Biology · Homeostasis, Nutrition & Biological Organisation
Homeostasis, regulated variables and negative or positive feedback
A pool has a steady amount even though material continually enters and leaves it. A larger input then raises the amount. Whether a response restores the earlier range depends on what the system senses and how its output affects that same pool.
Animal Diversity & Body Plans supplies the varied arrangements used in selected examples. Cells, Membranes & Organisation explains boundaries and transport; Energy, Enzymes & Metabolism explains energy conservation and coupling. The first lesson reintroduces cells, tissues and local exchange before using them. Gene Regulation & Cell Signalling is a useful return path for feedback, and Chemistry’s Solutions, Concentration & Solubility for the distinction between amount and concentration.
Back to the Biology mapBy the end, you can
Identify what is regulated, trace a feedback loop and distinguish opposing a disturbance from amplifying a process.
Name the quantity before naming its controller
Homeostasis is active maintenance of relatively stable internal conditions despite changing activity and surroundings. Stable does not mean motionless, and it is not thermodynamic equilibrium: an animal exchanges matter and energy while maintaining gradients. A steady reading can conceal continuous turnover. Regulation must be explained for a particular variable rather than attributed to 'balance' in general.
A regulated variable is a quantity whose value is sensed and influenced by the response. A reference range describes the values the system tends to maintain in the relevant context. The term set point is useful shorthand, but it need not be a single fixed number stored in one anatomical location. Biological rhythms, development and changing demands can alter the relevant range.
Distinguish a pool's amount from its concentration. Concentration is amount divided by volume. The teaching pool has a fixed volume of one volume unit, so an amount of five substance units corresponds to concentration five per volume unit. If volume changed, the same amount could give a different concentration. Our simplified comparison deliberately removes that additional variable.
A sensed pool with changing removal
Read the diagram in words
Input adds substance to the pool and removal takes it out. The sensor reports concentration to the comparison process; its output changes removal activity. The selected response opposes a rise in the sensed pool without transporting the substance through the sensor.
| Identifier | Structure or interpretation | Drawing x | Drawing y |
|---|---|---|---|
| supply | Material input | 0 | 2 |
| pool | Regulated pool | 3 | 2 |
| sensor | Concentration sensor | 3 | 0 |
| comparison | Reference comparison | 6 | 0 |
| effector | Removal effector | 8 | 4 |
| outside | Removed material | 11 | 2 |
| From identifier | From structure or process | To identifier | To structure or process | Relationship code | Meaning |
|---|---|---|---|---|---|
| supply | Material input | pool | Regulated pool | material-input | Substance enters |
| pool | Regulated pool | outside | Removed material | material-output | Substance leaves |
| pool | Regulated pool | sensor | Concentration sensor | measurement | Current concentration is sensed |
| sensor | Concentration sensor | comparison | Reference comparison | information-signal | Measured value |
| comparison | Reference comparison | effector | Removal effector | regulatory-signal | Greater excess increases removal |
| effector | Removal effector | pool | Regulated pool | negative-regulatory-effect | Removal tends to reduce the pool |
| From identifier | To identifier | Intermediate point order | Drawing x | Drawing y |
|---|---|---|---|---|
| comparison | effector | 1 | 8 | 0 |
| effector | pool | 1 | 3 | 4 |
| Quantity | Assigned value | Unit or counting convention |
|---|---|---|
| Fixed pool volume | 1 | Assigned volume units |
| Reference amount | 5 | Substance units |
| Reference concentration | 5 | Substance units per assigned volume unit |
| Identifier | Meaning |
|---|---|
| reference | Reference concentration: 5 |
| amount | Substance amount |
| amountUnit | Substance units |
| volume | Fixed volume: 1 |
| concentration | Amount ÷ volume |
| material | Material movement |
| regulation | Information and regulation |
| notEquilibrium | Stable conditions can include continuous flow |
Constructed pool with volume fixed at one volume unit and reference concentration five substance units per volume unit. No particular organ or substance is represented.
The response acts back on what was sensed
In negative feedback, a disturbance changes a variable, sensing alters a regulatory signal, and an effector response tends to oppose the change. The effector is the component doing the work; the response is its altered activity. A control diagram does not require every sensor, comparison and effector to be separate cells, or every comparison to occur in a brain.
The selected loop senses pool concentration and increases removal when it rises above five. Input and removal arrows carry material. Sensor and controller arrows carry information about the pool and alter activity; they do not transport the substance through the sensor. Closing the loop means that the response changes the quantity the system subsequently senses.
At each boundary in the discrete model, removal is 1 + 0.5 × (current amount − 5) substance units during the next interval. The rule is assigned only for the displayed amounts at or above five. Basal input is one unit per interval. A single larger input first lifts the pool; subsequent removal brings it progressively closer to five. A real delayed or capacity-limited loop can overshoot, oscillate or fail, so negative feedback alone does not promise instant restoration.
