Homeostasis

toc

Introduction

This topic covers a human body system, Homeostasis, which literally means “same state” and it refers to the process of keeping the internal body environment in a steady state, when the external environment is changed.

The importance of this steady state cannot be over-stressed, as it allows enzymes etc to be ‘fine-tuned’ to a particular set of conditions,and so to operate more efficiently. When enzymes can't work efficiently cell processes don't work properly. What actually happens to the whole body when the cells can't function properly? This is why homeostasis is central to survival.

Play one of these simple games - [|Body Control Center]

[|Bens Bad Day]

just to demonstrate how much has to be co-ordinated.

There are simple steps to keep homeostasis in body cells. This video http://www.youtube.com/watch?v=QKT47A-LBj4 takes 20 minutes, it is a good documentary beginning for our topic. For your assessment you will need a lot more detail than is covered here but it is a great starting point.

Negative feedbackOne mechanism used to maintain a constant value (called the set point) is called negative feedback. This is the most important point in this topic! Negative feedback means that whenever a change occurs in a system, this automatically causes a corrective mechanism to start, which reverses the original change and brings the system back towards the set point (i.e. ‘normal’).

http://www.youtube.com/watch?v=bV1oY6V105g This is a short lecture (3minutes)about stabilising systems from using negative feedback.

It also means that the bigger the change the bigger the corrective mechanism. Negative feedback applies to electronic circuits and central heating systems as well as to biological systems. When your oven gets too hot, the heating switches off; this allows the oven to cool down. Eventually it will get too cold, when the heating will switch back in, so raising the temperature once again. So, in a system controlled by negative feedback, the set level is never perfectly maintained, but constantly oscillates about the set point. An efficient homeostatic system minimises the size of the oscillations. Some variation must be permitted, however, or both corrective mechanisms would try to operate at once! This is particularly true in hormone-controlled homeostatic mechanisms (and most are controlled by hormones), where there is a significant time-lag before the corrective mechanism can be activated. This is because it takes time for protein synthesis to commence and build the hormone, and the hormone to diffuse into the blood-steam, and for it to circulate around the body and take effect. http://www.biology-online.org/4/1_physiological_homeostasis.htm has more information about negative feedback.

Key Terms A negative feedback loop works by giving information (feedback) back to the organ measuring the environment (the receptor), if it senses an increase it causes a body part (an effector) to make a change which lowers the increasing effect, then it measures this decrease and to stop it going too low (from that previous feedback) the receptor causes that change to stop. It is a continuous measuring and responding process.

There are key words describing specific parts of a feedback loop, the receptor, the stimulus, the effector and the response. These parts work together in the loop.

Mr Anderson does a great job of explaining feedback loops at his site, open these next two links in another tab. Watch them both, it will take a while but it is worth it. Then you can make your own examples for your own power point explanation of homeostasis feedback mechanisms. This is a group project to explain the key terms.

http://www.youtube.com/watch?v=CLv3SkF_Eag Positive and Negative feedback Loops.

http://www.youtube.com/watch?v=q_e6tNCW-uk Elements of a feedback Loop.

__Human body systems of interest. __There are five main systems that are managed by a negative feedback system, body temperature, body fluid composition, blood glucose, gas concentrations in the blood, blood pressure.

Temperature Homeostasis (thermoregulation) One of the most important examples of homeostasis is the regulation of body temperature. Not all animals can do this physiologically.Others rely on the environment (which restricts their choice of habitat a lot) or on behavioural changes.

Endotherms or ectotherms Animals that maintain a fairly constant body temperature (birds and mammals) are called endotherms, while those that have a variable body temperature (all others) are called ectotherms. Endotherms normally maintain their body temperatures at around 35 - 40°C, so are sometimes called warm-blooded animals, but in fact ectothermic animals can also have very warm blood during the day by basking in the sun, or by extended muscle activity (e.g. bumble bees, tuna). So don't use warm and cold blooded terms anymore, okay. It gives an incorrect impression.

The difference between the two groups is that endothermic animals use internal corrective mechanisms, whilst ectotherms use behavioural mechanisms (e.g. lying in the sun when cold, moving into shade when hot). These behavioural or external mechanisms can be very effective, and when coupled with internal mechanisms ensure that the temperature of the blood going to vital organs (brain, heart) is kept constant. We use both! Think about how you react when the day gets too hot at lunch time, you start to sweat and you move to sit in a shady place.

Hypothalamus In humans, body temperature is controlled by the thermoregulatory centre in the hypothalamus.

Follow this link to http://endocrine101.wikispaces.com/Hypothalamus

There is a video here that describes the action of the hypothalamus, it is one of the most primitive areas of the brain, essential for core functions of survival.

