Endocrine System
Endocrine communication occurs when the endocrine system uses chemical messengers called hormones that are
secreted into the bloodstream to relay messages and instructions to target cells all over the body. Target cells have
receptors on them that allow hormones to bind and read the hormonal message when it arrives.
Classes of hormones
Hormones can be divided into three classes based on their chemical structure
1. Amino acid derivatives - synthesised from amino acids (A) tyrosine and (B) tryptophan
a. Thyroid hormones, adrenaline and noradrenaline, dopamine (catecholamine)
b. Melatonin
2. Peptide hormones - chains of amino acids. can be divided into two groups: (A) glycoproteins and (B) short
polypeptides/proteins
a. Tsh, lh, fsh
b. Adh, oxytocin
3. Lipid derivatives - there are two groups: (A) eicosanoids and (B) steroid hormones
a. Signalling molecules and include leukotrienes, prostaglandins, thromboxanes and prostacyclins
b. Lipids structurally similar to cholesterol (androgens and corticosteroids from adrenal cortex
Transport AND INACTIVATION OF HORMONES
HORMONES ENTER THE BLOODSTREAM FOR DISTRIBUTION THROUGHOUT THE BODY. WITHIN THE BLOODSTREAM HORMONES MAY
CIRCULATE FREELY OR TRAVEL BOUND TO SPECIAL CARRIER PROTEINS. FREE HORMONES ARE INACTIVATED WHEN:
1. It diffuses out the bloodstream and binds to receptors on target cells
2. Absorbed and broken down by kidney or liver cells
3. Broken down by enzymes in blood or interstitial fluid
When free hormones are removed, bound hormones are released to replace them
Mechanism of hormone action
The binding of a hormone to its specific receptor may alter gene activity - stimulate synthesis of enzymes or
proteins by activating genes in the cell nucleus; alter the rate of protein synthesis by changing the rate of
transcription or translation; change membrane permeability. To affect a target cell a hormone must first interact
, with a specific receptor. Receptors are protein molecules to which hormones can bind to. If a cell has a receptor that
can bind a particular hormone, that cell will respond to that hormone.
● Receptors for catecholamines (A, NA, and dopamine) are extracellular receptors. Catecholamines cannot
penetrate a plasma membrane because they are not lipid soluble so the bind to the receptor proteins on the
outside of the cell to activate a second messenger system
○ cAMP, cGMP, IP3 and DAG second messenger system
● Steroid and thyroid hormones can diffuse across the lipid part of plasma membrane and bind to receptors
inside the cell in the cytoplasm or nucleus
Control of hormone secretion
Hormone secretion can be triggered by:
1. Humoral stimuli - changes in composition of extracellular fluid
2. Hormonal stimuli - changes in levels of circulating hormones
3. Neural stimuli - arrival of neurotransmitters at neuroglandular junctions
Hormones are secreted to adjust the activities of target cells and restore homeostasis. Hormone secretion or
activity is mainly controlled by negative feedback. An example is when blood glucose increases (humoral stimuli) the
pancreas will secrete insulin that will stimulate uptake of glucose and use. As the insulin level increases the glucose
level will decrease until it returns to normal and as a result insulin secretion will also return to resting levels.
This regulatory pattern is called negative feedback.
Describe the control of anterior pituitary hormone secretion
The hypothalamus regulates the functions of both the anterior and posterior pituitary gland and integrates the
activities of the nervous and endocrine system in three ways:
1. Hypothalamus acts as an endocrine organ and synthesises and releases hormones along the neurohypophysis
of the pituitary gland - adh and oxytocin
2. Secretes regulatory hormones that controls the secretory activity of the adenohypophysis
3. contains autonomic centres that exert direct neural control on the endocrine cells of adrenal medulla
The hypothalamus controls the production of hormones in the anterior lobe / adenohypophysis of the pituitary gland
by secreting specific regulatory hormones. Hypophyseal portal system ensures that when the hypothalamus releases
hormones into its bloodstream it enters portal vessels that link two capillary networks (hypothalamus and
Endocrine communication occurs when the endocrine system uses chemical messengers called hormones that are
secreted into the bloodstream to relay messages and instructions to target cells all over the body. Target cells have
receptors on them that allow hormones to bind and read the hormonal message when it arrives.
