Download Endocrine System Revision Notes

Endocrine System – Revision notes
 Endocrine system consists of glands widely separated from each other with no direct links
 These glands consist of groups of secretory cells surrounded by an extensive network of
capillaries that facilitates diffusion of hormones (chemical messengers) from secretory cells
to the bloodstream
 Ductless glands – hormones diffuse directly into the bloodstream
 Hormones are then carried by the blood to target tissues and organs – they influence cellular
growth and metabolism
 Homeostasis of the internal environment is maintained partly by the autonomic nervous
system and partly by the endocrine system
 Autonomic nervous system = rapid changes
 Hormones of endocrine system = slower and more precise adjustments
 This system consists of a number of distinct glands and some tissues in other organs
 The hypothalamus is classified as part of the brain – it controls the pituitary gland and has an
indirect effect on many other glands within the body
 Endocrine glands
o Pineal body
o Pituitary glans
o Thyroid gland
o Parathyroid gland
o Thymus gland
o Adrenal glands
o Pancreatic islets
o Ovaries (female)
o Testes (male)
 When a hormone arrives at its target cell it binds to a specific area (receptor)
o Here it acts as a switch – influencing chemical/metabolic reactions inside the cell
o Receptors for peptide hormones are situated on the cell membrane
o Receptors for lipid-based hormones are inside the cell
 The level of a hormone in the blood is variable and self-regulating within normal range
 Hormones are released in response to a specific stimulus
o Usually action reverses the stimulus through a negative feedback system
o Can be controlled
 Indirectly – release of hormones by the hypothalamus and anterior pituitary
glans
 Steroid
 Thyroid hormones
 Directly – by blood levels of the stimulus
 Insulin
 Glucagon
 Effect of a positive feedback mechanism is amplification of the stimulus
o Increase of a specific hormone until a particular process is complete and the
stimulus ceases
 Oxytocin during labour
Pituitary Gland and Hypothalamus
 Pituitary gland and the hypothalamus act as a unit
o Regulate activities of most other endocrine glands
 Pituitary gland lies in the hypophyseal fossa of the sphenoid bone below the hypothalamus
o Attached to hypothalamus by a stalk
o Size of a pea
o Weight about 500mg
o Consists of three distinct parts that originate from different types of cells
 Anterior pituitary (adenohypophysis)
 Up-growth of glandular epithelium from the pharynx
 Posterior pituitary (neurohypophysis)
 Down-growth of nervous tissue from the brain
 Network of nerve fibres between the hypothalamus and posterior
pituitary
 Between these lobes there is a thin strip of tissue (intermediate lobe) –
function is unknown in humans
Blood Supply
 Arterial blood
o Supplied by branches from the internal carotid artery
o Anterior lobe is supplied indirectly by blood that has already passed through a
capillary bed in the hypothalamus
o Posterior lobe is supplied directly
 Venous drainage
o Comes from both lobes
o Containing hormones
o Leaves the gland in short veins that enter the venous sinuses between layers of dura
mater
The Influence of the Hypothalamus on the Pituitary Gland
 Anterior pituitary
o Supplied with arterial blood that has already passed through a capillary network in
the hypothalamus
o This network forms part of the pituitary portal system
 Transports blood from the hypothalamus to the anterior pituitary where it
enters thin-walled vascular sinusoids
 Is in very close contact with the secretory cells
o This blood provides
 Oxygen
 Nutrients
 Releasing and inhibiting hormones
 Secreted by the hypothalamus
 Influence secretion and release of other hormones formed in the
anterior pituitary
o Some hormones secreted by the anterior lobe stimulate/inhibit secretion by other
endocrine glands (target glands)
o Other hormones have a direct effect on target tissues
o Growth hormone (GH)
 Regulates metabolism
 Promotes tissue growth – especially of bones and muscles
o Thyroid stimulating hormone (TSH)
 Stimulates growth and activity of the thyroid gland
o Adrenocorticotrophic hormone (ACTH)
 Stimulates the adrenal cortex to secrete glucocorticoids
o Prolactin (PRL)
 Stimulates milk production in the breasts
o
o
Follicle stimulating hormone (FSH)
 Stimulates production of sperm in testes
 Stimulated secretion of oestrogen by the ovaries
 Maturation of ovarian follicles
 Ovulation
Luteinising hormone (LH)
 Stimulates secretion of testosterone by testes
