The pictures on top are all related to homeostasis. Write down what you think homeostasis means. 

Homo = same, Stasis = to remain still or steady

HOMEOSTASIS is the regulation of conditions inside the body to maintain a constant internal environment despite fluctuations in the external environment.

Humans have homeostatic control systems that regulate:

• Body temperature

• Blood pressure

• Level of blood glucose

• Levels and balance of respiratory gases in tissues.

Homeostasis is the maintenance of a stable internal environment within narrow limits, despite external changes. It occurs through negative feedback mechanisms.

Example 1: Thermoregulation (Body Temperature)

Variable controlled:

• Core body temperature (~37°C)
  
  

Why it matters:

Enzymes work best at an optimum temperature. Too high → denaturation, too low → slowed metabolism.

* Body temperature rises (hot day/exercise)

Stimulus:

• Core temperature increases above normal
  
  

Receptors:

• Thermoreceptors in skin and hypothalamus detect change
  
  

Control centre:

• Hypothalamus compares temperature to the set point
  
  

Effectors:

• Sweat glands activated
  
  

• Blood vessels near skin dilate (vasodilation)
  
  

Response:

• Sweat evaporates, removing heat
  
  

• More heat lost from skin surface
  
  

• Temperature returns to normal
  
  

Negative feedback:

As temperature falls, sweating decreases.

* Body temperature drops (cold environment)

Effectors include:

• Shivering (muscle contractions generate heat)
  
  

• Vasoconstriction (reduces heat loss)
  
  

2. Blood Glucose Regulation

Variable controlled:

• Blood glucose concentration (~4–7 mmol/L)
  
  

Importance:

Cells need glucose for respiration. Too high damages vessels, too low starves brain cells.

Example: After a carbohydrate-rich meal

Stimulus:

• Blood glucose rises
  
  

Receptor + Control centre:

• Pancreas detects increase (β-cells)
  
  

Effector hormone:

• Insulin released
  
  

Target organs:

• Liver and body cells
  
  

Response:

• Cells take up glucose
  
  

• Liver converts glucose → glycogen (glycogenesis)
  
  

• Blood glucose decreases back to normal
  
  

Negative feedback:

Insulin secretion reduces once glucose normalises.

Example: During fasting/exercise

Stimulus:

• Blood glucose drops
  
  

Pancreas releases:

• Glucagon (α-cells)
  
  

Response:

• Liver breaks down glycogen → glucose (glycogenolysis)
  
  

• Blood glucose rises
  
  

3. Osmoregulation (Water Balance)

Variable controlled:

• Water potential of blood / osmotic concentration
  
  

Importance:

Cells must not shrink (hypertonic) or burst (hypotonic).

Example: Dehydration

Stimulus:

• Blood becomes more concentrated (low water potential)
  
  

Receptors:

• Osmoreceptors in hypothalamus
  
  

Control centre:

• Hypothalamus signals pituitary gland
  
  

Effector hormone:

• ADH (antidiuretic hormone)
  
  

Target organ:

• Kidneys (collecting ducts)
  
  

Response:

• More aquaporins inserted
  
  

• More water reabsorbed
  
  

• Small volume of concentrated urine produced
  
  

• Blood water levels return to normal
  
  

Negative feedback:

ADH decreases once hydration restored.

Example: Excess water intake

• ADH decreases
  
  

• Kidneys reabsorb less water
  
  

• Large volume of dilute urine produced
  
  

4. CO₂ and Blood pH Regulation

Variable controlled:

• Blood pH (~7.35–7.45)
  
  

Importance:

Enzymes are sensitive to pH. CO₂ forms carbonic acid → affects acidity.

Example: Exercise increases CO₂ production

Stimulus:

• CO₂ rises → pH drops (more acidic)
  
  

Receptors:

• Chemoreceptors in medulla and arteries
  
  

Control centre:

• Respiratory centre in brainstem
  
  

Effectors:

• Diaphragm/intercostal muscles increase ventilation rate
  
  

Response:

• More CO₂ exhaled
  
  

• Blood pH rises back to normal
  
  

Negative feedback:

Breathing slows once CO₂ levels fall.

5. Thermoregulation in Plants (Stomatal Control)

(Less common but useful extension)

Variable controlled:

• Water loss and gas exchange
  
  

Example: Hot/dry conditions

• Guard cells lose turgor
  
  

• Stomata close
  
  

• Transpiration decreases
  
  

• Water conserved
  
  

⭐ Excellence-Level Linking Statement (NCEA)

At Excellence, students must explain how the system contributes to survival:

Homeostasis operates through negative feedback loops that maintain internal conditions close to an optimum set point. This ensures enzyme-controlled metabolic reactions continue efficiently, supporting survival despite environmental fluctuations.

What are the Parts of a Homoeostatic Control System?

Homeostatic control systems have 3 functional components:

1. A RECEPTOR - located in various organs (e.g. skin, eyes, brain) to detect a change / a stimulus

2. A CONTROL CENTRE - located in the brain to compare this change against the set point, and give out instructions to effectors.

3. An EFFECTOR: various organs that direct the appropriate response to correct the change.

Communication systems are vital in ensuring instructions from the control centre are relayed to various effector organs.

There are two communication systems involved:

1. Nervous system

   • Sends very fast and precise electrical signals through nerves

2. Endocrine system

   • Sends slow, long-lasting hormonal signals that are sent everywhere in the body.

The mechanism for homeostasis is called NEGATIVE FEEDBACK LOOP.

The response of a homeostatic control system actively opposes or dampens any changes to the environment so as to maintain a stable, constant internal environment. In other words, a negative feedback loop acts as a regulatory mechanism that helps to keep a system in balance by opposing any changes to the system's input.  It  reverses the change and restore normal conditions (set point).

👉 It keeps the internal environment stable.