5.9 Endocrine System

Introduction

The body needs to coordinate and control many functions, such as growth, metabolism, reproduction, and response to stress. While the nervous system provides rapid, short-term control, the endocrine system provides slower, longer-lasting control. The endocrine system uses chemical messengers called hormones to regulate body activities. This topic introduces the endocrine system, its glands, and their hormones.

Definition

Endocrine system is a complex network of glands and organs that uses hormones to control and coordinate metabolism, energy level, reproduction, growth and development, and response to injury, stress, and mood.

Main Idea

• Hormones are chemicals produced by the ductless endocrine glands.
• Small amounts of these chemicals are carried around the body in the blood.
• The organ on which a particular hormone acts is called the target organ.
• Hormones speed up or slow down or alter the activities of these organs.
• Hormones are chemical messengers which regulate and control a number of activities of the body.
• The hormonal system is slower acting and regulates processes that occur over days or even months.

Why it is beneficial for an animal to have two different systems for coordinating activities?

Having two systems allows an animal to respond to two different types of stimuli:

• Those requiring immediate action (nervous system).
• Those that depend on a long-term response (endocrine system).

Core Concept

Endocrine Glands and Their Hormones

Detailed Explanation

1. What is the Endocrine System?

The endocrine system is a complex network of glands and organs. It uses hormones to control and coordinate the metabolism, energy level, reproduction, growth and development, and response to injury, stress, and mood.

Simple explanation: The endocrine system is like a chemical messaging system. It uses hormones to tell different parts of the body what to do.

Why it is important:

• It regulates long-term processes like growth and development.
• It controls metabolism and energy balance.
• It helps the body respond to stress.
• It regulates reproduction.

2. Hormones

Hormones are chemicals produced by the ductless endocrine glands. Small amounts of these chemicals are carried around the body in the blood. The organ on which a particular hormone acts is called target organ.

Hormones speed up or slow down or alter the activities of these organs.

Simple explanation: Hormones are like chemical messengers that travel in the blood to tell specific organs what to do.

Characteristics of hormones:

• Produced by endocrine glands.
• Secreted directly into the blood.
• Travel in the blood to target organs.
• Act on specific target cells with receptors.
• Effective in very small amounts.

3. Exocrine vs. Endocrine Glands

The gland that secretes a substance out through a duct is called exocrine gland. The secretion is either inside the body or on a surface of the body.

Simple explanation: Exocrine glands have ducts (tubes) to carry their secretions. Endocrine glands do not have ducts; they release hormones directly into the blood.

4. Major Endocrine Glands

Following major endocrine glands are present in man:

• Pituitary gland
• Thyroid gland
• Parathyroid glands
• Pancreas
• Adrenal glands
• Gonads

5. Pituitary Gland

Location: Attached to the hypothalamus; about the size of a pea.

Structure: Consists of two distinct lobes, the anterior and the posterior lobe.

Function: It is called the master gland because it controls the activity of other endocrine glands.

Anterior Lobe:

• Mainly affects, regulates, and maintains the development and functions of the endocrine glands.
• Secretes many hormones:
• Thyroid stimulating hormone (TSH): Controls development and maintenance of the thyroid gland including production of thyroid hormone.
• Growth hormone (GH) or somatotropic hormone: Affects height of the individual.
• Less GH during childhood → pituitary dwarf.
• Too much GH → pituitary giant.
• Overproduction in adult → acromegaly.
• Other hormones influence gonads and adrenal glands.

Posterior Lobe:

• Secretes two hormones:
• Oxytocin: Causes powerful contractions of uterus during birth. Causes the ejection of milk.
• Antidiuretic hormone (ADH): Causes reabsorption of water from the nephrons. Under-secretion results in large quantity of dilute urine. This disease is called diabetes insipidus.

Simple explanation: The pituitary gland is the master gland. It controls other glands and produces hormones that affect growth, water balance, and childbirth.

6. Thyroid Gland

Location: In the neck; fits closely around and sides of trachea just below larynx.

Structure: Has two lobes.

Hormones:

• Thyroxin: Regulates basic metabolism. Regulates process of growth especially maturation, mental and skeletal development in children.
• Deficiency of iodine in diet causes goitre (enlargement of thyroid gland).
• Calcitonin: Secreted when blood calcium level is high. Stimulates the deposition of extra calcium in bones.

Simple explanation: The thyroid gland controls metabolism. It needs iodine to work properly. Without iodine, the thyroid gland enlarges (goitre).

7. Parathyroid Glands

Location: Located on the thyroid glands; two pairs of very small structures.

Hormone: Parathormone.

Function: Parathormone has function opposite to the function of calcitonin. It controls the balance of calcium ions and phosphate in body.

Simple explanation: The parathyroid glands control calcium balance in the body.

8. Pancreas

Location: Present in the abdominal cavity near the stomach.

Function: It is an endocrine as well as exocrine gland.

Endocrine part: Within the pancreas are certain cells called Islets of Langerhans.

The islets contain two special groups of cells called alpha and beta cells.

Insulin:

• Secreted when the level of blood sugar rises (e.g., after a meal).
• Most important effect: facilitates glucose transport across cell membrane.
• Enhances conversion of glucose to glycogen.

