AS91604/BIO 3.4:

Demonstrate understanding of how an animal maintains a stable internal environment

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

Pass NCEA Biology - Homeostasis Key Words Hi, I was emailed this list by a teacher in the South Island. If you know who made this please email me and I will add credits as it would have taken a long time!! Acetylcholine Neurotransmitter produced by an enzyme in the body that stimulates muscle tissue. Adrenaline hormone produced by the

WHAT IS HOMEOSTASIS?

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:

A RECEPTOR - located in various organs (e.g. skin, eyes, brain) to detect a change / a stimulus
A CONTROL CENTRE - located in the brain to compare this change against the set point, and give out instructions to effectors.
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:

Nervous system
- Sends very fast and precise electrical signals through nerves.
Endocrine system
- Sends slow, long-lasting hormonal signals that are sent everywhere in the body by hormones in the blood.

WHAT IS THE MECHANISM OF A HOMEOSTATIC CONTROL SYSTEM?

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.

Homeostasis (stable internal environment) is possible due to a mechanism called a NEGATIVE FEEDBACK LOOP.

In thermoregulation, any changes to the CORE TEMPERATURE away from the SET POINT (37°C) triggers the negative feedback loop to COUNTERACT that change, to return the core temperature back to the set point of 37°C.

The thermoregulatory negative feedback loop serves to return the core temperature back to the set point (37°C) as quickly as possible.

The Negative Feedback Loop is made up of 5 key parts, found in various areas of the body:

1) Stimulus

The STIMULUS is a change in body temperature that triggers the need for thermoregulation.

2) Receptors (thermoreceptors)

THERMORECEPTORS are specialized cells located in the skin, hypothalamus, and other parts of the body that can detect the stimulus (changes in body temperature). These receptors respond to changes in temperature by sending signals to the control center.

3) Control centre

The CONTROL CENTRE is responsible for receiving the signals from the receptors and determines and coordinates the appropriate response to the stimulus. This center is located in the HYPOTHALAMUS at the base of the brain.

4) Effectors

EFFECTORS are the cells, tissues, and organs that respond to signals from the control center and causes a response to regulate body temperature.

In the case of temperature regulation, effectors include blood vessels, sweat glands, and muscle fibers throughout the body.

5) Response

The RESPONSE is the effect that the body has on the core temperature, in response to the stimulus.

For example, if the body temperature is too high, the effectors may dilate blood vessels in the skin to increase heat loss, or activate sweat glands to increase evaporative cooling. If the body temperature is too low, the effectors may constrict blood vessels in the skin to reduce heat loss, or activate muscle fibers to generate heat through shivering.

The response is designed to restore the body temperature to its normal set point, which is the desired level of body temperature. The greater the change from the set point, the greater the response will be to correct it.

This negative feedback loop continues to operate until the body's internal temperature has stabilized at the set point. The response of the body to the stimulus can be thought of as "opposing" the change in temperature, hence the term "negative feedback loop."

How do the 5 Parts Work Together in a Negative Feedback Loop?

The combination of these 5 parts work together in a feedback loop, where the response of the effectors leads to a change in the core temperature, which is then detected by the receptors, which sends signals to the control center to coordinate a new response.

This process continues until the core temperature returns to the set point and is maintained within a narrow range.

1. Thermoreceptors detect the stimulus (a change in body temperature).

There are two types of receptors in the body that detect changes in temperature.

Peripheral thermoreceptors in the skin

The skin is the largest organ in the body. It plays a vital role in thermoregulation in two ways:

Contains THERMORECEPTORS that detect changes in environmental temperature.
Contains EFFECTORS that can vary the rate of heat loss from the body.

Thermoreceptors in the hypothalamus

The HYPOTHALAMUS also contains thermoreceptors which are sensitive to the temperature deep in the brain (CORE TEMPERATURE).

2. The control centre compares the core temperature to the set point (37°C)

The control centre constantly monitors and compares the core temperature (detected by thermoreceptors) to the set point of 37°C.

3. The control centre sends messages to the effectors.

If the core body temperature is not the same as the set point (37°C), the hypothalamus (control centre) sends messages to EFFECTOR organs via the:

Nervous system (ACTION POTENTIALS and NEUROTRANSMITTERS)
Note: an action potential is an electrical signal that is rapidly transmitted along the neuron, while a neurotransmitter is a chemical signal that is released by neurons to communicate to another cell over very short distances.
Endocrine system (HORMONES)
Note: a hormone is a chemical signal released by glands that travel great distances through the bloodstream to target cells.

