
(ii)Thyroid stimulating hormone (TSH) from thyrotropes stimulates release of thyroid hormones
from the thyroid gland follicular cells.
(iii)Adrenocorticotropic hormone (ACTH) from corticotropes
(iv)Follicle stimulating hormone and luteinizing hormone
Diagram showing release of cortisol upon the hypothalamus receiving a stress stimuli

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The pituitary secretions are majorly controlled by the hypothalamus through hormonal or neural
signals. Hypothalamus receives signals from the central nervous system that are from stimuli
such as changes in electrolyte concentration balance and pain and uses this information to control
the anterior pituitary through release of secretory or inhibitory hormones into it (Siragy and
Carey, 2010). The hormones released by anterior pituitary gland through signals from the
hypothalamus are mainly either tropic hormones that stimulate other endocrine glands or
gonadotropins for their actions on the gonads.

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Feedback mechanisms are used to bring about homeostasis in the body, they are positive or
negative. Negative feedback mechanisms lower the production of the desired hormone while
positive feedback mechanisms increase the production of the desired hormone. Signals received
by the hypothalamus from the central nervous system influence the release of secretory or
inhibitory stimuli from the hypothalamus to either increase or decrease secretion and release of
hormones from the pituitary gland (Guérineau, 2018). The hormones released in the pituitary

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gland from the hypothalamus signals act on other endocrine glands to secrete hormones that
bring the net desired effects on the target tissue or organ.
QUESTION 2
Posterior lobe also known as the neurohypophysis acts as a storage for antidiuretic hormone
(ADH) and oxytocin which are peptide hormones secreted in the magnocellular neurons found in
the hypothalamus. It has its origin from neural tissue outgrowth from the hypothalamus making
connections between the two neural. The posterior pituitary is a collection of nerve axons whose
cell bodies are found in the hypothalamus. Transport to the posterior lobe of this two hormones
is through the axoplasm of the neurons’ nerve fibers that pass from the hypothalamus. Posterior
pituitary secretions are controlled by neural signals from the hypothalamus ending in the
posterior lobe (Granger et al.,2018) The two nuclei found in the hypothalamus that is involved in
the production of both oxytocin and antidiuretic hormones are supraoptic and paraventricular
nuclei.
Through its connections to the hypothalamus by the neural tissue the posterior pituitary can be
directly stimulated by neural signals coming from the hypothalamus (Lanciotti et al,2018). This
neural linkage with the hypothalamus that is stimulated by stimuli from the central nervous
system provides a connection between the posterior pituitary and the nervous system.
TASK 3
There are two adrenal glands in the human body located in the retroperitoneal cavity above each
of the kidneys. They consist of two zones, the adrenal cortex which is the outer zone and the
adrenal medulla the inner zone. The adrenal cortex occupies a larger portion of the zones totaling
to eighty percent while the adrenal medulla is about twenty percent and it is functionally related
to the sympathetic nervous system of the body (Herbison, 2016). The adrenal cortex which is

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further sub- divided into three zones namely; zona glomerulosa (the outermost) zona fasciculata
and zona reticularis (the innermost) secretes corticosteroids that consist of mineralocorticoids
such as aldosterone, glucocorticoids such as cortisol and androgens such as androstenedione. The
adrenal secretes catecholamines in response to sympathetic stimulation namely epinephrine and
nor-epinephrine which exhibit almost the same effects in the whole body as sympathetic nerve
stimulation (Herbison, 2016).
Corticosteroids synthesis and secretion depends entirely on adrenalcorticotropin hormone that
stimulates cholesterol desmolase that is involved in the first step of cholesterol breakdown to
pregnenolone. When there is low blood glucose levels as happens during fasting the body
produces increased amounts of cortisol that promotes gluconeogenesis in the liver thereby
increasing the amounts of glucose in the blood (Hook et al., 2019). The hypothalamus produces
corticotropin releasing hormone that acts on anterior pituitary to produce adrenalcorticotropin
hormone that in turn acts on the zona fasciculata and reticularis to produce cortisol. The primary
regulator of aldosterone synthesis in contrast is the changes in the extracellular fluid through the
renin-angiotensin II – aldosterone system (Hall, 2016). The mediator is angiotensin II which acts
on cholesterol desmolase and is produced in fall of extracellular fluid levels to produce
aldosterone that acts on kidney tubules to increase sodium ions reabsorption. Catecholamines are
released when there is sympathetic stimulation of the chromaffin cells of the adrenal medulla
such as in the case of fall in blood pressure. They cause increased myocardial muscle
contractility and vasoconstriction increasing the total peripheral resistance thereby causing an
increase in blood pressure.
Adrenaline has both positive and negative impacts in the human body. When there is a fall in the
blood pressure as in the case of an accident due to blood loss, the adrenal medulla is stimulated

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to release adrenaline which increases blood pressure leading to continued perfusion of tissues
despite low blood volume which is a positive impact( Stückenschneider et al.,2013). . Adrenaline
release due to various stimuli in an already hypertensive patient would be dangerous as it could
lead to death due to heavily increased blood pressure that can cause burst of small blood vessels
in the brain (Eiden and Jiang, 2018).
Comparatively speaking it is better to damage the adrenal medulla than the cortex in the sense
that the adrenal cortex hormone actions can substitute though not fully for the actions of
hormones secreted by adrenal medulla (Onuigbo, 2008). Aldosterone produced by the adrenal
cortex help increase lowered blood pressure in blood loss by increasing the re-absorption of
sodium ions and water thereby water loss through the kidneys increasing the extracellular fluid
volume (Hyde, Mezulis and Abramson, 2008). The actions of the catecholamines produced by
adrenal medulla are limited compared to the diverse actions of the corticosteroids from adrenal
cortex hence the adrenal medulla cannot compensate for the damage of adrenal cortex.
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