 Stimulates secretion of progesterone by the corpus luteum
 The release of an anterior pituitary hormone follows stimulation of the gland by a specific
releasing hormone produced by the hypothalamus and carried to the gland through the
pituitary portal system of blood vessels
 Whole system is controlled by a negative feedback mechanism
o When there Is a low level of a hormone in the blood supplying the hypothalamus it
produces the appropriate releasing hormone
 Stimulates release of a trophic hormone by the anterior pituitary
o This then stimulates the target gland to produce and release its hormone
o Results in the blood level of that hormone rising – inhibiting the secretion of
releasing factor by the hypothalamus
 Posterior pituitary
o Formed from nervous tissue and consists of nerve cells surrounded by supporting
cells (pituicytes)
o These neurones have their cell bodies in the supraoptic and paraventricular nuclei of
the hypothalamus and their axons
 Then stored in vesicles within the axon terminals within the posterior
pituitary
 Their release is by exocytosis is triggered by nerve impulses from the
hypothalamus
o Oxytocin
 Stimulates uterine smooth muscle and muscles of lactating breast during
childbirth
o Antidiuretic hormone (ADH)
 Reduces urine output
Thyroid Gland
 Situated in the neck in front of the larynx and trachea at the level of the 5th-7th cervical and
1st thoracic vertebrae
 Highly vascular gland
 Weighs about 25 grams
 Surrounded by a fibrous capsule
 Resembles a butterfly in shape
 Consists of two lobes – one either side of the thyroid cartilage and upper rings of the trachea
 The lobes are joined by a narrow isthmus
o Lies in front of the trachea
 Lobes are roughly cone shaped
o About 5cm long
o About 3cm wide
 The arterial blood supply to the gland is through the superior and inferior thyroid arteries
o Superior thyroid artery is a branch of the external carotid artery
o Inferior thyroid artery is a branch of the subclavian artery
 Venous return is by the thyroid veins
o Drain into the internal jugular veins
 Two parathyroid glands lie against the posterior surface of each lobe and are sometimes
embedded in thyroid tissue
 Recurrent laryngeal nerve passes upwards close to the lobes of the gland and on the right
side it lies near the inferior thyroid artery
 Gland is composed of cuboidal epithelium that forms spherical follicles
o Secrete and store colloid
 Thick sticky protein material
o Between the follicles are other cells found either singly or in small groups
 Parafollicular cells (C-cells)
 Secrete the hormone calcitonin
 This glad secretes
o Thyroxine
o Tri-iodothyronine
o Calcitonin
Parathyroid Glands
 Four small parathyroid glands
 Embedded in posterior surface of each lobe of the thyroid gland
 Surrounded by fine connective tissue capsule
 Cells forming the glands are spherical in shape and are arranged in columns with channels
containing blood between them
Function
 Parathyroid glands secrete parathyroid hormone (PTH parathormone)
 Secretion is regulated by the blood level of calcium
o When this falls secretion of PTH is increased
o Opposite happens if levels increase
 Main function of PTH is to increase the blood calcium level when it is low
o Achieved by indirectly increasing the amount of calcium absorbed from the small
intestine and reabsorbed from the renal tubules
o If these sources are inadequate then PTH stimulates osteoclasts (bone-destroying
cells) and reabsorption of calcium from bones
 Parathormone and calcitonin from the thyroid gland act in a complementary manner to
maintain blood calcium levels within the normal range
o Needed for
 Muscle contraction
 Blood clotting
 Nerve impulse transmission
Adrenal Glands
 Two adrenal glands
 One situated on the upper pole of each kidney
 Enclosed within the renal fissure
 About 4cm long
 About 3cm thick
 Arterial blood supply to the glands is by branches from the abdominal aorta and renal
arteries
 Venous return is by suprarenal veins
o Right gland drains into the inferior vena cava
o Left gland drains into the left renal vein
 The glands are composed of two parts which have different structures and functions
o Outer part is the cortex
o Inner part is the medulla
o Adrenal cortex is essential to life – the medulla is not
Adrenal Cortex
 Produces three types of steroid hormones from cholesterol
 Collectively called adrenocorticocoids
o Glucocorticoids
o Mineralocorticoids
o Androgens (sex hormones)
 Hormones in each group have different characteristic actions
