 How do we warm ourselves up when we are cold? And how do plants maintain the correct balance of water? Let's take a look at homeostasis and tolerance limits to find out. Homeostasis is the outcome of the process by which an organism is able to maintain its internal state in response to changing external or environmental conditions. This includes maintaining blood glucose levels, carbon dioxide levels, water balance in both plants and animals, and regulating body temperature. The exact mechanisms for these processes will be explored in later videos in this module. This process is only able to be conducted when they are within the organism's tolerance limits, the minimum and maximum limit of unfavorable conditions within which the organism can survive. This varies for all species, and to an even smaller extent from organism to organism. For example, an Arctic fox will have a much lower maximum temperature for its tolerance limit than a camel, who is usually found in warmer desert conditions. The underlying purpose of homeostasis is to allow the optimum functioning of metabolic processes, or otherwise the chemical reactions occurring within the body to maintain life, such as those required to provide the organism with energy. There are two types of metabolic reactions that occur within our bodies. The first are known as catabolic reactions, which are reactions that break down complex molecules, releasing energy in the process. This would be a reaction such as the breakdown of glucose in the food you eat to create carbon dioxide and water during cellular respiration. The second type of reactions are the anabolic reactions. These use smaller building blocks and require the input of energy to create a more complex molecule. This occurs when carbon dioxide and water are combined in photosynthesis, using light energy to create glucose. The process of maintaining homeostasis is often known as a stimulus response model. This model acts as a cycle, where a stimulus will either increase or decrease in the internal environment of an organism. This can be sometimes due to changing external conditions such as temperature or an increase in glucose within the body. A receptor within the organism will detect this change in stimuli and the information is sent to a control centre. Here at the control centre, the change is evaluated and a signal is sent to the effector to coordinate a response. In humans, the control centre is generally in the brain, or spinal cord, and invertebrates, receptors and effectors are linked by nervous and hormonal pathways, depending on the stimulus response pathway at hand. The effector, typically a muscle or a gland, will receive the signal from the control centre and will change its output to coordinate a response. This response is the action taken to return the internal conditions to their optimum, or at least to a level which is more suitable for organism functioning. This overall process is a continuous cycle using feedback mechanisms in order to ensure the optimum conditions for the organism are continuously maintained. There are two types of response which determine how the organism responds to the change in stimuli. The first is negative feedback, which is where a change in stimuli is detected and the organism initiates a response which reverses the change in order to maintain internal conditions within their tolerance limits. This is what we see in thermoregulation, insulin release in blood glucose regulation, as well as in osmoregulation which will be explained in later videos. The second type of response is positive feedback, which is less commonly seen in biological systems. Positive feedback initiates a response which amplifies the stimulus, causing it to vary even more from the initial deviation. For example, we see this in childbirth with the release of oxytocin promoting the release of even more oxytocin, allowing for faster and stronger contractions in order to speed up the process of labour. So in summary, homeostasis allows both plants and animals to maintain their internal environment, including blood glucose and carbon dioxide levels, water balance and regulating body temperature. This is to ensure that metabolism can occur at an appropriate rate for the functioning of the organism within that particular organism's tolerance limits. The process of maintaining homeostasis is known as a stimulus response model which outlines the steps required for the organism to initiate a response. The cycling of this system produces either positive feedback systems which amplify the stimulus or negative feedback systems which reduce it.