1. Chapter 6 - Homeostasis

2. 6.1 The Importance of Homeostasis
You should be able to:
define homeostasis and explain its importance in living organisms; and
explain the concept of negative feedback

3. 6.1 The Importance of Homeostasis
What is Homeostasis?
Homeostasis refers to the maintenance of a constant internal environment.
Our daily activities are all controlled by the chemical reactions of different cells in our bodies, which are bathed in tissue fluid – the internal environment.
The composition of tissue fluid has to be kept within narrow limits.

4. Why is homeostasis important?
6.1 The Importance of Homeostasis
Why is homeostasis important?
Body cell functions can be upset by small fluctuations in:
Temperature pH
Carbon dioxide levels
Oxygen levels
Osmotic pressure
Chemical ions

5. 6.1 The Importance of Homeostasis
Homeostasis helps to maintain stability in an organism

6. Cells are made of biological molecules such as proteins, which make them very sensitive to slight fluctuations in temperature and pH.
E.g. a rise in carbon dioxide levels in the blood causes pH levels to fall, which affects the activity of enzymes and thus alter the rate of cellular reactions.
Water content of blood also needs to be tightly regulated to avoid drastic changes to the shape of a cell which will affect the normal function of the cell.

7. It functions to maintain stability in our body system.
6.1 The Importance of Homeostasis
Negative Feedback
Negative feedback is the basis of the homeostatic control mechanisms in our bodies.
It functions to maintain stability in our body system.

8. A negative feedback system consists of the following components:
6.1 The Importance of Homeostasis
Negative Feedback
A negative feedback system consists of the following components:
Stimulus – change introduced to the system
Receptor – detects the stimulus
Regulator – receives information from the receptor about the change in the system
Corrective mechanism – counteracts the change (stimulus) and brings the system back to the norm
Effector – carries out the corrective mechanism

9. 6.1 The Importance of Homeostasis
A general negative feedback mechanism

10. 6.2 Homeostatic Control in Man
You should be able to:
describe the maintenance of blood glucose levels; and
describe the process of osmoregulation.

11. 6.2 Homeostatic Control in Man
When blood glucose level rises, the hormone insulin is secreted to decrease the glucose level in blood
When the blood glucose level is low, glucagon is secreted to raise blood glucose level to normal
Diabetics require insulin injection due to lack of insulin secretion by their bodies

12. 6.2 Homeostatic Control in Man
Regulation of blood glucose concentration

13. 6.2 Homeostatic Control in Man
The hypothalamus in the brain is the regulator of many important processes, including osmoregulation (regulation of water and ion concentrations in the body) and temperature control.

14. 6.2 Homeostatic Control in Man
Osmoregulation – Regulation of blood water molecule concentration

15. 6.3 Temperature Control in Mammals
You should be able to:
describe the maintenance of a constant body temperature in humans, including the role of the skin.

16. 6.3 Temperature Control in Mammals
Thermoregulation (temperature control) is one of the most important example of homeostasis.
It helps to maintain body temperatures at a constant 37oC for mammals and humans.

17. Heat Gain and Heat Loss Sources of heat gain Sources of heat loss
6.3 Temperature Control in Mammals
Heat Gain and Heat Loss
Sources of heat gain
From the sun and atmosphere
Intake of hot food and drinks
Actively respiring organs such as the liver
Sources of heat loss
From exposed surfaces of the body via conduction, convection and radiation
Evaporation of sweat in humans
Expired air from lungs
Faeces and urine

18. 6.3 Temperature Control in Mammals
Birds and mammals are homeothermic, able to keep their body temperatures consistent by losing heat and producing heat internally.
Fish, amphibians, reptiles and insects are poikilothermic, their temperatures vary, matching with that of their surroundings.
Poikilotherms generally do not possess internal thermoregulatory mechanisms, but can still control their body temperature by varying their behaviour.

19. Hypothalamus regulates body temperature in humans
6.3 Temperature Control in Mammals
Hypothalamus regulates body temperature in humans

20. The skin is made up of three main layers:
6.3 Temperature Control in Mammals
The skin is made up of three main layers:
Epidermis - cornified layer (dead cells, deposits of keratin) - Malpighian layer (living cells, pigmented with melanin)
Dermis - blood capillaries, hair, sebaceous glands, sweat glands, sense receptors
Hypodermis - blood vessels and adipose tissue

21. The dermis layer is made up of :
6.3 Temperature Control in Mammals
The dermis layer is made up of :
Blood capillaries – Supply blood to skin
Sebaceous glands – secrete sebum into hair follicle, keeping skin soft and moist, and hair from turning brittle
Hair – attached to epidermis by hair erector muscle
Sweat glands – secrete sweat continuously
Sense receptors – detect pain, pressure and temperature changes

22. Cross section of human skin
6.3 Temperature Control in Mammals
Cross section of human skin

23. Role of the skin in temperature regulation
6.3 Temperature Control in Mammals
Role of the skin in temperature regulation
The effectors in the skin involved in temperature regulation are:
Smooth muscles of arterioles in skin
Sweat glands
Hair erector muscles

24. Role of the skin in temperature regulation
6.3 Temperature Control in Mammals
Role of the skin in temperature regulation
Smooth muscles in arterioles of skin

25. Role of the skin in temperature regulation Sweat glands
6.3 Temperature Control in Mammals
Role of the skin in temperature regulation
Sweat glands

26. Role of the skin in temperature regulation Hair erector muscles
6.3 Temperature Control in Mammals
Role of the skin in temperature regulation
Hair erector muscles

27. 6.3 Temperature Control in Mammals
In addition to thermoregulatory responses in the skin, other mechanisms help to maintain the temperature in mammals.
They are
Increasing/decreasing metabolic rate
Shivering
Voluntary actions (e.g. putting on clothing, moving to a colder/warmer place, drinking cold water on a hot day)

28. Thermoregulation – how our body controls its own temperature

29. Key Concepts
Homeostasis is the maintenance of a constant internal environment.
Homeostatic control mechanisms rely on negative feedback to maintain a constant value (norm/set point).
Negative feedback refers to a process that reduces the deviation from the set point.
The control centre of temperature regulation is the hypothalamus in the brain.
The main difference between homoiotherms and poikilotherms is that the hormer can maintain their own body temperature using internal mechanisms while the latter relies on behavioural means.

30. Key Concepts
Arterioles increase heat loss by dilating and reduce heat loss by constricting.
When hair erector muscles contract, they raise the hairs which help to trap an insulating layer of air.
Effectors in the skin (either muscles or glands) respond to messages from the hypothalamus to regulate body temperature.
Thermoregulation is achieved by controlling the heat loss and heat gain mechanisms in the body.