Responding to stimuli and homeostasis

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Homeostasis – Keeping the Internal Environment Stable

Homeostasis is the maintenance of a stable internal environment despite external changes.
It ensures optimal conditions for enzyme activity and cell function.
Key variables regulated include body temperature, blood glucose, pH, and fluid balance.
Homeostatic mechanisms involve receptors, control centres, and effectors.
Most homeostatic control uses negative feedback to reverse a change.

Components of a Homeostatic Control System

Receptor – detects a change (stimulus) in the environment (e.g., thermoreceptors detect temperature change).
Control centre – processes the information and determines the response (e.g., hypothalamus in the brain).
Effector – carries out the response to restore balance (e.g., muscles, glands, or organs).
The response feeds back to the receptor to reduce or stop the stimulus (negative feedback).

Negative Feedback – The Core Mechanism

Negative feedback counteracts deviations from the set point, bringing the variable back to normal.
Example: Body temperature rises → receptors send signal to brain → effectors (sweat glands) cool the body → temperature drops.
Example: Blood glucose rises after eating → pancreas releases insulin → cells take up glucose → glucose level falls.
Most homeostatic systems rely on negative feedback to maintain stability.

A corrective response opposes the original deviation

The Nervous System – Rapid Responses

The nervous system detects stimuli and sends electrical signals (nerve impulses) for rapid, short-lived responses.
Sensory neurones carry impulses from receptors to the central nervous system (CNS – brain and spinal cord).
Motor neurones carry impulses from the CNS to effectors (muscles or glands).
Reflex arcs are automatic, rapid responses that bypass the brain for speed (e.g., pulling hand from a hot object).

CNS, PNS and the structure of a peripheral nerve

The Endocrine System – Slower, Longer-Lasting Responses

The endocrine system uses hormones (chemical messengers) released into the blood to target cells.
Hormones act more slowly than nerve impulses but have longer-lasting effects.
Example: Adrenaline prepares the body for 'fight or flight' – increases heart rate, breathing rate, and blood glucose.
Example: Insulin and glucagon regulate blood glucose levels via negative feedback.

Locations and roles of major endocrine glands

Coordination of Nervous and Endocrine Systems

The hypothalamus links the nervous and endocrine systems; it controls the pituitary gland.
The pituitary gland releases hormones that regulate other endocrine glands (e.g., thyroid, adrenal).
Together, the two systems coordinate responses to internal and external stimuli to maintain homeostasis.

Examples of Homeostatic Regulation

Thermoregulation: Body temperature kept around 37°C via sweating, shivering, and blood vessel dilation/constriction.
Osmoregulation: Water and salt balance maintained by the kidneys; antidiuretic hormone (ADH) controls water reabsorption.
Blood glucose regulation: Insulin lowers glucose; glucagon raises glucose; both are produced by the pancreas.
pH regulation: Buffers in blood and respiration control CO₂ levels to keep pH near 7.4.

Skin structures involved in thermoregulation

Stimuli and Receptors

A stimulus is any change in the internal or external environment that triggers a response.
Receptors are specialised to detect specific stimuli: thermoreceptors (temperature), mechanoreceptors (pressure), chemoreceptors (chemicals), photoreceptors (light).
Receptors convert stimulus energy into electrical signals (action potentials) for the nervous system.

A spinal reflex arc showing the pathway: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector (muscle). This is an example of a rapid, involuntary response.

Structure of a motor neurone: dendrites, cell body, axon, myelin sheath, nodes of Ranvier, and axon terminals. Neurones transmit electrical impulses for rapid responses.

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Practice questions

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1.What term describes the maintenance of a stable internal environment in living organisms?
Easy
- AHomeostasis
- BMetabolism
- CEvolution
- DAdaptation
2.Which component of a homeostatic control mechanism detects changes in the environment?
Easy
- AReceptor
- BControl center
- CEffector
- DStimulus
3.Which of the following is NOT a variable regulated by homeostasis in humans?
Easy
- ABody temperature
- BBlood sugar level
- CHeight
- DpH of extracellular fluid
4.In the reflex arc, which type of neurone carries the action potential from the receptor to the spinal cord?
Easy
- ASensory neurone
- BMotor neurone
- CRelay neurone
- DInterneurone
5.Homeostasis keeps the internal environment perfectly constant at all times.
Easy
True or false?
6.Negative feedback mechanisms work to reverse a change and bring the variable back to its set point.
Easy
True or false?
7.Match each component of a homeostatic control system with its correct description.
Medium
- Receptor
- Control center
- Effector
- Detects changes in the environment
- Determines the appropriate response
- Carries out the response
8.Arrange the following events in the correct order for a homeostatic response:
Medium
- Effector responds
- Receptor detects change
- Control center sends signal
- Variable returns to normal

