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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