HOMEOSTASIS Objectives
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Internal environment: outside of cells but inside of the body.
Interstitial compartment and plasma
Heart is an internal organ but the cells that make up the heart are not considered internal
environment.
NOTE: digestive, urinary and respiratory are all external environment because they are not controlled
by our body.
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Plasma: fluids and cells inside circulation
Interstitial compartment (ISF): fluids surrounding all cells of the body
Intracellular compartment (ICF): Fluids inside cells. Biggest volume
ISF and Plasma make up ECF
Blood vessel wall is permeable to small substances but not protein, viruses,
cells.
Cell membrane is permeable to hydrophobic hormones, gases but impermeable to polar and ions
such as glucose and charged particles.
Water and urea cross barriers.
1. Understand what homeostasis is, and how it is maintained
Homeostasis is the body’s ability to maintain a relatively stable but not static conditions in
internal environment.
stable but not static internal environment
Body must shift back and forth between homeostasis and allostasis (altered state of body
function which causes wear and tear).
** Homeostasis is controlled by negative feedback loops only
2. Know the homeostatic parameters listed in the table in lecture
No need to memorize but draw conclusions.
Ex. Glucose: the non-lethal limit is high and in large range which means that it must not be
tightly regulated in comparison to pH or potassium which has a very small range for
fluctuations indicating that it is important to be in range for them.
3. Understand the difference between a negative feedback loop and a positive feedback loop.
Pg. 8
Negative feedback: initial increase in the variable leads to subsequent decrease in the variable
to bring it back to normal. Stable
Positive feedback: an initial increase in the variable triggers further increase in that variable.
Unstable
There are more negative feedback loops than positive feedback loops physiologically.
4. Understand how negative feedback loops control homeostatic parameters
External change leads to internal change  internal sensors detect changes (ex. pH, O2, CO2,
pressure sensors etc.) sensors are triggered, and the information is sent to three main
controlling systems (Nervous (brain and spinal cord), endocrine, immune)  send their own
signal to organ system to offset the change
Ex: Glucose Homeostasis and negative feedback: INPUT=OUTPUT; absorption into gut +
release from liver = uptake into cells
Input: food and release from liver
Output: uptake into cells for energy storage and use, maintain ion gradient, respiration
and circulation
Eat increase in plasma glucose levelsignal to pancreas which releases insulin insulin
increases the rate of glucose transport to cells (skeletal muscle cells mostly, adipose, and liver
cells)  brings plasma glucose levels back down.