o Due to structural similarity actions may overlap
Adrenal Medulla
 Completely surrounded by the adrenal cortex
 Develops from nervous tissue in the embryo and is part of the sympathetic division of the
autonomic nervous system
 Stimulated by its extensive sympathetic nerve supply to produce the hormones
o Adrenaline (epinephrine)
o Noradrenaline (norepinephrine)
Pancreatic Islets
 Cells that make up the pancreatic islets are found in clusters; are irregularly distributed in
the substance of the pancreas
 There are no ducts leading from the cluster of islet cells
 Pancreatic hormones are secreted directly into the bloodstream and circulate throughout
the body
 Three main types of cells in the pancreatic islets
o Alpha cells – secrete glucagon
o Beta cells – secrete insulin
o Delta cells – secrete somatostatin
 Normal blood glucose level is between 3.5 and 8mmol/litre
 Blood glucose levels are controlled mainly by the opposing actions of insulin and glucagon
o Glucagon – increases blood glucose levels
o Insulin – reduces blood glucose levels
Insulin
 Polypeptide
 50 amino acids
 Main function – to lower raised blood nutrient levels
o Glucose
o
o
Amino acids
Fatty acids
 When these nutrients are in excess of immediate needs insulin promotes their storage by
o Acting on cell membrane and stimulating uptake and use of glucose by muscle and
connective tissue cells
o Increasing conversion of glucose to glycogen (glycogenesis) especially in the liver
and skeletal muscles
o Accelerating uptake of amino acids by cells, and the synthesis of protein
o Promoting synthesis of fatty acids and storage of fat in adipose tissue (lipogenesis)
o Decreasing glycogenolysis (breakdown of glycogen into glucose)
o Prevents breakdown of protein and fat and gluconeogenesis (formation of new
sugar from e.g. protein)
 Secretion of insulin is stimulated by increased blood glucose levels and to a lesser extent by
parasympathetic, raised blood amino acid and fatty acid levels, and gastrointestinal
hormones
o Gastrin
o Secretin
o Cholecystokinin
 Secretion is decreased by
o Sympathetic stimulation
o Glucagon
o Adrenaline
o Cortisol
o Somatostatin
Glucagon
 Increase blood glucose levels by stimulating
o Conversion of glycogen to glucose in the liver and skeletal muscles (glycogenolysis)
o Glucogenolysis
 Secretion of glucagon is stimulated by a low blood glucose level and exercise
 It is decreased by somatostatin and insulin
Somatostatin (GHRIH)
 Also produced by the hypothalamus
 Effect of this hormone is to inhibit the secretion of both insulin and glucagon in addition to
inhibiting the secretion of GH from anterior pituitary
Pineal Gland or Body
 Small body attached to the roof of the third ventricle and is connected to it by a short stalk
containing nerves, many of which terminate in the hypothalamus
 About 10mm long
 Reddish-brown in colour
 Surrounded by a capsule
 Melatonin is the hormone secreted by this gland
 Secretion is controlled by daylight and levels fluctuate during each 24-hour period – highest
at night and lowest around midday
 Secretion is also influenced by the number of daylight hours
o Seasonal variations
 Functions of this hormone are not fully understood it is believed to be associated with
o Coordination of the circadian and diurnal rhythms of many tissues – possibly by
influencing the hypothalamus
o Inhibition of growth and development of the sex organs before puberty – possibly
preventing synthesis or release of gonadotropins
 The gland tends to atrophy after puberty and may become calcified in later life
Thymus Gland
 Thymosin – hormone secreted by the thymus gland
 Required for the development of T-lymphocytes for cell-mediated immunity
Local Hormones
 Histamine
o Synthesised by mast cells in the tissues and basophils in blood
o Released as part of the inflammatory process
 Increasing capillary permeability and causing vasodilatation
 Also causes contraction of smooth muscle of the bronchi and alimentary
tract
 Stimulates secretion of gastric juice
Serotonin (5-hydroxytptamine, 5-HT)
 Present in platelets, in brain, and in intestinal wall
 Causes intestinal secretion and contraction of smooth muscle
 Has a role in haemostasis (blood clotting)
Prostaglandins
 Lipid substances found in most tissues that act as local hormones and have wide-ranging
physiological effects in
o Inflammatory response
o Potentiating pain
o Fever
o Regulating blood pressure
o Blood clotting
o Uterine contractions during labour
Gastrointestinal Hormones
 Gastrin, secretin and cholecystokinin (CCK) influence the secretion of digestive juices