Human Biology

The Endocrine System

5th Year · 6th Year (Leaving Cert)

✓By the end of this lesson students will be able to identify the major endocrine glands and their locations in the human body.
✓By the end of this lesson students will be able to name the hormones produced by these glands and state their principal functions.
✓By the end of this lesson students will be able to explain the roles of insulin and glucagon in blood glucose regulation and describe the causes and management of diabetes mellitus.
✓By the end of this lesson students will be able to describe the principle of negative feedback in hormonal control and provide relevant examples.
✓By the end of this lesson students will be able to outline the roles of reproductive hormones (FSH, LH, oestrogen, progesterone, testosterone) in human reproduction (Higher Level).

Key concepts

Endocrine System

The endocrine system is a system of ductless glands that secrete chemical messengers called hormones directly into the bloodstream. These hormones travel through the blood to target cells or organs, where they exert specific effects, regulating various body functions and maintaining homeostasis.

Hormone

A hormone is a chemical messenger produced by an endocrine gland, transported in the blood, and acts on specific target cells or organs to produce a particular physiological response. Hormones are effective in very small concentrations.

Major Endocrine Glands and Hormones

The principal endocrine glands and their primary hormones include: - **Pituitary Gland (often called the 'master gland'):** Located at the base of the brain. Produces hormones like Growth Hormone (GH), Thyroid Stimulating Hormone (TSH), Antidiuretic Hormone (ADH), Follicle Stimulating Hormone (FSH - HL), Luteinising Hormone (LH - HL), and Prolactin. - **Thyroid Gland:** Located in the neck. Produces Thyroxine, which regulates metabolic rate. - **Adrenal Glands:** Located on top of the kidneys. Produce Adrenaline (epinephrine), which prepares the body for 'fight or flight', and Aldosterone and Cortisol, involved in stress response and electrolyte balance. - **Pancreas:** Located behind the stomach. Produces Insulin and Glucagon, which regulate blood glucose levels. - **Ovaries (HL):** Female gonads. Produce Oestrogen and Progesterone, responsible for female sexual characteristics and menstrual cycle regulation. - **Testes (HL):** Male gonads. Produce Testosterone, responsible for male sexual characteristics and sperm production.

Insulin and Glucagon (Blood Glucose Regulation)

The pancreas plays a crucial role in regulating blood glucose levels through two hormones: - **Insulin:** Secreted by the beta cells of the islets of Langerhans in the pancreas when blood glucose levels are high (e.g., after a meal). Insulin lowers blood glucose by increasing the uptake of glucose by body cells (especially muscle and liver cells) and promoting the conversion of glucose into glycogen for storage in the liver and muscles. - **Glucagon:** Secreted by the alpha cells of the islets of Langerhans when blood glucose levels are low (e.g., during fasting). Glucagon raises blood glucose by stimulating the liver to break down stored glycogen (glycogenolysis) into glucose and to convert non-carbohydrate substances into glucose (gluconeogenesis).

Diabetes Mellitus

Diabetes mellitus is a chronic condition characterised by high blood glucose levels due to either insufficient insulin production or the body's inability to effectively use the insulin it produces. - **Type 1 Diabetes:** An autoimmune condition where the body's immune system attacks and destroys the insulin-producing beta cells in the pancreas. It typically develops in childhood or adolescence and requires daily insulin injections. - **Type 2 Diabetes:** Occurs when the body either doesn't produce enough insulin or the body's cells become resistant to insulin's effects (insulin resistance). It is often associated with lifestyle factors (obesity, lack of exercise) and typically develops in adulthood. Management often involves diet, exercise, oral medications, and sometimes insulin. Symptoms include frequent urination, increased thirst, unexplained weight loss, fatigue, and blurred vision.

Negative Feedback

Negative feedback is a control mechanism in the endocrine system where the output of a process inhibits or reduces the original stimulus, thereby maintaining homeostasis. When the level of a hormone or its effect reaches a certain point, it triggers a response that reduces its further production or release. This ensures that hormone levels do not become excessively high or low. Example: When thyroxine levels in the blood are high, they inhibit the release of TSH (Thyroid Stimulating Hormone) from the pituitary gland, which in turn reduces the production of thyroxine by the thyroid gland.

