PHYS20008 Lecture Notes - Lecture 2: Tonicity, Homeostasis, Osmotic Concentration
21 Jun 2018
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Lecture 2: Homeostasis
What is homeostasis?
Homeostasis is the maintenance of a relatively constant internal environment, and is
essential for the survival of each cell (and hence, the individual)
*Note the adjective ‘relatively’ which suggests that our system is biological and does not function as
or the way a robot does- we vary our ‘constant internal environment’ based on external influence
Positive vs. Negative feedback (For maintenance of the relatively constant internal environment):
Negative feedback is responsible for the constant monitoring and adjustment of our
internal environment. It works in fluctuations to get the body’s system back to the
set point, rather than suddenly reverting to the set point.
A positive feedback loop can’t maintain homeostasis as its goal is to worsen the
stimulus and, therefore, cause a system crash in the body
oEg. Child birth, lactation, ovulation, fighting infection and blood clotting
Afferent vs. Efferent pathways:
The afferent and efferent pathways are what passes the message that requires a response
(from the brain/to the brain or from the target/to the target area)
These pathways are either nerves or hormones:
oNerves: Fast, but specific
oHormones: Can communicate with any cell that has a receptor, although is
slower (in comparison to nerves)
*Afferent vs. Efferent (Remember E for exit)
o ‘Fight or Flight’ uses both nerves and hormones to signal
Cell membrane:
The cell membrane separates the cell from its environment, allowing the intracellular
environment to differ to the extracellular environment. It needs control and requires energy
to maintain differences.
Diffusion:
Diffusion moves substances against a concentration gradient. It concerns the movement
across a membrane from an area of higher concentration to an area of lower concentration
(Flows downhill)
*Charged ions ad larger particles can’t move across membranes easily
Osmolarity and tonicity:
Osmolarity is the total concentration of solutes, penetrating and non-penetrating.
Tonicity is the concentration of only the non-penetrating solutes. Tonicity can be further
described as either:
Isotonic
Hypotonic
Hypertonic
*Example: “(Extracellular environment) is isotonic/hypertonic/hypotonic to the cell”
The body is in osmotic equilibrium where water moves between intra- and extracellular
space, diluting the more concentrate solution (osmosis). Net movement stops at equilibrium
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