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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