Communication and homeostasis

All living things need to maintain a certain limited set of conditions inside their cells. These include:

• a suitable temperature
• a suitable pH
• an aqueous environment that keeps the substrates and products in solution
• freedom from toxins and excess inhibitors

Without these, the cells will become inactive and die. In multicellular organisms, cells are specialised and rely upon another; therefore they must be able to communicate in order to coordinate their activities.

All living things have an external environment that consists of the air, water or soil around them. This external environment changes, which may place stress on the living organism. For instance, a cooler environment will cause greater heat loss. If the organism is to remain active and survive, the changes in the environment must be monitored and the organism must change its behaviour or physiology to reduce the stress. The environmental change is a stimulus and the way in which the organism changes its behaviour or physiology is its response.

The environment may change slowly as the seasons pass. These changes elicit a gradual response. For example the arctic fox has a much thicker white coat in the winter, and a thinner grey/brown coat in the summer. The change in the coat provides greater insulation and camouflage in the winter, ensuring the animal can survive. Yet in the summer, the animal doesn't overheat.

However, the environment may change much more quickly. The appearance of a predator or moving from a burrow into the sunlight are rapid changes. Again, the stimulus must be monitored and the organism must respond to the change.

Most multicellular organisms have a range of tissues and organs. Many of the cells and tissues are not exposed to the external environment- they are protected by epithelial tissues and organs such as skin or bark. In many animals the internal cells and tissues are bathed in tissue fluid- this is the environment of the cells. As cells undergo their various metabolic activities, they use up substrates and create new products. Some of these compounds may be unwanted or even toxic. These substances move out of the cells into the tissue fluid. Therefore, the activities of the cells alter their own environment.

For example, one waste product is CO2. If this is allowed to build up in the tissue fluid outside cells, it will alter the pH of the tissue fluid and could disrupt the action of enzymes and other proteins. The accumulation of excess waste of toxins in this internal environment must act as a stimulus to cause removal of these waste products so that the cells can survive. In this example, the reduced pH of the blood stimulates greater breathing activity that expels CO2 from the body.

This build up of waste products in the tissue fluid may also act directly on cells, which respond by reducing their own activities so that less waste is produced. However, this response may not be good for the whole organism.

A multicellular organism is more efficient than a single celled organism, because its cells are differentiated. This means that its cells are specialised to perform particular functions. Groups of cells specialised to perform particular function form tissues and organs.

The cells that monitor the blood may be in a different part of the body well away from the source of the waste product. They may also be some distance from the tissue or organ specialised to remove the waste from the body. Therefore, a good communication system is required to ensure that these different parts of the body work together effectively.

A good communication system will:

•cover the whole body

•enable cells to communicate with each other

•enable specific communication

•enable rapid communication

•enable both short-term and long-term responses

The 2 major systems of communication that work by cell signalling are:

• neural system- an interconnected network of neurones that signal to each other across synapse junctions. The neurones can conduct a signal very quickly and enable rapid responses to stimuli that may be changing quickly.

• hormonal system- a system that uses the blood to transport its signals. Cells in an endocrine organ release the signal (a hormone) directly into the blood. The hormone is transported throughout the body, but is only recognised by specific target cells. The hormonal system enables longer-term responses to be coordinated. 

Homeostasis is used in many living organisms to maintain conditions inside the body, around a set point within a narrow range, despite changes in external and internal factors.

Aspects maintained by homeostasis may include:

•body temperature

•blood glucose concentration

•blood salt concentration

•water potential of the blood

•blood pressure

•carbon dioxide concentration

Any response to changes in the environment requires a complex mechanism, which may involve a series of tissues and organs that are coordinated through cell signalling. The standard pathway is:

Stimulus --> receptor --> communication pathway --> effector --> response

Sensory Receptors: Thermoreceptors, Baroreceptors, etc. One of these receptors detect a change it will be stimulated to send a message to an effector.

Communication System: Acts by signalling between cells. It is used to transmit a message from the receptor cells to the effector cells via a coordination centre which is usually in the brain. The messages from the receptor to the coordination centre are known as the input. The messages sent to the effectors are known as the output.

