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CIE Categories Archives: 14 Homeostasis

Summary of Homeostasis and Co-ordination

3 Homeostatic equilibrium requires receptors that detect changes in physiological
parameters such as the temperature, water potential and pH of the blood. Effectors are
the cells, tissues and organs that carry out the functions necessary to restore those
parameters to their set points. Homeostatic control systems use negative feedback in
which any change in a parameter stimulates actions by effectors to restore the
parameter to its set point.
4 Excretion is the removal of toxic waste products of metabolism, especially carbon
dioxide and urea. The deamination of excess amino acids in the liver produces
ammonia, which is converted into urea, the main nitrogenous waste product. Urea is
excreted in solution in water, as urine.
5 The kidneys regulate the concentration of various substances in the body fluids, by
excreting appropriate amounts of them. Each kidney is made up of thousands of
nephrons and their associated blood vessels. The kidneys produce urine by ultrafiltration
and reabsorption, plus some secretion of unwanted substances. Different regions of a
nephron have different functions, and this is reflected in the structure of the cells that
make up their walls.
6 Blood is brought to the glomerulus in an afferent arteriole. High hydrostatic pressure
in the glomerulus forces substances through the capillary walls, the basement
membrane and inner lining of Bowman‟s capsule. The basement membrane acts as a
filter, allowing only small molecules through. This filtrate collects in Bowman‟s capsule
and then enters the proximal convoluted tubule, where most reabsorption occurs by
diffusion and active transport; substances are also reabsorbed in the distal convoluted
tubule and collecting duct. The loop of Henle acts as a counter-current multiplier,
producing high concentrations of sodium and chloride ions in the tissue fluid in the
medulla. This tissue has a very low water potential. Water is reabsorbed from fluid in the
collecting duct by osmosis if the body is dehydrated.
7 The water content of the blood is controlled by changing the amount of water excreted
in the urine by the kidneys. This is done by regulating the permeability of the walls of the
collecting ducts to water, and hence the volume of water reabsorbed from the collecting
ducts into the blood. The permeability is increased by the hormone ADH, which is
secreted by the posterior pituitary gland in response to stimulation of osmoreceptors in
the hypothalamus.

8 Neurones are cells adapted for the rapid transmission of electrical impulses. Sensory
neurones transmit impulses from receptors to the central nervous system (brain and
spinal cord); motor neurones transmit impulses from the central nervous system to eff
ectors; intermediate neurones transmit impulses within the central nervous system. The
three neurones are found in series in reflex arcs that control fast, automatic responses to
stimuli. In vertebrates, the axons of many neurones are insulated by a myelin sheath,
which speeds up transmission.
9 Neurones have a resting potential, which is a potential difference across their
membranes, with the inside having a negative potential compared with the outside; this
potential difference is about −65 mV. An action potential is a rapid reversal of this
potential, caused by changes in permeability of the cell surface membrane to potassium
and sodium ions. Action potentials are always the same size. Information about the
strength of a stimulus is given by the frequency of action potentials produced.
10 Action potentials are propagated along axons by local circuits that depolarise regions
of membrane ahead of the action potential. This depolarisation stimulates sodium ion
voltage-gated channels to open, so that the permeability to sodium increases and the
action potential occurs further down the axon. Axons are repolarised by the opening of
potassium ion voltagegated channels that allow potassium ions to diffuse out of the
axon. After a short refractory period when the sodium channels cannot open, the
membrane is able to respond again. Refractory periods determine the maximum speed
of impulses.
11 Action potentials may be initiated within the brain or at a receptor. Receptors
respond to information from the environment. Environmental changes result in
permeability changes in the membranes of receptor cells, which in turn produce changes
in potential diff erence across the membrane. If the potential difference is sufficiently
great and above the threshold for the receptor cell, this will trigger an action potential in
a sensory neurone. Receptors are transducers converting the energy of stimuli into
electrical impulses.
12 A synapse is a junction between two neurones or between a motor neurone and a
muscle cell. At cholinergic synapses, a transmitter substance, acetylcholine, is released
when action potentials arrive. Impulses pass in one direction only, because transmitter
substances are released by exocytosis by the presynaptic neurone to bind to receptor
proteins that are only found on the postsynaptic neurone.
13 Any one neurone within the central nervous system is likely to have at least several
hundred synapses with other neurones, some of which will be stimulatory and some
inhibitory. This allows integration within the nervous system, resulting in complex and
variable patterns of behaviour, and in learning and memory.
14 Hormones are chemicals that are made in endocrine glands and transported in
blood plasma to their target cells, where they bind to specific receptors and so affect the
behaviour of the cells.
15 The concentration of glucose in the blood is controlled by the action of insulin and
glucagon, which are secreted by the islets of Langerhans in the pancreas and aff ect