More questions

9.Which of the following is an example of a homeostatic mechanism in humans?
Easy
- AShivering when cold
- BGrowing taller during puberty
- CLearning a new language
- DDigesting food
10.Which of the following is an example of a stimulus that triggers a response in the human body?
Easy
- AA sudden drop in external temperature
- BThe colour of a flower
- CThe shape of a cloud
- DThe taste of a fruit
11.Arrange the following events in the correct order for a homeostatic response to a rise in blood glucose level.
Medium
- Pancreas releases insulin
- Blood glucose level rises
- Cells take up glucose from blood
- Blood glucose level returns to normal
12.Which of the following are regulated by homeostasis in humans? (Select all that apply)
Medium
- ABody temperature
- BBlood pH
- CBlood glucose level
- DEye colour
13.Negative feedback mechanisms work to amplify a change away from the set point.
Easy
True or false?
14.Which of the following is an example of negative feedback in the human body?
Medium
- AIncreased blood glucose stimulates insulin release, lowering blood glucose
- BOxytocin release during childbirth increases contractions
- CPlatelet accumulation at a wound attracts more platelets
- DFruit ripening releases ethylene, which accelerates ripening
15.A student measures the heart rate of a person before and after drinking a caffeinated drink. The heart rate increases from 70 bpm to 95 bpm, then gradually returns to 70 bpm over the next hour. Which homeostatic mechanism is demonstrated?
Medium
- APositive feedback
- BNegative feedback
- CFeedforward regulation
- DReinforcing feedback
16.In a homeostatic control system for body temperature, which component is responsible for comparing the actual temperature to the set point and initiating a response?
Easy
- AReceptor
- BEffector
- CControl centre
- DStimulus
17.Homeostasis ensures that internal conditions remain absolutely constant at all times.
Easy
True or false?
18.Which of the following are regulated by homeostatic mechanisms in humans? (Select all that apply)
Medium
- ABody temperature
- BBlood glucose concentration
- CBlood pH
- DHeight
- EBlood oxygen levels
19.Arrange the following events in the correct order for a homeostatic response to a rise in blood carbon dioxide levels.
Medium
- Receptors in blood vessels detect increased CO₂
- Control centre in brain signals breathing muscles
- Breathing rate increases, expelling more CO₂
- Blood CO₂ level returns to normal
20.The diagram below shows a homeostatic control system. Which labelled part is the effector?
Medium
- AA
- BB
- CC
- DD
21.The graph below shows the change in blood glucose concentration after a meal. Which letter indicates the point at which the homeostatic control system reverses the rise?
Medium
- AA
- BB
- CC
- DD
22.In an experiment to investigate the effect of temperature on heart rate in Daphnia, a student places a Daphnia in a drop of water on a microscope slide and counts heartbeats per minute. The water temperature is changed by adding warm or cold water. Which variable should be kept constant to ensure a fair test?
Medium
- ASize of the Daphnia
- BTemperature of the water
- CHeart rate of the Daphnia
- DVolume of water on the slide
23.Which of the following are examples of stimuli that can trigger a response in the human body? (Select all that apply)
Medium
- AA drop in external temperature
- BAn increase in blood glucose concentration
- CA decrease in blood pH
- DThe colour of a person's eyes
24.Homeostasis is the maintenance of a stable internal environment despite changes in the external environment.
Easy
True or false?
25.In an experiment, a scientist measures the blood glucose concentration of a person after a meal. The results show a rapid increase followed by a gradual decrease to the normal range. Which homeostatic mechanism is responsible for the decrease?
Medium
- APositive feedback
- BNegative feedback
- CFeedforward regulation
- DReinforcing feedback
26.Which of the following is the best definition of homeostasis?
Easy
- AThe maintenance of a stable internal environment despite external changes
- BThe process by which organisms respond to stimuli
- CThe ability of organisms to reproduce
- DThe regulation of body temperature only
27.Who coined the term 'homeostasis'?
Easy
- AClaude Bernard
- BWalter Bradford Cannon
- CJoseph Barcroft
- DCharles Darwin
28.Homeostasis is an almost exclusively biological term.
Easy
True or false?
29.Which component of a homeostatic control system detects changes in the environment?
Easy
- AEffector
- BControl centre
- CReceptor
- DStimulus
30.Negative feedback mechanisms amplify a change away from the set point.
Easy
True or false?
31.Match each component of a homeostatic control system with its correct function.
Medium
- Receptor
- Control centre
- Effector
- Detects changes in the environment
- Compares input to set point and determines response
- Carries out the response to restore balance
32.Arrange the following events in the correct order for a homeostatic response to a rise in body temperature.
Medium
- Receptor detects temperature increase
- Control centre sends signals to effectors
- Effectors cause sweating and vasodilation
- Body temperature decreases back to normal
33.Homeostasis is the maintenance of a constant internal environment without any fluctuations.
