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Temperature Homeostasis (Thermoregulation)
Blood Glucose Homeostasis
Blood Water Homeostasis
Excretion and Homeostasis
literally means “same state” and it
refers to the process of keeping the internal body environment in a steady
state. The importance of this cannot be over-stressed, and a great deal of the
hormone system and autonomic nervous system is dedicated to homeostasis. In
module 3 we saw how the breathing and heart rates were maintained. Here we shall
look at three more examples of homeostasis in detail: temperature, blood glucose
and blood water.
homeostatic mechanisms use negative feedback to maintain a constant value
(called the set point). Negative feedback means that whenever a change
occurs in a system, the change automatically causes a corrective mechanism
to start, which reverses the original change and brings the system back to
normal. It also means that the bigger then change the bigger the corrective
mechanism. Negative feedback applies to electronic circuits and central heating
systems as well as to biological systems.
in a system controlled by negative feedback the level is never maintained
perfectly, but constantly oscillates about the set point. An efficient
homeostatic system minimises the size of the oscillations.
of the most important examples of homeostasis is the regulation of body
temperature. Not all animals can do this. Animals that maintain a fairly
constant body temperature (birds and mammals) are called homeotherms
(also spelt homoiotherms), while those that have a variable body
temperature (all others) are called poikilotherms. The homeotherms
maintain their body temperatures at around 37°C, so are sometimes called
warm-blooded animals, but in fact piokilothermic animals can also have very warm
blood during the day by basking in the sun.
humans temperature homeostasis is controlled by the thermoregulatory centre
in the hypothalamus. It receives input from two sets of thermoreceptors:
receptors in the hypothalamus itself monitor the temperature of the blood as it
passes through the brain (the core temperature), and receptors in the
skin monitor the external temperature. Both pieces of information are needed so
that the body can make appropriate adjustments. The thermoregulatory centre
sends impulses to several different effectors to adjust body temperature:
thermoregulatory centre is part of the autonomic nervous system, so the various
responses are all involuntary. The exact responses to high and low temperatures
are described in the table below. Note that some of the responses to low
temperature actually generate heat (thermogenesis), while others just
conserve heat. Similarly some of the responses to cold actively cool the body
down, while others just reduce heat production or transfer heat to the surface.
The body thus has a range of responses available, depending on the internal and
to low temperature
to high temperature
muscles in peripheral arterioles in the skin.
contract causing vasoconstriction. Less heat is carried from the
core to the surface of the body, maintaining core temperature.
Extremities can turn blue and feel cold and can even be damaged
relax causing vasodilation. More heat is carried from the core to
the surface, where it is lost by convection and radiation. Skin turns
secrete sweat onto surface of skin, where it evaporates. This is an
endothermic process and water has a high latent heat of evaporation, so
it takes heat from the body.
muscles in skin
to skin hairs)
contract, raising skin hairs and trapping an insulating layer of still,
warm air next to the skin. Not very effective in humans, just causing
relax, lowering the skin hairs and allowing air to circulate over the
skin, encouraging convection and evaporation.
contract and relax repeatedly, generating heat by friction and from
and thyroid glands
secrete adrenaline and thyroxine respectively, which increase the
metabolic rate in different tissues, especially the liver, so generating
stop releasing adrenaline and thyroxine.
up, huddling, finding shelter, putting on more clothes.
out, finding shade, swimming, removing clothes.
thermoregulatory centre normally maintains a set point of 37.5 ± 0.5 °C in
most mammals. However the set point can be altered is special circumstances:
Chemicals called pyrogens released by white blood cells raise the set
point of the thermoregulatory centre causing the whole body temperature to
increase by 2-3 °C. This helps to kill bacteria and explains why you shiver
even though you are hot.
Some mammals release hormones that reduce their set point to around 5°C
while they hibernate. This drastically reduces their metabolic rate and so
conserves their food reserves.
Bats and hummingbirds reduce their set point every day while they are
inactive. They have a high surface area:volume ratio, so this reduces heat
is the transport carbohydrate in animals, and its concentration in the blood
affects every cell in the body. Its concentration is therefore strictly
controlled within the range 80-100 mg 100cm-3, and very low
level (hypoglycaemia) or very high levels (hyperglycaemia) are
both serious and can lead to death.
glucose concentration is controlled by the pancreas. The pancreas has glucose
receptor cells, which monitor the concentration of glucose in the blood, and it
also has endocrine cells (called the islets of Langerhans), which secrete
hormones. The a
cells secrete the hormone glucagon,
while the b
cells secrete the hormone insulin.
These two hormones are antagonistic, and have opposite effects on blood glucose:
stimulates the uptake of glucose by cells for respiration, and in the liver
it stimulates the conversion of glucose to glycogen (glycogenesis).
