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Chapter 1

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BIOL 273
Norman Scott

Chapter 1 Homeostasis depends on mass balance (pg. 13) 2 options to maintain mass balance: 1. excretion a. through urine, feces, lungs, or skin e.g. CO2 –lungs, drugs/artificial food additives-liver and kidneys foreign substance in the body = xenobiotic 2. convert the substance to a different substance through metabolism a. convert original nutrient to a diff. molecule through metabolic pathway aka metabolite Excretion Clears Substances from the Body Clearance: volume of blood cleared of substance x per unit of time Hepatocytes: liver cells, metabolize diff types of molecules, esp. xenobiotics > resulting metabolites may be secreted to intestine for excretion in the feces/into the blood for excretion by kidneys Homeostasis does not mean equilibrium Stability of the extracellular fluid compartment (ECF) - Easy to monitor by taking a blood sample - plasma: fluid compartment of blood - dynamic steady state: materials constantly moving back and forth between the two compartments (ECF and ICF) , no net movement of materials b/t the compartments - steady state does not mean equilibrium(same composition in two compartments but diff. concentrations of substances) e.g. Na+ and Cl- more oncentrated in ECF, K+ more in ICF = ECF and ICF exist in a state of disequilibrium Control Systems and Homeostasis - regulated variables: key functions that has to be kept within acceptable range by physiological control if the variable strays from setpoint Two patterns of control mechanisms: 1) local control o restricted to the tissue or cell involved /region where change took place (local) o sense change and responds, usually by releasing a chemical o e.g. cells lining small blood vessels sense low concentration of oxygen> secrets chemical signal>signal molecule diffuses to nearby muscles in blood vessel wall> message to relax which dilates blood vessel increasing blood flow into tissue (more oxygen to area) 2) long-distance reflex control o widespread changes require long distance communication and coordination o e.g. blood pressure which is body-wide o reflex control: any long-d pathway that uses nervous system, endocrine system, or both  response loop: has an input signal, integrating center, output signal -> expanded to 7 steps: stimulus(change that occurs when regulated variable out of range) -> sensor (monitors the variable, activated by stimulus then sends input signal to integrating center) -> input signal -> integrating center (usually nervous and endocrine system, decides whether a response is needed) -> output signal (chemical signals, electrical signals, or both) -> target (can be any cell of the body) -> response  feedback loop  negative feedback: a pathway which response opposes or removes the signal  response removes the stimulus thus response loop shuts off  negative feedback loops can restore the normal state but can’t prevent the initial disturbance  Positive Feedback loops are not homeostatic o Reinforces the stimulus rather than decreasing or removing it o Sends regulated variable farther from its normal value -> system
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