The Endocrine System

• The endocrine system controls the body's homeostasis.

Exocrine vs. Endocrine Glands

• Exocrine glands release secretions, such as enzymes, into ducts; examples include sweat, tears, and digestive juices.
• Endocrine glands release secretions directly into the bloodstream and do not have ducts.
• Hormones are released by endocrine glands.

Hormones

• Hormones are classified as organic chemical messengers.
• Most hormones are proteins, but some are steroids (lipid-like).
• Hormones are secreted in small quantities directly into the bloodstream.
• Hormones are transported to target organs via blood.
• Hormones are short-lived and broken down by enzymes.
• Hormones regulate bodily functions, either stimulating or inhibiting processes.
• Hormone secretion is regulated by negative or positive feedback mechanisms.

Negative Feedback

• Hormone A stimulates the secretion of Hormone B, causing Hormone B levels to rise.
• Hormone B inhibits the secretion of Hormone A, leading to a decrease in Hormone A levels.
• Lower levels of Hormone A reduce the stimulation for the secretion of Hormone B, causing Hormone B levels to decrease.
• Lower levels of Hormone B release the inhibition on Hormone A, causing Hormone A levels to rise.
• This cycle repeats to maintain hormonal balance.

Glands and Hormones

• Glands release products directly into the bloodstream.
• Hormones deliver messages to the body.
• Target cells possess specific receptors for specific hormones.
• Endocrine glands regulate various functions. These include growth which is regulated by the pituitary gland, water balance which is regulated by the ovaries, reproduction which is regulated by the testes, metabolism which is regulated by the thyroid, calcium and glucose levels which are regulated by the pancreas and parathyroids, and response to stress, which is regulated by the adrenals.

Endocrine Glands

• The hypothalamus produces hormones that control the pituitary gland; it also stores hormones that are released by this gland.
• The pineal gland releases melatonin, which is involved with sleep.
• The thyroid gland produces thyroxine for metabolism regulation.
• The pituitary gland produces hormones and helps to regulate the other endocrine glands.
• The parathyroid glands release parathyroid hormone and regulate the levels of calcium in the blood.
• The thymus releases thymosin during childhood and stimulates T-cell development.
• The pancreas produces insulin and glucagon for glucose regulation.
• The ovaries produce estrogen and progesterone and maintain the female characteristics.
• The adrenal glands release epinephrine and norepinephrine to aid in dealing with stress.
• The testes produce testosterone and maintain sperm production with the help of male secondary sex characteristics.

Pituitary Grand / Hypophysis

• The pituitary gland, also known as the hypophysis, acts as the master gland b/c of its regulation of the other endocrine glands.
• It is located at the base of the brain and attached to the hypothalamus.
• It consists of two lobes: the anterior lobe (adenohypophysis) and the posterior lobe (neurohypophysis).

Anterior Lobe Hormones from the Pituitary Gland

• TSH (thyroid-stimulating hormone): Stimulates the thyroid to secrete thyroxin.
• Growth hormone (STH or somatotropic hormone): Promotes the growth of skeleton and muscles, too much of this causes gigantism; too little can cause dwarfism.
• FSH (follicle-stimulating hormone):
• In females: Stimulates the development of primary follicles into mature Graafian follicles.
• In males: Activates germinal epithelium that produces sperm.
• LH (luteinizing hormone):
  • In females: Stimulates ovulation and the development of corpus luteum in the ovary.
  • In males: Stimulates cells of Leydig to produce testosterone.
• Prolactin stimulates mammary glands to produce milk after birth.

Posterior Lobe Hormones from the Pituitary Gland

• ADH (antidiuretic hormone) controls water reabsorption by making walls of renal tubules and collecting ducts more permeable to water.
• Oxytocin is a hormone that causes contractions necessary for birth.
• It is a positive feedback loop in that the release of oxytocin, increases the release of more oxytocin.

Hypothalamus

• It is part of the brain and is attached to the pituitary gland.
• It controls pituitary secretions

Thyroid Gland

• It regulates metabolism and energy balance.
• It is composed of two lobes, with one on either side of the trachea.

Thyroid Hormones – Thyroxin

• Thyroxin regulates body energy usage.
• It increases breathing rate and heart rate.
• It is essential for normal growth, development of organs, and functioning of nervous system.