liver and muscle cells. The use of negative feedback keeps the blood glucose
concentration near the set point.
16 Plants produce several chemicals known as plant growth substances that are
involved in the control of growth and responses to environmental changes. Auxin is
synthesised mainly in growing tips of shoots and roots, and appears to be involved in
preventing the growth of lateral buds when an intact and active apical bud is present.
Gibberellin is synthesised in young leaves and in seeds. It stimulates growth of stems
and germination of seeds such as those of wheat and barley. Abscisic acid is
synthesised by any cells in a plant that contain chloroplasts or amyloplasts, especially in
stress conditions. The presence of large concentrations of abscisic acid in leaves
causes stomata to close.
1. End-of-chapter questions
1 Which of the following is an incorrect statement about the endocrine system?
A All hormones bind to receptors on the cell surface of their target cells.
B Endocrine glands are ductless.
C Endocrine glands secrete hormones into the blood.
D Hormones are transported in the blood plasma.
2 Glucose is small enough to be filtered from the blood in glomeruli in the kidney, but
is not normally found in the urine. This is because glucose is:
A reabsorbed in distal convoluted tubules
B reabsorbed in proximal convoluted tubules
C reabsorbed along the whole length of the nephrons
D respired by cells in the kidney
3 Which of the following is responsible for saltatory conduction in myelinated
neurones?
A axon membranes
B nodes of Ranvier
C Schwarm cells
D voltage-gated channel proteins
4 Which of the following correctly identifies the effects of the three plant
hormones,abscisic acid (ABA), auxin and gibberellin?

An investigation was carried out to determine the response of pancreatic cells to an
increase in the glucose concentration of the blood. A person who had been told not to
eat or drink anything other than water for 12 hours took a drink of a glucose solution.
Blood samples were taken from the person at one hour intervals for five hours, and the
concentrations of glucose, insulin and glucagon in the blood were determined. The
results are shown in the figure.

9 Gibberellin is a plant growth regulator.
a Outline the role of gibberellin in the germination of seeds such as those of wheat
and barley. [5] In an investigation of the effects of gibberellin, plants of short-stemmed and longstemmed varieties of five cultivated species were grown from seed. The young plants of
each species were divided into two groups. One group of plants was sprayed with a
solution of gibberellin each day. A control group was sprayed with the same volume of
water. After eight weeks, the stem length of each plant was measured and means
calculated for each group of plants. A statistical test was carried out to determine
whether the difference between the treatments for each species was significant.
The results are shown in the figure. The p value for each species is given.

a Using all the information provided, predict what happens to the pH in the stroma in
the light.[1] b i When light shines on the chloroplast, dissociation of ABA-H is stimulated. Explain
why this happens. [2] ii Explain the effect that this will have on diffusion of ABA-H into or out of the
chloroplast. [2] When the mesophyll cells of leaves become dehydrated, some of the ABA stored in
the chloroplasts is released into the transpiration stream in the apoplast,
c ABA travels in the apoplast pathway to the guard cells. Explain why this is an
advantage when the leaf is dehydrated. [2] [Total: 7] 11 a Explain how a nerve impulse is transmitted along a motor neurone. [9] b Describe how an impulse crosses a synapse. [6] [Total: 15] 12 a Describe a reflex arc and explain why such reflex arcs are important. [7] b Describe the structure of a myelin sheath and explain its role in the speed of
transmission of a nerve impulse. [8] [Total: 15] [Cambridge International AS and A Level Biology Paper 41, Question 10,
October/November2009] 13 a Compare the roles of the endocrine and nervous systems in control and
coordination in animals.

