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8: The Endocrine System - Biology

8: The Endocrine System - Biology



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  • 8.1: Introduction
    An animal’s endocrine system controls body processes through the production, secretion, and regulation of hormones, which serve as chemical “messengers” functioning in cellular and organ activity and, ultimately, maintaining the body’s homeostasis. The endocrine system plays a role in growth, metabolism, and sexual development. In humans, common endocrine system diseases include thyroid disease and diabetes mellitus.
  • 8.2: Types of Hormones
    There are three basic types of hormones: lipid-derived, amino acid-derived, and peptide. Lipid-derived hormones are structurally similar to cholesterol and include steroid hormones such as estradiol and testosterone. Amino acid-derived hormones are relatively small molecules and include the adrenal hormones epinephrine and norepinephrine. Peptide hormones are polypeptide chains or proteins and include the pituitary hormones, antidiuretic hormone (vasopressin), and oxytocin.
  • 8.3: How Hormones Work
    Hormones cause cellular changes by binding to receptors on target cells. The number of receptors on a target cell can increase or decrease in response to hormone activity. Hormones can affect cells directly through intracellular hormone receptors or indirectly through plasma membrane hormone receptors. Lipid-derived (soluble) hormones can enter the cell by diffusing across the plasma membrane and binding to DNA to regulate gene transcription.
  • 8.4: Regulation of Body Processes
    Hormones have a wide range of effects and modulate many different body processes. The key regulatory processes that will be examined here are those affecting the excretory system, the reproductive system, metabolism, blood calcium concentrations, growth, and the stress response.
  • 8.5: Regulation of Hormone Production
    Hormone production and release are primarily controlled by negative feedback. In negative feedback systems, a stimulus elicits the release of a substance; once the substance reaches a certain level, it sends a signal that stops further release of the substance. In this way, the concentration of hormones in blood is maintained within a narrow range.
  • 8.6: Endocrine Glands
    Both the endocrine and nervous systems use chemical signals to communicate and regulate the body's physiology. The endocrine system releases hormones that act on target cells to regulate development, growth, energy metabolism, reproduction, and many behaviors. The nervous system releases neurotransmitters or neurohormones that regulate neurons, muscle cells, and endocrine cells.
  • 8.E: The Endocrine System (Exercises)

How Does the Endocrine System Maintain Homeostasis

The endocrine system consists of ductless glands that secrete hormones into the blood stream. These hormones drive the homeostatic feedback loops that keep the body healthy and in equilibrium. The endocrine system is intimately integrated into physiological processes in order for it to carry out its functions.

The major endocrine glands in humans are the pituitary gland, pineal gland, ovaries, testes, pancreas, thyroid, parathyroid, and the adrenals. The hypothalamus-pituitary complex in the brain is the main neural control center for all endocrine glands and systems. Several organs in the body have secondary endocrine functions such as the heart, gonads, bones, kidneys, and liver. In addition to their homeostatic functions, hormones coordinate growth, reproduction and many other processes.


Types of Glands

Exocrine Glands are those which release their cellular secretions through a duct which empties to the outside or into the lumen (empty internal space) of an organ. These include certain sweat glands, salivary and pancreatic glands, and mammary glands. They are not considered a part of the endocrine system.

Figure 1. Major endocrine glands. (Male left, female on the right.) 1. Pineal gland 2. Pituitary gland 3. Thyroid gland 4. Thymus 5. Adrenal gland 6. Pancreas 7. Ovary 8. Testis

Endocrine Glands are those glands which have no duct and release their secretions directly into the intercellular fluid or into the blood. The collection of endocrine glands makes up the endocrine system.

  1. The main endocrine glands are the pituitary (anterior and posterior lobes), thyroid, parathyroid, adrenal (cortex and medulla), pancreas and gonads.
  2. The pituitary gland is attached to the hypothalamus of the lower forebrain.
  3. The thyroid gland consists of two lateral masses, connected by a cross bridge, that are attached to the trachea. They are slightly inferior to the larynx.
  4. The parathyroid glands are four masses of tissue, two embedded posterior in each lateral mass of the thyroid gland.
  5. One adrenal gland is located on top of each kidney. The cortex is the outer layer of the adrenal gland. The medulla is the inner core.
  6. The pancreas is along the lower curvature of the stomach, close to where it meets the first region of the small intestine, the duodenum.
  7. The gonads (ovaries and testes) are found in the pelvic cavity.