Effector Cells: These cells will bring about a response e.g. the liver cells and muscle cells.

When the effectors respond to the output from the coorindation centre, they bring about a response that will change the conditions inside the body.

Such changes will be detected by the receptors.

This will have an effect upon the response pathway.

In effect, the input will change.

This is known as feedback.

In order to maintain a constant internal environment, any change away from optimum must be reversed. In this way, conditions inside the body will be returned to the optimum. This mechanism that brings the condition back towards the optimum is known as negative feedback.

1. When conditions change, the receptors detect this stimulus and send an input to the coordination centre.

2. The coordination centre sends an output to the effectors and the effectors respond to this output.

3. When the effectors bring about a change that reverses the initial change in conditions, the system moves closer to the optimum and the stimulus is reduced.

4. The receptors detect a reduction in the stimulus and reduce the input to the coordination centre.

5. The output from the coordination centre to the effectors is also reduced, so the effectors reduce their activity. As the system gets closer to the optimum, the response is reduced.

When positive feedback occurs, the response is to increase the original change. This destabilises the system and is usually harmful.

For example, below a certain core body temperature enzymes become less active and the exergonic reactions that release heat are slower and release less heat. This allows the body to cool further and slows the enzyme-controlled reactions even more. This causes the body to spiral downwards. 

There are some occasions when positive feedback can be beneficial, it is used to stimulate an increase in a change. 

An example is seen at the end of pregnancy to bring about dilation of the cervix. As the cervix begins to stretch this causes the posterior pituitary gland to secrete the hormone oxytocin. Oxytocin increases the uterine contractions which stretch the cervix more, which causes secretion of more oxytocin. Once the cervix is fully dilated, the baby can be born. The birth ends the production of oxytocin. The activity of neurones also relies on positive feedback.

Endotherms control their body temperature within very strict limits. They use a variety of mechanisms to control body temperature and are largely independent of external temperatures.

Ectotherms are not able to control their body temperature as effectively as endotherms. They rely on external sources of heat and their body temperature fluctuates with the external temperature. However, using various behavioural mechanisms, some ectotherms are able to control their body temperature in all but the most extreme conditions.

Ectotherms do not use internal energy sources to maintain their body temperature. However, once they are active their muscle contractions will generate some heat from increased respiration. Temperature regulation relies upon behavioural responses that can alter the amount of heat exchanged with the most extreme conditions.

If ectotherms are not warm enough, they try to absorb more heat from the environment. They may:

•move into a sunny area

•lie on a warm surface

•expose a larger surface area to the sun

If they are too hot:

•move out of the sun

•move underground

•reduce the body surface exposed to the sun

Basks in the sun. In the UK, adders can often be found lying on an exposed path beside vegetation.

Absorbs heat directly from the sun.

In the early morning, locusts sit side-on to the sun exposing a large surface area, but at midday they face the sun head-on exposing a smaller surface area. They may also climb to the top of a plant at midday to get away from the soil surface.

Increases the rate and depth of breathing when it is hot.

In the cool morning they can absorb more heat, but at midday when the sun is hotter they absorb less heat. The soil surface gets hot and radiates heat; if the locust moves away from the soil it gains less heat from the soil.

Lizards- Many lizards use burrows or crevices between rocks. They will hide in the burrow during the hottest part of the day and the coolest part of the night. 

An underground burrow tends to have a more stable temperature than the air. In the hottest part of the day it will be cooler in the burrow, but at night the burrow may be warmer than the air outside.

Horned lizard- Can change its shape by expanding or contracting its ribcage.

Expanding the ribcage increases the surface area exposed to the sun, so more heat can be absorbed.

Ectotherms rely on external sources of heat to keep warm. They do not use up energy to keep warm therefore:

•Less of their food is used in respiration

•More of the energy and nutrients gained from food can be converted to growth

•They need to find less food

•They can survive for long periods without food

They are less active in cooler temperatures.

This means that they are at risk from predators while they are cold and unable to escape.

They can not take advantage of food that is available while they are at cold.

