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THE HUMAN ENDOCRINE SYSTEM

IB Biology

E. McIntyre

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Introduction to The Human Endocrine System

• The purpose of the Endocrine system is to aid in maintaining homeostasis.

• The endocrine system produces hormones. Hormones are chemical regulators produced by cells in one part of the body that affect cells in another part of the body.

• The endocrine system consists of a series of glands having no direct connection to other parts of the body, i.e.. they are ductless.

• Note: Exocrine Glands have a duct to connect them directly to a specific site in the body, e.g.. the salivary glands.

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Endocrine Glands

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n of a brain!

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Hormones

• Hormones are produced and secreted into the blood stream by the endocrine glands. The circulatory system carries hormones to other parts of the body via the blood.

• Hormones are chemical messengers responsible for co-ordinating certain body functions. In general they are stimulatory.

• The word hormone comes from the Greek hormon, meaning “to excite or set into motion”.

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...Hormones

Function of Hormones

• Hormones exert their effects on specific locations in the body called target tissues (this could be a whole organ or just a few cells). Their effects are long term, controlling functions such as growth, metabolism, maturity and the balance of chemicals in the body.

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...Hormones

The Mode of Action of Hormones• Hormones act at the molecular level by

causing changes in cell metabolism, for example:

• protein synthesis• enzyme activity• permeability of cell membranes• DNA translation

• there are two basic ways in which hormones do this, based on the type of hormone:

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(What’s Metabolism?)

• The sum of the chemical reactions occurring within a cell or a whole organism; includes the energy-releasing breakdown of molecules (catabolism) and the synthesis of new molecules (anabolism).

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...Hormones

Steroid Hormones• The hormone diffuses

directly into the cell. • Here it fuses with a

receptor molecule.• This receptor molecule is

only present in the target cells for that particular hormone.

• The hormone/receptor combination effects the translation of DNA in the nucleus.

steroid_hormone animation

mcgraw-hill animation steroid hormones

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...Hormones

Protein Hormones:• The hormone binds to a receptor

molecule located on the surface of the cells of the target tissue.

• This causes the production of a secondary messenger molecule within the cell, e.g.. cyclic AMP.

• The secondary messenger molecule causes a change in cell metabolism, e.g.. activates an enzyme.

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Negative Feedback

• In a negative feedback system some factor, such as blood pressure, changes. The change is detected by a sensor. The sensor sends a message to an integrating center which in turn stimulates an effector. The effector will do something to alter the factor that changed.

• In the example to the right blood pressure has increased. Receptors in the carotid arteries detect the change in blood pressure and send a message to the brain. The brain will cause the heart to beat slower and thus decrease the blood pressure. Decreasing heart rate has a negative effect on blood pressure.

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The Pituitary Gland: The Master Gland

• An outgrowth from the base of the fore-brain and in direct nervous contact with it.

• This gland secretes a range of hormones;– Some of these hormones have a direct effect on their target organs...– while others have an indirect effect by causing other glands to secrete

further hormones.

• It is for this reason that the pituitary gland is often called the master gland, as it regulates the secretions of a number of other endocrine glands.

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... The Pituitary GlandStructure• The pituitary

gland consists of two lobes, the anterior and the posterior lobes. Each releases specific hormones under the direction of the hypothalamus.

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• The posterior lobe of the pituitary stores and releases hormones which have been produced by the hypothalamus. The hormones travel by way of specialized nerve cells from the hypothalamus to the pituitary.

The POSTERIOR Pituitary

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Antidiuretic Hormone (ADH)

– regulates water balance in the body by increasing water absorption by the kidneys

• Oxytocin:

• stimulates uterine contractions during birth

• also stimulates milk production

The Hormones of the POSTERIOR Pituitary

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The ANTERIOR Pituitary

• The anterior lobe of the pituitary, unlike the posterior lobe, produces its own hormones.

• Like the posterior lobe, the anterior lobe is richly supplied with nerves from the hypothalamus.

