Summary
Biology OCR A level Homeostasis Summary Notes
Biology OCR A level Homeostasis Summary Notes
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1 of 9B15- Homeostasis
5.1.1 Communication and homeostasis
(a) The need for communication systems in multicellular organisms.
Key words:
Homeostasis- the maintenance of a stable equilibrium in the conditions inside the body.
• Homeostasis is essential in order to monitor changes in internal environment (pH of the blood,
core body temperature, and concentrations of ureas and sodium ions in the blood).
(b) The communication between cells by cell signalling.
• Nervous and hormonal systems coordinate the activities of whole organisms.
• Coordination relies on communication at a cellular level through cell signalling.
• Occurs through one cell releasing a chemical which has an effect on another cell, known as
target cell.
Through this process, cells can:
• Transfer signals locally, using a neurotransmitter.
• Transfer signals across large distances, using hormones.
(c) The principle of homeostasis.
To include the differences between receptors and effectors, and the differences between negative
feedback and positive feedback.
Key words:
Effectors- the muscles or glands that react to the motor stimulus to bring about a change in
response to a stimulus.
Receptors- detect changes in the internal and external environment of an organism.
• Receptors detect changes in the internal and external environment of an organism whereas
effectors are the muscles or glands that react to the motor stimulus to bring about a change in
response to a stimulus.
Positive feedback Negative feedback
• A change in the internal environment of the body is • A small change to ideal conditions is detected by
detected by sensory receptors, and effectors are sensory receptors. Effectors work to reverse the
stimulated to reinforce change and increase the change and restore conditions to their base level.
response. • Works to reverse initial stimulus.
• Conditions change- change detected- responses • Increase/decrease detected- responses raise/
reinforce change lower levels- ideal conditions met
• E,g, Blood clotting cascade • E.g. Blood sugar levels by insulin and glucagon
• E.g. Child birth, head of the baby presses against
the cervix, stimulating the production of oxytocin.
(d) The physiological and behavioural responses involved in temperature control in ectotherms
and endotherms.
Key words:
Endotherms- animals that rely on their metabolic processes to warm their bodies and maintain
their core temperature.
Ectotherms- animals that use their surroundings to warm their bodies so their core temperature
is heavily dependent on the environment.
Thermoregulation- the maintenance of a relatively constant core temperature.
Vasodilation- the cooling down of core body temperature by arterioles near the the surface of the
skin dilating.
Vasoconstriction- the warming of core body temperature by arterioles near the surface of the
skin constricting.
Aestivation- opposite of hibernation. A period of prolonged or deep sleep during extreme heat/
dryness in order to prevent heat loss.
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• Endotherms use their internal exothermic metabolic activities to keep them warm and energy
requiring physiological responses to keep them cool.
Detecting temperature changes
• Peripheral temperature receptors in the skin detect changes in the surface temperature.
• Temperature receptors in the hypothalamus detect the temperature of the blood in the body.
• Combination of the two allows the body to have great sensitivity and allows it to respond to
changes in temperature of the blood and preempt possible problems from changes in the
external environment.
Cooling down
• Vasodilation: arterioles near the surface of the skin dilate when the temperature rises. The
vessels provide a direct connection between the arterioles and the venules constrict. Forcing
blood through the capillary networks close to the surface of the skin. Skin flushes and cools as
a result of increased radiation.
• Increased sweating: sweat spreads out across the surface of the skin. As the sweat evaporates
from the surface of the skin, heat is lost, cooling the blood below the surface. Some animals
open their moves and pant when they get hot losing heat as the water evaporates.
• Reducing the insulating effect of hair or feathers: the erector pili muscles (hair erector muscles)
in the skin relax- as a result, the hair or feathers of the animal lie flat to the skin, avoiding
trapping an insulating layer of air. Those that live in hot climates often have anatomical
adaptations.
Warming up
• Vasoconstriction: arterioles near the surface of the skin constrict. The arteriovenous shunt
vessels dilate, so very little blood flows through the capillary networks close to the surface of
the skin. Skin looks pale and little radiation takes place.
• Decreasing sweating: rates of sweating decrease and sweat production will stop entirely. This
greatly reduces cooling by the evaporation of water from the surface of the skin, although some
evaporation from the lungs still continues.
• Raising the body hair or feathers: erector pili muscles in the skin contract, pulling the hair or
feathers of the animal erect. This traps an insulating layer of air and so reduces cooling through
the skin.
• Shivering: rapid, involuntary contracting and relaxing of the large voluntary muscles in the body.
The metabolic heat from the exothermic reactions warms up the body instead.
• Endotherms living in the cold often have specific adaptations designed to reduce energy loss
including a low SA:V, e.g. polar bears have small ears, hollow hairs to permanently trap heat.
Changes to heating up/cooling down depend upon:
• Exothermic chemical reactions.
• Latent heat of evaporation- objects cool down as water evaporates from the surface.
• Radiation- the transmission of electromagnetic waves to and from the air, water or ground.
• Convection- the heating and cooling by currents of air/water, warm air/water rises and cooler
air/water sinks setting up convection currents around an organism.
• Conduction- heating as a result of the collision of molecules.
Endotherms- vertebrate Ectotherms- invertebrate (fish/
(mammals and birds) reptiles)
Physiological • Peripheral temperature • Body temperature is heavily
receptors dependent on their
• The role of the hypothalamus environment.
• Effectors in skin and muscles • Those living in water don’t need
• Vasodilation/constriction to thermoregulate as the high
• Increasing/decreasing sweating heat capacity of water means
• Raising/lowering hair to temperature doesn’t change.
increase/reduce insulating
effect.
• Shiveing
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