Regulation of Biological Systems

Physiological Homeostasis

Negative Feedback Control

In animals such as ourselves, the internal environment of our bodies must have certain conditions within tolerable limits to continue the healthy functioning of us.

This is done by a process called negative feedback control, where various receptors and effectors bring about a reaction to ensure that such conditions remain favourable. In this tutorial, we investigate the control of blood sugar concentrations, water concentrations and temperature.

The principle of negative feedback control is illustrated by the diagram below

Principle of Negative Feedback Control

This occurrence is known as physiological homeostasis, translating in layman's terms to the physical equilibrium. It is essentially a corrective mechanism, consider the following scenario in a person

Requirement of Negative Feedback Control

Because mammals are warm blooded, the enzymes that are part of their make-up as a warm blooded animal require a certain temperature to operate optimally. Also, the water concentration of a cell and its chemical concentration must remain at a certain level to allow normal cellular processes to occur.

In light of this, the feedback mechanism in such warm blooded animals is essential in regards to allowing the body to work in optimal conditions - so any change in from the norm in temperature is corrected by the feedback mechanism.

Advantages of Homeostasis

Homeostasis has survival value because it means an animal can adapt to a changing environment. It can deal with the temperature difference you face when you step our your front door.

The body will attempt to maintain a norm, the desired level of a factor to achieve homeostasis. However, it can only work within tolerable limits, where extreme conditions can disable the negative feedback mechanism

In these instances, death can result, unless medical treatment is executed to bring about the natural occurrence of these feedback mechanisms

The following page looks at regulation of blood sugar concentration and temperature regulation in detail.


Homeostasis of Organism Water Regulation

Osmoregulation

Osmoregulation is the regulation of water concentrations in the bloodstream, effectively controlling the amount of water available for cells to absorb.

The homeostatic control of water is as follows

Negative Feedback Control of Water in Animals

Evolutionary Adaptations in Water Regulation

Some of the tutorial pages in the adaptation tutorial investigate some of the evolutionary adaptations that organisms have achieved through natural selection. This looks at

More Homeostasis

The body also contains negative feedback control mechanisms for the control of blood sugar concentration and temperature regulation. These types of homeostasis are described on the next page.


Sugar Homeostasis

Blood / Sugar Regulation

As described in the cell biology tutorial, the body requires volumes of glucose in order to create ATP. The amount of ATP demanded will fluctuate, and therefore the body regulates the availability of glucose to maximise its energy making potential.

Two hormones are responsible for controlling the concentration of glucose in the blood. These are insulin and glucagon. The diagram illustrates the principle of negative feedback control in action involving blood/sugar levels.

Blood Sugar Regulation

Pancreas Receptors

The receptors of the pancreas are responsible for monitoring glucose levels in the blood, since it is important in every cell for respiration.

Two types of cell release two different hormones from the pancreas, insulin and glucagon. These hormones target the liver, one or the other depending on the glucose concentration

The Liver

The liver acts as a storehouse for glycogen, the storage form of glucose. When either of the above hormones target the liver, the following occurs

Diabetes

Diabetes insipidus is a condition where excess urine is excreted caused by the sufferers inability to produce ADH and promote the retention of water.

Diabetes Mellitus is another form of diabetes where the sufferer does not have the ability to produce sufficient insulin, meaning that glucose cannot be converted into glycogen. Anyone who has this condition usually has to take injections of insulin after meals and snacks to maintain their storage of glucose needed in emergencies.

Fight or Flight

In emergencies, adrenaline is released by the body to override the homeostatic control of glucose. This is done to promote the breakdown of glycogen into glucose to be used in the emergency. These emergencies are often known as 'fight or flight reactions'.

Adrenaline is secreted by the adrenal glands. The secretion of it leads to increased metabolism, breathing and heart rate. Once the emergency is over, and adrenaline levels drop, the homeostatic controls are once again back in place

The next page investigates temperature regulation in homeotherms


Temperature Regulation in Animals

Control of Temperature in Homeotherms

Animals capable of temperature regulation within a given range are deemed homeotherms (alternatively homiotherms or homotherms). They have the ability to regulate temperature via negative feedback control.