b Describe the part played by auxins in apical dominance in a plant
shoot. [7] [Total:15] [Cambridge International AS and A Level Biology 9700/04, Question 10, May/June
2008] 2. End-of-chapter answers
Cambridge International Examinations bears no responsibility for the example answers
to questions taken from its past question papers which are contained in this publication.
1 A 2 B 3 B 4 C
5 a C: depolarisation/the inside of the membrane becomes more positive/less negative;
sodium ions/Na+ , flow in;
D: repolarisation/inside of the membrane becomes more negative/less positive;
potassium ions/K+ , flow out;
E: hyperpolarisation/refractory period;
more negative than resting
potential; [6] 6 a excretion: removal from the body; of waste products of metabolism;
carbon dioxide/nitrogenous waste/urea/uric acid/ any other example;
substances in excess of requirements;
water/salts/sodium ions/potassium ions/any other
example; [max. 3] b i A: distal convoluted tubule;
B: Bowman‟s capsule;
C: glomerulus/capillary;
D: proximal convoluted
tubule; [4] ii cortex; glomeruli/convoluted tubules, are only found in the
cortex;

7 a hypothalamusb 1555 cm3 (or any answer within the range 1150 to 1160 cm3 or equivalent in dm3
); [1] c any four from:
water was absorbed into the blood;
water increases the water potential of the plasma;
any effect of an increase in water potential of the plasma on, cells/tissues, e.g. water
enters cells by osmosis/cells will swell/decreases efficiency of reactions inside
cells/cells may burst; osmoreceptors detect increase in water potential; do
not, secrete/release, ADH; collecting ducts remain impermeable to water; excess
water lost in urine; until water potential returns to normal/
set point;
[max. 4] d (after absorption of dilute salt solution) no change in water potential of blood plasma;
water and salt is not lost in the urine, so must remain in the body; giving an increase in
volume, of blood or body fl uids; body tolerates changes in blood volume, but not its
water potential; [max. 2] e homeostasis is maintenance of constant internal conditions;
negative feedback: a deviation from the set point;
is detected by a receptor; a control centre instructs eff ector to carry out an action;
to reverse the change/return factor to set point;
positive feedback: any (small) deviation in a factor leads to an increase in the
change (not a reversal); e.g. opening of voltage-gated sodium ion channels in rising
phase of action potential; [max. 5] [Total: 13] 8 a animals are multicellular/complex organisms;
cells are long distances apart;
coordination;
of cells/tissues/organs, so that they work together;
regulation of internal environment/ refer to homeostasis;
response to, changes in the environment/external stimuli;
any
example; [max.
4] b islets of Langerhans;
small groups of cells;
scattered among the exocrine tissue;
blood spaces/ capillaries, in between the cells;
α cells;
β cells;
cells full of vesicles containing (molecules of), hormones/protein;
cells with rough endoplasmic
reticulum;

c i glucose concentration may already be high;
if person had eaten within 12 hours;
effect of sudden increase would not be seen/so there was a sudden increase;
may already be a high concentration of
insulin; [max. 3] ii β cells secrete insulin; concentration of insulin increases over first hour after taking the
glucose solution;
insulin concentration increases from 60 to 300 pmol dm−3 ;
α cells do not secrete glucagon;
glucagon concentration, remains constant/decreases;
from 42 to 36 pmol dm−3
; [max. 4] iii insulin: stimulates, liver/muscle, cells;
increase in uptake of glucose from the blood;
stimulates enzymes; to increase conversion of glucose to glycogen;
brings about a decrease in the blood glucose
concentration; [5] [Total: 21] 9 a gibberellin secreted by embryo;
stimulates cells in the aleurone layer;
stimulates protein synthesis;
to make amylase;
to break down starch in the endosperm;
mobilises glucose;
for respiration;
to provide energy for
germination; [max. 5] b gibberellin increases the mean length of the stem in the short-stemmed varieties of
all species;
figures for any one species;
increase is significant in all but one species;
gibberellin increases the mean length of the stem in the long-stemmed varieties of
four of the species; not tomato;
increases are not signifi cant; [max. 5] c short-stemmed plants do not make (much) gibberellin;
(because) they do not have the allele for the enzyme that makes gibberellin;
long-stemmed variety has dominant allele for gibberellin synthesis;
gibberellin supplied each day promotes growth of
stems; [max. 3] d less energy used for growth of stem; (therefore) more energy in, peas/beans/seeds;
plants do not need (as much) support; less likely to be damaged by wind; less plant
material to harvest/less wastage at harvest ;