Reproductive Hormones (Higher Level)

Reproductive hormones are crucial for sexual development, puberty, and the regulation of the reproductive cycles. - **Follicle Stimulating Hormone (FSH):** Produced by the pituitary gland. - Females: Stimulates the development of ovarian follicles and oestrogen production. - Males: Stimulates sperm production in the testes. - **Luteinising Hormone (LH):** Produced by the pituitary gland. - Females: Triggers ovulation (release of an egg) and stimulates the formation of the corpus luteum, which produces progesterone. - Males: Stimulates the Leydig cells in the testes to produce testosterone. - **Oestrogen:** Produced primarily by the ovaries. - Responsible for the development of female secondary sexual characteristics (e.g., breast development, widening of hips) and the repair and thickening of the uterine lining after menstruation. It also inhibits FSH release. - **Progesterone:** Produced primarily by the corpus luteum in the ovary. - Maintains the uterine lining, preparing it for implantation of a fertilised egg. It also inhibits the release of FSH and LH, preventing further follicle development and ovulation during pregnancy.

Testosterone (Higher Level)

Testosterone is the primary male sex hormone, produced mainly by the testes. - Responsible for the development of male secondary sexual characteristics (e.g., deepening of voice, growth of facial and body hair, increased muscle mass). - Essential for the production of sperm (spermatogenesis).

Key facts to remember

1The endocrine system uses hormones as chemical messengers, secreted by ductless glands directly into the bloodstream.
2Hormones act on specific target cells or organs to regulate various physiological processes and maintain homeostasis.
3The pituitary gland is often referred to as the 'master gland' due to its control over many other endocrine glands.
4Insulin lowers blood glucose by promoting glucose uptake and storage, while glucagon raises blood glucose by stimulating glucose release from the liver.
5Diabetes mellitus is characterised by persistently high blood glucose levels due to insulin deficiency or resistance.
6Negative feedback is the primary mechanism for regulating hormone levels, ensuring they remain within a narrow, healthy range.
7Reproductive hormones (FSH, LH, oestrogen, progesterone, testosterone) are vital for sexual development, secondary sexual characteristics, and reproduction (HL).
8The pancreas has both endocrine (insulin, glucagon) and exocrine (digestive enzymes) functions.

Worked examples

Example 1

Describe the role of insulin in regulating blood glucose levels, explaining the sequence of events when blood glucose is high.

I**Stimulus:** After a meal, carbohydrates are digested into glucose, leading to an increase in blood glucose concentration.

II**Detection:** The high blood glucose level is detected by the beta cells in the islets of Langerhans within the pancreas.

III**Hormone Release:** The beta cells respond by secreting insulin directly into the bloodstream.

IV**Target Cells/Actions:** Insulin travels to target cells throughout the body, particularly muscle and liver cells. It increases the permeability of these cells to glucose, allowing them to take up more glucose from the blood. In the liver and muscle cells, insulin promotes the conversion of excess glucose into glycogen for storage (glycogenesis). It also promotes the conversion of glucose into fat and increases protein synthesis.

V**Outcome:** As glucose is removed from the blood and stored, blood glucose levels decrease, returning to the normal homeostatic range. This reduction in blood glucose then acts as a negative feedback signal, inhibiting further insulin release.

Answer

When blood glucose levels rise, typically after a meal, the beta cells in the pancreas detect this increase and release insulin. Insulin then acts on target cells, primarily in the liver and muscles, to increase their uptake of glucose from the blood. It also stimulates the liver and muscles to convert excess glucose into glycogen for storage. These actions lead to a decrease in blood glucose levels, restoring them to the normal range, which in turn reduces further insulin secretion via negative feedback.

Remember to link the stimulus, hormone, target cells, and the resulting physiological effect.

Example 2

(Higher Level) Outline the hormonal control of the menstrual cycle, detailing the roles of FSH, LH, oestrogen, and progesterone.

I**Early Follicular Phase (Day 1-14):** The pituitary gland releases Follicle Stimulating Hormone (FSH). FSH stimulates the development of several ovarian follicles, one of which typically matures. As follicles develop, they produce increasing amounts of oestrogen.

II**Mid-Follicular Phase:** Rising oestrogen levels cause the uterine lining (endometrium) to thicken and repair. Oestrogen initially exerts negative feedback on FSH, but as it reaches a high threshold, it switches to positive feedback on the pituitary, leading to a surge in Luteinising Hormone (LH).

III**Ovulation (around Day 14):** The LH surge triggers ovulation, where the mature follicle ruptures and releases an egg from the ovary.

IV**Luteal Phase (Day 14-28):** After ovulation, the ruptured follicle transforms into the corpus luteum, which produces large amounts of progesterone and some oestrogen. Progesterone maintains the thickened uterine lining, preparing it for possible implantation of a fertilised egg. Progesterone also exerts strong negative feedback on both FSH and LH, preventing the development of new follicles and further ovulation.

