Hormonal Coordination and Endocrine System

The endocrine system is made up of glands that produce and secrete hormones directly into the bloodstream.

Key Endocrine Glands

The pituitary gland is called the "master gland" because it produces hormones that regulate many other endocrine glands.

Maintaining a constant blood glucose level is an example of homeostasis — keeping internal conditions stable.

The pancreas monitors blood glucose levels and responds by releasing hormones:

When Blood Glucose Is TOO HIGH (e.g., after eating)

1. The pancreas detects high blood glucose
2. Beta cells in the pancreas secrete insulin
3. Insulin causes:
   • Body cells (especially liver and muscle cells) to absorb glucose from the blood
   • Glucose to be converted to glycogen for storage in the liver and muscles
   • Increased cellular respiration (glucose is used up)
4. Blood glucose level falls back to normal

When Blood Glucose Is TOO LOW (e.g., during exercise or between meals)

1. The pancreas detects low blood glucose
2. Alpha cells in the pancreas secrete glucagon
3. Glucagon causes:
   • The liver to convert stored glycogen back into glucose
   • Glucose to be released into the blood
4. Blood glucose level rises back to normal

Negative Feedback

This system works by negative feedback — when a change is detected, the body acts to reverse it and return to the normal level.

$\text{Blood glucose rises} \xrightarrow{\text{insulin}} \text{Glucose absorbed/stored} \xrightarrow{} \text{Blood glucose falls}$ $\text{Blood glucose falls} \xrightarrow{\text{glucagon}} \text{Glycogen → Glucose released} \xrightarrow{} \text{Blood glucose rises}$

Memory tip: Insulin makes blood glucose go In to cells (decreases blood glucose). Glucagon makes Glucose go into the blood (increases blood glucose).

Diabetes is a condition where the body cannot properly control blood glucose levels.

Type 1 Diabetes

• Cause: The immune system destroys the beta cells of the pancreas
• The pancreas produces no insulin (or very little)
• Onset: Usually in childhood or young adulthood
• Treatment: Regular insulin injections and careful monitoring of diet and blood glucose levels
• Not linked to lifestyle — it is an autoimmune condition

Type 2 Diabetes

• Cause: Body cells become resistant to insulin (they no longer respond properly), and/or the pancreas doesn't produce enough insulin
• Onset: Usually in adults; increasingly common in younger people
• Risk factors: Obesity, lack of exercise, poor diet (high sugar/fat), genetic factors, age
• Treatment: Controlled by diet and exercise (low sugar, regular exercise); medication may be needed; insulin injections in severe cases
• Prevention: Maintaining a healthy weight, regular exercise, balanced diet

Adrenaline

• Produced by the adrenal glands in response to stress, fear, or excitement
• Prepares the body for the "fight or flight" response:
  • Increases heart rate and breathing rate
  • Diverts blood flow to muscles (away from non-essential organs like the gut)
  • Stimulates the liver to convert glycogen to glucose (increases blood sugar for energy)
  • Dilates pupils

Thyroxine

• Produced by the thyroid gland
• Controls the basal metabolic rate (how fast chemical reactions occur in cells)
• Important for growth and development
• Regulated by negative feedback: high thyroxine levels signal the pituitary gland to reduce TSH (thyroid-stimulating hormone) production
• Hypothyroidism (too little thyroxine): slow metabolism, weight gain, fatigue
• Hyperthyroidism (too much thyroxine): fast metabolism, weight loss, anxiety

Question: Explain how blood glucose levels are controlled after eating a meal high in carbohydrates. (4 marks)

Solution:

After eating a carbohydrate-rich meal, blood glucose levels rise as glucose is absorbed from the small intestine into the blood. The pancreas detects this increase and beta cells secrete insulin into the bloodstream. Insulin causes cells (particularly liver and muscle cells) to absorb glucose from the blood. In the liver, glucose is converted to glycogen for storage. This causes blood glucose levels to fall back to normal. This is an example of negative feedback — the response (insulin release) reverses the change (high blood glucose).

• Insulin and glucagon have OPPOSITE effects — learn them as a pair.
• Remember: Insulin = IN (glucose goes into cells). Glucagon = glucose goes out (of liver into blood).
• When describing Type 2 diabetes, always mention insulin resistance — it's not just "not enough insulin."
• The pituitary gland is the "master gland" — it controls other glands. This is a favourite exam fact.

• The endocrine system uses hormones (chemical messengers) carried in the blood to coordinate slower, longer-lasting responses.
• Key glands: pituitary (master gland), thyroid (thyroxine), adrenal (adrenaline), pancreas (insulin and glucagon), ovaries/testes.
• Blood glucose is regulated by insulin (lowers glucose) and glucagon (raises glucose) through negative feedback.
• Type 1 diabetes: no insulin produced (autoimmune); treated with insulin injections. Type 2 diabetes: insulin resistance; linked to lifestyle; treated with diet/exercise.
• Adrenaline triggers the "fight or flight" response; thyroxine controls metabolic rate.