Endocrine System Overview

• An "unimpressive" system in terms of size – to collect 1kg of endocrine tissue, you’d need all of the endocrine organs from 8 or 9 adults
• Unlike other systems, there is no true continuity between endocrine organs; they communicate ( if they communicate) using the circulatory system
• The major endocrine, or ductless, glands include the hypophysis , thyroid, parathyroid, adrenal, pineal and thymus glands (all of which are strictly endocrine), the hypothalamus (a neuroendocrine organ); and the pancreas (gastroendocrine) and gonads (ovaries or testes).
• Minor endocrine glands – those with "pockets" of hormone-secreting cells – include the small intestine, the stomach, the kidney and the heart
• Some tumors synthesize and secrete hormones as well
• The functions of the endocrine system as quite diverse : processes of reproduction, growth and development, mobilization of body defenses against stressors, maintenance of electrolyte, water, and nutrient balances in the blood, and regulation of cellular metabolism and energy balance.

Hormones

• Organic substances, secreted by cells into extracellular fluid, that regulate the metabolic function of other cells.
• Hormones usually fall into one of two major types : steroids, which are synthesized from cholesterol, and amino-acid based, the more numerous type, which includes proteins and glycoproteins
• Hormones circulate via the bloodstream throughout the body, but only affect its target cells . The target cells must also have a receptor for the hormone.
• Hormones don’t last long; a typical hormone "half-life" is only a fraction of a minute to 30 minutes in duration.
• Hormones are released in response to one of three types of stimuli:

• Humoral
• the concentration of a nutrient or ion in the blood

• Neural
• nerve fibers

• Hormonal
• concentration of a hormone in the blood

The Major Endocrine Organs and Their Hormones

Hypophysis (aka – "pituitary gland")

• Connected by the infundibulum to the hypothalamus
• It has two lobes, one made of neural tissue (the neurohypophysis, or posterior lobe) and one made of glandular tissue (the adenohypophysis, or anterior lobe). The pituitary is well supplied with blood by means of the hypophyseal arteries, which are branches of the internal carotid.
• The hypophysis is closely connected to the hypothalamus, and in fact, the posterior lobe is actually a downgrowth of the hypothalamus.
• The hypothalamus stimulates the secretion of neurohypophyseal hormones directly by neural stimuli, and this controls the adenohypophysis indirectly through releasing and inhibiting hormones
• Hormones of the adenohypophysis
• TSH
– thyroid stimulating hormone or thyrotropin, stimulates development and secretory activity of the thyroid
• GH
– growth hormone, stimulates most body cells (esp. bone and skeletal muscle) to grow and divide. It is an anabolic hormone, promoting protein synthesis. Hyposecretion can lead to pituitary dwarfisn in children and possibly progeria in adults. Hypersecretion can lead to gigantism in children and acromegaly in adults.
• ACTH
– adrenocorticotropic hormone, stimulates the adrenal cortex to release corticosteroids
• FSH
– follicle stimulating hormone, is a gonadotropin. It stimulates gamete production.
• LH
– luteinizing hormone, is also a gonadotropin. In females, it causes maturation of the egg and ovulation, as well as production of ovarian hormones. In males, it promotes production of testosterone.
• PRL
– prolactin. Structurally similar to GH. In some animals, it stimulates ovaries, but in humans, it stimulates milk production.
• Hormones of the neurohypophysis
• OT
– oxytocin, stimulates uterine contraction and milk secretion. It possibly plays a role in sexual arousal and orgasm, and is sometimes called a "cuddle hormone."
• ADH
– antidiuretic hormone, prevents dehydration or excessive fluid volumes. Alcohol inhibits ADH secretion, causing copious urine output. In high blood concentrations, ADH can cause vasoconstriction, usually in visceral blood vessels. Diabetes insipidus is a disease caused by an ADH imbalance. Symptoms include intense thirst and a high urine output. It can be caused by a blow to the head that damages the hypothalamus or neurohypophysis, causing ADH deficiency. It can be life threatening if dehydration occurs.

