Homeostasis Flashcards
What is homeostasis?
Homeostasis is when the body prevents any disturbances in the system and maintains an optimum internal environment for cells to function.
Homeo- similar
Stasis- condition
Why is homeostasis important?
Out bodies aren’t tolerant of change in the internal environment so failure to adequately correct imbalances can result in pathology.
What is negative feedback control in homeostasis and how does it work?
It is essentially a trigger that causes a series of events and these switch off the trigger. A change in a monitored variable is sensed by a receptor which feeds to an integrating centre that compares it to a reference value. The difference in these values generates another signal which is fed to an effector mechanism which produces a response to correct the change. The magnitude of change from reference value is proportional to the response.
Describe two characteristics of negative feedback control
There is oscillation around the set point.
It restores the regulated variable after its initial displacement but cannot prevent it from happening.
What is feed forward control and how does it work?
In feed forward Control additional receptors allow the system to detect change and activate a response earlier. So for example temperature receptors and activate responses before the core temperature can change.
What is a positive feedback mechanism and how does it work?
Positive feedback involves a disturbance setting off a train of events that leads to a greater disturbance. They are rare in physiology and are common in pathophysiology. They do however occur in nerve action Potentials (self propagation)
Why is homeostasis of water important?
It is important as water affects the concentration of everything else in the body
What are the four compartments that water is split in the body?
Intracellular Fluid
Interstitial fluid (between cells) Plasma (fluid component of blood) These are classed together as extracellular fluid. Transcellular fluid (CSF around brain and spinal cord, joint fluid and fluid round the eye)
What factors affect total body water content?
It will vary with age and sex. Women and elderly have less water content. Water content of fat is 10% compared to muscle, 70%. Women generally have more fat than men and elderly people have less muscle.
This means that lipid and water soluble drugs will work differently on different people.
What is the dilution principle equation?
C=m/v
So
V=m/c
What and how can compartments of water be measured?
Plasma Volume can be measured directly as plasma proteins can’t cross the capillary wall so dyes or radioactive labels that attach to plasma proteins can be used (Evans blue)
Extracellular volume can be measured with things that can cross capillary walls but not cell walls such as sucrose, inulin or mannitol which are too large or sodium or chloride which are actively extruded from cells.
Total body water can be measured directly using a loading dose of radioactive water-deuterium Oxide, D2O or heavy water.
What are the equations to measure water indirectly?
ISF=ECF-PV
ICF=TBW-ECF
How do you calculate volumes of water in the body?
Inject a substance that will only stay in one compartment then calculate the volume of distribution by taking the amount injected and dividing it by the concentration of the sampled fluid.
