29 Homeostasis

Question 1

what is the definition of homeostasis?

Answer 1

the maintenance of a stable internal environment

Question 2

what are the internal factors that must be controlled?

Answer 2

pH (arterial: 7.35 - 7.45)

temperature (36.5 - 37.5)

plasma glucose concentration (3.6 - 7.8 mmol dm^3)

plasma water potential (275 - 295 mOsm kg-1)

ion balance (Na+: 135 - 145 mmoldm-3)

blood pressure (120/80 - 140/90 mmHg)

Question 3

what are the external factors that we respond to?

Answer 3

humidity

air temperature

light intensity

new/sudden sounds

Question 4

what is the importance of homeostasis?

Answer 4

optimum conditions for enzyme-controlled reactions

control of rate and efficiency of metabolic pathways

Question 5

outline the negative feedback mechanism

Answer 5

a stimulus produces a change in a variable

change detected by receptor

input info sent along afferent pathway to control centre

output info sent along efferent pathway to effector

corrective action is made

variable returns to set point

Question 6

what is the set point?

Answer 6

the desired value that the negative feedback mechanism operates around

Question 7

what is the feedback loop?

Answer 7

the pathway which returns an altered factor back to its set point, and informs receptors of the change to the system

Question 8

outline the role of the positive feedback mechanism

Answer 8

to enhance the original stimulus

Question 9

give an example of PFM

Answer 9

blood clotting cascade

voltage gated Na+ channels and action potentials

Question 10

outline the role of oxytocin during labour

Answer 10

release of oxytocin –> increased speed and intensity of uterine contractions (due to increased prostaglandin production)

further increased secretion of oxytocin

positive feedback continues until childbirth is complete

oxytocin secretion stops

Question 11

what is the role of the SAN in controlling heart rate?

Answer 11

generates electrical impulses/waves of excitation –> contraction of cardiac muscle

Question 12

what is the role of the medulla oblongata in controlling heart rate?

Answer 12

two regions of cardiac centre, linked to SNA by motor neurones:

• incr. h.r. = symp. NS –> accelerator nerve -> SAN
• decr. h.r. = para. NS –> vagus nerve -> SAN

Question 13

what is the function of baroreceptors?

Answer 13

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

Question 14

what is the function of chemoreceptors?

Answer 14

to detect O2/CO2 levels in plasma and plasma pH in the aorta, carotid artery and medulla oblongata

Question 15

how does high CO2 blood concentration lead to and increase in breathing rate?

Answer 15

increased CO2 from aerobic respiration

increased blood pH (from H+ ions)

detected by chemoreceptors - impulse travels along sensory neurones to medulla oblongata

medulla oblongata sends impulse down accelerator nerve to SAN

noradrenaline released and binds to receptors on surface of SAN –> increased h.r.

increased ventilation rate to compensate and maintain efficient gradients for gas exchange

Question 16

where is the adrenal gland located?

Answer 16

on top of the kidneys

Question 17

how does adrenaline increase h.r.?

Answer 17

binds to specific receptors on cells of SAN

G protein activated

–> incr. cAMP production

–> incr. permeability of Ca2+ channels

SAN depolarises more quickly (lower threshold)

cardiac muscle contracts more frequently (and with more force)

Question 18

what are the effects of adrenaline during the F/F/F response?

Answer 18

incr. h.r. ∴ blood pumped around body faster

muscles around bronchioles relax –> deeper breathing

glycogen hydrolysed to glucose and used as a respiratory substrate for aerobic respiration

vasodilation of arterioles supplying lungs and skeletal muscle –> incr. delivery of oxygen –> incr. aero. resp.

vasoconstriction of arterioles supplying the gut

Question 19

what is the definition of thermoregulation?

Answer 19

the ability to regulate internal body temperature, irrespective of external temperature changes

Question 20

where are thermoreceptors found?

Answer 20

hypothalamus - internal blood plasma temperature

skin (peripheral) - external temperature of skin

Question 21

what are the mechanisms employed to reduce body temperature?

Answer 21

increased sweating - evaporation of sweat uses heat

erector pili muscles relax

vasodilation of arterioles supplying the skin

Question 22

what is the definition of hypothermia?

Answer 22

when internal body temperature is below 35.C

Question 23

what are the mechanisms employed to increase body temperature?

Answer 23

decreased sweating

erector pili muscles contract

vasoconstriction of arterioles supplying the skin

Question 24

outline the effect of thyroxine secretion on internal body temperature

Answer 24

hypothalamus secretes thyrotropin-releasing hormone (TRH)

TRH stimulates production of thyrotropin (thyroid stimulating hormone, TSH) from the anterior p.g.

TSH stimulates secretion of thyroxine

thyroxine diffuses into nucleus of cells and causes transcription of specific genes

increased production of mitochondria and respiratory enzymes

increased metabolism of lipids and glucose

∴ rate of respiration increases

more thermal energy released

body temperature increases

Question 25

how does secretion of ADH cause more water to be reabsorbed?

Answer 25

osmoreceptors shrink due to lower w.p. of blood plasma

ADH released from posterior p.g. into blood stream

binds to cells in the walls of the collecting duct

increase in selective reabsorption of water