It therefore decreases blood glucose.
stimulates the breakdown of glycogen to glucose in the liver (glycogenolysis),
and in extreme cases it can also stimulate the synthesis of glucose from
pyruvate. It therefore increases blood glucose.
a meal, glucose is absorbed from the gut into the hepatic portal vein,
increasing the blood glucose concentration. This is detected by the pancreas,
which secretes insulin from its b
cells in response. Insulin causes glucose to be taken up by the liver and
converted to glycogen. This reduces blood glucose, which causes the pancreas to
stop secreting insulin. If the glucose level falls too far, the pancreas detects
this and releases glucagon from its a
cells. Glucagon causes the liver to break down some of its glycogen store to
glucose, which diffuses into the blood. This increases blood glucose, which
causes the pancreas to stop producing glucagon.
negative feedback loops continue all day, as shown in this graph:
is a disease caused by a failure of glucose homeostasis. There are two forms of
the disease. In insulin-dependent diabetes (also known as type 1 or
early-onset diabetes) there is a severe insulin deficiency due to autoimmune
killing of b cells
(possibly due to a virus). In non insulin-dependent diabetes (also known
as type 2 or late-onset diabetes) insulin is produced, but the insulin receptors
in the target cells don’t work, so insulin has no effect. In both cases there
is a very high blood glucose concentration after a meal, so the active transport
pumps in the proximal convoluted tubule of the kidney can’t reabsorb it all
from the kidney filtrate, so much of the glucose is excreted in urine (diabetes
mellitus means “sweet fountain”). This leads to the symptoms of diabetes:
thirst due to osmosis of water from cells to the blood, which has a low
urine production due to excess water in blood.
vision due to osmotic loss of water from the eye lens.
due to loss of glucose in urine and poor uptake of glucose by liver and
wasting due to gluconeogenesis caused by increased glucagon.
Diabetes can be treated by injections with insulin or by careful diet. Until the discovery of insulin in 1922 by Banting and Best, diabetes was an untreatable, fatal disease.
The water potential of the blood must be regulated to prevent loss or gain of water from cells. Blood water homeostasis is controlled by the hypothalamus. It contains osmosreceptor cells, which can detect changes in the water potential of the blood passing through the brain. In response, the hypothalamus controls the sensation of thirst, and it also secretes the hormone ADH (antidiuretic hormone). ADH is stored in the pituitary gland, and its target cells are the endothelial cells of the collecting ducts of the kidney nephrons. These cells are unusual in that water molecules can only cross their membranes via water channels called aquaporins, rather than through the lipid bilayer. ADH causes these water channels to open. The effects of ADH are shown in this diagram:
means the removal of waste products from cells. There are five important
excretory organs in humans:
sweat, containing water, ions and urea
carbon dioxide and water
bile, containing bile pigments, cholesterol and mineral ions
mucosa cells, water and bile in faeces. (The bulk of faeces comprises
plant fibre and bacterial cells, which have never been absorbed into the
body, so are not excreted but egested.)
urine, containing urea, mineral ions, water and other “foreign”
chemicals from the blood.
section is mainly concerned with the excretion of nitrogenous waste as urea. The
body cannot store protein in the way it can store carbohydrate and fat, so it
cannot keep excess amino acids. The “carbon skeleton” of the amino acids can
be used in respiration, but the nitrogenous amino group must be excreted.
acid metabolism takes place in the liver, and consists of three stages:
this reaction an amino group is transferred from an amino acid to a keto acid,
to form a different amino acid.
acid 1 + Keto acid 2 ↔
Keto acid 1 + Amino acid 2
this way scarce amino acids can be made from abundant ones. In adult humans only
11 of the 20 amino acids can be made by transamination. The others are called essential
amino acids, and they must be supplied in the diet.
this reaction an amino group is removed from an amino acid to form ammonia and a
keto acid. The most common example is glutamate deamination:
reaction is catalysed by the enzyme glutamate dehydrogenase. Most other
amino acids are first transaminated to from glutamate, which is then deaminated.
The NADH produced is used in the respiratory chain; the a-ketoglutarate
enters the Krebs cycle; and the ammonia is converted to urea in the urea cycle.
this reaction ammonia is converted to urea, ready for excretion by the kidney.
is highly toxic, due to the reversal of the glutamate dehydrogenase reaction
that would use up all the a-ketoglutarate
and so stop the Krebs cycle. Urea is less toxic than ammonia, so it is safer to
have in the bloodstream. The disadvantage is that it “costs” 3 ATP molecules
to make one urea molecule. This is not in fact a single reaction, but is a
summary of another cyclic pathway, called the urea cycle (or orthnthine
cycle). It was the first cyclic pathway discovered.