10
a increases; [1] b i concentration of protons in the stroma decreases (as enter grana); shifts
equilibrium to the
right; [2] ii increases diff usion into the chloroplast; as concentration of ABA–H decreases;
so maintaining a concentration gradient into the
chloroplast; [max. 2] c ABA stimulates closure of stomata; less water vapour is lost; [2] [Total: 7] 11 a resting potential;
anything in the range −60 to −70 mV;
sodium–potassium pump uses ATP to pump Na+ out and K+ in;
many anions inside the neurone; resting potential is due to leakage of K+ out;
action potential is depolarisation of membrane;
up to +40 mV;
opening of voltage-gated sodium ion channels/ sodium ions flow in;
closing of voltage-gated sodium ion channels;
voltage-gated potassium ion channels open;
K+ flow out; resting potential restored;
local circuits depolarise next part of, membrane/ axon;
refractory period ensures impulse does not travel backwards;
saltatory conduction in myelinated neurones;
action potential only at nodes of Ranvier;
[max. 9] b action potential arrives at presynaptic membrane; voltage-gated calcium ion channels
open; calcium ions enter to stimulate vesicles to move to membrane;
vesicles fuse with membrane/ exocytosis, to release (named) neurotransmitter;
(named) neurotransmitter diff uses across (synaptic) cleft;
binds with receptor on postsynaptic membrane;
stimulates opening of sodium ion channel proteins;
sodium ions flow in through postsynaptic membrane/depolarisation of postsynaptic
membrane;
[max. 6] [Total: 15] 12 a reflex arc: to max 5 – these points may be on a diagram:
strong stimulus in receptor/AW;
action potential/impulses, along sensory neurone;
dorsal root of spinal nerve; into spinal cord; synapse with intermediate neurone;
(then) motor neurone;
action potential/impulses, to eff ector;
action potential/impulses, to brain;
response; e.g. knee jerk;
other points which may also be given on a diagram: fast/immediate;

stops/limits, damage/danger;
automatic/no conscious thought;
innate/stereotyped/instinctive; [max. 7] b myelin sheath: Schwann cells;
wrap around axon;
sheath mainly lipid;
(sheath) insulates axon (membrane);
Na+ /K+ , cannot pass through sheath/can only pass through membrane at nodes;
depolarisation (of axon membrane) cannot occur where there is sheath/only at nodes of
Ranvier;
local circuits between nodes;
action potentials „jump‟ between nodes;
saltatory conduction;
increases speed/reduces time, of impulse transmission;
up to 100 m s−1 ;
speed in non-myelinated neurones about 0.5 m s−1 [max. 8] [Total: 15] 13 a endocrine system: uses hormones;
which are chemical messengers/chemicals that transfer messages;
secreted/released, into blood by ductless glands;
influence target/organs/cells;
which have receptors on cell membranes;
an example of named hormone and eff ect;
e.g. insulin, stimulates decrease in blood glucose concentration nervous system: use
impulses/action potentials;
not electrical, signals/current along neurones/nerve fi bres;
not nerves synapse (at target cell)/neuromuscular junction;
named neurone;
e.g. receptor/sensory/motor/ eff ector/intermediate/relay
differences between the two systems: endocrine has slow effect/nervous is fast;
endocrine has long-lasting eff ect/nervous has short-term eff ect;
endocrine has widespread eff ect/nervous has very localised effect;
any other detail;
e.g. extra detail of synapse, such as
neurotransmitters [max. 8] b auxins: indole 3-acetic acid (IAA/plant growth regulator);
synthesised in, growing tips/apical buds/meristems;
moves by diff usion;
from cell to cell;
also, by mass flow/in phloem;
stimulates cell elongation;
not cell enlargement inhibits, side/lateral, buds/ growth accept inhibits branching plant
grows, upwards/ taller;
accept stem elongates interaction between IAA and other plant growth regulators;
AVP; e.g. role of abscisic acid (ABA) and lateral bud inhibition
AVP; e.g. cytokinins antagonistic to IAA/ gibberellins enhance IAA [max. 7] [Total: 15]