Temperature is controlled in a variety of ways in these animals.

The hypothalamus once again acts as a receptor in regulation, by detecting fluctuations in temperature. These receptors are better known as thermoreceptors.

Skin also possesses thermoreceptors which can detect the temperature of the external environment. This information is relayed to the hypothalamus which can in turn transmit nerve pulses for corrective mechanisms to occur

Corrective Mechanisms in Temperature Control

Increased sweating is a corrective response aimed to reduce the temperature of the organism. 

Vasodilation is a corrective response where the blood vessels close to the skin surface become more dilated, meaning their is a larger surface area for heat to be lost of the external environment from the blood vessel carrying over-heated blood.

Vasoconstriction is the opposite of this and occurs when temperatures in an organism drop. The blood vessels become constricted so that minimal heat loss occurs.

The hairs on your body also play an important role in temperature regulation. A corrective response can occur where the hairs 'stand on end', and trap a layer of air between the hair and the skin. This insulation of warmer air next to the skin reduces heat lost, while a thin layer of insulation would increase heat loss.

Other corrective mechanisms are involved, such as a drop in metabolic rate and shivering when temperatures drop. 

The previous 4 pages have described the importance for regulation and equilibrium internal to organisms. The next pages look at factors affecting populations as a whole by looking at how populations are affected


Population Regulation in an Ecosystem

Darwin focused some of this work in regards to the population size of a species, and what factors may affect them. He definedthese factors as environmental resistance, which can be split into two areas;

Density Dependant Factors

These types of factors are a direct result of a species' population size. Usually these factors arise because the population of the species have to compete for the same resources in a limited geographical niche. They can be as follows 

Shortage of Food

Members of a species will require a minimum amount of food in order to survive. These members may have to compete with fellow members of the same species (intraspecific competition), or may have to compete with other species (interspecific competition). As food will become limited when the population reaches a size, the death rate will increase until the food supply is sustainable for the population sizes continued survival.

Predators

An increase in population size will result in more prey for the species' predators, therefore more chance of that species' members of being killed. This is also the case if the predators' species population increases, as their will be a higher occurrence of both species coming across one another
Disease - A disease can spread rapidly across the densely populated area, especially across the same members of a species (Foot and Mouth for example). Disease it at its' most deadly when a species population is densely dispersed, and the distance between each of the diseases host means that it is easier for the disease to spread

Toxic Waste

Particularly important for organisms living in a small-enclosed ecosystem, a large population in a species means that the waste produced by them begins to come into play, as the waste increases the risk of disease, and reduces hygiene. This is still the case in some human societies, such as villages in Africa, which are over-populated, as sewage systems flow right through the living area, and is one of the main causes of death in these areas. 

Density Independent Factors

These factors occur regardless of a population size in a species. Usually these are environmental disasters, such as a forest fire, tidal wave etc that destroy the natural ecosystem that a species survives in. The result is that ecosystem can no longer sustain the species' population size and they begin to die out until the population size is at a more sustainable level. Human intervention also has increasingly played a part in determining the population of species, either via domestication or consequence from our actions. Pollution is such a factor that is not determined by a species' population, but by its external environment.

However, in Darwin's work, he identified that species tend to produce more offspring than the external environment can support, so effectively, homeostatic control comes into place as the maximum population of a species is the 'norm', taking into account that one of a species primary reasons for being alive is to reproduce.

However, with this in hand, some species are unable to increase their population levels through breeding. This is most usually because of climatic change, or environmental damage brought about by the Industrial Age and human intervention since then. For example, some rare birds do produce offspring, but since they are rare, the material value of their eggs is high, so opportunistic humans steal them in the attempt to profit. Species living in the tropical rain forests may also face danger soon, because of the alarming rate that their ecosystem is being chopped down for the material value that wood has to humans.

Such actions endanger species towards extinction, where the population size of a species becomes critically low. It is essential to keep the balance of an ecosystem, and the extinction of a species can unbalance the food chain in an area, and directly put other species in the ecosystem under threat, and possibly become the beginning of a chain reaction. As we begin the 21st century, humans have realised the importance of maintaining and preserving the world we live in, and to ensure that we are not unintentionally destroying natures' species, scientists use indicator species which indicate the state of an ecosystem. 

