Homeostatic Control of Body Systems
How do internal and external influences affect homeostatic feedback mechanisms, which keep the
body's internal environment stable?
The body must maintain a state of equilibrium, which is otherwise known as ‘homeostasis’. Homeostasis
maintains the body’s internal conditions, in response to both internal and external factors. If the body does
not maintain optimum, internal conditions, such as temperature, pH or blood glucose, the body will not be
able to function as it should. Homeostasis consists of ‘loops’ of positive or negative feedback.
Positive feedback in homeostasis involves the receptor detecting a change in stimulus and the effector
increasing the stimulus to amplify the changes in the environment. Essentially, positive feedback aims to
move a system away from the equilibrium that homeostasis within the body is trying to maintain. These
loops of positive feedback can result in unstable conditions in some circumstances. However, they can also
be useful. An example of useful positive feedback is the onset of contractions during labour. During labour,
the baby pushes against the cervix which results in the stretching of the cervix. This causes nerve impulses
to be sent to the brain, and subsequently, the brain stimulates the posterior pituitary in order to secrete
oxytocin. The oxytocin causes the smooth muscle lining in the uterus to contract.
Oppositely, the negative feedback system involves the receptor detecting a change in stimulus and the
effector decreasing the stimulus in order to decrease the effector or stop it completely. The negative
feedback loop aims to move a system closer to the equilibrium that homeostasis is trying to maintain. With
negative feedback, the receptor detects a change in internal or external environment. Information is then
sent by the receptor through the nervous system, to either the brain or the spinal cord. The central control
(brain or spinal cord) instructs the effector to act accordingly. This could mean that hairs on the body stand
up in order to trap heat if the internal body temperature is too low. The stimulus is constantly monitored
by receptors in the body so that is remains at a set point.
Set Point (i.e. Feedback
Receptor Controller Effector
temperature) Fig. 1: The process of a feedbackLoop
loop
Why does the body need to maintain a constant internal environment?
If the body is to function properly, the conditions inside it must be carefully managed. The well-being of both
individual cells and the entire body depends on maintaining a stable internal environment by giving the cells with
what they require to survive (oxygen, nutrition, and waste elimination). Internal conditions must be kept within a
certain range for organisms to function. Homeostasis maintains ideal conditions for enzyme functioning and all cell
activities throughout the body. It is the preservation of a constant internal environment in the face of changing
internal and external circumstances.
Enzymes are proteins that catalyse (fasten) important chemical reactions within the body. Enzymes can only work
properly in a narrow set of circumstances, such as a specific pH and temperature. The enzymes are unable to
function if any conditions are outside of their particular range. This may result in the organism's death. Homeostasis
relies on automatic control systems that include neurological reactions (nervous system) or chemical responses
(endocrine system) (endocrine system). Also required are receptors and effectors.
,Thermoregulation
Thermoregulation in homeostasis is the regulation and control of the internal body temperature in response to both
internal and external environmental changes. It is vital that the internal body temperature is maintained at around
36.1 and 37.2 degrees Celsius. Biological and biochemical processes within the body usually have an optimum
temperature at which they work properly and as they should. For example, enzymes in the body perform as they
should when the body’s internal temperature is maintained at around 36 – 37 degrees Celsius. If the body
temperature suddenly drops or rises, enzymes required by the body to break down substances may denature,
leaving substances and molecules as they are. This can cause problems when it comes to digestion, breaking down
waste products within cells and much more.
Essential components which are used during thermoregulation are receptors, the coordinator, and the effectors. The
receptors are the components which detect the change in temperature, such as thermoreceptors in the skin and
thermoreceptors in the hypothalamus. Thermoreceptors within the skin are nerve cells which are specialised to
detect changes in temperature. There are heat receptors, which can be found nearer to the surface of the skin, and
there are cold receptors which are found deeper in the dermis of the skin. The location and quantity of
thermoreceptors determine the sensitivity of the skin to changes in the temperature. Therefore, our skin is more
sensitive to heat (encountering hot surfaces) and less sensitive to the cold as there are more heat receptors on the
surface of our skin than cold receptors.
What is the role of the hypothalamus in
thermoregulation?
The hypothalamus is where the thermoregulatory centre
is located. The hypothalamus is vital for controlling the
internal body temperature. The hypothalamus is divided
into the heat loss centre and the heat gain centre. When
hot environments are encountered, the heat loss centre
is activated in order to assist with cooling down the
body. When colder environments are encountered, the
heat gain centre is activated in an effort to warm up the
body. As well as receiving information from the skin’s
thermoreceptors, the hypothalamus has its own Fig. 2: The location of the hypothalamus
thermoreceptors.
Rather than detecting changes in external environments, the thermoreceptors within the hypothalamus detect
changes in the temperature of the blood. After receiving this information from the thermoreceptors, nerve impulses
are sent to the effectors by the hypothalamus to return the body temperature back to normal.
Effectors include the hair erector muscles, sweat glands, skeletal muscles, glands, and arterioles which supply skin
capillaries with blood. These effectors are involved in the cooling and heating mechanisms which occur during
thermoregulation.
When thermoreceptors in the skin detect an increase in temperature (heat), the hypothalamus must send a nerve
impulse to the effectors to engage in a cooling mechanism. One way in which the body cools down is by relaxing the
muscles within the walls of arterioles which allows for vasodilation (blood vessels dilating). This results in an
increased blood flow to the capillaries on the surface of the skin which leads to increased heat loss. Hair erector
muscles will also relax. This makes them lie flat which prevents them from trapping any heat. Sweat glands produce
more sweat. This is because heat is absorbed by sweat and is carried away from the body when the sweat
evaporates.
When thermoreceptors in the skin detect a decrease in temperature (cold), the hypothalamus must send a nerve
impulse to the effectors to engage in a heating mechanism. One way in which the body increases its temperature is
by contracting the muscles within the walls of arterioles which allows for vasoconstriction (blood vessels narrow).
This results in a decreased blood flow to the capillaries on the surface of the skin which leads to decreased heat loss.
, Hair erector muscles will contract. This makes them stand up which allows them to trap heat. Sweat glands produce
less or no sweat. This way, there is less sweat to absorb heat, allowing the body to retain more heat.
Fig. 3: Thermoregulation Process