 

 

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Homeostasis in plants

Abscisic acid and stomatal closure
Abscisic acid (ABA) is a stress hormone that is secreted in response to difficult
environmental conditions such as very high temperatures or much reduced water
supplies. ABA triggers the closure of stomata to reduce transpiration and prevent
water loss.
ABA binds to cell surface receptors
 inhibits proton pumps: stop H
+
pumped out
 stimulates movement of Ca2+ through the cell surface membrane and tonoplast
Ca2+ acts as a 2nd messenger to activate channel proteins to open that allow
negatively charged ions to leave the guard cell. This in turn
 opens channel proteins that allow K
+
to leave the cell
 closes channel proteins that allow K
+
to enter the cell
 –> net movement: K
+
leaves cell
Loss of ions = higher water potential inside cell = water passes out by osmosis = guard
cells become flaccid –> stomata close

 

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Osmoregulation

 

Osmoregulation is the control of the water content of body fluids. It is part of
homeostasis, the maintenance of a constant internal environment.
It is important that cells are surrounded by tissue fluid of a similar water potential to their
own contents, to avoid too much water loss or gain which could disrupt metabolism. You
have seen that water is lost from the fluid inside a nephron as it flows through the
collecting duct. The permeability of the walls of the distal convoluted tubule and
collecting duct can be varied.
 If they are permeable, then much water can move out of the tubule and the urine
becomes concentrated. The water is taken back into the blood and retained in the body.
 If they are made impermeable, little water can move out of the tubule and the
urine remains dilute. A lot of water is removed from the body.
ADH
ADH is antidiuretic hormone. It is secreted from the anterior pituitary gland into the
blood.
When the water potential of the blood is too low (that is, it has too little water in it), this is
sensed by osmoreceptor cells in the hypothalamus. The osmoreceptor cells are
neurones (nerve cells). They produce ADH, which moves along their axons and into the
anterior pituitary gland from where it is secreted into the blood.
The ADH travels in solution in the blood plasma. When it reaches the walls of the
collecting duct, it makes them permeable to water. Water is therefore reabsorbed from
the fluid in the collecting duct and small volumes of concentrated urine are produced.
When the water potential of the blood is too high (that is, it has too much water in it), this
is sensed by the osmoreceptor cells and less ADH is secreted. The collecting duct walls
therefore become less permeable to water and less is reabsorbed into the blood. Large
volumes of dilute urine are produced.
Negative feedback

The mechanism for controlling the water content of the body, using ADH, is an example
of negative feedback.
When the water potential of the blood rises too high or falls too low, this is sensed by
receptor cells. They cause an action to be taken by effectors which cause the water
potential to be moved back towards the correct value.
In this case, the receptors are the osmoreceptor cells in the hypothalamus, and the
effectors are their endings in the anterior pituitary gland that secrete ADH.

 

 

 

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Production of urine in a nephron – Ultrafiltration and reabsorption

The blood in a glomerulus is separated from the space inside the renal capsule by:

 

• the capillary wall (endothelium) which is one cell thick and has pores in it;
• the basement membrane of the wall of the renal capsule;
• the layer of cells making up the wall of the renal capsule, called podocytes; these cells
have slits between them.

