1. Homeostasis

2. Learning Objectives
define homeostasis as the maintenance of a constant internal environment
explain the basic principles of homeostasis in terms of stimulus resulting from a change in the internal environment, a corrective mechanism and a negative feedback
identify on a diagram of the skin: hairs, sweat glands, temperature receptors, blood vessels and fatty tissue
describe the maintenance of a constant body temperature in humans in terms of insulation and the role of: temperature receptors in the skin, sweating, shivering, blood vessels near the skin surface and the co-ordinating role of the brain

3. Definition
Homeostasis is the maintenance of a constant internal environment.

4. The internal environment is made up of blood and tissue fluid.

5. What factors within the internal environment must be kept constant?
The concentration of glucose
The concentration of ions, e.g. sodium and potassium
The concentration of carbon dioxide
The osmotic pressure, determined by the relative concentrations of water and solutes (osmoregulation)
Temperature (thermoregulation)
The pH (acid-base balance)
Nitrogenous waste products and other toxic substances, which are either eliminated or at least kept to a minimum

6. Importance of Homeostasis
Homeostasis keeps the body environment under control and keeps the conditions right for cells to live and function.
For eg. Enzymes in our body can only work at a narrow range of pH and temperature. Changes in these conditions can lead to enzyme denaturation or inactivation that can subsequently affect the workings of our body.

8. Negative Feedback
Homeostasis involves an important principle called negative feedback which is the reverse effect of a stimulus.
An increase in temperature will induce a feedback to decrease the temperature.
A decrease in temperature will induce a feedback to increase the temperature.

9. NEGATIVE FEEDBACK CONTROL LOOP
A norm or set point to be maintained
A stimulus which is a change in the internal environment.
A receptor which can detect the stimulus.
A control centre
A corrective mechanism to bring about the reverse effect of the stimulus.
A feedback to the receptor when the set-point is reached. This causes corrective mechanism to stop.

10. Examples of homeostasis in Man:
Regulation of blood glucose concentration
Body cells need glucose for tissue respiration to provide energy. The concentration in blood plasma remains relatively constant. (70-90mg per cubic cm)
Glucose level may rise after a meal. It falls during exercise or starvation.
How is the concentration of glucose in blood plasma regulated?
?? If BGL rises – can lead to cells shrinking…

13. Regulation of blood water potential.
Recall the functions of kidney to regulate water potential.

14. Regulation of carbon dioxide level.
Changes in carbon dioxide concentration of blood automatically affects the rate of breathing.
Exercising increase the concentration of carbon dioxide in blood hence also the rate of breathing.

15. Regulation of blood temperature

17. Functions of the Mammalian Skin
Regulates body temperature
Protects the body against damages
Prevents water loss
Excretory organ
Sensory organ
Produces vitamin D

18. Structure of the Human Skin

19. Structure of the Mammalian Skin:
Largest organ of the body
Made up of 3 parts: epidermis, dermis and subcutaneous(hypodermis) tissue.
The hypodermis is a few layers of adipose cells (adipose tissue) where fat is stored.
The fat serves as an insulation layer as well as a food storage organ.

20. The Epidermis Outer cornified layer Granular layer
Innermost Malpighian layer

22. Parts of the Mammalian Skin
Epidermis
Outer cornified layer has:
Dead, dry, flat cells filled with keratin which makes the skin water resistant and impenetrable.
Protects body from mechanical injury
Prevents water loss through evaporation
Granular layer
Living cells synthesizing keratin
Flatter and ceases division
Give rise towards cornified layer
Innermost Malpighian layer
Pigmented living cells(melanin- protects against harmful UV rays)
Undergoes cell division- cells are pushed towards granular layer-change shape and structure and pushed towards granular layer.

24. Parts of the Mammalian Skin
Dermis
Blood capillaries
Arterioles that supply blood to the capillaries are controlled by nerves
Vasodilation: arterioles dilate; more blood sent to skin; heat lost
Vasoconstriction: arterioles constrict; less blood sent to skin; heat retained
Constriction and dilation helps to increase or decrease amount of blood brought to the surface hence amount of heat lost to environment

25. Parts of the Mammalian Skin
Dermis
Hair
Formed in hair follicle (hollow tube-like structure)
Base of follicle is the hair papilla where hair develops; cells in papilla are nourished by surrounding blood capillaries
Hair erector muscle attached to hair follicle
When contract, it cause the hair to stand forming “goosebumps”
Sweat gland
Coiled tubular gland producing sweat (water, salts NaCl and urea)
Surrounded by blood capillaries
Sweat evaporates from skin for thermoregulation

26. Parts of the Mammalian Skin
Dermis
Sebaceous gland
At least 2 per follicle
Secrete sebum into hair follicle
Sebum
lubricates hair
maintain softness of skin
prevents dehydration
Antiseptic ( prevents growth of bacteria)
Sense receptors
Nerve endings (found in epidermis too)
Detect changes in external environment e.g. temp, touch(pressure), pain

27. Parts of the Mammalian Skin
Fatty tissues (adipose tissues)
Insulating layer – reduce heat loss

30. Temperature Receptors in Skin
Thermoregulation
Ext. Temp
Ext. Temp
Body Temp
Body Temp
Temperature Receptors in Skin
Blood Temp
Blood Temp
Hypothalamus
Heat Loss Centre
Heat Gain Centre
Vaso-constriction
Vasodilation
Sweat Pdtn
Rapid Breathing
Sweat Pdtn
Shivering
Negative Feedback
Normal

31. Heat is lost from the body to prevent overheating by:
Through the skin by convection, radiation and a little conduction.
Evaporation of sweat from the surface of the skin.
In the faeces and urine.
Expired air from lungs.

32. Advantages of constant body temperature:
Can remain active throughout the day and year unchanged with temperature variation.
Enzymes work best at a constant optimum temperature.
Need not hibernate hence able to feed throughout the seasons.
Exploit and colonize areas with different climate conditions.

34. To Summarise

35. What else?
Glycemic index ( measures how quickly BGL increases after a meal.
Test yourself 12.1, 12.2 (discuss) and review qs
Frost bite and heat stroke, melanoma( find out). Why do you shiver when you’re having fever?
Test yourself 12.3 for homework in your practical notebook.