Homeostasis pt 2
Monacia Williams, Contributor

Did you have a good week? Some of you are now in the final stages of the preparation of your SBAs. I sincerely hope that you have given them your best effort because that is the only way to ensure that you do well. I need to remind you again that the first question on Paper 02 always relates to the practical part of your course so you need to be comfortable with handling practical data.

Last week we looked at the homeostatic me-chanisms that were responsible for the control of:

water content of the blood - osmoregulation

blood glucose level.

These two are regulated by different hormones produced by the endocrine system. This week, as promised, we will look at the third regulatory system, the one responsible for the control of body temperature. This system is, however, not under the control of the endocrine system but under the control of the nervous system.

Controlling body temperature

How does this happen?

The organ responsible for the regulation of body temperature is the skin. Mammals are warm-blooded. This means that they are capable of keeping their body temperature constant at 370C regardless of the surrounding temperature. The skin utilises a combination of factors to maintain the body temperature. These include:

Warm-blooded animals usually have hair or fur on their skin. This helps to trap a layer of air next to the skin. Air is a poor conductor of heat so this layer acts as an insulator. In cold weather, the hair stands up, causing more air to be trapped next to the skin, hence less heat is lost. In hot weather, the hair lies flat, trapping less air, hence causing more heat to be lost, cooling down the body.

The brain monitors the temperature of the blood in the blood vessels running through its tissues while nerves from the skin bring information about its temperature to the brain.

If it is hot:

• Blood vessels at the surface of the skin dilate, that is, they become wider. This means that more blood will be brought to the skin by these vessels. The heat carried by the vessels will then be lost by radiation.
• Sweat glands in the skin will make sweat. The sweat is lost from the skin by evaporation, and this cools down the body, reducing the body temperature.

If it is cold:

• Blood vessels to the skin constrict, that is, they become narrower. This means that the amount of blood going to the skin will be less, hence less heat will be lost by radiation.
• Sweat glands will stop producing sweat, so more heat will be conserved.
• Shivering begins. Shivering is due to rapid contraction of the muscles. The energy for the contractions is provided by respiration, respiration produces heat energy as well as energy stored in ATP. Hence, shivering helps to warm the body.

Unfortunately, these mechanisms cannot prevent the risks associated with exposure to extremes of temperature. Exposure to very low temperatures can cause hypothermia, which can cause the individual to go into a coma leading to eventual death, while exposure to high temperatures can cause heat stroke, dehydration and eventual death. Old people and babies are particularly susceptible to the effects of these extremes.

This marks the end of our study of homeostasis. I sincerely hope that you found it interesting and that things are somewhat clearer for you now. As is customary, we will now look at a few questions.

Question 1

The regulation of body temperature is achieved by balancing energy release against energy loss. Energy release is greater during exercise.

a. The rate of metabolism changes during exercise. Describe how this influences energy release. (2 marks)

b. Explain why, during exercise, the body attempts to lose more energy. (2 marks)

c. What role does negative feedback play in regulating body temperature? (1 mark)

d. During rest, excess sugar is stored, how is this achieved? (2 marks)

e. The graph below shows how the blood glucose and the concentration of hormone involved in glucose storage vary over a 12-hour time period.

i. Using these graphs, explain why a single daily dose of hormone would not adequately control blood glucose concentration in diabetes. 2 marks

see graph

ii. Explain how physical exercise would influence the production of the hormone involved in glucose storage. (2 marks)

Next week we will go through the answers and we will begin a new topic: coordination and control. Have a good week!

Monacia Williams teaches at Glenmuir High School. Send questions and comments to kerry-ann.hepburn@gleanerjm.com