Humans rely on many species for our own survival, most notably as a food source. Although we wish to yield an economy of scale when we are selling these foodstuffs', part of the population must be left to reproduce in order to preserve the long term survival of the species, and stay as a in-exhaustible source of food. Species can not be exploited too much, as excessively hunting them can damage the genetic diversity of them, which poses a greater risk to their long term survival.


Indicator Species and Endangered Species

Indicator Species

Certain species are capable of expressing characteristics that can indicate the state of the ecosystem they currently occupy. These species that can leave clues about the state of the ecosystem are known as indicator species, because they indicate the state of the local environment.

Biston betularia, otherwise known as peppered moth, is a species that can adapt to polluted environments more suitably as a result of an adaptation changing the colour of them to suit their environment, as explained in the natural selection page of the genetics tutorial. Consider the following;

In light of this, various species exhibit characteristics that give us insight into the local environment without having to study the local environment itself. In the case of the peppered moth being an indicator species, the presence of pollution (and dark moth) would indicate that additional abiotic stress is being placed on the organisms who live in that polluted (and usually less favourable) environment.

Endangered Species

When the last of a species dies out, the gene pool of the species is lost forever. To protect species, we must monitor their population levels. If one species dies out, those who rely on it in one way or another (i.e. protection or food) will also be affected. In light of this, humans aim to preserve genetic diversity and the diversity of species alive today.

The Green Movement

Modern social values have led us to recognise the damage we have done to the environment and the species that occupy it. In light of this, government legislation is bound to environmental measures to prevent further damage to the environment. Green belts have also been established outside major cities, where no buildings can be erected. These green belts are basically segments of countryside that are preserved to 'compensate' the local environment for all the man-made interference that results from urban areas.

Ecosystem Succession

If the balance of nature is left untouched, landscapes can change dramatically over time, where a previous ecosystem is superceded by the arrival of a newer ecosystem. This is known as ecosystem succession, and is elaborated upon in the next page.


Ecosystem Succession

Just one of the amazing aspects of life on Earth is that it spreads to all areas where the habitat will allow it to survive. In hostile environments, where the soil is infertile, and therefore cannot sustain life in a sustainable fashion, small plants and bacteria begin to colonise the area, which recognise the area as a new ecological niche without competition.

While these small organisms begin to occupy the area, the pioneers, the organisms that first colonised the area begin to die out. This essential process allows the dead mass to decompose into the ground, and essentially provide nutrients to plants that continue to live in the habitat. Now that their is more nutrients available in the local environment, larger species are able to be supported, and this process can have a multiplying effect over the longer term. This process is called succession.

Succession usually occurs in areas where no other species offer competition in the area. Succession, however, can occur in changing climates, where less suited species give way to the more evolutionary adapted species of the area. In a more stable environment, a climax community develops, the climax being that the community stays in a relative equilibrium over a long period of time. Succession is no longer an issue as the most advanced species have established their presence and lesser organisms cannot compete.

The type of organisms' that occupy areas in such circumstances depends on a number of factors;

Climate

Mainly temperature and precipitation, varying species require various amounts of sunlight and temperature to operate at their optimum level. The organisms' most suited to a particular environment, and the ones that can be found in the vicinity are most likely to be the pioneers, or at the climax of succession

Soil

Soil also plays a major factor in what organisms' occupy an ecosystem. pH is one major factor, as some plants prefer and acidic environment to alkaline, or vice versa. The composition of the soil also plays a role in determining which species' are suitable, as the soil may be sandy, loamy or clay.

Human Intervention 

Humans' have altered the worldwide environment on a huge scale, particularly since the industrial age. Factors such as pollution, like acid rain, can alter the way an ecosystem operates and effect factors within it. With this in light, humans play a major role in which organisms' are likely to succeed in an ecosystem.


http://www.biology-online.org/4/1_physiological_homeostasis.htm