6.4.1 Principles of homeostasis and negative feedback

Question 1

What is homeostasis?

Answer 1

Maintenance of a constant internal environment within a living organism

Question 2

Outline the 5 steps the process of homeostasis follows:

Answer 2

1. Stimulus: produces change in variable
2. Change detected by receptor
3. Input: information sent along different pathways to control center
4. Output: information sent along different pathway to control center
5. Response of effector feeds back to influence magnitude of stimulus and returns variable to homeostasis

Question 3

What is homeostasis responsible for maintaining constant and give examples:

Answer 3

• Blood pH: optimum enzyme activity, maintaining shape of antibodies
• Core temperature: optimum enzyme activity
• Blood glucose concentration: effective energy transfer and maintaining water potential of blood
• Water potential of the blood - maintain correct cellular water potential and structure

Question 4

How do homeostatic mechanisms operate?

Answer 4

• Via a feedback loop that may involve either the nervous or endocrine system

Question 5

What happens when specialised receptors detect an internal change to conditions?

Answer 5

A response is generated to correct the change

Question 6

What do most homeostatic responses involve?

Answer 6

An effect that is antagonistic to the detected stimulus (negative feedback)
- When levels return to equilibrium, the effector ceases to generate a response and an internal balance is maintained

Question 7

What does negative feedback involve?

Answer 7

Involves a response that is the reverse of the change detected

Question 8

What are examples of processes that utilise negative feedback loops?

Answer 8

1. Thermoregulation: if body temperature changes, mechanisms are induced to restore normal levels
2. Blood sugar regulation (insulin lowers blood glucose when levels are high; glucagon raises blood glucose when levels are low)
3. Osmoregulation (ADH is secreted to retain water when dehydrated and its release is inhibited when the body is hydrated)

Question 9

What is positive feedback?

Answer 9

When the change stimulates a further change in the same direction.

Question 10

When is positive feedback useful?

Answer 10

When a stimulus needs to be amplified to produce the desired effect eg/ an action potential

Question 11

What are 4 ways in maintaining a stable core temperature?

Answer 11

• Maintaining optimum temperature for enzyme reactions
• Outcompete ectothermic prey
• Live in different environments
• Remain active at night

Question 12

How do ectotherms gain heat?

Answer 12

From the environment, so their body fluctuates with that of the environment

Question 13

How do ectotherms control their body temperature?

Answer 13

• Exposing themselves to the sun
• Taking shelter
• Gaining warmth from the ground

Question 14

How do endotherms gain heat?

Answer 14

Endotherms gain most of their heat from internal metabolic activities. Their body temperature remains relatively constant despite fluctuations in the external temperatures

Question 15

What are the different ways in conserving and gaining heat in response to a cold environment?

Answer 15

• Vasoconstriction
• Shivering (muscles of body undergo involuntary rhythmic contractions that produce metabolic heat)
• Raising of hair
• Increased metabolic rate
• Decrease in sweating
• Behavioural mechanisms

Question 16

How does the raising of hair conserve and gain heat in response to a cold environment?

Answer 16

The hair erector muscles in the skin contract, raising the hairs on the body. This enables a thicker layer of still air, which is a good insulator, to be trapped next to the skin, insulation and conserving heat in mammals with thick fur

Question 17

How does increasing metabolic rate conserve and gain heat in response to a cold environment?

Answer 17

In cold conditions more of the hormones that increase metabolic rate are produced. As a result metabolic activity, including respiration, is increased and so more heat is generated

Question 18

What are the different ways in losing heat in response to a warm environment?

Answer 18

• Vasodilation
• Increased sweating
• Lowering of body hair
• Behavioural mechanisms

Question 19

How does the lowering of hair lose heat in response to a warm environment?

Answer 19

Hair erector muscles in the skin relax and the elasticity of the skin causes them to flatten against the body. This reduces the thickness of the insulating layer and allows more heat to be lost to the environment when the internal temperature is higher than the external temperature

Question 20

How does increasing sweating lose heat in response to a warm environment?

Answer 20

To evaporate water from the skin surface requires energy in the form of heat.
- In relatively hairless mammals, water in sweat absorbs heat from body as it evaporates. sweat is secreted onto surface of skin so increases the rate of heat loss from the body therefore cooling effect
- In mammals with fur, cooling is achieved by the evaporation of water from the mouth and tongue, during panting. The high latent heat of vaporisation of water makes sweating an efficient way of losing heat.

Question 21

Describe what happens when body temperature rises and what does it triggers?

Answer 21

VASODILATION
- Smooth muscle in wall of arteriole relaxes and arteriole dilates
- More blood flows through capillaries close to surface of the skin
- Less blood flows through the deep vessel
- Rate of heat loss from body increases

Question 22

Describe what happens when body temperature falls and what does it triggers?

Answer 22

VASOCONSTRICTION
- Smooth muscle in wall of arteriole contracts which constricts the lumen
- Less blood flows close to surface of skin
- More blood is shunted away from surface of skin
- Rate of heat loss from body decreases

Question 23

How do endotherms maintain body temperature at a set point?

Answer 23

• Generate heat energy through respiration
• Change cellular and physiological processes

Question 24

How do ectotherms mostly rely on external heat energy?

Answer 24

• Warm or cool their body by changing their behaviour in response to changing conditions