The Thyroid Gland

• A butterfly shaped organ located in the anterior neck, inferior to the larynx, on the trachea. It is the largest pure endocrine organ.
• Hormones of the thyroid :
• T3
– triiodothyronine
• T4
– thyroxine
• T3 and T4 together work as TH
, thyroid hormone, which stimulates hormones concerned with glucose oxidation in order to increase the base metabolic rate and heat production. It also plays a role in maintaining blood pressure.
• Calcitonin
– regulates blood calcium levels (works against PTH). It targets the skeleton – inhibits osteoclasts and stimulates calcium uptake and incorporation into the bone matrix.
• Hypothyroidism
results from deficiencies of the thyroid or TSH. Symptoms include a slow metabolism, chilly feeling, constipation, thick dry skin, puffy eyes, edema, lethargy and slow thinking. If the condition (also called myxedema ) is the result of iodine deficiency, the thyroid swells (goiter ). Cretinism is severe hypothyroidism in infants, resulting in mental retardation, a short, disproportionate body and a thick tongue and neck.
• Graves’ disease
is a type of hyperthyroidism. The patient’s serum contains abnormal antibodies that mimic TSH, tricking the thyroid to make more and more TH. Graves’ disease is considered by many to be an autoimmune disease. Symptoms include an elevated metabolic rate, sweating, rapid breathing and heartbeat, nervousness, weight loss, and protrusion of the eyes.

The Parathyroid Glands

• Tiny, yellow-brown glands (usually two pairs) found on the posterior aspect of the thyroid. Sometimes they are found in the thorax or regions of the neck other than where they are supposed to be.
• Hormone : PTH – parathyroid hormone, releases ionic calcium into the blood by targeting the skeleton, kidneys and intestine. Osteoclasts go to work on the skeleton. Hyperparathyroidism is rare and usually only the result of a tumor. It results in soft bones, nervous system depression and kidney stones. PTH deficiency results in muscle twitches, loss of sensation and convulsions, possibly death.

Adrenal Glands (Suprarenal Glands)

• Paired "pyramid" shaped organs found on top of the kidneys. They are enclosed in a fibrous capsule and a cushion of fat
• The outer cortex and inner medulla work as two separate glands, but all adrenal hormones are secreted to help with stressful situations
• Adrenal cortex hormones :
• Corticosteroids
: a collection of over two dozen steroid hormones. The mineralocorticoids regulate electrolyte concentrations (particularly sodium and potassium).
• Aldosterone
is the strongest and most prevalent mineralocorticoid; it decreases sodium excretion and is stimulated by increasing concentrations of potassium, low concentrations of sodium, and/or decreasing blood volume and pressure.
• Aldosteronism
(hypersecretion) results in hypertension and edema due to sodium retention and excretion of too many potassium ions, which can result in paralysis
• Addison’s disease
(hyposecretion) is also a hyposecretion of glucocorticoids. Victims tend to lose weight, plasma glucose and sodium ion levels drop and potassium ion levels rise. Dehydration and hypotension are common.
• Glucocorticoids
: of which cortisol is the most common, influence metabolism of most body cells and provide resistance to stressors. Secretion is regulated by a negative feedback system.
• Cushing’s syndrome
is an excess of cortisone, and is characterized by hyperglycemia, muscle and bone protein loss and water and salt retention.
• Gonadocorticoids
: are the sex hormones. Most are androgens , or male sex hormones. Most sex hormones are made by the gonads.
• Adrenal medulla hormones :
• Epinephrine
and norepinephrine: prolong and reinforce the sympathetic nervous system response. ~80% of the medulla hormones released are epinephrine.

Pancreas

• A soft, "triangular" organ that is a mixture of endocrine and exocrine cells
• Most of the pancreas is made of acinar cells, which make digestive juices. Among the acinar cells are the clusters of endocrine tissue called pancreatic islets or islets of Langerhans.
• The islets are made of two hormone producing cells – alpha and beta cells.
• Pancreatic hormones :
• Glucagon
– made by the alpha cells, this 29 amino acid protein is a potent hyperglycemic agent. That is, one molecule of glucagon can cause up to 1 million molecules of glucose to be released into the blood. Most of glucagon’s liberated glucose comes from the breakdown of glycogen in the liver. Glucagon also promotes synthesis of glucose from lactic acid in the liver. Glucagon secretion is prompted by low blood sugar.
• Insulin
– made by the beta cells, this 51 amino acid protein is a hypoglycemic agent. It lowers blood sugar levels and promotes protein and fat metabolism. Blood sugar levels are lowered because insulin "enhances" membrane transport of glucose into body cells (esp. muscle, but not liver, kidney or brain).
• Diabetes mellitus
is the result of hyposecretion or hypoactivity of insulin

Other Organs

• Gonad

the ovaries ( estrogens, progesterone) or testes ( testosterone )

• Pineal gland

secretes melatonin, which functions in circadian rhythms

• Thymus

decreases in size from infancy to old age (and by that time, it is nothing more than adipose and fibrous tissue!). Plays a role in the growth and development of T lymphocytes.