5.3.1 The principles and importance of homeostasis

what is the definition of homeostasis?

the maintenance of a stable internal environment

what are the internal factors that must be controlled

• pH - 7.35-45

• temperature - 36.5-37.5

• blood glucose conc - 3.6-7.8

• water potential of plasma

• blood pressure - 120/80 or 140/90

what is the importance of homeostasis?

• ensure optimum conditions for enzyme-controlled reactions

• control the rate and efficiency of metabolic pathways

outline the negative feedback mechanism

• stimulus produces a change from the set point (desired value that -ve feedback mech operates around)

• receptors detects stimulus and converts stimulus to a nerve impulse

• nerve impulse travels to control centre which sends impulse to the effector

• effector is a target tissue or organ that brings about the response (secretion of hormone or muscle contraction)

• variable returns to set point

what is the role of negative feedback

bring about a counter response to changes in the environment

what’s the role of positive feedback

• to enhance the original stimulus —> amplifies any changes

• effector triggers a response that moves the change further away from set point

what are general examples of positive feedback

• blood clotting cascade

• voltage gated Na+ channels and action potentials

• hyperthermia/hypothermia

how is oxytocin in labour an example of positive feedback

• release of oxytocin –> increased speed and intensity of uterine contractions

• contractions stimulates placenta to release prostaglandins

• prostaglandins stimulate more uterine contractions

• this stimulates more oxytocin to be secreted

• stops when baby is born

what role does the medulla oblongata have in controlling heart rate

Has 2 centres that are linked to SAN by motor neurones:

• INCREASES HR = impulses sent through SYMPATHETIC NS where they’re transmitted by the accelerator nerve to the SAN

• DECREASES HR = impulses sent through PARASYMPATHETIC NS where they’re transmitted by vagus nerve to SAN

EQ: name the structure in the brain where the cardiovascular centres are located

medulla oblongata

what is the function of baroreceptors?

detect changes in blood pressure in walls of aorta, carotid artery and vena cava

what is the function of chemoreceptors?

• detect changes in pH in walls of aorta and carotid artery

• detect changes in O2/CO2 levels of plasma

how does the body respond to increased blood pressure

• baroreceptors detect change in BP

• stimulus converted to nerve impulse and sent to M.O

• M.O sends impulses to parasympathetic NS via vagus nerve which secretes acetylcholine

• ACh binds to specific receptors on SAN

• causes heart rate to decrease

• BP reduced back to normal level

(OPPOSITE for decreased BP - sympathetic NS - accelerator nerve - noradrenaline - increase HR)

what neurotransmitter does the accelerator nerve secrete

acetyl choline (ACh) - sympathetic NS

what neurotransmitter does the vagus nerve secrete

noradrenaline - parasympathetic NS

how does the body respond to decreased pH / O2 levels (inc CO2)

• chemoreceptors detect change

• nerve impulses sent to MO

• MO sends impulses to sympathetic NS via accelerator nerve which secretes noradrenaline

• this binds to receptors on SAN which causes heart rate to inc —> returns O2/CO2/ph levels back

how does adrenaline increase heart rate

• adrenaline is secreted by adrenal glands into blood plasma

• adrenaline travels to SAN and binds to specific receptors on cells of SAN

• this activates G proteins which increases production of cAMP

• cAMP increases permeability of Ca2+ channels

• more Ca2+ can leave membrane —> membrane becomes more negative than outside

• depolarisation occurs quicker (threshold for AP is lowered)

• cardiac muscles contract more frequently and harder

• heart rate increases

• more blood pumped around body

when is adrenaline secreted

• fight or flight —> excitement/stress

structure of adrenaline

• hydrophilic hormone —> cannot diffuse through csm

importance of adrenaline (in flight or flight)

• inc HR

• more blood pumped around body

• muscles around bronchioles relax —> breathe deeper

• adrenaline stimulates glycogenolysis —> glycogen is hydrolysed to glucose —> more respiratory substrate available for aerobic respiration

• vasodilation of arterioles supplying lungs and skeletal muscle - more O2 delivered- more aerobic respiration

what is the definition of thermoregulation?

the ability to regulate internal body temperature regardless of external temperature changes

which part of the brain contains the thermoregulatory centre that controls body temp

hypothalamus

role of peripheral temperature receptors

• found in skin

• detect changes in temp

• send impulses along sensory neurones to hypothalamus

• impulses sent along motor neurones to effectors to restore the core body temp

what mechanisms reduce body temp

• increase sweating - more heat lost by evaporation from surface of skin

• erector pili muscles relax - hairs lie flat so less air trapped between hairs + skin - less insulation - more heat loss by radiation

• vasodilation of arterioles - more blood flows to surface of skin - more heat loss via radiation

what mechanisms increase body temp

• less sweating - less heat loss by evaporation

• erector pili muscles contract - hairs stand erect - air trapped between hair and skin - skin is insulated more

• vasoconstriction - less blood flow to surface of skin - less heat loss by radiation

• increase adrenaline secretion

• increase thyroxine secretion

• shivering - more muscle contraction - more thermal energy released from respiration

role of thyroxine in regulating body temp (TOO LOW)

• hypothalamus secretes thyrotropin-releasing hormone (TRH)

• TRH stimulates production of thyrotropin (a thyroid stimulating hormone, TSH) from the anterior pituitary gland

• TSH stimulates secretion of thyroxine

• thyroxine stimulates transcription of specific genes that’s involved in metabolism

• metabolic rate increases - more respiration substrates are available

• more respiration

• more thermal energy released

• body temp increases

what sites can u measure core body temp

1. oral - mouth

2. tympanic - ear

3. axillary - armpit

4. rectal

pros and cons of each

Oral - easiest, cannot do vigorous exercise, smoke/drink before

tympanic - most accurate (eardrum shares blood supply with thermoregulatory system), must be done carefully so no damage done to eardrum

axillary - least accurate, must have good skin contact

rectal - invasive, v accurate

what is the definition of hypothermia?

when internal body temperature is below 35.C

Causes and symptoms of hypothermia

CAUSES:

• prolonged exposure to extreme cold - large temp gradient between skin + environment, so heat is conducted to warm the cold air next to it - strong winds increase heat loss

• wet

• fuel poverty - heating not used to save money

Symptoms:

• shivering

• shallow breathing

• weak pulse

treatment for hypothermia

• move person indoors

• remove wet clothes

• warm compress to neck, chest, groin

Causes and symptoms of hyperthermia

CAUSES:

• prolonged exposure to extreme high temp

• over-exertion

• high BI

• infection - some bacteria produce toxins that affect hypothalamus

SYMPTOMS:

• migraine

• vomiting

• muscle cramps

Treatment for hyperthermia

• move person to cool conditions

• keep hydrated

• apply cold water to skin

what is hyperthermia

internal body temp is above 38oc —> PYREXIA is when body temp is above 41oc