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Endocrine system
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The endocrine system forms part of the regulatory system of the body
...
The nervous system provides short – term
control
...
A series
of endocrine glands synthesise and secrete chemical messengers directly into the
blood stream to their target organs
...
What are the chemicals known as? Hormones
...
All other organs remain
unaffected
...

Metabolism is the amount of chemical reactions that occur in the body
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These changes in activity are balanced to maintain homeostasis
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Not all hormones are secreted by the endocrine glands
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It stimulates the release of gastric juices
...
It stimulates the secretion of
pancreatic juices
...
It helps maintain the corpus luteum in the ovary
throughout gestation
...
It stimulates red bone marrow to produce more erythrocytes
...
Stimuli may be
...

A stimulating or releasing hormone - thyroid stimulating hormone (TSH) from
the anterior pituitary gland stimulates the release of thyroid hormone
...

Feedback loops – prevent over secretion or reduced secretion once the effect
of the hormone has been achieved
...

The rate of secretion is carefully regulated by several feedback mechanisms
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The level of thyroid hormone within the circulation increases
and causes the suppression of the thyroid stimulating hormone
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The contractions of the uterus
during birth stimulate the secretion of oxytocin which further increases the
uterine contractions
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The Pituitary Gland:
Responsible for activating the other endocrine glands in the body
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It lies on the ventral surface of the brain and it is connected to the
hypothalamus
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Somatotrophin (growth hormone) – controls epiphyseal growth in young animals, the
production of protein and it regulates the use of energy during periods of poor food
supply
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Prolactin – stimulates the development of
the mammary glands and the secretion of milk during the latter half of pregnancy
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This secretes
gonadotrophin releasing hormone which stimulates the anterior hypophysis to
secrete FSH
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Luteinising

hormone (LH) – secretion is stimulated by the presence of oestrogen in the blood
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The remaining tissue forms the corpus luteum
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Stimulates the interstitial cells or cells of Leydig in the testes to secrete
testosterone
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The TSH, ACTH,
FSH and LH hormones are all ‘trophic’ hormones that stimulate other endocrine
glands
...

Posterior Hypophysis:
The hormones associated with this part of the pituitary gland are secreted by the
hypothalamus and only stored in the posterior hypophysis
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Acts on the collecting ducts of the kidney and stimulates reabsorption of
water
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Pineal gland:
A small gland on the ventral surface of the brain between the cerebral hemispheres
and the cerebellum
...
It functions as a biological
clock, affecting the production of gonadotrophins
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The Thyroid Glands:
Paired glands
...
Secretions are regulated and
controlled by the release of thyroid stimulating hormone (TSH) from the anterior
pituitary gland
...
Synthesis of the thyroid hormones requires
iodine; therefore, a lack of iodine in the diet can have a dramatic effect
...
Thyroxin (T4) and
Tri – iodothyronine (T3) have two main effects; Metabolic – control the basal
metabolic rate (BMR)
...

Therefore, both hormones affect the uptake of oxygen by the cells and are essential
for normal growth
...
This is the
rate energy must be used to maintain vital functions such as respiration, peristalsis
and heart rate
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Calcitonin lowers the levels of calcium by decreasing the
rate of bone resorption
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g
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Calcitonin has
an opposite effect to parathormone
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Closely associated with the thyroid
glands
...
Main function is to regulate
metabolism and calcium homeostasis in the body
...
A low blood plasma calcium
concentration (hypocalcaemia) stimulates the parathyroids to release parathormone
to raise the plasma concentration
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Increases the absorption of calcium from the small intestine (in the presence
of active vitamin D)
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Primary hyperparathyroidism – due to neoplasia of the parathyroid glands
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The calcium: phosphorus ratio in the blood is altered due to impaired kidney
function
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Resorption is in particular from the mandible and maxilla, this leads to a
condition known as ‘rubber jaw’ – the jaw becomes pliable and the teeth may
fall out
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The daily requirement can not be met and parathormone is produced to raise
the blood calcium levels by bone resorption
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The control is a series of reactions that begins with the parathyroid glands
detecting a change in the blood concentration of either calcium or
phosphorus
...

PTH acts on bone to increase resorption of calcium from the bone
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The lower level of calcitonin in the blood triggers the kidney to increase the
rate of activation of vitamin D
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Blood calcium level is normalised and the mechanisms are shut off
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Calcitonin acts on bone to decrease resorption of calcium
...

The kidney produces less active vitamin D and so less calcium is absorbed
from the intestines
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Adrenal Glands:
Paired glands which lie close to the cranial part of each kidney
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The two parts function separately
...
There are three groups:
Minerolocorticoids, Glucocorticoids, Adrenal sex hormones
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This acts on the distal convoluted tubule of the
kidney where it regulates acid base balance of plasma and extracellular fluid by the
excretion of hydrogen ions
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It controls the absorption of sodium by the kidney
tubules
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The hormones are
called corticosteroids; The most important are cortisol and corticosterone
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They have two main actions:

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They increase blood glucose levels by reducing the amount of glucose taken
up by the cells in the body
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When present in large quantities they suppress the inflammatory reaction,
which delays healing and repair
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Over secretion of glucocorticoids - Leads to Cushing’
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Under secretion of aldosterone - Leads to Addison’s disease
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They may be the reason why some neutered animals continue to show signs
of sexual behaviour
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The hormones prepare the body for emergencies, known as the ‘fear, flight,
fight’ syndrome
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They act to:

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
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Increase the heart rate and the rate and depth of respiration to increase the
O2 reaching the tissues
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This process will increase the energy levels
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Decreases the activity of the gastrointestinal tract & bladder as their functions
are less important in an emergency
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Pancreatic secretions are vital for
digestion; however the pancreas is described as a mixed gland as it contains an
endocrine section and an exocrine section
...

The endocrine function:
The function is to secrete hormones directly into the blood stream
...

The Islets of Langerhans secrete the following hormones:


Insulin; Secreted by the beta cells, which make up the majority of the cells
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It lowers the blood glucose



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levels by; increasing the uptake of glucose into the cells to provide energy
...

The process is called glycogenesis
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Secreted in response to low blood
glucose levels
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Breaking down glycogen
stores in the liver to release the glucose
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Somatostatin; Secreted by the delta cells
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It also helps to decrease
the gut motility and the secretion of digestive juices which acts to reduce the
efficiency of the digestive and absorptive processes

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