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Summary 14 Homeostasis
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EVERYTHING you need to know about "Homeostasis" for A-level Biology CIE
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Homeostasis10 September 2021 09:18
• Homeostasis is the maintaining of a relatively constant internal environment for cells in the body
despite changes in the external environment
• In mammals the internal environment is the circulating blood and the tissue fluid which bathes the
cells
• Body temperature, blood glucose concentration, water potential, ion levels, and CO2 levels are all
regulated to keep cells at their optimal conditions
• At an organism level, homeostasis allows mammals to occupy more niches and out-compete 'non-
regulators' who cannot control their internal environment
How is Homeostasis Achieved?
• Negative feedback, the mechanism behind homeostasis
• A process in which a change in some parameter, such as blood glucose level, brings about processes
which move its level back towards normal again
• Eg, conditions are returned to a set value having deviated from it
How Does it Work?
• Each condition (eg, temperature) has a set point
• Receptors detect any change away from the set point
• The receptor sends a nerve impulse (or releases a hormone) to the coordinator, which compares the
data from the receptor to the set point
• If the condition deviates too far from the set point, the co-ordinator sends a nerve impulse (or
releases a hormone which travels in the blood) to an effector (a muscle or gland)
• When the impulse or hormone reaches the effector, the effector creates a response (a corrective
action) which returns the temperature to the set point
• How precisely the variable is maintained depends on the sensitivity of the receptor, so there is usually
some level of fluctuation about the set point
Nervous vs Hormonal Control
• Homeostasis requires different cells and organs in the body, such as receptors, co-ordinators, and
effectors, to communicate with each other. Information passes between these areas by nerve
impulses through the nervous system or by hormones released by organs of the endocrine system
Coordinating Nervous and Endocrine Control:
• Although the nervous and endocrine system work in different ways, many of their actions are
coordinated by the pituitary gland
• The pituitary glands sits underneath the hypothalamus and the hypothalamus secretes hormones that
regulate the functioning of the pituitary
• The hypothalamus also monitors the levels of hormones in the blood and uses negative feedback to
regulate
Thermoregulation
• Mammals are endotherms ('heat inside')
• A healthy human maintains core body temperature in a small range around 37°C
• Most heat comes from metabolic reactions which generate heat as a waste product, which is then
carried by the blood throughout the body
• At rest, about 70% of our heat comes from abdominal organs, eg, heart, liver, kidneys, lungs. The brain
is also very 'active'
The thyroid gland produces a hormone called thyroxine, which increases basal metabolic rate, so by
altering thyroxine secretion, the body can change temperature
• Increased adrenaline secretion increases the rate of heat production in the liver
• In addition during intense exercise, muscle contraction of the skeletal muscles releases heat
• Shivering involves using muscle contraction to release heat too, but in a non-coordinated manner
• The control centre for thermoregulation is the hypothalamus
• It consists of a 'heat loss centre' and a 'heat gain centre'
• It contains temperature sensitive neurones known as thermoreceptors which are able to detect
changes in the temperature of the blood as it flows through the brain (internal stimuli)
• The hypothalamus also receives nerve impulses via sensory neurones from thermoreceptors in the
skin (external stimuli)
• The hypothalamus can then send nerve impulses to effectors in the rest of the body
• Heat is transferred between body and environment by conduction, convection, and radiation (plus
evaporation of water)
• Thermoregulation involves controlling the passage of heat across the body surface
• When the body temperature drops, the heat gain centre both inhibits the heat loss centre and sends
nervous impulses to the skin and elsewhere in the body to bring about the appropriate responses
The Skin as an Effector;
• The arterioles supplying the capillary networks in the skin can widen (vasodilation), allowing more
blood to flow through the capillaries, and so more heat is lost by radiation
• When the body needs to retain heat, the arterioles narrow (vasoconstriction), so less blood flows into
capillaries near the skin's surface, so less heat is lost by radiation
• When heat needs to be conserved, hair erector muscles contract, trapping a layer of insulating air.
They relax to flatten hairs when heat needs to be lost
• The sweat glands secrete sweat on the skin's surface when heat needs to be lost, but do not do so
when heat must be retained. The evaporation of sweat removes heat energy due to the high latent
heat of vaporisation of water
• Other effectors which contribute to thermoregulation:
- Enhanced respiration in brown adipose tissue (fat cells)
- Skeletal muscle contraction (shivering)
- Behavioural responses
Excretion
• The chemical reactions in cells produce by-products, many of which are toxic at high concentrations,
so need to be removed as part of homeostasis
• Excretion is defined as the removal of toxic or waste products of metabolism
• Amino acids which are excess to immediate requirements are carried to the liver in the blood plasma.
They are absorbed by liver cells and a series of chemical reactions known as the urea cycle (ornithine
cycle) begins:
- Under the control of an enzyme catalyst the amino acid is oxidised
- The amine group, -NH2, and a hydrogen atom, -H, are removed from the main structure of the amino
acid
- The amine group is reduced to ammonia (NH3), by the addition of this hydrogen atom
- This whole process is called deamination
- The non-nitrogenous portion of the molecule can be respired, or converted to carbohydrate or lipid
- Ammonia is highly toxic so, catalysed by enzymes in liver cells, carbon dioxide reacts with the
ammonia molecule
- Through a series of reactions the less toxic nitrogenous compound, urea, is produced together with
water
- CO2 + 2NH3 -> CO(NH2)2 + H2O
• Urea (carbamide) is the main nitrogenous excretory product of humans, but others can be made such
as uric acid, and creatinine
, Kidney Structure
17 September 2021 11:57
The renal artery enters the kidney, where it forms into
smaller arterioles and enters millions of nephrons. The
afferent arteriole enters the nephron which is held by
the Bowman's capsule and forms the glomerulus, a
capillary network. The narrower efferent arteriole runs
closely along the proximal convoluted tubule and loop of
Henle.
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