They do not rely on external heat sources of heat- they can use physiological and behavioural adaptions means to control their body temperature.

Temperature regulation on effectors in the skin and muscles. The skin is the organ in contact with the external environment. Therefore, many of the physiological adaptions to control body temperature involve the skin. The changes that take place in the skin alter the amount of heat being lost to the environment.

Many chemical reactions in the body are exergonic- they release energy in the form of heat. Endotherms can increase respiration in the muscles and liver simply to release heat- they are using some of their energy intake to stay warm. They also have other physiological mechanisms, such as directing blood towards or away from the skin to alter the amount of heat lost to the environment.

Too hot:

•Sweat glands secrete fluid onto the skin surface; as this evaporates it uses heat from the blood as the latent heat of vaporisation.

•Hairs and feathers lie flat to reduce insulation and allow greater heat loss.

•Vasodilation of arterioles and precapillary sphincters directs blood to the skin surface so more heat can be radiated away from the body.

Too cold:

•Less sweat is secreted, so less evaporation means less heat is lost.

•Hairs and feathers stand erect to trap air, which insulates the body.

•Vasoconstriction of arterioles and precapillary sphincters leading to skin surface. Blood is directed away from the surface of the skin and less heat is lost.

Too hot:

•Some animals pant, increasing evaporation of water from the surface of the lungs and airways. Evaporation uses heat from the blood as the latent heat of vaporisation.

Too cold:

•Less panting, so less heat is lost.

Too hot:

•Less respiration takes place, so less heat is released.

Too cold:

•Increased respiration in the liver cells means that more energy from food is converted to heat.

Too hot:

•Fewer contractions means that less heat is released.

Too cold:

•Spontaneous muscle contractions (shivering) release heat.

Too hot:

•Dilation to direct blood to the extremities so that more heat can be lost.

Too cold:

•Constriction to limit blood flow to the extremities, so that blood is not cooled too much- this can lead to frostbite in extreme conditions.

Too hot:

• hide away from sun in the shade or in a burrow
• orientate body to reduce surface area exposed to sun
• remain inactive and spread limbs out to enable greater heat loss
• wet skin to use evaporation to help cool the body. Cats lick themselves and elephants spray water over their bodies.

Too cold:

• Lie in sun
• orientate body towards sun to increase surface area exposed
• move about to generate heat in the muscles or, in extreme cases, roll into a ball shape to reduce surface area and heat loss
• remain dry

•Maintain a fairly constant body temperature whatever the external temperature.

•Remain active even when external temperatures are low, which means they can take advantage of prey that may be available or escape from potential predators.

•Inhabit colder parts of the planet.

They use a significant part of their energy intake to maintain body temperature in the cold.

•Need more food.

•Use for growth a lower proportion of the energy and nutrients gained from food.

•May overheat in hot weather.

The maintenance of a core body temperature is important, if the core temperature changes this alters the temperature of the blood.

Temperature receptors in the hypothalamus of the brain detect this change. The hypothalamus then sends out an impulse that causes different responses to reverse the change. Some responses need to be quick in order to prevent further change in the body temperature- the neuronal system transmits the output from the hypothalamus in order to make these responses rapid. Other responses may need to be longer term; the hormonal system transmits the output to cause these reactions.

If the core temperature is too low, the hypothalamus will bring about:

• changes in the skin to reduce heat loss
• relrease of heat through extra muscular contraction
• increased metabolism in order to release more heat from exergonic reactions.

If the core temperature rises above the optimum the hypothalamus will bring about the opposite changes, this is an example of negative feedback.

The thermoregulatory centre in the hypothalamus monitors blood temperature and detects changes in the core body temperature.

However, an early warning that the body temperature may change could help the hypothalamus to respond more quickly and reduce variation in the core body temperature.

Peripheral temperature receptors in the skin monitor the temperature in the extremities.

This information is fed to the thermoregulatory centre in the hypothalamus.

If the thermoregulatory centre signals to the brain that the external environment is very cold or very hot, the brain can initiate behavioural mechanisms for maintaining the body temperature, such as moving to shade.