Pituitary Gland

The pituitary gland is located at the base of the brain . It is controlled by the nervous system via the brain structure called the hypothalamus , to which it is connected by a thin stalk. The pituitary gland consists of two lobes, called the anterior (front) lobe and posterior (back) lobe. The posterior lobe stores and secretes hormones synthesized by the hypothalamus. The anterior lobe synthesizes and secretes its own endocrine hormones, also under the influence of the hypothalamus. One endocrine hormone secreted by the pituitary gland is growth hormone, which stimulates cells throughout the body to synthesize proteins and divide. Most of the other endocrine hormones secreted by the pituitary gland control other endocrine glands. Generally, these hormones direct the other glands to secrete either more or less of their hormones, which is why the pituitary gland is often referred to as the “master gland” of the endocrine system.


Endocrine Glands

The endocrine glands secrete hormones into the surrounding interstitial fluid those hormones then diffuse into blood and are carried to various organs and tissues within the body. The endocrine glands include the pituitary, thyroid, parathyroid, adrenal glands, gonads, pineal, and pancreas.

The pituitary gland , sometimes called the hypophysis, is located at the base of the brain ([Figure 1]a). It is attached to the hypothalamus. The posterior lobe stores and releases oxytocin and antidiuretic hormone produced by the hypothalamus. The anterior lobe responds to hormones produced by the hypothalamus by producing its own hormones, most of which regulate other hormone-producing glands.

Figure 1: (a) The pituitary gland sits at the base of the brain, just above the brain stem. (b) The parathyroid glands are located on the posterior of the thyroid gland. (c) The adrenal glands are on top of the kidneys. d) The pancreas is found between the stomach and the small intestine. (credit: modification of work by NCI, NIH)

The anterior pituitary produces six hormones: growth hormone, prolactin, thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-stimulating hormone, and luteinizing hormone. Growth hormone stimulates cellular activities like protein synthesis that promote growth. Prolactin stimulates the production of milk by the mammary glands. The other hormones produced by the anterior pituitary regulate the production of hormones by other endocrine tissues ([Figure 1]). The posterior pituitary is significantly different in structure from the anterior pituitary. It is a part of the brain, extending down from the hypothalamus, and contains mostly nerve fibers that extend from the hypothalamus to the posterior pituitary.

The thyroid gland is located in the neck, just below the larynx and in front of the trachea ([Figure 1]b). It is a butterfly-shaped gland with two lobes that are connected. The thyroid follicle cells synthesize the hormone thyroxine, which is also known as T4 because it contains four atoms of iodine, and triiodothyronine, also known as T3 because it contains three atoms of iodine. T3 and T4 are released by the thyroid in response tothyroid-stimulating hormone produced by the anterior pituitary, and both T3 and T4 have the effect of stimulating metabolic activity in the body and increasing energy use. A third hormone, calcitonin, is also produced by the thyroid. Calcitonin is released in response to rising calcium ion concentrations in the blood and has the effect of reducing those levels.

Most people have four parathyroid glands however, the number can vary from two to six. These glands are located on the posterior surface of the thyroid gland ([Figure 1]b).

The parathyroid glands produce parathyroid hormone. Parathyroid hormone increases blood calcium concentrations when calcium ion levels fall below normal.

The adrenal glands are located on top of each kidney ([Figure 1]c). The adrenal glands consist of an outer adrenal cortex and an inner adrenal medulla. These regions secrete different hormones.