The blood in a glomerulus is at a relatively high pressure, because the efferent arteriole
is narrower than the afferent arteriole. This forces molecules from the blood through
these three structures, into the renal capsule.
The pores in the capsulary endothelium and the slits between the podocytes will let all
molecules through, but the basement membrane acts as a filter and will only let small
molecules pass through.
– Substances that can pass through include water, glucose, inorganic ions such as Na+,
K+ and Cl- and urea.
– Substances that cannot pass through include red and white blood cells and plasma
proteins (such as albumen and fibrinogen).
– The liquid that seeps through into the renal capsule is called glomerular filtrate.

The cells in the walls of the tubule have many mitochondria, to provide ATP for active
transport. Their surfaces facing the lumen of the tubule have a large surface area
provided by microvilli.
 Active transport is used to move Na+ out of the outer surface of a cell in the wall
of the proximal convoluted tubule, into the blood.

This lowers the concentration of Na+ inside the cell, so that Na+ ions diffuse into
the cell from the fluid inside the tubule. The Na+ ions diffuse through protein transporters
in the cell surface membrane of the cell
 As the Na+ions diffuse through these transporter proteins, they
carry glucose molecules with them. This is called co-transport. The glucose molecules
move through the cell and diffuse into the blood.
 The movement of Na+ and glucose into the blood decreases the water potential
in the blood. Water therefore moves by osmosis from the fluid inside the tubule, down a
water potential gradient through the cells making up the wall of the tubule and into the
blood.

 

Aa a result, the fluid inside the nephron now has:
 no glucose
 a lower concentration of Na+than the filtrate originally had
 less water than the filtrate originally had
About 50% of the urea is also reabsorbed in the proximal convoluted tubule.
The loop of Henle
Some, but not all, nephrons have long loops of Henle that dip down into the medulla and
then back up into the cortex. The function of the loop of Henle is to build up a high
concentration of Na+ and CI- in the tissues of the medulla. This allows highly
concentrated urine to be produced. Note that the loop of Henle itself does not produce
highly concentrated urine.

As fluid flows down the descending limb of the loop of Henle, water moves out of it by
osmosis. By the time the fluid reaches the bottom of the loop, it has a much lower water
potential than at the top of the loop. As it flows up the ascending limb, Na+ and Cl- move
out of the fluid into the surrounding tissues, first by diffusion and then by active
transport.
This creates a low water potential in the tissues of the medulla. The longer the loop, the
lower the water potential that can be produced.

Role of loop of Henle
 Creating a Salt Gradient in the Medulla
 The function of the loop of Henle is to create a salt bath concentration in the fluid
surrounding the tubule
 The descending limb of the loop of Henle is permeable to water, but relatively
impermeable to Na+Cl-.
 The ascending limb of the loop of Henle is permeable to salts, but impermeable to
water
 This means that as the loop descends into the medulla, the interstitial fluid
becomes more salty (and less salty as it ascends into the cortex)
 As the vasa recta blood network that surrounds the loop flows in the opposite
direction (counter-current exchange), this further multiplies the effect.

The distal convoluted tubule and collecting duct
The fluid inside the tubule as it leaves the loop of Henle and moves into the collecting
duct has lost a little more water and more Na+ than it had when it entered the loop.
Because more water has been lost, the concentration of urea has increased.
Now, in the distal convoluted tubule, Na+ is actively transported out of the fluid.
The fluid then flows through the collecting duct. This passes through the medulla, where
you have seen that a low water potential has been produced by the loop of Henle. As
the fluid continues to flow through the collecting duct, water moves down the water
potential gradient from the collecting duct and into the tissues of the medulla. This
further increases the concentration of urea in the tubule. The fluid that finally leaves the
collecting duct and flows into the ureter is urine.

