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Year 12 Science

Homeostasis

Understand how the body maintains a stable internal environment through feedback loops, thermoregulation, blood glucose regulation, and osmoregulation.

Feedback Loops

Homeostasis is the maintenance of a relatively stable internal environment despite changes in external conditions. The body uses feedback mechanisms involving a stimulus, receptor, control centre, and effector to regulate variables like temperature, blood glucose, and water balance.

Negative Feedback Loop

Stimulus

A change away from the set point (e.g., body temp rises)

↓

Receptor

Detects the change (e.g., thermoreceptors in skin/hypothalamus)

↓

Control Centre

Processes information, sends signals (e.g., hypothalamus)

↓

Effector

Carries out response to return variable to set point (e.g., sweat glands)

Key point: Negative feedback opposes the change and returns the variable to its set point (most homeostatic mechanisms). Positive feedback amplifies the change and moves the variable further from the set point (e.g., childbirth contractions, blood clotting).

Thermoregulation and Blood Glucose

Humans are endotherms that maintain core body temperature near 37 degrees C. The hypothalamus acts as the thermostat, receiving input from peripheral and central thermoreceptors. Blood glucose is regulated primarily by the hormones insulin and glucagon, secreted by the pancreas.

Response to Temperature Changes

Too Hot

  • • Vasodilation (blood vessels near skin widen)
  • • Sweating increases (evaporative cooling)
  • • Behavioural responses (seek shade, remove clothing)
  • • Hair lies flat (less insulation)

Too Cold

  • • Vasoconstriction (blood vessels narrow)
  • • Shivering (muscle contractions produce heat)
  • • Behavioural responses (add clothing, seek warmth)
  • • Increased metabolic rate (thyroid hormones)

Blood Glucose Regulation

High blood glucose: Beta cells of the pancreatic islets of Langerhans secrete insulin, which stimulates cells to take up glucose and the liver to convert glucose to glycogen (glycogenesis).

Low blood glucose: Alpha cells secrete glucagon, which stimulates the liver to break down glycogen to glucose (glycogenolysis) and produce glucose from non-carbohydrate sources (gluconeogenesis).

Diabetes: Type 1 -- autoimmune destruction of beta cells (no insulin); Type 2 -- cells become resistant to insulin.

Osmoregulation

Osmoregulation is the control of water and solute balance in the body. The kidneys play a central role, adjusting urine concentration in response to the hormone ADH (antidiuretic hormone), which is released from the posterior pituitary gland.

ADH and Water Balance

Dehydrated (high osmolarity)

More ADH released → collecting ducts more permeable → more water reabsorbed → concentrated urine

Over-hydrated (low osmolarity)

Less ADH released → collecting ducts less permeable → less water reabsorbed → dilute urine

Kidney Structure Summary

  1. 1. Glomerulus: High-pressure filtration of blood produces filtrate.
  2. 2. Proximal tubule: Reabsorbs most glucose, amino acids, and salts.
  3. 3. Loop of Henle: Creates a concentration gradient in the medulla.
  4. 4. Distal tubule & Collecting duct: Fine-tune water and ion reabsorption under hormonal control (ADH, aldosterone).

Key Vocabulary

Homeostasis

The maintenance of a relatively stable internal environment within narrow limits, despite changes in the external environment.

Negative Feedback

A regulatory mechanism where the response opposes and reduces the initial stimulus, returning the variable to its set point.

Thermoregulation

The process by which the body maintains its core internal temperature at approximately 37 degrees C through vasodilation, vasoconstriction, sweating, and shivering.

Osmoregulation

The regulation of water and dissolved solute concentrations in body fluids, primarily controlled by the kidneys and the hormone ADH.

Worked Examples

1

Explain how the body responds to a rise in core temperature after vigorous exercise.

Step 1: Thermoreceptors in the hypothalamus detect the increase in blood temperature above the set point of ~37 degrees C.

Step 2: The hypothalamus sends nerve impulses to effectors: sweat glands increase sweat production for evaporative cooling; arterioles near the skin undergo vasodilation, increasing blood flow to the surface for radiation of heat.

Answer: This is a negative feedback mechanism -- the response (cooling) opposes the stimulus (heating) until core temperature returns to the set point.

2

Describe the hormonal regulation of blood glucose after eating a carbohydrate-rich meal.

Step 1: Digestion raises blood glucose levels above the normal range (4-8 mmol/L).

Step 2: Beta cells in the islets of Langerhans detect the rise and secrete insulin into the blood.

Answer: Insulin binds to receptors on target cells (liver, muscle, adipose tissue), stimulating glucose uptake and glycogenesis (conversion of glucose to glycogen for storage). Blood glucose returns to normal via negative feedback.

3

Explain why a person who drinks a large amount of water produces more dilute urine.

Step 1: Excess water intake decreases blood osmolarity (makes blood more dilute).

Step 2: Osmoreceptors in the hypothalamus detect the low osmolarity and reduce ADH release from the posterior pituitary.

Answer: With less ADH, the collecting ducts of the kidneys become less permeable to water, so less water is reabsorbed and a large volume of dilute urine is produced, restoring normal blood osmolarity.

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

In a negative feedback loop, the effector's response:

Question 2

When blood glucose is too low, which hormone is released and from which cells?

Question 3

Vasodilation in the skin during hot weather helps cool the body because:

Question 4

Increased ADH secretion would result in:

Question 5

Which of the following is an example of positive feedback?

Key Concepts Summary

Year 12: Human Immune System Year 12: Evidence for Evolution