Characteristics of Living Organisms » Homeostasis and Control Systems

What you'll learn this session

Study time: 30 minutes

The concept of homeostasis and why it's essential for survival
How the body maintains a stable internal environment
Key homeostatic control systems including temperature regulation
Blood glucose regulation and diabetes
Osmoregulation and water balance
The role of hormones and nerves in homeostatic control

Introduction to Homeostasis

Imagine your body as an incredibly complex machine that needs specific conditions to work properly. Too hot, too cold, too much sugar, too little water - any of these can cause serious problems. That's where homeostasis comes in!

Key Definitions:

Homeostasis: The maintenance of a constant internal environment despite changes in the external environment.
Control system: A mechanism that monitors and adjusts conditions to maintain stability.
Negative feedback: When a change triggers a response that reverses that change, bringing conditions back to normal.

💪 Why Homeostasis Matters

Cells in our body need specific conditions to function properly. Enzymes work best at certain temperatures and pH levels. Too much variation can cause enzymes to denature (change shape), stopping essential chemical reactions. Without homeostasis, our cells would struggle to:

Get enough oxygen
Remove waste products
Generate energy efficiently
Repair damage
Grow and reproduce

🛠 How Control Systems Work

Homeostatic control systems typically have three main components:

Receptors: Detect changes in the internal environment
Control centre: Processes information from receptors and determines the response (often in the brain)
Effectors: Carry out the response (muscles, glands, etc.)

This creates a feedback loop that maintains stability.

Temperature Regulation

Humans are endotherms (warm-blooded), meaning we maintain a nearly constant body temperature regardless of the external environment. Our core body temperature needs to stay around 37°C for optimal enzyme function.

How We Maintain Temperature

Your body has several mechanisms to respond to temperature changes:

🔥 When Too Hot

Sweating: Water evaporates from skin, removing heat energy
Vasodilation: Blood vessels near the skin widen, increasing heat loss
Reduced metabolism: Less heat generated internally
Behavioural changes: Seeking shade, removing clothes

❄ When Too Cold

Shivering: Rapid muscle contractions generate heat
Vasoconstriction: Blood vessels near the skin narrow, reducing heat loss
Increased metabolism: More heat generated internally
Behavioural changes: Putting on clothes, seeking warmth

Case Study: Hypothermia

When body temperature drops below 35°C, hypothermia sets in. The body's control systems start to fail, leading to confusion, slurred speech and eventually unconsciousness. In severe cases, the heart may stop. This shows how crucial temperature homeostasis is for survival. Mountain climbers and winter hikers are particularly at risk, which is why proper clothing and shelter are essential in cold environments.

Blood Glucose Regulation

Your cells need glucose for energy, but too much or too little in the bloodstream can be dangerous. The body maintains blood glucose levels between 4-7 mmol/L through hormonal control.

The Role of Insulin and Glucagon

Two hormones work together to regulate blood glucose:

👇 Insulin (Lowers Blood Glucose)

When blood glucose rises (e.g., after eating):

Beta cells in the pancreas release insulin
Insulin makes cells absorb more glucose from the blood
Excess glucose is converted to glycogen in the liver and muscles
If glycogen stores are full, glucose is converted to fat

👆 Glucagon (Raises Blood Glucose)

When blood glucose falls (e.g., during exercise):

Alpha cells in the pancreas release glucagon
Glucagon makes the liver convert glycogen back to glucose
Glucose is released into the bloodstream
If needed, glucagon can trigger conversion of proteins and fats to glucose

Case Study: Diabetes

Diabetes occurs when blood glucose regulation fails:

Type 1 Diabetes: The immune system attacks insulin-producing cells. The body can't produce insulin, so blood glucose remains high. Treatment requires regular insulin injections.
Type 2 Diabetes: Cells become resistant to insulin, or the pancreas doesn't produce enough. Often linked to lifestyle factors like obesity. Can sometimes be managed with diet, exercise and medication.

Untreated diabetes can lead to serious complications including nerve damage, kidney failure and vision problems.

Osmoregulation: Water Balance

Maintaining the right amount of water in your body is crucial. Too much or too little can disrupt cell function and even be life-threatening.

How Water Balance Works

Your kidneys play a key role in osmoregulation by controlling how much water is excreted in urine:

💦 Too Much Water

When blood becomes too dilute:

Less ADH (antidiuretic hormone) released from pituitary gland
Kidneys become less permeable to water
More water excreted in dilute urine

🏜 Too Little Water

When blood becomes too concentrated:

More ADH released from pituitary gland
Kidneys become more permeable to water
Less water excreted in concentrated urine

🧠 The Thirst Response

The brain also triggers thirst when:

Blood becomes too concentrated
Blood volume decreases
This behavioural response helps restore water balance

Nerves vs Hormones in Homeostasis

The body uses two main communication systems for homeostatic control:

⚡ Nervous System Control

Characteristics:

Very rapid response (milliseconds)
Short-lasting effects
Highly specific target areas
Uses electrical impulses along neurons
Example: Pulling your hand away from a hot surface

📰 Hormonal Control

Characteristics:

Slower response (seconds to hours)
Longer-lasting effects
Can affect multiple target tissues
Uses chemical messengers in bloodstream
Example: Insulin regulating blood glucose

Putting It All Together: The Stress Response

The stress response (fight-or-flight) shows how nervous and hormonal systems work together:

Stressor detected (e.g., danger)
Nervous system: Immediately increases heart rate and breathing
Hormonal system: Adrenaline and cortisol released, sustaining the response
Blood diverted to muscles, glucose released for energy
After danger passes, negative feedback returns body to normal

This temporary disruption to homeostasis prepares the body for action, but chronic stress can lead to health problems by keeping the body in this state too long.

Exam Tips for Homeostasis Questions

When answering questions about homeostasis in your IGCSE exam:

Always mention the normal range/set point the body is trying to maintain
Describe the receptor, control centre and effector involved
Explain how negative feedback brings conditions back to normal
Use specific examples like insulin/glucagon or vasodilation/vasoconstriction
Link failures in homeostasis to specific conditions (e.g., diabetes, hypothermia)

🧠 Test Your Knowledge!