4. What do effectors do to bring the temperature back to the set point of 37°C?

To lower body temperature: sweating (sweat glands), vasodilation (smooth muscle of skin arterioles), hormonal response (thyroid gland), behavioural response (skeletal muscle).

To increase body temperature: shivering (skeletal muscle), vasoconstriction (smooth muscle of skin arterioles), piloerection, hormonal response (thyroid gland), behavioural response (skeletal muscle).

Positive Feedback Mechanism

Positive feedback is a physiological mechanism where a change in the body triggers responses that amplify or increase the original change rather than reversing it. The process continues until a specific outcome or endpoint is reached.

👉 Unlike negative feedback (which stabilises conditions), positive feedback pushes a process to completion.

⭐ Key Examples (Level 3 Biology / NCEA)

1️⃣ Childbirth (Labour Contractions)

Stimulus: Baby’s head presses against the cervix.
Response:

Oxytocin hormone released
Uterine contractions increase
More pressure on cervix

👉 This leads to even more oxytocin release until birth occurs.

Outcome: Baby is delivered → feedback loop stops.

2️⃣ Blood Clotting

Stimulus: Blood vessel damage.
Response:

Platelets release chemicals
More platelets attracted
Clot forms faster

👉 The response intensifies until bleeding stops.

3️⃣ Lactation (Milk Let-down Reflex)

Stimulus: Baby suckling.
Response:

Oxytocin released
Milk released
More suckling encouraged

👉 Continues while feeding occurs.

⭐ Excellence-Level Statement (NCEA Style)

Positive feedback mechanisms amplify physiological changes to rapidly achieve a specific biological outcome, such as childbirth or blood clotting. Although not stabilising like negative feedback, they are essential for completing certain biological processes.

Why Homeostasis Is Important? | Examples of HomeostasisDuring the study of life, one of the most important attribute to be aware of is the concept of internal balance or homeostasis. But what exactly is homeostasis, how does it occur, and why homeostasis is important in living organisms? Learn what it is with good examples in detail here.

What Is Homeostasis in Biology? Definition and ExamplesLearn about homeostasis in biology. Get the homeostasis definition and examples and see the importance of these processes in the human body.

GLUCOREGULATION

Negative Feedback Loop - Glucoregulation

Glucoregulation is keeping glucose levels in the blood at a set point. Glucose is a type of sugar that comes from the food we eat. Glucose is used by the body during cellular respiration and is needed by every cell in the body.

Glucose concentration in the blood is controlled by the pancreas. The pancreas has receptor cells which monitor the level of glucose in the blood. It also has cells that secrete hormones – Alpha cells secrete the hormone glucagon and Beta cells secrete the hormone insulin. These two hormones have an opposite effect on blood glucose.

Purpose of the Blood Glucose Regulation System

The purpose of the blood glucose regulation system is to maintain blood glucose levels within a narrow, healthy range (homeostasis).

Glucose is the body’s main source of energy and is essential for:

Cellular respiration (ATP production)
Brain function (the brain relies heavily on glucose)
Muscle activity
Normal organ function

Why Regulation Is Important?

1️⃣ Prevents Hypoglycaemia (Low Blood Sugar)

If blood glucose drops too low:

Dizziness
Confusion
Fainting
In severe cases, coma

2️⃣ Prevents Hyperglycaemia (High Blood Sugar)

If blood glucose stays too high:

Damage to blood vessels
Kidney damage
Nerve damage
Increased risk of diabetes complications

🔁 How It Works

The pancreas monitors blood glucose and releases:

Insulin → lowers blood glucose
Glucagon → raises blood glucose

This negative feedback system keeps levels close to a set point.

🔹 Define

Define means to give the precise meaning of a term.

Short
Clear
Accurate
No extra explanation needed

Example:
Define homeostasis:
Homeostasis is the maintenance of a stable internal environment.

🔹 Describe

Describe means to give characteristics or features of something.

What it is like
What happens
Key steps or components
No need to explain why (yet)

Example:
Describe blood glucose regulation:
Blood glucose is controlled by the pancreas. When glucose rises, insulin is released. When glucose falls, glucagon is released.

🔹 Explain

Explain means to give reasons and show how or why something happens.

Use cause and effect
Link ideas together
Include biological mechanisms
Often required for Merit/Excellence

Example:
Explain blood glucose regulation:
When blood glucose rises, beta cells in the pancreas release insulin. Insulin binds to receptors on target cells, increasing glucose uptake and glycogen formation. This lowers blood glucose back to the set point through negative feedback.

Define = What does it mean?
Describe = What happens?
Explain = Why and how does it happen?

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