BIOLOGY CHAPTER 10 HOMEOSTASIS 1. Homeostasis is defined as the maintenance of a constant internal environment. 2. The cells in our body are bathed in fluid (blood, lymph and tissue fluid). These fluids make up the internal environment. 3. This environment must be kept constant for healthy functioning of the body cells. 4. Homeostasis involves constant adjustments by various organs working together to achieve this internal stability. 5. The control center of homeostasis is in the brain. The liver, kidneys, skin, lungs and hormones also play important roles in homeostasis. 6. Homeostasis involves: a. A change in the internal environment (stimulus) b. A receptor that can detect the stimulus c. A corrective mechanism a system that return the conditions to normal, and d. Concept of control by negative feedback. (A correction/change in the opposite direction, hence the term negative). 7. Examples of homeostatic processes are: a. Carbon dioxide level: Changes in the carbon dioxide concentration of blood automatically affect the rate of breathing. b. Water balance: The urine composition is adjusted to ensure that the osmotic pressure of blood remains constant. c. Regulation of body temperature: This is kept at about 37 C. The skin plays an important role in this homeostatic control. 1
8. Skin A. THE HUMAN SKIN The skin is made up of a thin outer layer, the epidermis and an inner thicker layer, the dermis. B. Functions of the skin structure Structure Description Function 1. Epidermis a) Horny layer (cornified layer) Outer layer, made up of dead keratinised cells. b) Granular layer Layer of cells that begin to die due to the increasing distance from the blood supply of the dermis. c) Malpighian layer Made up of living cells and pigmented with melanin. Makes the skin waterproof hence reducing the water loss. Protects against mechanical, thermal and chemical injury. Protects against invasion by germs. Replaces dead cell in the cornified layer. Produces new cells. Pigment melanin prevents entry of harmful ultraviolet rays. 2
2. Dermis a) Hair Made up of dead cells and keratin. Projects from the hair follicle. b) Erector muscle Attached to the base of each hair follicle. Traps air in order to form an insulating layer when cold. May act as a camouflage. Contracts to make hair stand upright. c) Sebaceous gland Opens into the hair follicle. Secretes sebum (oil) for waterproofing skin and keep hair supple. d) Sweat glands Surrounded by blood capillaries and connected to sweat pores by sweat ducts. e) Sense receptors Nerve endings found in the epidermis and dermis. Receptors connected to nerve fibres. f) Nerve fibres Lead to and from the central nervous system (CNS). Excrete sweat. When sweat evaporates, it cools the body. Detect sensations of touch, pain, pressure, or temperature changes in the external environment. Carry impulses from the sense organs to the CNS. g) Blood vessels Network of capillaries. Arterioles dilate or constrict, to supply more or less blood to the capillaries in the skin for body temperature control. h) Adipose tissue Layers of fat cells found at the base of the dermis. Provides insulation against cold. Store energy in the form of fat. [Note: Another important role played by the skin is the of vitamin D when exposed to sunlight. Vitamin D is necessary for the absorption of calcium to form strong bones and teeth.] 3
9. Regulation of body temperature The thermo-receptors in the skin detect the change in the temperature. Messages are sent to hypothalamus in the brain. Hypothalamus is the temperature control center. Summary of homeostatic process that regulates the body temperature: A. Regulation of body temperature on a HOT DAY. Blood temperature returns to normal levels Decreased metabolic rate reduces heat Stimulus Blood and skin temperatures rise Corrective Mechanism Increase in heat loss and decrease in heat Negative feedback Receptor Temperature receptors detect changes and send nerve impulses to the brain Hypothalamus of brain is stimulated and sends nerve impulses to relevant body parts Corrective mechanism: 1. Vasodilation: widening of arterioles in the skin allows more warm blood to flow through the capillaries near the surface of the skin resulting in more heat lost by radiation. 2. Sweat glands become active and cause sweating. Evaporation of sweat brings about heat loss and cools body. 3. Hair lies flat because the erector muscles relax. The layer of air trapped is very thin. Thus heat is easily lost from the body by conduction and convection. 4
B. Regulation of body temperature on a COLD DAY. Blood temperature returns to normal levels Increased metabolic rate increases heat Stimulus Blood and skin temperatures fall Negative feedback Corrective Mechanism Decrease in heat loss and increase in heat Receptor Temperature receptors detect changes and send nerve impulses to the brain Hypothalamus of brain is stimulated and sends nerve impulses to relevant body parts Corrective mechanism: 1. Vasoconstriction: Narrowing of arterioles in the skin reduces the amount of warm blood flowing near the surface of the skin so resulting in reduced heat loss by radiation. 2. Sweat glands are less active thus reducing sweating. Less evaporation resulting in reduced heat loss from body 3. Erector muscles contract causing hairs to stand on end. The layer of air trapped between the hairs become thicker. Since air is a poor conductor of heat, it insulates the body. 5