Easy
True or false?
34.Which of the following is an example of an effector in a homeostatic control system?
Easy
- AThermoreceptor in the skin
- BHypothalamus in the brain
- CSweat gland
- DSensory neurone
35.Which of the following is an example of a stimulus that triggers a homeostatic response?
Medium
- ARise in blood glucose after a meal
- BIncrease in body temperature during exercise
- CDecrease in blood oxygen levels at high altitude
- DAll of the above
36.Which of the following are effectors in the homeostatic control of body temperature? (Select all that apply)
Medium
- ASweat glands
- BBlood vessels in the skin
- CHypothalamus
- DSkeletal muscles (shivering)
37.Arrange the following events in the correct order for a homeostatic response to a decrease in blood oxygen concentration.
Medium
- Receptors detect low oxygen
- Control centre in the brain sends signals
- Breathing rate and depth increase
- Blood oxygen concentration rises back to normal
38.A person runs a marathon on a hot day. Their body temperature rises above 37°C. Which of the following is an effector response triggered by the homeostatic control centre to lower body temperature?
Medium
- AVasoconstriction of skin arterioles
- BIncreased sweat production
- CShivering
- DPiloerection (hair standing up)
39.Which of the following variables are regulated by homeostatic mechanisms in humans? (Select all that apply)
Medium
- ABlood glucose concentration
- BBody temperature
- CBlood pH
- DHeight
- EBlood oxygen concentration
40.Homeostasis is the maintenance of a perfectly constant internal environment with no fluctuations.
Easy
True or false?
41.Complete the sentence:
Medium
In a homeostatic control system, the ____ detects the stimulus, the ____ determines the response, and the ____ carries out the response.
42.A student measures the body temperature of a healthy person over 2 hours and finds it ranges from 36.5°C to 37.5°C. What is the midpoint (set point) of this homeostatic range?
Medium
43.Complete the sentence: The three main components of a homeostatic control system are the receptor, the control centre, and the ___.
Easy
The three main components of a homeostatic control system are the receptor, the control centre, and the ____.
44.A person's body temperature is measured as 37.2°C. If the normal homeostatic range is 36.5°C to 37.5°C, by how many degrees Celsius is this temperature above the midpoint of the range?
Medium
45.What term is used to describe the maintenance of a stable internal environment in living organisms?
Easy
46.A person's blood glucose level is measured as 5.8 mmol/L. The normal homeostatic range is 4.0–6.0 mmol/L. How many mmol/L above the lower limit of the normal range is this value?
Medium
47.Complete the sentence about homeostasis.
Easy
The three main components of a homeostatic control system are the ____, the ____, and the ____.
48.Complete the sentence about body temperature regulation.
Medium
When body temperature rises, the hypothalamus (control centre) sends signals to the skin to increase blood flow and produce sweat. In this example, the effector is the ____.
49.A person's blood glucose concentration is 7.8 mmol/L. The normal homeostatic range is 4.0–6.0 mmol/L. By how many mmol/L is this value above the upper limit of the normal range?
Medium
50.What term is used to describe a response that reverses a change to bring a variable back to its set point?
Easy
51.Complete the sentence about the control of blood carbon dioxide levels.
Easy
When blood carbon dioxide levels rise, the respiratory centre in the brainstem sends signals to increase the rate and depth of breathing. In this example, the receptor is the ____ that detects changes in CO₂ concentration.
52.A healthy person's body temperature is 37.0°C. If the set point for body temperature is 37.0°C and the acceptable range is ±0.5°C, what is the maximum temperature (°C) that is still within the homeostatic range?
Easy
53.Explain why negative feedback is important for homeostasis. Provide an example.
Medium
54.Describe the role of the receptor in a homeostatic control system.
Medium
55.A student measures the body temperature of a healthy person over 2 hours and finds it ranges from 36.5°C to 37.5°C. What is the midpoint (set point) of this homeostatic range?
Easy
- A41 °C
- B45 °C
- C37 °C
- D33 °C
56.A person's body temperature is measured as 37.2°C. If the normal homeostatic range is 36.5°C to 37.5°C, by how many degrees Celsius is this temperature above the midpoint of the range?
Medium
- A-0.8 °C
- B0.2 °C
- C1.2 °C
- D2.2 °C
57.A person's blood glucose level is measured as 5.8 mmol/L. The normal homeostatic range is 4.0–6.0 mmol/L. How many mmol/L above the lower limit of the normal range is this value?
Easy
- A1.8 mmol/L
- B2.8 mmol/L
- C3.8 mmol/L
- D0.8 mmol/L
58.A person's blood glucose concentration is 7.8 mmol/L. The normal homeostatic range is 4.0–6.0 mmol/L. By how many mmol/L is this value above the upper limit of the normal range?
Easy
- A1.8 mmol/L
- B2.8 mmol/L
- C3.8 mmol/L
- D0.8 mmol/L
59.A healthy person's body temperature is 37.0°C. If the set point for body temperature is 37.0°C and the acceptable range is ±0.5°C, what is the maximum temperature (°C) that is still within the homeostatic range?
Easy
- A37.5 °C
- B45.1 °C
- C33.7 °C
- D41.3 °C

