2.44 Homeostasis→ To be able to define the term homeostasis→ To understand why the body must keep constant conditions around cells→ To remember that tissue fluid surrounds the cells→ To understand the principle of negative feedback→ To know the organs involved in homeostasis

Keywords• External environment: conditions outside the body• Internal environment: conditions inside the body• Optimum: Best value for cells to function• Osmoregulation: the regulation of water and salt levels• Excretion: the removal of the waste products from metabolism and

other reactions from an organism• Sensory receptors: They feed the brain with info about the body’s

conditions

The body has mechanisms of controlling both external and internal environments, so both conditions remain constant

Keeping internal environment constant• Cells in blood plasma and others are surrounded by tissue fluid• Conditions in blood (& therefore tissue fluid) kept at optimum• So conditions around cells are constant• This is called HOMEOSTASIS

Homeostasis• Keeps a basic principleExternal environment → varying conditions → homeostatic systems → blood → tissue fluid(detail on page 128)

Cells in homeostasis• Cells change the composition of tissue fluid• Remove food and oxygen• Add CO2 and other wastes

Heart in homeostasis• supplies constant pressure needed to:

→ deliver blood to the tissues → form the tissue fluid

Skin in homeostasis• The main organ for control of heat exchange

Kidneys in homeostasis• For osmoregulation• For excretion

Lungs in homeostasis• Regulate exchange of carbon dioxide and oxygen

Liver in homeostasis• Regulates levels of many solutes in the blood• Removes poisons

Intestines in homeostasis• Supply soluble foods and water

How is homeostasis brought about• Work of all organs coordinates to achieve homeostasis• Sensory receptors work along with organs• Therefore if something is out of place, they inform the brain which

commands a change• Homeostasis depends on this “feedback”

summary• Homeostasis is the maintenance of the body in a constant internal

environment• Homeostasis involves control by negative feedback• In negative feedback, a change sets off a response that cancels out in

change