Ch.14-Endocrine System Flashcards
Briefly describe the endocrine system.
The endocrine system works alongside the nervous system to regulate the functions of the human body to maintain homeostasis.
The endocrine system functions much slower than the nervous system.
The endocrine system, like the nervous system, uses chemical signals that bind to receptor molecules.
The glandular cells of the endocrine system release hormones that diffuse from the interstitial fluid into the bloodstream, carrying messenger molecules throughout the body.
Endocrine glands are ductless glands, their functions include: Regulate metabolism Control chemical reactions Transport substances Regulate water and electrolyte balance Reproduction Growth Development
The major endocrine glands include: Pituitary gland Thyroid gland Parathyroid glands Adrenal glands Pancreas Pineal gland Thymus gland Reproductive glands
The endocrine functions of the hypothalamus include the production and release of hormones, so it is considered a neuroendocrine organ.
Several other organs may secrete hormones, including the stomach, small intestine, kidneys and heart, but it is not their primary function.
Define the term ‘hormone’.
Hormones are chemical messengers, that act on specifically targeted cells, regulating their metabolic function, even if only in low concentrations.
Hormones affect most body cells, regulating growth and development, balancing the components of blood, body defences, cellular metabolism, energy balance and even reproduction.
Hormones are considered ‘long distance’ chemical signals.
Most hormones are either steroids or steroid-like substances, which are made from cholesterol
Or
Are nonsteroidal, including amines, glycoproteins, peptides or proteins made from amino acids.
Describe the sites of hormone production.
Pituitary gland(hypophysis):
- located at the base of the brain, attached to the hypothalamus superiorly.
- About 1cm in diameter.
- Lies in the sella turcica of the sphenoid bone and secretes a number of different hormones.
- Arterial blood is delivered to the pituitary via hypophyseal branches of the internal carotid arteries.
- Veins leaving the gland drain into the dural sinuses.
- Anterior and Posterior lobes have different function.
- The Anterior pituitary gland is composed of glandular tissue, manufactures/releases a variety of hormones, and is split into three regions: Pars distalis, pars tuberalis, pars intermedia.
- Releasing actions of the anterior pituitary are mostly stimulatory.
- Hormone secretion from the anterior pituitary is regulated by hormones coming directly from the nearby hypothalamus.
Posterior pituitary and the hypothalamus.
- The posterior pituitary gland is actually a part of the brain, formed by a down growth of hypothalamus.
- Neural connection between the 2 via a nerve bundle called the ‘hypothalamic-hypophyseal tract’, this runs through the infundibulum
- Made up mostly of nerve fibres and neuroglial cells.
- neurons of the hypothalamus produce, respectively, the posterior pituitarys hormones - oxytocin and antidiuretic hormone(ADH)/Vasopressin.
- Nerve impulses from the hypothalamus release these hormones into the bloodstream.
Thyroid gland.
- Located just below the larynx on either side and in front of the trachea.
- 2 lobes connected by a broad isthmus, forming a butterfly like shape.
- covered by a capsule of connective tissue with secretory parts called follicles.
- lumen/central cavity is filled with a clear, amber coloured substance called colloid which store hormones produced by the follicles.
- follicles produce thyroglobulin from which thyroid hormone is synthesised, this is body’s primary metabolic hormone ‘thyroxine’ and affects almost every cell in the body
Parathyroid Glands.
- located on posterior thyroid gland surface.
- usually 4, superior and inferior laterally on thyroid glands.
- covered in thin connective tissue capsules and yellowish brown in colour.
- produce parathyroid hormone - increases blood calcium concentration, decreases blood phosphate concentration, affecting bones, intestines and kidneys
Adrenal Glands.
- pyramid in shape, sitting atop each kidney, embedded in the adipose tissue.
- have central ‘medulla’ and outer ‘cortex’, each secreting different hormones.
- both are well supplied with blood vessels.
- medulla is closely connected sympathetic division of ANS. Appears more like mass of tissue than a gland.
- cortex consists of layers of cells, including the outer ‘zona glomerulosa’, the middle ‘zona fasiculate’ and the inner ‘zona reticularis’.
Pancreas.
- functions as both exocrine gland, secreting digestive juices, and as endocrine gland, releasing hormones.
- elongated, slightly flatted organ posterior to stomach, behind parietal peritoneum.
- joined to duodenum of small intestine.
- endocrine part consists of groups of cells called pancreatic islets/islets of langerhans.
- of these cells alpha cells secrete the hormone glucagon, and beta cells secrete the hormone insulin, delta cells produce a peptide hormone which supresses production of the other 2.
Pineal gland.
- small cone shaped gland located deep in cerebral hemisphere and attached to thalamus near upper part of the third ventricle.
- secretes the hormone melatonin in response to light conditions in the external environment from its pinealocytes which are arranged in tight cords and clusters.
Thymus.
- located deep inside mediastinum posterior to the sternum, between the lungs.
- larger in children than in adults, shrinks with age, important in early immunity.
Describe actions of Oxytocin.
Stimulates uterine contraction during childbirth and milk letdown.
Oxytocin receptors peak in number near the end of pregnancy.