• The hypothalamus regulates the release of hormones from the anterior pituitary. Hormones are secreted from the nerve ends of the cells of the hypothalamus and transported in the blood to the pituitary gland. Most of these hormones activate specific cells in the pituitary, causing the release of pituitary hormones, which are then carried by the blood to target tissues.

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The Hormones of the ANTERIOR Pituitary

• Somatotropin (STH) or Growth Hormone (GH)

• has a direct effect on the growth of tissues

• is produced in greater quantities during childhood

• Thyroid Stimulating Hormone (TSH)

• has an indirect effect on metabolic rate, by affecting the thyroid gland

• stimulates the thyroid gland, causing it to produce the hormone thyroxin

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• Adrenocorticotropic Hormone (ACTH)• stimulates the adrenal cortex to produce a range of hormones

responsible for stress relief

• Gonadotropic Hormones:• Follicle Stimulating Hormone (FSH)

• Females: stimulates the development and

production of ova, causes the release of estrogens

• Males: stimulates the production of sperm cells

• Lutinizing Hormone (LH)• Females: causes the development of the corpus luteum. (A structure

that develops in the ovary and secretes progesterone )

• Males: (called interstitial cell stimulating hormone - ICSH) stimulates the production of testosterone from the interstitial cells of the testes

...The Hormones of the ANTERIOR Pituitary

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• Prolactin

• stimulates milk production after birth

• Melanocyte Stimulating Hormone (MSH)

• stimulates melanin (a brown pigment) production in the skin

• responsible for changing the skin colour of certain reptiles and amphibians

...The Hormones of the ANTERIOR Pituitary

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Control of Pituitary Hormone Secretions:

• Brought about by the hypothalamus, which monitors the levels of hormones and other chemicals in the blood and forms a connection between the nervous and endocrine systems.

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Control of Secretions from the Anterior Pituitary

• The hypothalamus contains neurosecretory cells which produce and release pituitary hormone releasing factors into the capillaries in the upper part of the anterior pituitary

• these releasing factors stimulate specific cells in the anterior pituitary to produce and release the various tropins and other hormones of the anterior pituitary into the blood leaving the pituitary gland, to be carried to their target organs.

• Certain releasing factors inhibit the release of hormones from the anterior pituitary e.g.. dopamine inhibits the secretion of prolactin

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Control of the secretions from the posterior pituitary

• Unlike the anterior pituitary, the posterior pituitary does not actually produce its own hormones.

• The hormones released from the posterior pituitary are produced in the hypothalamus and pass down neurosecretory cells into the cells of the posterior pituitary. They are stored here and released into the blood leaving the pituitary gland when needed.

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The Thyroid Gland:

•Produces three hormones;–Thyroxine which controls the basic metabolic rate and also influences growth rate by controlling the growth and differentiation of cells.–Triiodothyroxine, which appears to have the same functions as thyroxine.

–Calcitonin which effects bone development and muscle contraction (both

need calcium)

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Thyroid disorders• Thyroid absorbs iodine from blood.• Iodine is assimilated into thyroglobin.• When stimulated, thyroglobin becomes thyroxine.• Goiter is when there is no iodine, therefore the

precursor to thyroglobin builds up in the thyroid gland.• Iodine is added to table salt, i.e.. iodized salt, to

overcome the problem of dietary deficiency of iodine.

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Over secretion of thyroxine:

• Referred to as hyperthyroidism.

• In adults this condition causes over-activity, decreased body weight, increased heart rate and blood pressure, which can lead to heart failure - called thyrotoxicosis.

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Under secretion of thyroxine:

• Referred to as hypothyroidism

• In infants: results in retarded mental and physical development called cretinism. This can be easily treated by early administration of thyroid extract.

• In adults: causes sluggishness and over-weight. This condition is called myxoedema.

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Control of Thyroxine Production

• This provides an example of negative feedback control of the endocrine system:

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The Parathyroid Glands:• These are located as two pairs of small glands on

either side of the thyroid gland• They secrete parathormone which increases plasma

calcium concentrations (opposite effect to calcitonin)• Parathormone stimulates the breakdown of calcium

in the bones and causes it to be released into the blood.

