12.6 The Endocrine System
The coordination system in the human body relies on close cooperation between the nervous system and the endocrine system to maintain homeostasis. While the nervous system coordinates rapid actions through electrical impulses, the endocrine system complements this by secreting chemical substances called hormones directly into the bloodstream. These chemical messengers travel to specific target cells throughout the body to regulate reproduction, growth, and internal balance.
Learning Objectives
- State the definition and general characteristics of the endocrine system and hormones.
- Identify the main endocrine glands and their anatomical positions in the human body.
- Describe the hormones secreted by the anterior and posterior lobes of the pituitary gland and their functions.
- Explain the functions of hormones secreted by the thyroid gland, pancreas, adrenal glands, ovaries, and testes.
- Describe how the nervous system and endocrine system coordinate during a 'fight or flight' situation.
- Compare and contrast the characteristics of the nervous system with the endocrine system.
Comparison and Contrast Between the Nervous System and the Endocrine System
A dual-circle Venn diagram comparing the structural and functional similarities and differences between nervous coordination and hormonal coordination.
Nervous System Specific Traits
Endocrine System Specific Traits
Shared Features
- Both contain target tissues or organs.
- Both produce a response to a stimulus.
- Both function to regulate all activities of the body.
Short Explanation
The Endocrine System of Humans
The human endocrine system works like a wireless chemical delivery network. Because endocrine glands are ductless, they release hormones directly into the blood circulation. While hormones reach almost every corner of the body, they act only on target cells that possess the matching receptors on their cell membranes.
Main Functions of Hormones
Hormones coordinate three main bodily functions: reproduction, growth, and homeostasis.
Mechanism of Action
Hormones are transported throughout the body via the bloodstream. They bind with specific molecule receptors located on the membrane surface of target cells to produce specific physiological responses.
Main Endocrine Glands in Humans
Hypothalamus
Located in the brain; secretes gonadotrophin-releasing hormone (GnRH) which stimulates the anterior pituitary gland to release FSH and LH into the blood.
Pituitary Gland
Located at the bottom of the hypothalamus in the brain. Known as the main gland of the endocrine system because it secretes hormones that control the secretion of other endocrine glands.
Thyroid Gland
Secretes thyroxine, which increases the metabolism rate, increases body temperature, and regulates growth and development.
Adrenal Gland
Secretes aldosterone and adrenaline. Adrenaline increases the level of sugar and fatty acids in the blood, increases respiratory rate and heartbeat, increases the rate of metabolism and contracts blood vessels.
Pancreas
Secretes insulin and glucagon to regulate blood glucose level.
Ovary (in females)
Secretes oestrogen and progesterone to regulate female secondary sexual characteristics and the uterus wall.
Testis (in males)
Secretes testosterone to stimulate the development of secondary sexual characteristics in males and spermatogenesis.
The Pituitary Gland — Master Gland
The pituitary gland is divided into the anterior lobe and the posterior lobe. The hypothalamus secretes GnRH, which stimulates the anterior pituitary gland to release FSH and LH into the blood.
| Hormone | Target Tissues/Organs | Function |
|---|---|---|
| Antidiuretic hormone (ADH) | Kidney tubule | Stimulates the reabsorption of water. |
| Oxytocin | Uterine muscles, mammary glands | Stimulates the contraction of uterine muscles during birth; stimulates the production of milk from the mammary gland. |
| Luteinizing hormone (LH) | Ovary, testis | Ovary: stimulates ovulation, development of corpus luteum, and secretion of oestrogen and progesterone. Testis: stimulates secretion of testosterone. |
| Growth hormone (GH) | Soft tissue, bone | Stimulates growth, protein synthesis, and fat metabolism. |
| Adrenocorticotropic hormone (ACTH) | Adrenal cortex | Stimulates the adrenal cortex to secrete hormones. |
| Thyroid-stimulating hormone (TSH) | Thyroid gland | Stimulates the thyroid gland to secrete thyroxine. |
| Follicle-stimulating hormone (FSH) | Ovary, testis | Ovary: stimulates the development of follicles in the ovary. Testis: stimulates spermatogenesis. |
Hormones Secreted by Other Endocrine Glands
| Gland | Hormone | Target Tissues/Organs | Functions |
|---|---|---|---|
| Thyroid Gland | Thyroxine | All tissues | Increases the metabolism rate, increases body temperature, regulates growth and development. |
| Pancreas | Insulin | Liver, muscles, and adipose tissues | Reduces blood glucose level and promotes the conversion of excess glucose to glycogen. |
| Glucagon | Liver, muscles, and adipose tissues | Increases the blood glucose level and promotes the conversion of glycogen to glucose. | |
| Adrenal Glands | Aldosterone | Kidney | Increases the reabsorption of salt in kidneys. |
| Adrenaline | Muscle tissues, liver, and heart | Increases the level of sugar and fatty acids in the blood; increases the respiratory rate and heartbeat; increases the rate of metabolism and contracts blood vessels. | |
| Ovaries (Female) | Oestrogen | Gonad, skin, muscles, and bones | Stimulates the development of secondary sexual characteristics in females and maturity of the follicles; repairs and stimulates the thickening of the uterus wall. |
| Progesterone | Gonad | Stimulates the development of the uterus wall and the formation of the placenta. | |
| Testes (Male) | Testosterone | Gonad, skin, muscles, and bones | Stimulates the development of secondary sexual characteristics in males and spermatogenesis. |
Involvement in a 'Fight or Flight' Situation
In an emergency or dangerous situation, the hypothalamus transmits nerve impulses directly to the adrenal medulla. Adrenal medulla cells are stimulated to secrete adrenaline and noradrenaline. These hormones act rapidly to cause increased heart rate, increased respiratory rate, increased blood pressure, increased blood glucose level, and increased metabolic activity. The heart pumps more oxygen and glucose to the brain and skeletal muscles to supply the additional energy needed to fight or run quickly. Both nervous and endocrine systems work together to deliver an immediate, effective survival response. Once the crisis is resolved, bodily mechanisms return conditions to the normal range.
