Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Describe the roles of the pituitary, thyroid, adrenal glands, and pancreas in homeostasis.
• Explain negative feedback control for blood glucose and water balance.
• Discuss causes of diabetes and thyroid disorders.
• Explain aldosterone's role in controlling blood pressure.

Lesson summary

The endocrine system is crucial for maintaining homeostasis, the body's stable internal environment. This week, we will delve deeper into how hormones regulate various bodily functions. Understanding the endocrine system is vital because hormonal imbalances can lead to diseases like diabetes (prevalent in South Africa), thyroid disorders, and growth problems.

Furthermore, the endocrine system interacts significantly with the nervous system, influencing behaviour, stress responses, and reproduction. This knowledge empowers you to understand the physiological basis of health and illness and to make informed decisions about your well-being.

Lesson notes

2. 1.

The Pituitary Gland: ADH and Water Balance The pituitary gland, often called the "master gland," controls other endocrine glands. One of its key functions is secreting Antidiuretic Hormone (ADH), also known as vasopressin. ADH plays a crucial role in regulating water balance.

Mechanism: When the body is dehydrated (e.g., after strenuous exercise on a hot day in Gauteng), the hypothalamus detects the increase in blood osmolarity (concentration of solutes in the blood). This stimulates the posterior pituitary to release ADH. ADH travels in the bloodstream to the kidneys. In the kidneys, it increases the permeability of the collecting ducts, allowing more water to be reabsorbed from the filtrate back into the bloodstream. This reduces urine volume and increases blood volume, restoring water balance.

Negative Feedback: As blood water levels increase, the hypothalamus detects the decrease in blood osmolarity. This reduces the signal to the pituitary gland, leading to a decrease in ADH secretion. This completes the negative feedback loop, preventing over-hydration.

Example: Thandi runs a 10km race in Durban. She sweats a lot, leading to dehydration. Her hypothalamus triggers the release of ADH, causing her kidneys to reabsorb more water, resulting in concentrated urine. 2.

2. The Thyroid Gland: Thyroxin and Metabolism The thyroid gland, located in the neck, produces thyroxin. This hormone regulates metabolism, which is the rate at which the body uses energy.

Mechanism: Thyroxin increases the metabolic rate in nearly all cells of the body. This means cells burn more glucose and oxygen, generating more heat and energy. This is essential for growth, development, and maintaining body temperature, especially important in colder regions of South Africa.

Negative Feedback: Low thyroxin levels stimulate the hypothalamus to release Thyrotropin-Releasing Hormone (TRH). TRH stimulates the anterior pituitary to release Thyroid-Stimulating Hormone (TSH)*. TSH stimulates the thyroid gland to produce and release thyroxin. When thyroxin levels are high enough, they inhibit the release of TRH and TSH, completing the negative feedback loop.

Example: In winter, the hypothalamus detects a decrease in body temperature. It signals the pituitary to release TSH, which stimulates the thyroid to release thyroxin. This increases metabolism, generating more heat to keep the body warm. 2.

3. The Adrenal Glands: Adrenaline and Aldosterone The adrenal glands, located on top of the kidneys, produce several hormones, including adrenaline and aldosterone.

Adrenaline: This hormone is released in response to stress or danger ("fight or flight" response).

Mechanism: Adrenaline increases heart rate, blood pressure, and breathing rate. It also diverts blood flow away from the digestive system and towards muscles, providing them with more oxygen and glucose for quick action. It also stimulates the liver to release glucose into the bloodstream, providing additional energy.

Example: Siya is walking home in Johannesburg at night when he sees someone approaching him suspiciously. His adrenal glands release adrenaline, causing his heart to race, his muscles to tense, and his senses to sharpen, preparing him to either fight or run away.

Aldosterone: This hormone regulates sodium and potassium levels in the blood, which affects blood pressure.

Mechanism: When blood pressure is low, the kidneys release renin, which activates angiotensinogen to angiotensin I, then angiotensin II. Angiotensin II stimulates the adrenal cortex to release aldosterone. Aldosterone acts on the kidneys to increase the reabsorption of sodium ions (Na+) from the filtrate back into the bloodstream. Water follows sodium by osmosis, increasing blood volume and, therefore, blood pressure. Aldosterone also promotes the excretion of potassium ions (K+) into the urine.

Example: A patient with severe diarrhea loses a lot of fluid and electrolytes, including sodium. This lowers blood pressure. The adrenal glands release aldosterone, causing the kidneys to reabsorb more sodium and water, helping to restore blood pressure. 2.

4. The Pancreas: Insulin and Glucagon The pancreas is both an endocrine and exocrine gland. Its endocrine function involves producing insulin and glucagon, which regulate blood glucose levels.

Insulin: This hormone is released when blood glucose levels are high (e.g., after a meal rich in carbohydrates).

Mechanism: Insulin stimulates cells, especially muscle and liver cells, to take up glucose from the blood. In the liver and muscles, glucose is converted into glycogen for storage. Insulin also inhibits the breakdown of glycogen into glucose. This lowers blood glucose levels.

Example: After eating a plate of pap and vleis, Zola's blood glucose levels rise. Her pancreas releases insulin, causing her liver and muscle cells to take up glucose from her blood and store it as glycogen.