homeostasis and negative feedback

homeostasis

- the maintenance of an organism’s internal environment within set limits
- body temp, blood glucose, pH, water potential

what are internal conditions regulated by?

the endocrine and nervous systems, as well as behavioural patterns

negative feedback

- where a change triggers a response which reduces the effect of a change
- when the feedback causes the corrective measures to be turned off to return system to normal
- deviation from normal set point → receptor detects the change → sensory neurone → controller co-ordinates the info → motor neurone (autonomic NS) → effector takes corrective measures → returns to normal set point → feedback loop (negative feedback)

thermoregulation (an example of a negative feedback loop)

normal body temp → increases → corrective mechanisms → normal body temp

normal body temp → decreases → corrective mechanisms → normal body temp

positive feedback

• when the feedback causes the corrective measures to stay on

• large deviation from the norm

behavioural patterns

taxis and kinesis are behavioural mechanisms to maintain homeostasis, both involve movement of organisms in response to external stimuli

taxis

directional response to a stimulus

kinesis

non-directional movement in response to a stimulus

homeostasis is controlled by a reflex arc

• stimulus (deviance from the optimum)

• receptor (cells)

• co-ordinator (CNS)

• effectors (muscle/gland)

• response (returning the condition back to optimum)

nervous vs endocrine

the nervous system and endocrine system are both involved in controlling the internal conditions of a mammal

nervous control signal type

electrical (action potential)

nervous control speed of signal

reach the target cells in milliseconds

nervous control duration of signal

short lasting

endocrine control signal type

chemical (hormones)

endocrine control speed of signal

relatively slow, travelling in the blood stream

endocrine control duration of signal

long lasting → makes them ideal for controlling conditions in a more gradual manner to avoid over-compensating

why is homeostasis essential?

- enzymes are sensitive to pH and temp → reactions should take place at a suitable rate
- water potential → blood and tissue fluid may cause cells to shrink or burst due to osmosis → maintaining blood glucose concentration ensures a constant water potential (and provides glucose for cellular respiration)

temperature

mammals are able to maintain their internal environment and therefore are successful in all conditions

thermoregulation in animals

• ectotherms

• endotherms

ectotherms

obtain most of their heat from environment, e.g. reptiles and amphibians

endotherms

obtain most of their heat from metabolic activities inside their bodies, e.g. birds and mammals

list 2 general heat gain methods

• heat produced by respiration

• heat gained from environment

list 2 general heat loss methods

• heat lost by evaporation (sweating)

• heat lost to environment (conduction/convection/radiation)

what method do ectotherms use to regulate their temperature?

- exposing themselves to sun
- taking shelter
- gaining warmth from ground
- generating metabolic heat
- colour variations

what methods do endotherms use to regulate their temperature?

heat gain:
- shivering
- vasoconstriction of superficial arterioles
- hair erector muscles contract (raise hair)
- increased metabolic rate

heat loss:
- sweating
- vasodilation of superficial arterioles hair erector
- hair erector muscles relax (lower hair)
- decreased metabolic rate

hair erector muscles

- raising of body hairs provides insulation and prevents heat loss
- lower reduces insulation

what happens when body temperature gets too high?

• detected by thermoreceptors in the hypothalamus and skin

• autonomic NS

• heat loss centre

• physiological mechanisms: sweating, vasodilation or superficial arterioles, hair erector muscles relax (lower hair), decreased metabolic rate

what happens when body temperature gets to low?

• detected by thermoreceptors in the hypothalamus and skin

• heat gain centre

• physiological mechanisms: shivering, vasoconstriction of superficial arterioles, hair erector muscles contract (raise hair), increased metabolic rate

explain how normal core body temperature is maintained when a person moves into a cold room

1. sensors in skin and hypothalamus (thermoreceptors) detect reduced temperature
2. heat gain centre activated
3. vasoconstriction - constriction of arterioles in skin surface
4. dilation of shunt vessels
5. less blood to skin surface
6. reduced heat loss by radiation
7. increased heat gain by increased metabolic rate
8. decreased heat loss by reduced sweating

how does maintaining a constant body temperature allow metabolic reactions in cells to proceed with maximum efficiency?

1. 37 degrees Celsius is optimum temp for enzymes

2. excess heat denatures enzymes active site

3. substrate cannot bind

4. reactions slowed

5. too little reduced kinetic energy of molecules

6. fewer collisions so fewer e-s complexes formed