Lecture 2: Homeostasis Flashcards
What is homeostasis?
The process by which an organism maintains a stable internal environment in response to external changes
Why is homeostasis important? Provide an example.
It allows for the maintence of the bodys’ internal environment despite changes in the external environment
e.g. Our bodies ability to maintain their internal temperature despite daily temperature changes in the environment
How does the body maintain homeostasis?
By using homeostatic control systems to maintain homeostatic variables within their normal range around a set point
What are homeostatic variables? Give examples.
Features of the body’s internal environment that need to be regulated
▪ e.g. pH, composition of body fluids, volume of body water, core body temperature, oxygen, glucose, etc.
What is the set point?
The optimal value for a homeostatic variable.
What is the normal range?
The range above and below the set point in which the homeostatic variable can fluctuate without the body responding to fix the change
What are the 3 homeostatic control systems?
- Negative feedback
- Positive feedback
- Feedforward
What do feedback systems respond to?
A CHANGE in the homeostatic variable and can be positive or negative
What is negative feedback?
The control system acts to make the deviation from the set point SMALLER
e.g. core temperature, blood glucose
What is positive feedback?
The control system acts to make a deviation of the variable BIGGER
e.g. childbirth, blood clotting
A ‘stop’ mechanism is required so that the process doesn’t continue forever
What do feedforward systems respond to?
A change in the environment which is likely to cause a CHANGE in the homeostatic variable
What are the 2 different types of feedforward responses?
- Physiological: anticipatory physiological adjustments and biological rhythms
e.g.
- Smelling or thinking about food stimulates saliva
- Insulin secretion in response to glucose in the mouth - Behavioural: both innate and learnt behaviours
e.g. putting on extra clothes if weather forecast is cold
What occurs in feedforward?
The control system acts to PREVENT CHANGE from the set point/normal range in a controlled variable
1. Change in the external environment
2. The sensor detects change
3. The integration centre compares the predicted value to the set-point
4. Control signals are sent to the effectors to prevent or reduce the predicted changes in the controlled variable
What happens if the feedforward system does not prevent changes in the controlled variable?
Those changes will be corrected by negative feedback control
What is the order of a homeostatic control system?
Controlled variable -> Sensor -> Integration centre -> Effector(s)
What is the controlled variable?
The homeostatic variable that the system tries to keep in a steady state (within the normal range)
What is the sensor?
The sensor detects changes in the controlled variable and sends this information to the integration centre
What is the integration centre?
The integration centre compares the detected level of the homeostatic variable to to the set point/normal range.
If a difference is detected, information is sent via communication pathways to activate the effectors
What is the effector(s)?
The actions of the effector(s) restore the controlled variable to the set-point/within the normal range
What are the relative ion conc. in the ICF and ECF?
ICF:
Na+ - Low
Cl- - Low
K+ - High
Ca2+ - Low
ECF:
Na+ - High
Cl- - High
K+ - Low
Ca2+ - High
What is the osmolarity of the ICF and ECF?
275-300 mosmol/L
Describe the body fluid compartments:
TBW: Male - 60% of body weight, Female - 55% of body weight
ICF: 2/3 of TBW
ECF: 1/3 of TBW
Plasma: 1/5 of ECF
ISF: 4/5 of ECF
What barrier separates the intra and extracellular compartments in humans?
The plasma membrane (aka cell membrane)
What are the 4 functions of the plasma membrane?
- Enclose the cell
- Regulate movement of substances between intra & extracellular environments
- Provide a site for anchorage to other cells and extracellular components
- Provide location for proteins that detect the presence of signalling molecules
Describe the structure of the plasma membrane:
- Composed of a phospholipid bilayer
- Proteins and cholesterol are embedded in the phospholipid bilayer
- Semipermeable
- Restricts movement of water-soluble (polar) substances e.g. ions, but not lipid-soluble (non-polar) substances e.g. steroid hormones
What are the 2 classes of membrane proteins in the plasma membrane?
- Integral Membrane proteins are integrated into the membrane
- Peripheral membrane proteins are located on the surface of the membrane
What type of proteins are transmembrane proteins? Describe the 3 roles of these proteins:
They’re a type of integral protein
- SOME form a selective channel for water soluble substances to pass in or out of the cell e.g. aquaporins, ion channels, solute carriers or ATPases
- SOME transmit chemical signals across the membrane
- SOME anchor extracellular and intracellular protein filaments to the plasma membrane
Describe the 3 roles of peripheral membrane proteins:
Associate with one side of the membrane
- MANY anchor the membrane to the cytoskeleton or extracellular environment
- SOME are enzymes that mediate metabolism of membrane components
- SOME transport small molecules along the membrane or between the membrane and cytosol