Try to Answer First
Answer in your mind, then press "Check Answer".
Why are endocrine glands known as ductless glands?
What is the difference in action between insulin and glucagon in regulating blood glucose levels?
Explain why heart rate and respiratory rate increase when a person faces a sudden 'fight or flight' danger.
Common Mistakes
- Confusing the lobes of the pituitary gland: Students often mistakenly state that the anterior lobe secretes ADH or oxytocin, whereas ADH and oxytocin are secreted by the posterior lobe.
- Swapping the functions of insulin and glucagon: Students frequently confuse which hormone raises blood sugar and which lowers it. Insulin lowers glucose (promotes glucose to glycogen), while glucagon raises glucose (promotes glycogen to glucose).
- Assuming hormones affect all body cells indiscriminately: Hormones travel everywhere in the blood, but they only trigger responses in specific target cells that possess complementary surface receptors.
- Forgetting that the 'fight or flight' response is a combined effort: Students often credit only adrenaline, overlooking that the hypothalamus initiates the response via nerve impulses to the adrenal medulla.
Concept Misunderstandings
The nervous system and endocrine system function completely independently of one another.
They closely cooperate to maintain homeostasis; for example, the hypothalamus in the brain controls the pituitary gland via GnRH and directly stimulates the adrenal medulla during emergencies.
Hormones produce effects immediately, just like nerve impulses.
Nerve responses are rapid and short-lived, whereas hormonal responses are generally slower to develop, prolonged, and have long-lasting effects across multiple organs.
All pituitary hormones stimulate other endocrine glands.
While stimulating hormones like TSH and ACTH control other glands, hormones like growth hormone (GH), oxytocin, and ADH act directly on target organs such as bones, uterine muscles, and kidney tubules.
Summary
- The endocrine system consists of ductless glands that release hormones directly into the bloodstream to bind with specific target cell receptors.
- Hormones coordinate three main bodily functions: reproduction, growth, and homeostasis.
- The pituitary gland is divided into the anterior lobe (secretes LH, GH, ACTH, TSH, FSH) and the posterior lobe (secretes ADH, oxytocin).
- Major peripheral endocrine glands include the thyroid (thyroxine), pancreas (insulin, glucagon), adrenal glands (aldosterone, adrenaline), ovaries (oestrogen, progesterone), and testes (testosterone).
- During emergencies, the hypothalamus and adrenal medulla work together to trigger the 'fight or flight' response via adrenaline and noradrenaline.
- The nervous system provides rapid, short-term, localised electrical coordination, whereas the endocrine system provides slower, prolonged, widespread chemical coordination.
Short Activity
Objective: test your understanding of endocrine glands, hormones, and the comparison between nervous and endocrine systems.
A. Objective Quiz
Which hormone is secreted by the posterior lobe of the pituitary gland to stimulate the contraction of uterine muscles during childbirth?
Which statement correctly describes a difference between the nervous system and the endocrine system?
B. Fill in the Blanks
Because endocrine glands lack ducts to transport their secretions, they are classified as glands.
In a 'fight or flight' situation, the adrenal medulla is stimulated to secrete adrenaline and .
C. Matching / Drag and Drop
Match each hormone to its correct physiological function in the human body. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Hormone to Its Function
Write the correct hormone next to each function.
| Physiological Function | Hormone |
|---|---|
| 1. Reduces blood glucose level and promotes the conversion of excess glucose to glycogen | |
| 2. Stimulates the reabsorption of water in the kidney tubules | |
| 3. Increases the metabolism rate and body temperature of all tissues | |
| 4. Increases the reabsorption of salt in the kidneys |
Answer choices: Insulin • Antidiuretic hormone (ADH) • Thyroxine • Aldosterone