Flashcards

Define homeostasis.

Homeostasis is the state of steady internal physical and chemical conditions maintained by living organisms, essential for optimal functioning.

What are the three interdependent components of a homeostatic control mechanism?

Receptor (sensor), control center, and effector.

What is the role of the receptor in homeostasis?

The receptor monitors and responds to changes in the environment (internal or external), sending signals to the control center.

What does the control center do in a homeostatic mechanism?

The control center sets the acceptable range for a variable and determines the appropriate response, sending signals to an effector.

What is the function of an effector in homeostasis?

The effector (muscle, organ, or gland) acts to bring the variable back to its normal state.

Give an example of a receptor type.

Thermoreceptors (detect temperature) and mechanoreceptors (detect mechanical changes).

Give an example of a control center.

The respiratory center and the renin-angiotensin system.

What is negative feedback?

Negative feedback is a process where the response counteracts the stimulus, reducing the deviation from the set point, thus maintaining homeostasis.

How does negative feedback stop further signaling?

When the effector acts and the variable returns to normal, the receptor stops sending signals to the control center.

Name two variables regulated by homeostasis.

Body temperature and blood sugar level (or pH, fluid balance, ion concentrations).

What is the homeostatic range?

The pre-set upper and lower limits within which a variable must be kept for optimal functioning.

Who coined the term 'homeostasis'?

Walter Bradford Cannon in 1926.

Who first described the concept of regulation of the internal environment?

Claude Bernard in 1849.

What does the prefix 'hyper-' indicate in homeostatic terms?

A level higher than normal, e.g., hyperthermia (high body temperature).

What does the prefix 'hypo-' indicate?

A level lower than normal, e.g., hypothermia (low body temperature).

How does the nervous system contribute to homeostasis?

The nervous system detects stimuli and sends rapid signals (action potentials) to coordinate responses, such as muscle contraction or gland secretion.

How does the endocrine system contribute to homeostasis?

The endocrine system releases hormones that act as chemical messengers to regulate slower, long-term changes like metabolism and growth.

What is the role of the cannabinoid receptor type 1 (CB1) in homeostasis?

CB1 receptors, located on presynaptic neurons, bind endocannabinoids to reduce neurotransmitter release, helping fine-tune homeostasis.

What are endocannabinoids?

Endocannabinoids are compounds like anandamide synthesized by postsynaptic neurons that travel back to presynaptic terminals to modulate neurotransmitter release.

How do polyunsaturated fatty acids relate to homeostasis?

They are precursors for endocannabinoids, which help fine-tune body homeostasis.

What is the etymology of 'homeostasis'?

From Greek 'homoios' (similar) and 'stasis' (standing still), meaning 'staying the same'.

What is cybernetics?

Cybernetics refers to technological control systems (e.g., thermostats) that function like homeostatic mechanisms, though defined more broadly.

Why is homeostasis important for cells?

Metabolic processes require specific physical and chemical conditions; homeostasis maintains the internal environment for cells to survive and function.

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