PHYS20008 Human Physiology
Notes: Homeostasis and Membrane
Potential Concepts 2026 University
of Melbourne
Human Physiology notes
Homeostasis
- Defined as the maintenance of a relatively constant internal
environment
- Homeo meaning “the same”
- Stasis meaning “stand or stay”
- Essential for the survival of each cell … and hence the individual
- Feedback loop
o Detect changes
▪ Sensor
o Controller or integrator
▪ Compare to set point
o Message to effector
▪ Efferent
o Response
▪ Return to within desired range
▪ => negative feedback
Integration
- Negative feedback
o Constant monitoring and adjustment
- Integration involves:
o Endocrine: hormones
o Neural
Positive feedback
- Positive feedback loops cannot maintain
homeostasis
- Reinforces the stimulus
- Escalates the response
- Rare but important
o E.g. birth, ovulation, Na channels in
action potential
Diffusion
- Passive process
, - Spontaneous
- Flowing downhill
- Net movement
- Eliminates concentration gradients
Cell membrane
- Separates cell from environment
- Intracellular different from extracellular
- Need control
- Energy needed to maintain differences
- Pass?
o Lipid soluble e.g. uncharged, small
o E.g. lipids, water, O2, CO2
- Stopped?
, o Lipid insoluble e.g. charged, large
o E.g. ions, proteins
Osmolarity and tonicity
- Osmolarity
o Osmolarity is the total concentration of solutes, penetrating and non penetrating
o Normal cell osmolarity is approximately 300 mOsm
- Tonicity
o Tonicity is the concentration of only the non penetrating solutes
o Tonicity is always in reference of the extracellular non penetrating solute concentration compared to
the cell’s non penetrating solutes
o A solution can be isotonic, hypertonic or hypotonic with respect to the cell
Osmotic equilibrium
- Body is in osmotic equilibrium
o Water moves between intra and extra cellular space
o Dilutes the more concentrated solution: osmosis
o Net movement stops when at equilibrium
o Osmolarity usually 300 mOsm
o Does not mean an even distribution of different types of particle
▪ E.g. Na+ higher outside than inside
▪ Chemical disequilibrium
Fick’s law
, Hypotonic solutions have lower concentrations of non penetrating solutes than isotonic solutes
A cell contains 300 mOsm of non penetrating solutes. If the extracellular fluid contains 250 mOsm non penetrating
solutes, the extracellular solution is hypotonic and the cell will swell.
Facilitated diffusion
Active transport
Membrane potential
- RMP – resting membrane potential
o Membrane potential of non
excitable cells and excitable cells
at “rest”
- >>90% of RMP is due to passive diffusion
of K+ and Na+ down their concentration
gradient
Notes: Homeostasis and Membrane
Potential Concepts 2026 University
of Melbourne
Human Physiology notes
Homeostasis
- Defined as the maintenance of a relatively constant internal
environment
- Homeo meaning “the same”
- Stasis meaning “stand or stay”
- Essential for the survival of each cell … and hence the individual
- Feedback loop
o Detect changes
▪ Sensor
o Controller or integrator
▪ Compare to set point
o Message to effector
▪ Efferent
o Response
▪ Return to within desired range
▪ => negative feedback
Integration
- Negative feedback
o Constant monitoring and adjustment
- Integration involves:
o Endocrine: hormones
o Neural
Positive feedback
- Positive feedback loops cannot maintain
homeostasis
- Reinforces the stimulus
- Escalates the response
- Rare but important
o E.g. birth, ovulation, Na channels in
action potential
Diffusion
- Passive process
, - Spontaneous
- Flowing downhill
- Net movement
- Eliminates concentration gradients
Cell membrane
- Separates cell from environment
- Intracellular different from extracellular
- Need control
- Energy needed to maintain differences
- Pass?
o Lipid soluble e.g. uncharged, small
o E.g. lipids, water, O2, CO2
- Stopped?
, o Lipid insoluble e.g. charged, large
o E.g. ions, proteins
Osmolarity and tonicity
- Osmolarity
o Osmolarity is the total concentration of solutes, penetrating and non penetrating
o Normal cell osmolarity is approximately 300 mOsm
- Tonicity
o Tonicity is the concentration of only the non penetrating solutes
o Tonicity is always in reference of the extracellular non penetrating solute concentration compared to
the cell’s non penetrating solutes
o A solution can be isotonic, hypertonic or hypotonic with respect to the cell
Osmotic equilibrium
- Body is in osmotic equilibrium
o Water moves between intra and extra cellular space
o Dilutes the more concentrated solution: osmosis
o Net movement stops when at equilibrium
o Osmolarity usually 300 mOsm
o Does not mean an even distribution of different types of particle
▪ E.g. Na+ higher outside than inside
▪ Chemical disequilibrium
Fick’s law
, Hypotonic solutions have lower concentrations of non penetrating solutes than isotonic solutes
A cell contains 300 mOsm of non penetrating solutes. If the extracellular fluid contains 250 mOsm non penetrating
solutes, the extracellular solution is hypotonic and the cell will swell.
Facilitated diffusion
Active transport
Membrane potential
- RMP – resting membrane potential
o Membrane potential of non
excitable cells and excitable cells
at “rest”
- >>90% of RMP is due to passive diffusion
of K+ and Na+ down their concentration
gradient