The hormones stimulatory effects are most effective on uterine smooth muscle.
When blood levels of oxytocin rise, uterine contractions increase until expulsion of the foetus occurs.
In the brain oxytocin acts as a neurotransmitter and is involved in affection and sexual behaviours, as well as promoting trust, nurturing behaviours, and bonding as ‘couples’.
Describe the actions of adrenaline and noradrenaline.
Adrenaline makes up 80% of adrenal medulla secretions, the rest being noradrenaline.
The hormones aid in coping with stressors and participate in the fight or flight response.
Adrenaline is stronger in its stimulation of bronchial dilation, metabolic activities, and increased blood flow to the heart and skeletal muscles.
Adrenaline is used clinically as a bronchodilator and heart stimulant.
Noradrenaline more greatly influences blood pressure and peripheral vasoconstriction.
In response to stress the hypothalamus releases impulses which control the adrenal medulla’s secretions which have long lasting effects including elevation of heart rate, cardiac muscle contraction force, breathing rate, and blood glucose level, whilst elevating blood pressure decreasing digestive activity.
Describe the actions of insulin.
Stimulates the liver to form glycogen from glucose and inhibiting conversion of noncarbohydrates into glucose.
Insulin decreases blood glucose concentrations, promotes amino acid transport into cells, increases protein synthesis, and stimulates adipose cells to make and store fat.
Insulin prevents high blood glucose concentrations by promoting glycogen formation.
Insulin secretion decreases as glucose concentrations fall.
After eating, insulin lowers blood glucose levels and influences protein and fat metabolism.
Insulin enhances membrane transport of glucose into primarily, fat and muscle cells, inhibits glycogen breakdown into glucose, and inhibits conversion of fats or amino acids to glucose.
Insulin is not required for glucose entry into the brain, kidneys and liver.
In the brain insulin plays a role in feeding behaviours, learning, memory and neuronal development.
Insulin functions alongside glucagon to maintain stable blood glucose concentrations.
Describe the actions of glucagon.
Stimulates the liver to break down glycogen in the process known as glycogenolysis, and to convert certain non-carbohydrates, like amino acids, into glucose in the process known as gluconeogenesis, raising blood sugar concentration more effectively than adrenaline is able to.
Glucagon secretion prevents hypoglycaemia from occurring when glucose concentration is relatively low.
just one molecule of glucagon can release 100 million glucose molecules into the bloodstream.
The release of glucagon is suppressed by insulin, somatostatin, and rising blood glucose levels.
Describe the actions of aldosterone and angiotensin.
Synthesised by the ‘outer’ adrenal cortex.
A mineralocorticoid that regulates mineral electrolyte concentrations.
Helps kidneys to balance sodium and potassium, and stimulates water retention via the process of osmosis.
If blood sodium decreases or blood potassium increases, the adrenal cortex secretes aldosterone.
The kidneys can stimulate aldosterone secretion if blood pressure falls.
Aldosterone enhances the absorption of sodium from gastric juice, perspiration, and saliva.
It has regulatory effects that occur within 20 minutes, allowing precise control of plasma electrolyte balance.
Aldosterone secretion is stimulated by decreased blood volume and pressure, and raised blood levels of potassium.
The 2 most important mechanisms that regulate aldosterone secretion are the renin-angiotensin-aldosterone mechanism and the plasma concentrations of potassium.
The renin-angiotensin-aldosterone mechanism regulates aldosterone release, helping to control blood volume, blood pressure, and the reabsorption of sodium and water by the kidneys.
When blood pressure or volume falls, cells in the juxtaglomerular complex of the kidneys are excited, which respond by releasing renin into the blood.
The renin cleaves off part of the plasma protein known as angiotensin.
This causes an enzymatic cascade to occur forming angiotensin II.
This substance stimulates cells of the glomerulosa to release aldosterone.
All the effects of this mechanism ultimately raise blood pressure.
Describe the actions of ANP(Atrial Natriuretic Peptide).
A hormone from the heart that is secreted when blood pressure rises.
It regulates blood pressure and sodium-water balance, and greatly inhibits the renin-angiotensin-aldosterone mechanism.
It also inhibits other mechanisms that enhance sodium and water reabsorption.
Overall decreasing blood pressure by allowing sodium and water to leave the body in the urine.
Desire the actions of ADH(AntiDiuretic Hormone)/Vasopressin.
Regulates water concentration of body fluids by reducing water excretion by the kidneys.
Prevents both water overload and dehydration from occurring.
Decreased levels of ADH cause polyuria and diabetes insipidus.
Osmoreceptors sense increases in osmotic pressure due to dehydration and use ADH to signal the kidneys to produce less urine.
If too much water is in the body, ADH release is inhibited and urine production increases.
Triggers that cause ADH release include low blood pressure, pain, morphine, barbiturates and nicotine.
In severe blood loss conditions, extremely high amounts of ADH are released, causing vasoconstriction mostly of visceral blood vessels, causing the blood pressure to rise.
Diuretics antagonise ADH effects, removing water from the body. They are used for certain types of hypertension and oedema, such as in congestive heart failure.