The adrenal cortex produces mineralocorticoids, glucocorticoids, and androgens. The main mineralocorticoid is aldosterone, which regulates the concentration of ions in urine, sweat, and saliva. Aldosterone release from the adrenal cortex is stimulated by a decrease in blood concentrations of sodium ions, blood volume, or blood pressure, or by an increase in blood potassium levels. The glucocorticoids maintain proper blood-glucose levels between meals. They also control a response to stress by increasing glucose synthesis from fats and proteins and interact with epinephrine to cause vasoconstriction. Androgens are sex hormones that are produced in small amounts by the adrenal cortex. They do not normally affect sexual characteristics and may supplement sex hormones released from the gonads. The adrenal medulla contains two types of secretory cells: one that produces epinephrine (adrenaline) and another that produces norepinephrine (noradrenaline). Epinephrine and norepinephrine cause immediate, short-term changes in response to stressors, inducing the so-called fight-or-flight response. The responses include increased heart rate, breathing rate, cardiac muscle contractions, and blood-glucose levels. They also accelerate the breakdown of glucose in skeletal muscles and stored fats in adipose tissue, and redirect blood flow toward skeletal muscles and away from skin and viscera. The release of epinephrine and norepinephrine is stimulated by neural impulses from the sympathetic nervous system that originate from the hypothalamus.

The pancreas is an elongate organ located between the stomach and the proximal portion of the small intestine ([Figure 1]d). It contains both exocrine cells that excrete digestive enzymes and endocrine cells that release hormones.

The endocrine cells of the pancreas form clusters called pancreatic islets or the islets of Langerhans. Among the cell types in each pancreatic islet are the alpha cells, which produce the hormone glucagon, and the beta cells, which produce the hormone insulin. These hormones regulate blood-glucose levels. Alpha cells release glucagon as blood-glucose levels decline. When blood-glucose levels rise, beta cells release insulin. Glucagon causes the release of glucose to the blood from the liver, and insulin facilitates the uptake of glucose by the body’s cells.

The gonads—the male testes and female ovaries—produce steroid hormones. The testes produce androgens, testosterone being the most prominent, which allow for the development of secondary sex characteristics and the production of sperm cells. The ovaries produce estrogen and progesterone, which cause secondary sex characteristics, regulate production of eggs, control pregnancy, and prepare the body for childbirth.

There are several organs whose primary functions are non-endocrine but that also possess endocrine functions. These include the heart, kidneys, intestines, thymus, and adipose tissue. The heart has endocrine cells in the walls of the atria that release a hormone in response to increased blood volume. It causes a reduction in blood volume and blood pressure, and reduces the concentration of Na + in the blood.

The gastrointestinal tract produces several hormones that aid in digestion. The endocrine cells are located in the mucosa of the GI tract throughout the stomach and small intestine. They trigger the release of gastric juices, which help to break down and digest food in the GI tract.

The kidneys also possess endocrine function. Two of these hormones regulate ion concentrations and blood volume or pressure. Erythropoietin (EPO) is released by kidneys in response to low oxygen levels. EPO triggers the formation of red blood cells in the bone marrow. EPO has been used by athletes to improve performance. But EPO doping has its risks, since it thickens the blood and increases strain on the heart it also increases the risk of blood clots and therefore heart attacks and stroke.

The thymus is found behind the sternum. The thymus produces hormones referred to as thymosins, which contribute to the development of the immune response in infants. Adipose tissue, or fat tissue, produces the hormone leptin in response to food intake. Leptin produces a feeling of satiety after eating, reducing the urge for further eating.

Endocrine Glands and Their Associated Hormones
Endocrine Gland Associated Hormones Effect
Pituitary (anterior) growth hormone promotes growth of body tissues
prolactin promotes milk production
thyroid-stimulating hormone stimulates thyroid hormone release
adrenocorticotropic hormone stimulates hormone release by adrenal cortex
follicle-stimulating hormone stimulates gamete production
luteinizing hormone stimulates androgen production by gonads in males stimulates ovulation and production of estrogen and progesterone in females
Pituitary (posterior) antidiuretic hormone stimulates water reabsorption by kidneys
oxytocin stimulates uterine contractions during childbirth
Thyroid thyroxine, triiodothyronine stimulate metabolism
calcitonin reduces blood Ca 2+ levels
Parathyroid parathyroid hormone increases blood Ca 2+ levels
Adrenal (cortex) aldosterone increases blood Na + levels
cortisol, corticosterone, cortisone increase blood-glucose levels
Adrenal (medulla) epinephrine, norepinephrine stimulate fight-or-flight response
Pancreas insulin reduces blood-glucose levels
glucagon increases blood-glucose levels


THE ENDOCRINE SYSTEM

The endocrine system is a collection of glands that secrete chemical messages we call hormones. These signals are passed through the blood to arrive at a target organ, which has cells possessing the appropriate receptor.