VIDEO
Homeostasis in humans

 

 

 

 

 

 

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Thermoregulation – The control of body temperature

Ectotherms
– Animals that have a variable body temperature.
– Use behavioural mechanisms (e.g. lying in the sun when cold, moving into shade
when hot). Such mechanisms can be very effective, particularly when coupled with
internal mechanisms to ensure that the temperature of the blood going to vital organs
(brain, heart) is kept constant.
We use both!
Thermoregulation
All mammals generate heat and have ways to retain it within their bodies. They also
have physiological methods to balance heat gain, retention of body heat and heat loss
so that they can maintain a constant body temperature. As a result, they are not
dependent on absorbing heat from their surroundings and can be active at any time of
day or night, whatever the external temperature. Most other animals (except birds) rely
on external sources of heat and are often relatively inactive when it is cold.
The heat that mammals generate is released during respiration. Much of the heat is
produced by liver cells that have a huge requirement for energy. The heat they produce
is absorbed by the blood flowing through the liver and distributed around the rest of the
body.
In humans, body temperature is controlled by the thermoregulatory centre in
the hypothalamus. It receives input from 2 sets of thermoreceptors:
– Receptors in the hypothalamus monitor the temperature of the blood as it passes
through the brain (the core temperature), that remains very close to the set point, which
is 37 °C in humans. This temperature fluctuates a little, but is kept within very narrow
limits by the hypothalamus.
– Receptors in the skin (especially on the trunk) monitor the external temperature.

Both sets of information are needed so that the body can make appropriate adjustments.
Our first response to encountering hotter or colder condition is voluntary:
– if too hot, we may decide to take some clothes off, or to move into the shade;
– if too cold, we put extra clothes on or turn the heating up!
It is only when these responses are not enough that the thermoregulatory centre is
stimulated. This is part of the autonomic nervous system, so the various responses are
all involuntary.

Response to low temperature
If the core temperature decreases, or if the temperature receptors in the skin detect a
decrease in the temperature of the surroundings, the hypothalamus sends impulses to
several different effectors to adjust body temperature:
 Vasoconstriction – muscles in the walls of arterioles that supply blood to
capillaries near the skin surface contract. This narrows the lumens of the arterioles and
reduces the supply of blood to the capillaries so that less heat is lost from the blood.
 Shivering – the involuntary contraction of skeletal muscles generates heat which
is absorbed by the blood and carried around the rest of the body.
 Raising body hairs – muscles at the base of hairs in the skin contract to increase
the depth of fur so trapping air close to the skin. Air is a poor conductor of heat and
therefore a good insulator. This is not much use in humans, but is highly effective for
most mammals.
 Decreasing the production of sweat – this reduces the loss of heat by
evaporation from the skin surface.
 Increasing the secretion of adrenaline – this hormone from the adrenal gland
increases the rate of heat production in the liver.
Response to high temperature
When an increase in environmental temperature is detectedby skin receptors or the
central thermoreceptors, thehypothalamus increases the loss of heat from the body
andreduces heat production.
 Vasodilation – the muscles in the arterioles in the skin relax, allowing more blood
to flow through the capillaries so that heat is lost to the surroundings.
 Lowering body hairs – muscles attached to the hairs relax so they lie flat,
reducing the depth of fur and the layer of insulation.
 Increasing sweat production – sweat glands increase the production of sweat
which evaporates on the surface of the skin so removing heat from the body.
Behavioural responses
The behavioural responses of animals to heat include resting or lying down with the
limbs spread out to increase the body surface exposed to the air. We respond by
wearing loose fitting clothing, turning on fans or air conditioning and taking cold drinks.
When the environmental temperature decreases gradually:
– The hypothalamus releases a hormone which activates the anterior pituitary gland to
release thyroid stimulating hormone (TSH).
– TSH stimulates the thyroid gland to secrete the hormone thyroxine into the blood.
– Thyroxine increases metabolic rate, which increases heat production especially in
the liver.
When temperatures start to increase again, the hypothalamus responds by reducing the
release of TSH by the anterior pituitary gland so less thyroxine is released from the
thyroid gland.

VIDEO
Controlling body temperature

Homeostasishttps://www.youtube.com/watch?v=e4YbdGBvFAE

 

 

 

 

 

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