UNIT 4 HOMEOSTASIS

I. INTRODUCTION A.Definition – a process of maintaining a constant internal environment despite changes in the external environnment

B. Feedback Loop 1. a feedback loop is any system in which part of an output of the system is connected back into one of its inputs 2. eg. Ecosystems if the weasels eat more mice then there are fewer mice to eat which will result in fewer weasels over time if there are fewer weasels, there will be more mice  more food for weasels and more weasels in an ecosystem in which weasels eat mice we have a feedback loop

3. eg. thermostat

Cool room Thermostat switches Warm room Furnace on Furnace off Thermostat switches Co-ordinatorregulator result Co-ordinatorregulator result Sensor Bimetallic strip Sensor

4. Parts of a feedback loop b. co-ordinator - part that manages the response c. regulator- part that affects a change d. result - the effect of the regulator change 5. negative feedback – a system that is activated to restore the original condition eg keep room at 20 o C 6. positive feedback loop – a system which is designed to amplify a small effect eg. contractions at childbirth a. sensor - part that detects the difference

Thermoregulation in humans

Thermoregulation in humans – maintaining the body at 37 o C regardless of outside conditions Skin blood vessels dilate so more blood flow to skin to lose heat Sweat glands initiate sweating Sweat evaporates and cools skin Hypothalamus reacts to high temperature in blood and switches on cooling Body temperature decreases Body temperature too high Co-ordinator regulators result Sensor Carotid artery sensors detect blood temp.

Skin blood vessels contract and so less heat is lost Skeletal muscles contract,. shivering generates heat Body temperature increases too low Hypothalamus reacts to low temperature in blood and switches on heating Co-ordinator regulators result Body temperature Body hair becomes erect, raised hair traps warm air Carotid artery sensors detect blood temp. sensors

-the blood passes through the carotid arteries in the neck on the way to the hypothalamus in the brain eg. a scarf keeps the neck warm & so you feel warm eg. when you are hot, a wet cloth around the neck makes you feel cool -if this blood is cooled, you feel cool and if this blood is warm, you feel warm

II) HORMONES & THE ENDOCRINE SYSTEM

A. Hormones  chemicals released by cells in one part of the body that affect cells in other parts of the body. 1. source - endocrine glands -these secrete hormones directly into the blood 2. transport - blood -hormones go to every cell 3. target cells -these are cells with receptors for the hormones -a hormone may have one specific target cell or a hormone may target every cell

Types of hormones a. Steroid hormones b. Peptide hormones

B. Steroid hormones 1. Hormone made in endocrine cell 2. hormone diffuses into blood 3. hormone circulates in blood 4. hormone enters target cell 5. hormone binds to hormone receptor 6. hormone-receptor complex. diffuses into nucleus 7. this binding activates a gene 8. mRNA is transcribed & a new protein is made 9. the new protein leaves the cell nucleus target cell DNA

C. Protein hormones 1. Hormone made in endocrine cell 2. hormone diffuses into blood 3. hormone circulates in blood 4. Hormone binds to receptor on cell surface 5. hormone-receptor complex causes ATP  cAMP cAMP ATP 6. cAMP activates existing enzyme / protein 7. activated protein leaves cell target cell

D. The Master Gland – the Pituitary Gland - the pituitary gland is located under the hypothalamus, the area of the brain associated with homeostasis - there are two lobes of the pituitary, - the posterior lobe stores & releases hormones - the anterior lobe produces many hormones

[pg 392, 424] Complete table of hormones Hormone assignment