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Over secretion of parathormone:

• Hyperparathyroidism causes the bones to soften due to excess calcium released from the bone.

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Under secretion of parathormone:

• Hypoparathyroidism causes muscle cramps due to lack of calcium for muscle contraction, it is all absorbed into bone.

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The Adrenal Glands:• These are located just above each kidney• They consist of two layers, an outer cortex

and an inner medulla.• Each layer secretes specific hormones.

Adrenal cortex clipAdrenal gland clip

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Secretions from the adrenal cortex:

• Glucocorticoids - the most important being cortisol (a steroid hormone). This hormone stimulates the conversion of proteins and fats to glucose for use in cellular respiration and makes available amino acids for protein synthesis necessary for the repair of cells. This is important in the recovery of the body from stress.

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• Cortisol also reduces the allergic and inflammatory responses caused in damaged tissues, by inhibiting the immune system. (Cortisone is a drug given in many instances to reduce inflammation or allergic response)

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• Mineralocorticoids - the most important being aldosterone. This hormone stimulates the absorption of sodium ions by the kidneys to maintain salt and water balance in the blood.

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HORMONES THAT AFFECT BLOOD SUGAR

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Abnormalities Associated with the Pituitary

• Abnormalities Related to the Secretion of Growth Hormone:

• Dwarfism– Caused by under secretion of GH during

childhood resulting in reduced over all growth.– Under secretion in adults has been linked to early

senility.

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• Gigantism

– caused by over secretion of GH during childhood.

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• Acromegaly

– caused by over secretion of GH during adulthood, leading to excessive growth of facial bones and those of the hands and feet.

Andre the Giant 7’ 4” 500 lbs

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• Gonadocorticoids (Sex hormones) - in fact these are hormones very similar to the sex hormones and have similar effects, to maintain the viability of the gonads and cause the development of secondary sex characteristics.

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Control of the secretions from the Adrenal Cortex:

• Secretions from the adrenal cortex are stimulated by ACTH from the anterior pituitary.

• The control of these secretions demonstrates the feedback mechanism:

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Secretions from the Adrenal Medulla:

• Unlike the adrenal cortex, which is under hormonal control, the release of hormones from the adrenal medulla is controlled by the nervous system, another illustration of the connections between the nervous and endocrine systems.

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• In stressful situations the sympathetic nervous system stimulates the release of two hormones from the adrenal medulla: adrenaline and noradrenaline (also called epinephrine and norepinephrine)– These two hormones essentially have the

same functions, to prepare the body for stressful or exciting situations when vigorous activity may be required.

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• Epinephrine & Norepinephrine serve to prolong and maintain the body in a state of preparation for activity longer than could be maintained by the nervous system alone.

• They cause the following effects:• heart rate increases

• breathing rate increases

• pupils dilate

• blood is diverted from non-essential tasks, such as digestion, to the muscles for activity

• anal and bladder sphincters relax.

• face becomes pale (blood diverted from here)

• hair stands on end, sweating increases.

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The Pancreas:• The pancreas has both endocrine and exocrine

functions;

• Glucose control

• Digestive enzyme production

• It also has patches of specialized cells which produce two hormones, insulin and glucagon, that are secreted into the blood stream.

• These patches of cells are referred to as the islets of Langerhans

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• The islets have two types of secretory cells within them, alpha cells which secrete glucagon and beta cells which secrete insulin.

• These two hormones are responsible for the control of glucose metabolism by controlling the breakdown and synthesis of glycogen, the carbohydrate store in the body.

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Insulin• Released into the bloodstream in response

to increasing blood glucose concentration, for example after a meal. It has two effects:1. It increases the permeability of the cells of the

liver and muscles to glucose so that glucose is readily absorbed by these cells.