Classes of hormones
Hormones can be divided into three classes based on their chemical structure
1. Amino acid derivatives - synthesised from amino acids (A) tyrosine and (B) tryptophan
a. Thyroid hormones, adrenaline and noradrenaline, dopamine (catecholamine)
b. Melatonin
2. Peptide hormones - chains of amino acids. can be divided into two groups: (A) glycoproteins and (B) short
polypeptides/proteins
a. Tsh, lh, fsh
b. Adh, oxytocin
3. Lipid derivatives - there are two groups: (A) eicosanoids and (B) steroid hormones
a. Signalling molecules and include leukotrienes, prostaglandins, thromboxanes and prostacyclins
b. Lipids structurally similar to cholesterol (androgens and corticosteroids from adrenal cortex
Transport AND INACTIVATION OF HORMONES
HORMONES ENTER THE BLOODSTREAM FOR DISTRIBUTION THROUGHOUT THE BODY. WITHIN THE BLOODSTREAM HORMONES MAY
CIRCULATE FREELY OR TRAVEL BOUND TO SPECIAL CARRIER PROTEINS. FREE HORMONES ARE INACTIVATED WHEN:
1. It diffuses out the bloodstream and binds to receptors on target cells
2. Absorbed and broken down by kidney or liver cells
3. Broken down by enzymes in blood or interstitial fluid
When free hormones are removed, bound hormones are released to replace them
Mechanism of hormone action
The binding of a hormone to its specific receptor may alter gene activity - stimulate synthesis of enzymes or
proteins by activating genes in the cell nucleus; alter the rate of protein synthesis by changing the rate of
transcription or translation; change membrane permeability. To affect a target cell a hormone must first interact
, with a specific receptor. Receptors are protein molecules to which hormones can bind to. If a cell has a receptor that
can bind a particular hormone, that cell will respond to that hormone.
● Receptors for catecholamines (A, NA, and dopamine) are extracellular receptors. Catecholamines cannot
penetrate a plasma membrane because they are not lipid soluble so the bind to the receptor proteins on the
outside of the cell to activate a second messenger system
○ cAMP, cGMP, IP3 and DAG second messenger system
● Steroid and thyroid hormones can diffuse across the lipid part of plasma membrane and bind to receptors
inside the cell in the cytoplasm or nucleus
Control of hormone secretion
Hormone secretion can be triggered by:
1. Humoral stimuli - changes in composition of extracellular fluid
2. Hormonal stimuli - changes in levels of circulating hormones
3. Neural stimuli - arrival of neurotransmitters at neuroglandular junctions
Hormones are secreted to adjust the activities of target cells and restore homeostasis. Hormone secretion or
activity is mainly controlled by negative feedback. An example is when blood glucose increases (humoral stimuli) the
pancreas will secrete insulin that will stimulate uptake of glucose and use. As the insulin level increases the glucose
level will decrease until it returns to normal and as a result insulin secretion will also return to resting levels.
This regulatory pattern is called negative feedback.
Describe the control of anterior pituitary hormone secretion
The hypothalamus regulates the functions of both the anterior and posterior pituitary gland and integrates the
activities of the nervous and endocrine system in three ways:
1. Hypothalamus acts as an endocrine organ and synthesises and releases hormones along the neurohypophysis
of the pituitary gland - adh and oxytocin
2. Secretes regulatory hormones that controls the secretory activity of the adenohypophysis
3. contains autonomic centres that exert direct neural control on the endocrine cells of adrenal medulla
The hypothalamus controls the production of hormones in the anterior lobe / adenohypophysis of the pituitary gland
by secreting specific regulatory hormones. Hypophyseal portal system ensures that when the hypothalamus releases
hormones into its bloodstream it enters portal vessels that link two capillary networks (hypothalamus and
