Homeostasis Flashcards Preview

Science for Medicine 16 > Homeostasis > Flashcards

Flashcards in Homeostasis Deck (17):

Define homeostasis

Homeostasis - the tendency towards a relatively stable equilibrium between interdependent elements, especially as maintained by physiological processes.


Explain the importance in maintaining the consistency of the internal environment

The body needs the extracellular and intracellular compartment composition to be maintained in a state compatible with the survival of the individual cells.

For example, 80% of ECF is ISF and 20% is plasma.


Describe the principles behind negative feedback control

Any change in the environment is detected by receptors, which is fed to a integrating centre and compared to a reference level. If there is a difference, a signal is sent to an effector and the change is negated, returning the environment to it's normal state e.g. temperature, glucose levels.

The magnitude of the error signal is proportional to the size of the response and the deviation.


Describe feed forward control

Feed forward control is a more sophisticated version of negative feedback, where more sensors are involved and therefore, changes can be anticipated before significant changes occur. E.g. Our skin detects minor changes in temperature before our core temperature is affected, preventing body temperature being too high or low.


Illustrate the concept of homeostasis by outlining daily water balance in man.

In a normal healthy 70kg man:

- water makes up ~60% body weight
- 1/3 of total body water is in the ECF
- 2/3 of total body water is in the ICF
- 80 % ECF = ISF, 20% ECF = plasma

If these values change, it can have a wide range of consequences e.g. cell bursting, cell shrinkage


Describe hypertonic and hypotonic conditions

Hypertonic - if the interstitial fluid has a higher concentration of solutes than the intracellular fluid, it will pull water out of the cell, causing the cell to shrink

Hypotonic - If the interstitial fluid has a lower concentration of solutes than the intracellular fluid, water will flow into the cell, causing the cell to swell and sometimes burst.


Common everyday challenges to our internal environment

External temperature

These factors impact on body fluid composition, energy stores and body temperature, and physiological mechanisms must act to counteract these potential threats to homeostasis.


Describe positive feedback

Positive feedback has, unsurprisingly, the opposite effect of negative feedback. Where negative feedback aims to restore conditions to optimum, with positive feedback initial disturbance set off a train of events that lead to an even greater disturbance.

Such cycles usually lead to instability, common in pathophysiology, rare in normal physiology. However, they do occur, e.g. in the nerve action potential, in childbirth, in ovulation and sexual behaviour.


How is homeostasis coordinated?

Maintenance of homeostasis requires overall regulation of numerous cells, tissues and organs, often located some distance apart.

- Integration of activity over long distances is the job of the endocrine (hormone) and nervous systems.
- Homeostatic mechanisms are represented by reflexes, which may be neural and/or hormonal.
- They may be simple or very complex and will maintain O2 and CO2 levels, water and ion balance, blood pressure and blood volume, nutrient levels etc.


Define the dilution principle in water balance

∴ v = m/c = Dilution Principle, used to measure body fluid volumes

Dilution is a reduction in the pH of a chemical. It is the process of decreasing the concentration of a solute in solution, usually simply by mixing with more solvent.
This can also be shown in the equation:

C1 x V1 = C2 x V2

C1 = initial concentration or molarity
V1 = initial volume
C2 = final concentration or molarity
v2 = final volume

When measuring body fluid compartments remember:
1. Only plasma can be sampled, ∴only compartments of which plasma is a component can be measured directly (plasma, ISF, ECF).
2. The nature of barriers which separate compartments is crucial in determining the test substance.


What proportion of body weight is made up by water?



Name physiological examples of feed forward control

Heart rate
Salivating before meals


Name some physiological examples of positive feedback control

Nerve signalling
Childbirth and ovulation
Sexual behaviour
Innappropriate control of glucose in diabetes


Describe how water is split in the body

In normal average 70kg male

Total BW = 42L
Water is split 2/3 in ICF and 1/3 in ECF

ECF comprises 80% ISF and 20% plasma


Who have "less water"?

Females and the elderly (fat content)


Which body fluid volumes can be measure directly?

Plasma volume
Extracellular volume (PV + ISF)
Total body water


How can you measure indirectly ISF and ICF?