2. It promotes the conversion of glucose to glycogen in these cells.

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Glucagon

• Has an antagonistic effect to insulin, it promotes the break down of glycogen to glucose and its release into the blood stream, therefore increasing the blood glucose level in the blood.

• It is released when the blood glucose level falls during periods of exercise or fasting between meals.

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Glucose Control

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Diabetes Mellitus:• When the beta cells of the pancreas deteriorate

and fail to produce adequate amounts of insulin and the blood sugar level rises above normal.

• This is known as hyperglycemia.

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Symptoms:1. Glucose in the urine (glycosuria) caused by excretion of

glucose by the kidneys as the blood glucose level rises above renal threshold (160mg/100mL of blood)

2. Production of large volumes of urine (diuresis) in order to excrete the glucose results in dehydration and excessive thirst. (This symptom is usually the first noticed by new diabetics)

3. Low energy levels due to the unavailability of glucose for cellular respiration. Despite the abundance of glucose in the blood, little moves into the cells and so the cells are starved of energy. This may cause unconsciousness.

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4. The smell of propanone (acetone) on the breath. Propanoneis a by product of fat metabolism.

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Treatment of diabetes:

• There are two forms of diabetes mellitus, juvenile, or early-onset diabetes and adult, or late-onset diabetes.

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Adult or late-onset diabetes• Around 90% of diabetics develop the condition as

adults.

• It is due to reduced insulin output by the beta cells of the pancreas.

• Sulfonamide drugs are also prescribed to stimulate the beta cells to produce a little more insulin.

• Not necessary to give insulin and so this condition is also known as non insulin dependent diabetes.

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Juvenile diabetes • Occurs usually before the person reaches about

20 years of age and is caused by the early degeneration of the beta cells in the pancreas.

• Daily injections of insulin are necessary to replace the missing insulin, for this reason this type of diabetes is also known as insulin dependent.

• Injections are administered by the patient subcutaneously, since insulin is a protein hormone and would be digested if given orally.

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The Gonads (reproductive organs)

• These produce the sex hormones which are responsible for the development of secondary sex characteristics in both males and females. They also maintain physiological, anatomical and behavioural factors leading to reproduction. Sex hormones in both males and females are steroids.

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Males:• Male sex hormones are referred to as androgens, the

principle one being testosterone.• Testosterone production is under the control of the

anterior pituitary gland, which cause the development of male secondary sex characteristics:

• increased facial and pubic hair• increased muscle development• deepening voice (due to lengthening of the vocal

cords)• sperm production

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Females:• The two principle female sex hormones are

estrogen and progesterone. Both are produced in the ovaries, which are located in the female pelvic cavity.

• The anterior pituitary also controls the release of these two hormones by producing FSH and LH during a cycle of events lasting approx. 28 days from the onset of puberty until menopause.

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• Estrogen is produced by the follicle cells in the ovary and is responsible for the development of female secondary sex characteristics and the preparation of the uterus for the possible implantation of a fertilized ovum.

• breast development• production of a fatty layer under the skin• widening if the hips• growth of pubic hair• ovulation (as part of the menstrual cycle)

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• Progesterone is produced from the cells of the corpus luteum after ovulation. It is responsible for the maintenance of the uterine lining and prevents ovulation during pregnancy by a negative feedback effect on the pituitary gland.

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Thymus Gland• Is located in the thoratic cavity behind the

sternum, close to the heart.

• It produces the hormone thymosin which is involved in the production and development of lymphocytes by the lymphatic tissue.

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Homeostasis• The term used to express the coordinating influence of the

endocrine system through hormonal action producing rhythmic patterns of activity, controlling long term physiological processes and responding to stressful situations.

• The endocrine system also fulfils a homeostatic function by regulating the internal environment of the body by controlling processes such as blood glucose level, water and mineral balance, temperature and metabolic rate.

• The endocrine system operates on a series of feedback mechanisms where the hormone “feeds back” to switch off the cause of its own production.

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Review of Hormones and Glands (Take the Quiz)

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Endocrine & Nervous System

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Bibliography

• Nelson Biology 12

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