Disorders of the Thyroid (Thyroxin)

• Hyperthyroidism (Over-secretion):
• Increased metabolic rate.
• Weight loss.
• Increased heart rate, respiration rate, and blood pressure.
• Increased irritability and sufferers have exophthalmic goitre.
• Hypothyroidism (Under-secretion):
• Low metabolism.
• Cretinism in children: Results in no physical growth and immature sexual development.
• Myxoedema in adults: Causes mental and physical sluggishness, low blood pressure, slow heart and respiratory rate, and low body temperature.

Control of Thyroxin Levels: Negative Feedback

• Thyrotropin-releasing hormone (TRH) is released in the hypothalamus.
• Thyroid-stimulating hormone (TSH) is released in the pituitary gland.
• Thyroxine goes towards the target regions, and uses negative feedback inhibition with the hypothalamus.

Pancreas

• It has two types of cells which include exocrine and endocrine.
• Exocrine secretes pancreatic cells into the pancreatic duct.
• Endocrine:
• Islets of Langerhans (alpha and beta).
• Secretes hormones directly into the bloodstream.
• Alpha cells secrete the hormone glucagon, which aids in increasing the glucose levels.
• Beta cells secrete the hormone insulin, which aids in decreasing glucose levels.

What Happens with the Pancreas Between Meals?

• Blood glucose is low, and the pancreas secretes glucagon.
• The liver changes glycogen to glucose, and then glucose is sent to target tissues.

What Happens with the Pancreas After a Meal?

• Blood glucose becomes too high which causes the pancreas to secrete insulin.
• Then glucose is sent to the liver in the form of glygogen and then sent to target tissues.

Diabetes Mellitus

• It is a chronic disease with no cure, and high glucose levels in the blood.
• This is caused by the abscence of a normal insulin production, or its failure to function effectively.
• It is due to insulin being absent or not functioning effectively, can cause hyperglycaemia.
• Body cells do not receive enough glucose

Symptoms of Diabetes Mellitus

• Frequent urination.
• Increased thirst, hunger.
• Weight loss and repeated infections.
• Wounds that heal more slowly and impaired vision.
• Exhaustion as well as dizziness
• In term, diabetes can lead to blindness, kidney failures as well as cardiovascular related disease. Two types.

2 Common Types of Diabetes Mellitus

• Type 1 diabetes occurs due to the loss of insulin-producing beta cells, where the body's immune system destroys its own beta cells causing the loss of insulin production.
• Type 1 diabetes is managed with daily insulin injections, a special diet, and regular testing of blood sugar levels.
• Type 2 diabetes occurs due to insulin, where the body cannot use it effectively.
• Type 2 diabetes is related to lifestyle factors such as overweight, inactivity, and age.
• It is managed through a balance diet, exercise, and/or medication.

Adrenal Glands

• They release horomones to deal with stress
• They have an outer cortex that releases aldosterone which regulates ionic balance of the tissue fluid.
• They have an inner medulla from a "fight or flight" response that creates adrenalin.

Effects of Adrenalin

• Accelerates and strengthens the heart rate.
• Increases blood pressure (vasoconstriction).
• Accelerates conversion of glycogen to glucose.
• The rate and depth of breathing are both increased.
• There is a increase in muscle tone and sweat production.
• Pupiles dilate

Gonads

• Involved in the production of gametes and secretion of sex hormones.
• In females, ovaries produce estrogen/progesterone.
• In males, testes produce testosterone.

Testosterone

• Secreted by interstitial cells (Leydig cells).
• Responsible for secondary male sexual characteristics.
• Needed for maturation of sperm and contributes to its quality.

Oestrogen

• Secreted by developing follicles in the ovaries.
• Responsible for secondary female sexual characteristics.
• Needed for thickening of endometrium and preparation of the uterus for implantation.
• Inhibits secretion of hormone FSH.

Progesterone

• Secreted by corpus luteum in the ovaries.
• Needed for thickening of the endometrium and maintenance of endometrium during pregnancy.
• Suppresses release of hormones FSH and LH.

Infertility

• Males: FSH, LH, and Testosterone used to treat infertility, which affects the production of sperm.
• Females: FSH, LH, and estrogen-suppressing hormones used to treat infertility and stimulate the functioning of the hypophysis, hypothalamus, and ovaries (ensure production of ova).