Chpt. 30Homeostasis

Two environments affect all organisms

External environmentSurrounds the outside of an

organism

Internal environmentSurrounds the

cells in an organism

Homeostasis: is the ability of an organism to maintain a constant internal environment.

• it involves a combination of processes acting together to control the internal environment of an organism.

• in humans it is the brain which co-ordinates the activities of organs and systems responsible for maintaining this controlled internal environment.

Example of homeostasis in humans and the organs and systems involved.Example Organ or System

Maintaining body temperature of 37oC.

Controlled by the skin

Maintaing pH of blood and tissue fluid close to 7.4

Controlled by the kidneys

Preventing build up of toxic waste

Controlled by kidneys, skin and lungs

Maintaing correct oxygen levels Controlled by the breathing system

Maintaing correct glucose levels Controlled by endocrine system

Diffusion and Homeostasis

Diffusion - is the movement of molecules from an area of high concentration to an area of low concentration.

• in many cases homeostasis is dependent on an organism exchanging materials with its environment by diffusion.

• in single–celled organisms there are no special organs required for exchange unlike in larger multicellular organisms where problems of diffusion and exchange are overcome by a range of methods.

Methods used to improve Diffusion and Exchange

• Organ/Organism may be flattened i.e. leaves of a plant.

• Respiratory Systems – provide increased surface area for the exchange of gases by diffusion.

• Respiratory and excretory systems – take materials from within the body to the body surface.

Note: Diffusion is only effective over short distances so large organisms require a circulatory system to

carry materials over long distances!!!

Homeostasis and Changing Internal Environments

Homeostasis allows internal conditions of an organism to change slightly at certain times e.g.

• 1o C drop in body temperature when we sleep.

• Rise in body temperature when body gets an infection.

• Hormonal changes during the menstrual cycle, puberty and the menopause alter the internal environment of the body.

Necessity for Homeostasis

• To allow normal body reactions to occur at their most efficient rate.

• To maintain constant conditions within the cells allowing organisms to function independently of external conditions.

• To help eliminate toxins.

• To permit slight changes in internal conditions when necessary.