[16.2] feedback mechanisms

Question 1

what is positive feedback?

Answer 1

when a deviation from an optimum causes changes that result in an even greater deviation from the normal

Question 2

what is an example of purposeful positive feedback?

Answer 2

• in neurones, where a stimulus leads to a small influx of sodium ions
• this influx increases the permeability of the neurone membrane to sodium ions, so more ions enter
• this causes further increase in permeability and even more rapid entry of ions
• in this way, a small stimulus can bring about a large and rapid response

Question 3

describe positive feedback in terms of body temperature

Answer 3

• body temperature increases and increases further = hyperthermia
• body temperature decreases and decreases further = hypothermia

Question 4

what is negative feedback?

Answer 4

if a level falls too high or low beyond the normal level, corrective mechanisms will work to bring it back to the norm level

Question 5

what corrective mechanisms can regulate body temperature? (7)

Answer 5

• vasodilation
• vasoconstriction
• shivering
• sweating
• raising body hair
• lowering body hair
• behavioural mechanisms

Question 6

thermoregulation: vasodilation

Answer 6

• diameter of the arterioles near the surface of the skin becomes larger
• warm blood passes close to the skin surface through the capillaries
• heat from blood is radiated away from the body

Question 7

thermoregulation: vasoconstriction

Answer 7

• diameter of arterioles near the surface of the skin decreases
• volume of blood reaching the skim surface through the capillaries decreases
• most of the blood entering the skin passes beneath the insulating layer of fat
• less heat is lost to the environment

Question 8

thermoregulation: shivering

Answer 8

• quick, random, involuntary rhythmic contractions of muscles of the body
• releases metabolic heat through respiration

Question 9

thermoregulation: sweating

Answer 9

• high latent heat of vaporisation of water makes sweating an efficient way of losing heat
• varies between species eg. effective in humans, plant equivalent is transpiration, furry mammals evaporate water from the mouth and tongue during panting

Question 10

thermoregulation: raising of body hair

Answer 10

• hair erector muscles in the skin contract, raising the hairs on the body
• this enables a thicker layer of still air, a good insulator, to be trapped next to the skin
• useful in insulation and conserving heat in mammals with thick fur, less so with humans

Question 11

thermoregulation: lowering of body hair

Answer 11

• hair erector muscles in the skin relax
• 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

Question 12

thermoregulation: behavioural mechanisms

Answer 12

• maintain core body temperature by: sheltering from the wind, basking in the sun, huddling together
• prevent body temperature from rising by avoiding the heat of the sun by sheltering, seeking out shade