7.1 Human Endocrine System
The human endocrine system is an essential coordination system that regulates various body functions using chemical substances known as hormones.
Learning Objectives
- State the definition and characteristics of the endocrine system and hormones.
- Identify the main endocrine glands and their positions in the human body.
- State the hormones secreted by each endocrine gland and explain their functions.
- Explain the causes and effects of hormonal imbalances on human health.
Human Endocrine System Overview: Glands, Hormones, and Targets
A clear anatomical map of the six major endocrine glands, their secreted hormones, and pathways into the bloodstream toward target organs.
Short Explanation
What is the endocrine system?
The endocrine system is a coordination system that regulates body functions using chemical substances called hormones.
Endocrine glands are ductless glands that release hormones directly into the bloodstream. Hormones travel to target organs, triggering specific and long-lasting physiological responses such as the "fight or flight" reaction during emergencies.
Characteristics of hormone action
Hormones are secreted directly into the blood circulation in small quantities. They act on specific target organs that possess suitable complementary receptors. A hormone can have more than one target organ. Hormonal responses are relatively slow compared to nerve impulses, but their effects are prolonged.
Main Endocrine Glands and Their Hormones
Each gland has a specific location, hormone and function.
| Gland | Location | Hormone | Function |
|---|---|---|---|
| Pituitary gland | Below the cerebrum in the brain | Antidiuretic hormone (ADH) | Controls the quantity of water reabsorbed by the kidneys. |
| Pituitary gland | Below the cerebrum in the brain | Growth hormone (GH) | Stimulates growth in children; maintains healthy body composition and adult muscle and bone mass. |
| Thyroid gland | In front of the trachea | Thyroxine | Controls the body's rate of metabolism; controls physical and mental development in children. |
| Adrenal glands | On top of each kidney | Adrenaline | Prepares the body to act in emergency situations by increasing metabolism, heartbeat, blood pressure, blood glucose level, breathing rate and dilating the pupils. |
| Pancreas | Behind the stomach | Insulin | Controls blood glucose level by converting excess glucose into glycogen to be stored in the liver. |
| Ovaries (female) | A pair in the pelvic space | Oestrogen | Controls female secondary sexual characteristics, stimulates production of ova, and prepares the uterus for embryo implantation. |
| Ovaries (female) | A pair in the pelvic space | Progesterone | Maintains the thickness of the uterine wall for embryo implantation. |
| Testes (male) | A pair held and protected inside the scrotum | Testosterone | Controls development of male secondary sexual characteristics and stimulates sperm production. |
Causes and Effects of Hormonal Imbalance
Deficiency or excess of a hormone disrupts homeostasis and causes specific disorders.
| Hormone | Deficiency effects | Excess effects |
|---|---|---|
| Thyroxine | Low metabolic rate, intolerance to cold, stunted physical and mental development in children (cretinism), fatigue in adults (myxedema), tendency to become fat, goitre. | High metabolic rate, excessive sweating, feeling hot, sleeping difficulties, very high appetite, weight loss, thyroid enlargement, protruding eyeballs, swollen neck. |
| Growth hormone (GH) | Dwarfism. | Uncontrolled growth, acromegaly or gigantism. |
| Antidiuretic hormone (ADH) | Poor water reabsorption at the collecting duct of kidneys, excessive urine production, extreme thirst, diabetes insipidus. | Headache and dizziness. |
| Insulin | Failure to convert excess glucose to glycogen, elevated blood glucose levels, diabetes mellitus. | Abnormally low blood glucose levels (hypoglycaemia) and excessive thirst. |
| Testosterone | Delayed puberty and low sperm count. | Development of male characteristics in women. |
| Oestrogen | Disrupted development of female secondary sexual characteristics. | Development of female characteristics in men. |
| Progesterone | Menstrual problems, headache, constipation, and miscarriage. | — |
Try to Answer First
Answer in your mind, then press “Check Answer”.
Why is the pituitary gland referred to as the 'master gland'?
What is the physiological difference between diabetes mellitus and diabetes insipidus in terms of hormonal cause?
How does the hormone adrenaline help an individual escape when confronted with danger?
Common Mistakes
- Confusing the symptoms and causes of diabetes mellitus (insulin deficiency) with diabetes insipidus (ADH deficiency).
- Believing hormones travel through dedicated anatomical ducts, rather than entering the bloodstream directly. Endocrine glands are strictly ductless.
- Thinking hormones affect every cell in the body equally. They only activate target cells/organs possessing complementary receptors.
- Swapping the functions of oestrogen and progesterone in the menstrual and pregnancy cycle.
Concept Misunderstandings
Hormonal responses act immediately and instantaneously like nerve impulses.
Hormonal coordination generally produces a slower response compared to the nervous system, but its physiological effects are more widespread and prolonged.
The thyroid gland only controls mental development.
The thyroid gland secretes thyroxine, which primarily regulates the general metabolic rate of the entire body in addition to supporting physical and mental growth in children.
Summary
- The endocrine system coordinates body functions via ductless glands that release hormones directly into the blood.
- The pituitary gland regulates other glands and controls water balance (ADH) and body growth (GH).
- The thyroid secretes thyroxine to control metabolism; the adrenal glands secrete adrenaline for emergencies.
- The pancreas produces insulin to regulate blood sugar; the gonads produce sex hormones governing secondary sexual characteristics and reproduction.
- Balanced hormone secretion is essential, as deficiency or excess results in serious metabolic and developmental disorders.
Short Activity
A. Objective Quiz
1 Which endocrine gland is located in front of the trachea and secretes a hormone that regulates the body's rate of metabolism?
2 A patient suffers from excessive urine production, continuous extreme thirst, and poor water reabsorption in the kidneys. Which condition is the patient most likely experiencing?
B. Fill in the Blanks
3 Endocrine glands are known as glands because they secrete hormones directly into the bloodstream without using a duct.
4 The hormone secreted by the pancreas that converts excess blood glucose into stored glycogen in the liver is .
C. Drag and Drop: Match the Disorder to Its Hormonal Cause
Drag each endocrine disorder to its correct underlying hormonal cause. On a phone, tap an answer first, then tap a matching box. Tap the Answer Choices area to return a selected item.
C. Match the Disorder to Its Hormonal Cause
Write the correct endocrine disorder next to each hormonal cause.
| Hormonal Cause | Endocrine Disorder |
|---|---|
| 1. Caused by excessive secretion of growth hormone (GH) | |
| 2. Caused by a deficiency in thyroxine secretion | |
| 3. Caused by a deficiency in insulin secretion | |
| 4. Stunted physical and mental development in children due to thyroxine deficiency |
Answer choices: Acromegaly • Goitre • Diabetes mellitus • Cretinism