V**If no fertilisation:** If the egg is not fertilised and implanted, the corpus luteum degenerates around Day 24-26. This causes a sharp drop in progesterone and oestrogen levels. The decline in these hormones leads to the breakdown and shedding of the uterine lining (menstruation) and removes the negative feedback on FSH, allowing the cycle to begin anew.

Answer

The menstrual cycle is regulated by a complex interplay of hormones. In the early follicular phase, FSH from the pituitary stimulates follicle development in the ovary, which in turn produces oestrogen. Oestrogen causes the uterine lining to thicken. A peak in oestrogen then triggers an LH surge from the pituitary, leading to ovulation. After ovulation, the ruptured follicle forms the corpus luteum, which secretes progesterone and some oestrogen. Progesterone maintains the uterine lining and, along with oestrogen, inhibits FSH and LH release via negative feedback. If fertilisation does not occur, the corpus luteum degenerates, causing a drop in progesterone and oestrogen, leading to menstruation and the start of a new cycle.

Pay attention to the feedback mechanisms (positive and negative) and the specific effects of each hormone on both the ovary and the uterus.

Example 3

Explain what is meant by negative feedback in the context of the endocrine system, and provide an example of how it regulates hormone levels.

I**Definition of Negative Feedback:** State that negative feedback is a regulatory mechanism where the output or product of a process inhibits or reduces the initial stimulus, thereby maintaining a stable internal environment (homeostasis).

II**Example Selection:** Choose a clear example, such as the regulation of thyroxine.

III**Explanation of Example (Stimulus):** When thyroxine levels in the blood are low, the hypothalamus releases Thyrotropin-Releasing Hormone (TRH).

IV**Explanation of Example (Response):** TRH stimulates the pituitary gland to release Thyroid Stimulating Hormone (TSH). TSH then travels to the thyroid gland, stimulating it to produce and release thyroxine.

V**Explanation of Example (Feedback):** As thyroxine levels in the blood rise, this increase is detected by the hypothalamus and pituitary gland. High thyroxine levels then inhibit the release of both TRH from the hypothalamus and TSH from the pituitary. This inhibition reduces the stimulation of the thyroid gland, causing thyroxine production to decrease, thus bringing the levels back to normal. This completes the negative feedback loop.

Answer

Negative feedback is a crucial regulatory mechanism in the endocrine system where the end product of a pathway inhibits the initial steps of that pathway, thereby maintaining homeostasis. For example, the regulation of thyroxine levels demonstrates negative feedback. When blood thyroxine levels are low, the hypothalamus releases TRH, which stimulates the pituitary to release TSH. TSH then stimulates the thyroid gland to produce thyroxine. As thyroxine levels in the blood increase, this rise is detected by the hypothalamus and pituitary. High thyroxine levels then inhibit the release of both TRH and TSH, reducing the stimulation of the thyroid gland and subsequently decreasing thyroxine production. This brings thyroxine levels back to their normal range, illustrating how the product (thyroxine) inhibits its own production.

Ensure your example clearly shows how the 'output' (e.g., high hormone level) leads to a reduction in the 'input' (e.g., stimulating hormone).

Common mistakes

✗Confusing endocrine glands (ductless) with exocrine glands (have ducts, e.g., salivary glands).
✗Mixing up the functions of insulin (lowers blood glucose) and glucagon (raises blood glucose).
✗Not understanding that hormones only affect specific 'target' cells or organs that have the correct receptors.
✗Misinterpreting negative feedback as a process that increases the original stimulus, rather than inhibiting it.
✗Failing to distinguish between Type 1 diabetes (autoimmune, no insulin production) and Type 2 diabetes (insulin resistance or insufficient production).
✗(HL) Confusing the roles of FSH and LH, or oestrogen and progesterone, particularly in the menstrual cycle.

Exam tips

★Learn the name, location, and primary function of the hormone(s) for each major endocrine gland.
★Practice drawing and labelling diagrams of the human endocrine system to aid recall of gland locations.
★For blood glucose regulation, understand the sequence of events for both high and low blood glucose levels, including the roles of the pancreas, insulin, and glucagon.
★Clearly define negative feedback and be prepared to provide a detailed example, such as thyroxine regulation or ADH regulation.
★(HL) Create a flow chart or diagram to visualise the hormonal interactions during the menstrual cycle, paying close attention to feedback loops.

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