Glucagon:

• Has function opposite to insulin.
• Most important job: increase glucose concentration by stimulating the liver to convert glycogen to glucose.

Diabetes Mellitus:

• Caused when beta cells do not produce enough insulin.
• Glucose accumulates in the blood and is excreted with urine.
• Requires treatment with regular injections of insulin or medicines prescribed by the physician.

Simple explanation: The pancreas produces insulin and glucagon, which control blood sugar levels. Insulin lowers blood sugar; glucagon raises it.

9. Adrenal Glands

Location: The two adrenal glands rest on the top of kidneys.

Structure: Each adrenal gland has two parts:

• Adrenal cortex
• Adrenal medulla

Adrenal cortex: Secretes several hormones collectively called corticosteroids.

Adrenal medulla: Produces hormones called adrenaline and nor-adrenaline.

Simple explanation: The adrenal glands produce adrenaline, which prepares your body for emergencies. It makes your heart beat faster and increases blood flow to your muscles.

10. Gonads

What they are: The gonads are testes and ovaries.

Function: Their hormones are required for reproduction.

Simple explanation: The gonads produce sex hormones that control sexual development and reproduction.

Summary Table of Hormones

Intermediate Level

The endocrine system uses hormones to control and coordinate body functions.

Key points:

• Hormones are chemical messengers.
• They are secreted by endocrine glands directly into the blood.
• They act on target organs.
• The pituitary gland is the master gland.
• Insulin lowers blood sugar; glucagon raises it.
• Adrenaline prepares the body for emergencies.

Important Points

• - The endocrine system is a complex network of glands and organs.
• - It uses hormones to control and coordinate the metabolism, energy level, reproduction, growth and development, and response to injury, stress, and mood.
• - Hormones are chemicals produced by the ductless endocrine glands.
• - Small amounts of these chemicals are carried around the body in the blood.
• - The organ on which a particular hormone acts is called target organ.
• - Hormones speed up or slow down or alter the activities of these organs.
• - Hormones are chemical messengers which regulate and control a number of activities of the body.
• - The hormonal system is slower acting and regulates processes that occur over days or even months.
• - Having two systems allows an animal to respond to two different types of stimuli.
• - The gland that secretes a substance out through a duct is called exocrine gland.
• - The major endocrine glands are: pituitary gland, thyroid gland, parathyroid glands, pancreas, adrenal glands, and gonads.
• - Pituitary gland is attached with the hypothalamus.
• - It is called master gland because it controls the activity of other endocrine glands.
• - Thyroid gland is located in the neck.
• - Parathyroid glands are located on the thyroid glands.
• - Pancreas is present in the abdominal cavity near the stomach.
• - Adrenal glands rest on the top of kidneys.
• - Gonads are testes and ovaries.
• - Their hormones are required for reproduction.

Remember

• - Endocrine system = network of glands that produce hormones
• - Hormones = chemical messengers
• - Target organ = organ on which hormone acts
• - Endocrine glands = ductless
• - Exocrine glands = have ducts
• - Pituitary = master gland
• - Thyroid = metabolism (thyroxin), calcium (calcitonin)
• - Parathyroid = calcium balance (parathormone)
• - Pancreas = insulin (lowers blood sugar), glucagon (raises blood sugar)
• - Adrenal = adrenaline (fight or flight)
• - Gonads = testosterone (male), oestrogen (female)

Common Mistakes

• 1. Confusing endocrine and exocrine glands.
• - Endocrine glands = ductless, secrete hormones into blood.
• - Exocrine glands = have ducts, secrete substances through ducts.
• 2. Thinking the pituitary gland is not important.
• - The pituitary gland is the master gland. It controls other endocrine glands.
• 3. Mixing up insulin and glucagon.
• - Insulin = lowers blood sugar.
• - Glucagon = raises blood sugar.
• 4. Forgetting the role of the thyroid gland.
• - The thyroid gland controls metabolism. It needs iodine to function.
• 5. Confusing the parathyroid and thyroid glands.
• - Thyroid = metabolism, calcium (calcitonin).
• - Parathyroid = calcium balance (parathormone).
• 6. Thinking adrenaline is only for emergencies.
• - Adrenaline is released during stress and emergencies, but also during exercise and excitement.
• 7. Believing hormones act on all cells.
• - Hormones act only on target cells with specific receptors.

Quick Revision

60-Second Revision

• Endocrine system = network of glands that produce hormones
• Hormones = chemical messengers
• Target organ = organ on which hormone acts
• Endocrine glands = ductless
• Exocrine glands = have ducts
• Pituitary = master gland
• Thyroid = metabolism (thyroxin), calcium (calcitonin)
• Parathyroid = calcium balance (parathormone)
• Pancreas = insulin (lowers blood sugar), glucagon (raises blood sugar)
• Adrenal = adrenaline (fight or flight)
• Gonads = testosterone (male), oestrogen (female)
• Warning: Do not confuse endocrine (ductless) with exocrine (have ducts) glands. Insulin lowers blood sugar; glucagon raises it.