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Homeostasis is the maintenance of certain key physiological variables within narrow limits despite changing conditions.
Physiological variables are always regulated within a range. As the text puts it, they are kept “relatively constant”. Why is it good to have some “slack” in homeostasis?
Some slack is useful, as more precise control requires greater energy expenditure, better sensors and better effectors (think of your parents’ role as controllers!). Some slack is also useful to allow for changing conditions without resetting a setpoint. The relative amount of “slack” (the “narrowness” of the limits) varies among systems– watch as we study various ones.
2. a. Because plasma and interstitial fluid mix readily with each other, physiologists often classify plasma volume plus interstitial fluid volume as ‘extracellular fluid volume’.
What percentage of total body water is in ‘extracellular fluid’?
Plasma (7% of total body water) + interstitial fluid (26%) = extracellular fluid (33%).
The rest of body water is, by definition, intracellular fluid (67%).
b. Assuming that 60% of body weight is total body water, what percentage of body weight is intracellular fluid?
67% of 60% = 40.2%, or approximately 40%.
c. One important difference between intracellular and extracellular fluid is their ion compositions. The primary cation in intracellular fluid is potassium (K+), but the main cation in extracellular fluid is sodium (Na+). A consequence of this difference is that cells can maintain a potential difference (voltage) between outside and inside (a ‘resting potential’) by controlling the permeability of their membranes to these two ions (soon to come in lecture). Reasoning from the fact that the inside of a cell at rest is generally more negative than the outside, do you think the cell membrane is more permeable to sodium or potassium ions?
Potassium runs down its concentration gradient, to make a slight excess of K+ ions outside the cell. Sodium also runs down its concentration gradient, to make a slight excess of Na+ ions inside the cell. As the resting potential is relatively negative inside, cell membranes at rest must be generally more permeable to potassium than to sodium.
a. Feedback control system (= closed loop control system)
A feedback control system is one in which the control action depends on the output.
b. Negative feedback control system
A negative feedback control system is one in which the controller acts to reduces the difference between the output and the set point.
c. Positive feedback control system
- A positive feedback control system is a closed loop control system in which the control action increases the effect of any
- disturbance. Therefore positive feedback is information sent to the controller from the output that is used to increase the effect of disturbances, thus driving the system to an extremum.
A feedforward (=openloop) control system
A feedforward control system is one in which the controller anticipates actions of system and disturbances. Feedforward control systems and negative feedback control systems are sometimes both used to maintain homeostasis, but for a feedforward control system, the control action need not depend on the output.
a. Explain why a negative feedback control system reduces the effects of any disturbance of the output.
Any perturbation of the output is sensed and fed back to the controller, which compares the perturbed output to the desired value of the controlled variable (the ‘set point’). The controller uses this difference between the actual output and the set point to determine the control action, which is to reduce the effect of the disturbance.
2. a. Explain why a negative feedback control system
reduces the effects of any disturbance of the output.
b. Does this also work for a disturbance of a sensor?
No. If the sensor is badly affected, the controller gets an incorrect measure of the output, so the control action can be erroneous, even increasing the effect of a disturbance. Think of the extreme case in which the sensor fails to work, or sends a signal unrelated to the output – then the controller can’t possibly do its job.
3. Explain why a negative feedback control system still works if properties of the controlled process change.
If the properties of the of the controlled process change, then the output will change with them. The system output will still be compared with the set point, and the controller will still act to reduce the effect of a disturbance. In this case, though, the ‘disturbance’ is due to the change in the controlled process.
4. Give an example of positive feedback in physiology. In your example, how does the controlled variable return from its extremum, after
the positive feedback control system has driven it there?
You might use any example in which the output is driven to an extremum -- bearing down in childbirth, sexual arousal, sneezing, voiding a lumen … In every physiological case I can think of, the positive feedback control ends once an extremum is reached, and a negative feedback controller restores homeostasis.
5. Multiple controls of a single system are common in physiology. Explain how the control of cardiac output when a person begins to
exercise is an example of the use of two different control systems, and why this multiple control is useful.
feedforward controller causes heart rate, blood pressure, ventilation and other variables to increase when exercise is anticipated. Either at rest or during exercise, all of these variables are under negative feedback control. So when one begins to exercise, the feedforward control is superimposed on the feedback control. During exercise, feedback control continues to operate, although the set points are different (appropriate to the level of power output). After exercise, the set points return to their resting values (the rate of their return depends on the variable being controlled).
1) What is the central dogma of genetic information?
- A: Transcription of DNA to RNA to protein: This dogma forms the backbone of molecular biology and is represented by four major
. The DNA replicates its information in a process that involves many enzymes: replication.
. The DNA codes for the production of messenger RNA (mRNA) during transcription.
. In Eukaryotic cells, the mRNA is processed (essentially by splicing) and migrates from the nucleus to the cytoplasm.
. Messenger RNA carries coded information to ribosomes. The ribosomes "read" this information and use it for protein synthesis. This process is called translation.
1) What dictates the three-dimensional structure of a protein?
A: its amino acid sequence.
1) What is the genetic information present in DNA described by?
A: the sequence of bases along a DNA strand.
1) List the key processes in a cell that can control the amount and level of activity of a protein.
A: Transcription of DNA, splicing of RNA, mRNA degradation, mRNA translation, post-transcriptional modifications, protein degradation.
1) How does a relatively limited set of genes and process gives rise to an infinite set of biological processes?
A: Each process allows variability in and of itself, and the combinatorial combination is what allows it
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