Comparison of MIPAMS with other initiatives

A uterus B cervix

C fallopian tube D vagina

2. How long is the average human pregnancy?

A 30 weeks B 35 weeks C 40 weeks D 45 weeks

3. Which of the following is NOT a way to help prevent the spread of HIV/AIDS?

A washing your hands after using the toilet.

B using a condom when you have sex.

C having only one sexual partner.

D not sharing needles for intravenous drug use.

3.5 Homeostasis

By the end of this section you should be able to:

• Define homeostasis as maintenance of a constant internal environment and explain its significance.

• Define poikilotherms as organisms whose temperature is governed by the external temperature.

• Define homoiotherms (homeotherms) as organisms with constant body temperatures.

• Explain the physiological methods of temperature regulation in homoiotherms.

• Explain the behavioural methods of temperature regulation in homoiotherms and poikilotherms.

• Label the structures of the kidney.

• State the functions of the structures of the kidney.

• Explain how the kidney regulates water and ionic balance.

• Explain how the skin helps in water and salt balance.

• Explain the role of the liver in regulation of the body.

Just stop and think for a moment about the different conditions in which people live around the world. From the warmth of Africa to the cold of the Arctic tundra, from the arid dryness of the Sahara desert to the risk of flooding in the Nile delta – there are few areas of the planet where people have not settled, survived and thrived.

What is more, think about the different conditions your own body finds itself in during a single day. You may eat lots of food, or you may eat very little, you may spend time training for sport in the sun, or swim in the river, or sit in the shade. You may be ill and have a fever. You may even take a flight to a country with a very different climate from your own land. But, however much your external environment may change, things inside your body need to stay the same.

Figure 3.53 The beautiful countryside of Ethiopia and our thriving cities bring pleasure to local people and tourists alike – and everyone maintains a constant internal environment, regardless of the external

conditions they are familiar with.

DID yoU kNow?

The word homeostasis comes from the Greek words homoios, which means ‘like’ or ‘the same’, and stasis, which means

‘state’. So the word tells you exactly what it means – keeping the conditions in the inside of your body (the internal environment) in the same state all the time.

KEY WORDS

homeostasis the body’s ability to maintain normal function and stability homoios like, or the same stasis state

Figure 3.54 In warm sunny weather like this, or cold snowy weather, and whatever clothes we wear, homeostasis makes sure conditions inside our bodies stay the same.

Here are some of the main threats to a stable state inside your body.

You eat several times a day, so sometimes products of digestion

•

are flooding into your blood and at other times little or no food is available from the gut, yet your cells need a constant supply of glucose for respiration.

When you respire, you produce a poisonous waste product –

• carbon dioxide. If levels build up in your body they change the pH of your tissues. This in turn could denature your enzymes and so stop your cell chemistry completely.

As you break down the products of digestion, poisonous wastes

•

are produced, such as the urea which comes from the breakdown of amino acids in your liver. A build-up of urea could poison and kill you.

Whenever you exercise you produce heat from your muscles.

• This can increase your core body temperature, as can spending too much time in the sun or having a fever. If your body

temperature gets too high, your enzymes will denature and so all your cell chemistry will come to a halt and you may die.

If you lose too much heat from your body because the external

•

conditions are very cold, or you lose a lot of body heat, the cellular reactions slow down and you may die.

The amounts of water and salt you take in vary greatly

• throughout the day and from day to day, and so does the amount of water and salt you lose through sweat and urine – yet the water balance inside your body needs to stay the same to keep the cells in osmotic balance.

As you can see, keeping your internal conditions in a stable state isn’t easy – yet this is what your body manages to do 24 hours a day, every day of your life. The nervous and hormonal systems which you discovered in sections 3.1, 3.2 and 3.3 play an enormous role in maintaining this important balance. Feedback mechanisms involving both the nervous system and hormonal systems play a very important part in maintaining homeostasis. Most of these control systems in the body are examples of negative feedback. This

Figure 3.55 People can live in conditions of extreme heat and extreme cold and still maintain a constant internal body temperature.

means that when levels of a substance in your body rise, changes are made which lower the levels again. Similarly, when levels of a substance fall, changes are made so that it rises again to the original levels. Look out for these feedback mechanisms as you learn about homeostasis.