Exocrine glands (not part of the endocrine system) secrete products that are passed outside the body. Sweat glands, salivary glands, and digestive glands are examples of exocrine glands.

Hormones are grouped into three classes based on their structure:

The Nervous and Endocrine Systems

The pituitary gland (often called the master gland) is located in a small bony cavity at the base

of the brain. A stalk links the pituitary to the hypothalamus, which controls release of pituitary hormones. The pituitary gland has two lobes: the anterior and posterior lobes.

Too little or two much GH(Growth hormone) can cause dwarfism or gigantism, respectively.

Prolactin is secreted near the end of pregnancy and prepares the breasts for milk production.

ADH(Antidiuretic hormone) controls water balance in the body and blood pressure. Oxytocin is a small peptide hormone that stimulates uterine contractions during childbirth.

Thyroid secretion is usually higher in winter than in summer.

Endocrines: The Postal System of Communication and Co-Ordination

  • Hormones are chemical substances manufactured by organs called endocrine glands or ductless glands. Ductless glands are also sometimes called ‘exocrine glands’.

ENDOCRINE GLAND OF THE BODY

Adrenal gland

The adrenal glands (also known as suprarenal glands) are endocrine glands that produce a variety of hormones including adrenaline.

They are found above the kidneys.

Hypothalamus

The hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions.

Function: link the nervous system to the endocrine system via the pituitary gland.

Pituitary gland

It is an endocrine gland about the size of a pea and weighing 0.5 grams in humans.

Hormones secreted from the pituitary gland help control:

  • growth,
  • blood pressure,
  • certain functions of the sex organs,
  • metabolism,
  • pregnancy,
  • childbirth,
  • nursing,
  • water/salt concentration,
  • temperature regulation
  • pain relief.

The thyroid gland, or simply the thyroid is one of the largest endocrine glands in the body.

It is found in the interior neck, below the Adam’s apple.

  • It secretes two hormones: triodothyro (T3) and tetraiodothysonine (T4), are called tyrosine. Both these hormones contain iodine.
  • Hypothyroidism (hypo, ‘under’)–diminished thyroid activity. Hypothyroi-dism in childhood gives rise to a conditions called cretinism.

It controls

  • rate of use of energy sources, protein synthesis, controls the body’s sensitivity to other hormones.

Goiter– is called enlargement of the thyroid gland. It manifests itself as a swelling in the neck.

A goiter may be associated with increased, normal or descreased activity of the thyroid gland.

Government of India launched the Universal salt iodisation programme in 1986.

The pancreas is a glandular organ in the digestive system and endocrine system of vertebrates.

In humans, it is located in the abdominal cavity behind the stomach.

It produce several important hormones

  • including insulin,
  • glucagon,
  • somatostatin, and
  • pancreatic polypeptide which circulate in the blood.

The pancreas is also a digestive organ, secreting pancreatic juice containing digestive enzymes that assist digestion and absorption of nutrients in the small intestine. These enzymes help to further break down the carbohydrates, proteins, and lipids in the chyme.

Reduction on the quantity of effective insulin gives rise to diabetes mellitus (diabetes, siphon, mellitus of honey) commonly called simply diabetes.

Saliva: Tylene, Maltase

Gastric Juice: Pepsin, Renin

Pancreatic Juice: Trypsin, Amylase, Lipase

Intestinal Juice: Erepsin, Maltase, Lactase, Sucrase, Lipase


Thyroid Problems

When the thyroid gland is not functioning properly, several thyroid disorders may develop. These disorders can range from a slightly enlarged gland to thyroid cancer. An iodine deficiency may cause the thyroid to become enlarged. An enlarged thyroid gland is referred to as a goiter.