The hypothalamus receives input from two sets of thermoreceptors: receptors in the hypothalamus itself monitor the temperature of the blood as it passes through the brain (the core temperature), and receptors in the skin (especially on the trunk) monitor the external temperature.

Both sets of information are needed so that the body can make appropriate adjustments.

The thermoregulatory centre sends impulses to several different effectors to adjust body temperature: strategies include vasoconstriction or vasodilation, shivering or sweating, hair raising or hair lowering, depending on which way the temperature needs to be adjusted.

Our first response to encountering hotter or colder conditions is voluntary - if too hot, we may decide to take some clothes off, or to move into the shade; if too cold, we put extra clothes on - or turn the heater up! It is only when these responses are not enough that the thermoregulatory centre is stimulated. This is part of the autonomic nervous system, so the various responses are all involuntary. When we get too hot, the heat loss centre in the hypothalamus is stimulated; when we get too cold, it is the heat conservation centre of the hypothalamus which is stimulated. Note that some of the responses to low temperature actually generate heat (thermogenesis), whilst others just conserve heat. Similarly some of the responses to heating actively cool the body down, while others just reduce heat production or transfer heat to the surface. The body thus has a range of responses available, depending on the internal and external temperatures.

What are the actual responses of the body?

This youtube clip explains vasoconstriction in simple terms.media type="custom" key="24942776"

__How does Homeostasis work right down at the cellular level?__ Cell chemical reactions are really a series of enzyme controlled pathways, enzymes require the best conditions to be able to function at the best rate. Other raw materials are the substrate for the chemical reactions so these need to be correct as well. These are called the physiology of the cell, this very good video explains how the cells are involved in your understanding of homeostasis.

media type="custom" key="24942798"

These metabolic pathways are what go wrong when people die of fever or hypothermia. The thermoregulatory centre normally maintains a set point of 37.5 ± 0.5 °C in most mammals. However the set point can be altered in special circumstances: • Fever. Chemicals called pyrogens released by white blood cells raise the set point of the thermoregulatory centre causing the whole body temperature to increase by 2-3 °C. This helps to kill bacteria, inhibits viruses, and explains why you shiver even though you are hot. A mild fever is an immune response to stop a bacteria based infection, although remember a fever that stays too high can damage the cell activity permanently. This link gives just a bit more information about how homeostasis works to protect the body and how sometimes a fever can be helpful.

http://www.youtube.com/watch?v=YWee_X63mEs
• Hibernation. Some mammals release hormones that reduce their set point to around 5°C while they hibernate. drastically reduces their metabolic rate and so conserves their food reserves e.g. hedgehogs. • Torpor. Bats and hummingbirds reduce their set point every day while they are inactive. They have a high surface area/volume ratio, so this reduces heat loss. Adaptive advantage

Homeostasis has survival value because it means an animal can adapt to a changing environment . When internal or external factors vary, 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.

Enzymes work at optimum temperatures. If the cell temperature is too high,the enzymes denature and will need to be rebuilt to be able to complete the task they are specifically designed to do.If the cell temperature is too low, the enzymes rate of reaction is lowered and the rate of the cell metabolism slows right down. These enzymes will work again if the temperature increases sufficiently. A rise of just 2 degrees can cause disruption to cell activity, up to43 - 45 degrees can kill, while below 23 degrees can cause death.

All homeostatic mechanisms work to restore balance.

At the cellular function level, one part of the cell biochemistry affected by the hormones is the permeability of the cell membrane. Think about why this would affect the rate of reactions being carried out by the cell organelles. What do you know about the organisation of the cell membrane? How do the pump action and the concentration of various ions on either side of the membrane affect the cells ability to do chemical reactions?



[[image:opotikicollegebiology/titanic.png width="447" height="312" align="right"]]
On Monday 15th April 1912 2207 people were on the Titanic when it hit an iceberg. 1502 people died, despite the weather being calm and many having life jackets on. Why did they die? The water was less than -2 degrees. People were in the water waiting to be rescued for over 1 hour. What effect did this have on cell activity?

[[image:opotikicollegebiology/triathlete distress.jpg width="392" height="268" align="left"]]
== This athlete has used several external mechanisms to protect themselves from excessive temperature variation. What does she risk if she continues to lose body heat? What systems will her body use to generate the heat to return her system to the normal situation? ==

This clip describes the experience of climbers who survived hypothermia. They were relying on equipment to keep their body temperature at normal levels. http://www.youtube.com/watch?v=0Y1Rog_d0dM

Your job is to understand the actual biology that happened in their body when the behavior and equipment they were expecting to use to maintain their body temperature failed them.

In New Zealand in 2013 a young couple died on Mt Taranaki. http://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=11147301 in another tragic example of failed equipment in extreme environments.

This website has the same stuff about Homeostasis, maybe a few different links. Use it. [|Pass Biology.co.nz]