Human Body Systems and Homeostasis Chapter 8.1 McGraw-Hill Ryerson Biology 12 (2011)

Homeostasis Maintenance of a relatively stable internal, fluid environment (internal milieu) despite a changing external environment http://www.lionden.com/homeostasis.htm Critical for survival since proteins, enzymes, etc, require optimal conditions for operation Temperature 37oC (98.6oF) Blood pH 7.35 Blood sugar level 0.1%

Homeostasis Example: Blood glucose levels rise after a meal thus endocrine system works to lower it to normal Blood glucose levels fall during a fast thus endocrine system works to raise it to normal by resorting to glycogen supply in the liver

Homeostatic Control Systems Homeostasis relies on a feedback system: cycle of events in which a variable* is monitored assessed and adjusted 3 main components of feedback system A sensor -> senses state of system A control centre-> receives incoming info about system and decides what the next steps should be A regulator or effector -> carries out the appropriate next step to keep system at homeostasis *Variable can be tempearture, blood pH, blood glucose level, etc.

Positive Feedback System A mechanism by which an output is enhanced Contractions in childbirth: contractions stimulate oxytocin which stimulates more muscle contractions Negative feedback mechanism consists of reducing the output or activity of any organ or system back to it's normal range of functioning. Blood vessels can sense resistance of blood flow against the walls when blood pressure increases. The blood vessels act as the receptors and they relay this message to the brain. The brain then sends a message to the heart and blood vessels, both are the effectors. The heart rate would decrease as the blood vessels increase in diameter. This change would result in the blood pressure to fall back to its normal range. The opposite would happen when blood pressure decreases.

Positive Feedback System What positive feedback actually does is push the variable even further away from the normal The system is pushed to the extreme until the ordeal is over Positive feedback is not very commonly seen Negative feedback systems are much more common in organisms Negative feedback mechanism consists of reducing the output or activity of any organ or system back to it's normal range of functioning. Blood vessels can sense resistance of blood flow against the walls when blood pressure increases. The blood vessels act as the receptors and they relay this message to the brain. The brain then sends a message to the heart and blood vessels, both are the effectors. The heart rate would decrease as the blood vessels increase in diameter. This change would result in the blood pressure to fall back to its normal range. The opposite would happen when blood pressure decreases.

Negative Feedback System a mechanism by which an output is diminished The system resists change thus will initiate a response to bring it back to normal range Negative feedback loops require a Receptor, A Control Center, and Effectors Negative feedback mechanism consists of reducing the output or activity of any organ or system back to it's normal range of functioning. Blood vessels can sense resistance of blood flow against the walls when blood pressure increases. The blood vessels act as the receptors and they relay this message to the brain. The brain then sends a message to the heart and blood vessels, both are the effectors. The heart rate would decrease as the blood vessels increase in diameter. This change would result in the blood pressure to fall back to its normal range. The opposite would happen when blood pressure decreases.

Negative Feedback System

Negative Feedback System Blood vessels have receptors which can measure the resistance of blood flow against vessel walls The brain (control center) receives info about change in body’s internal conditions, sends out signals via nerves Effectors - the muscles, organs, and other structures that receive signals from the control center and respond to correct the deviation. Sweating when hot Shivering when cold Negative feedback mechanism consists of reducing the output or activity of any organ or system back to it's normal range of functioning. Blood vessels can sense resistance of blood flow against the walls when blood pressure increases. The blood vessels act as the receptors and they relay this message to the brain. The brain then sends a message to the heart and blood vessels, both are the effectors. The heart rate would decrease as the blood vessels increase in diameter. This change would result in the blood pressure to fall back to its normal range. The opposite would happen when blood pressure decreases.

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