When the thyroid produces hormones in excess of the normal amount, it causes a condition called hyperthyroidism. Excess thyroid hormone production causes the body's metabolic processes to accelerate resulting in rapid heart rate, anxiety, nervousness, excessive sweating, and increased appetite. Hyperthyroidism occurs more commonly in women and individuals over sixty.

When the thyroid does not produce enough thyroid hormone, hypothyroidism is the result. Hypothyroidism causes slow metabolism, weight gain, constipation, and depression. In many cases, hyperthyroidism and hypothyroidism are caused by autoimmune thyroid diseases. In autoimmune disease, the immune system attacks the body's own normal tissues and cells. Autoimmune thyroid diseases can cause the thyroid to become overactive or to stop producing hormones entirely.


Other Animals Have Endocrine Systems

Humans and other vertebrates (e.g., cats, dogs, frogs, fish, birds, lizards) all have a hypothalamus-pituitary axis that serves as the basis for the endocrine system. Other vertebrates also have a thyroid, although it may serve a slightly different function. For example, in frogs, the thyroid regulates the transformation from a tadpole into an adult. All vertebrates have an adrenal gland, too.

Endocrine signaling isn't limited to vertebrates. All animals with a nervous system have an endocrine system.


Conditions and Disorders

What conditions and disorders affect the endocrine system?

Dozens of conditions can cause issues in the endocrine system. These conditions can lead to health problems all over the body. Some of the most common disorders are:

  • Diabetes: This endocrine disorder affects the way your body uses the energy from the food you eat. Diabetes develops when the pancreas doesn’t make enough of a hormone called insulin, or insulin doesn’t work as it should.
  • Thyroid disorders: Several conditions can affect the function of the thyroid gland. Hypothyroidism happens when the thyroid doesn’t produce enough hormones. Hyperthyroidism occurs when it creates too many hormones.
  • Hypogonadism (low testosterone): In men, hypogonadism can cause erectile dysfunction. It can also cause memory and concentration problems, changes in muscle strength and low sex drive. It happens when the testes do not produce enough of the sex hormone testosterone.
  • Polycystic ovary syndrome (PCOS): A hormonal imbalance causes women with PCOS to have irregular periods, abnormal hair growth, excess acne and weight gain. It can lead to diabetes, increased risk of metabolic syndrome and infertility.
  • Osteoporosis: When a woman’s ovaries don’t produce enough estrogen, bones become brittle and weak. Although it is more common in women, men sometimes have osteoporosis when testosterone levels get too low. People with an overactive parathyroid gland (hyperparathyroidism) may also have weak bones.

Chemicals called endocrine disrupters can also affect the endocrine system. These chemicals appear everywhere — in pesticides, plastics, cosmetics and even our food and water. Endocrine disrupters cause a wide range of problems throughout the body by changing how hormones send messages.

How common are these conditions?

  • Diabetes: This condition is widespread. Almost 10% of people in the United States have diabetes and 27% have prediabetes.
  • Thyroid disorders: About 20 million Americans have thyroid disease. Women are about five times more likely than men to develop the condition.
  • Hypogonadism: About 40% of men over 45 have low testosterone. Levels of this sex hormone naturally drop as men age. Other factors, such as a man’s diet, weight and other health problems also affect testosterone levels.
  • PCOS: This common condition affects about 5% to 10% of adult women in the U.S. It is a leading cause of infertility.
  • Osteoporosis: More than half of adults over age 50 have osteoporosis. It is more likely to occur in women than in men.

How can I keep my endocrine system healthy?

Your endocrine system needs the same things the rest of your body needs to stay healthy. You should exercise, eat right and see your healthcare provider regularly.

If you have a family history of diabetes, thyroid disorders or PCOS, talk to your provider. Managing these conditions can help you avoid a hormone imbalance that can lead to health problems.


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