A finite return toward the reference
Read the diagram in words
The amounts at boundaries 0-4 are 5, 7, 6, 5.5 and 5.25. Interval input/removal pairs are 3/1, 1/2, 1/1.5 and 1/1.25. Every ending amount equals the starting amount plus input minus removal.
| Interval boundary | Pool amount |
|---|---|
| 0 | 5 |
| 1 | 7 |
| 2 | 6 |
| 3 | 5.5 |
| 4 | 5.25 |
| Interval | Starting amount | Input during interval | Removal during interval | Ending amount |
|---|---|---|---|---|
| 1 | 5 | 3 | 1 | 7 |
| 2 | 7 | 1 | 2 | 6 |
| 3 | 6 | 1 | 1.5 | 5.5 |
| 4 | 5.5 | 1 | 1.25 | 5.25 |
| Quantity | Assigned value | Unit or counting convention |
|---|---|---|
| Reference amount | 5 | Substance units |
| Basal input | 1 | Substance units |
| Basal removal | 1 | Substance units |
| Assigned excess-removal coefficient | 0.5 | Dimensionless ratio |
| Quantity | Assigned value | Unit or counting convention |
|---|---|---|
| Reference amount | 5 | Substance units |
| Axis | Structure or interpretation | Minimum | Maximum | Unit or counting convention |
|---|---|---|---|---|
| x | Interval boundary | 0 | 4 | Assigned steps |
| y | Pool amount | 0 | 8 | Substance units |
| Axis | Tick value |
|---|---|
| x | 0 |
| x | 1 |
| x | 2 |
| x | 3 |
| x | 4 |
| y | 0 |
| y | 2 |
| y | 4 |
| y | 5 |
| y | 6 |
| y | 8 |
Removal = 1 + 0.5 × (current amount − 5)
Displayed amounts are at or above 5
| Identifier | Meaning |
|---|---|
| step | Interval |
| start | Starting amount |
| input | Input during interval |
| removal | Removal during interval |
| end | Ending amount |
| noExactReturn | Finite record approaches 5 without reaching it |
| noContinuous | Values between boundaries unassigned |
Exact assigned discrete recurrence for displayed amounts at or above five; no sampling noise, extra delay, changing volume or continuous curve is assigned.
Positive feedback needs a different question
Positive feedback reinforces a change: greater activity promotes further activity of the same process. Positive and negative describe the direction of feedback, not whether an effect is beneficial. Local reinforcement participates in processes such as recruitment of platelet activity during clot formation, while limiting mechanisms and finite resources constrain the overall process.
The separate activation model contains sixteen units that can be active or inactive. While its amplification gate is open, active units double at each assigned step by recruiting inactive units: 1, 2, 4, 8, 16. The total number remains sixteen. There is no creation of new units, and no continued doubling is possible after the inactive pool is exhausted.
An independently assigned reset then returns all sixteen to the inactive state and closes the gate. This is a teaching state model, not a reconstruction of clotting or a measured biological rate. Resource exhaustion and an external stopping or reset mechanism are distinct explanations. An amplifying arrow on its own supplies neither.
Amplification with a finite inactive pool
Read the diagram in words
Active/inactive inventories are 1/15, 2/14, 4/12, 8/8 and 16/0 while the gate is open. Each state contains the same sixteen identities. A separate reset yields 0/16 with the gate closed.
| Identifier | State | Interval boundary | Active count | Inactive count | Gate open | Recruitment possible |
|---|---|---|---|---|---|---|
| S0 | Initial activation | 0 | 1 | 15 | Yes | Yes |
| S1 | Assigned amplification step 1 | 1 | 2 | 14 | Yes | Yes |
| S2 | Assigned amplification step 2 | 2 | 4 | 12 | Yes | Yes |
| S3 | Assigned amplification step 3 | 3 | 8 | 8 | Yes | Yes |
| S4 | Inactive pool exhausted | 4 | 16 | 0 | Yes | No |
| S5 | Separate reset and closed gate | 5 | 0 | 16 | No | No |
| State | Identifier | Meaning |
|---|---|---|
| Initial activation | U1 | Active |
| Initial activation | U2 | Inactive |
| Initial activation | U3 | Inactive |
| Initial activation | U4 | Inactive |
| Initial activation | U5 | Inactive |
| Initial activation | U6 | Inactive |
| Initial activation | U7 | Inactive |
| Initial activation | U8 | Inactive |
| Initial activation | U9 | Inactive |
| Initial activation | U10 | Inactive |
| Initial activation | U11 | Inactive |
| Initial activation | U12 | Inactive |
| Initial activation | U13 | Inactive |
| Initial activation | U14 | Inactive |
| Initial activation | U15 | Inactive |
| Initial activation | U16 | Inactive |
| Assigned amplification step 1 | U1 | Active |
| Assigned amplification step 1 | U2 | Active |
| Assigned amplification step 1 | U3 | Inactive |
| Assigned amplification step 1 | U4 | Inactive |
| Assigned amplification step 1 | U5 | Inactive |
| Assigned amplification step 1 | U6 | Inactive |
| Assigned amplification step 1 | U7 | Inactive |
| Assigned amplification step 1 | U8 | Inactive |
| Assigned amplification step 1 | U9 | Inactive |
| Assigned amplification step 1 | U10 | Inactive |
| Assigned amplification step 1 | U11 | Inactive |
| Assigned amplification step 1 | U12 | Inactive |
| Assigned amplification step 1 | U13 | Inactive |
| Assigned amplification step 1 | U14 | Inactive |
| Assigned amplification step 1 | U15 | Inactive |
| Assigned amplification step 1 | U16 | Inactive |
| Assigned amplification step 2 | U1 | Active |
| Assigned amplification step 2 | U2 | Active |
| Assigned amplification step 2 | U3 | Active |
| Assigned amplification step 2 | U4 | Active |
| Assigned amplification step 2 | U5 | Inactive |
| Assigned amplification step 2 | U6 | Inactive |
| Assigned amplification step 2 | U7 | Inactive |
| Assigned amplification step 2 | U8 | Inactive |
| Assigned amplification step 2 | U9 | Inactive |
| Assigned amplification step 2 | U10 | Inactive |
| Assigned amplification step 2 | U11 | Inactive |
| Assigned amplification step 2 | U12 | Inactive |
| Assigned amplification step 2 | U13 | Inactive |
| Assigned amplification step 2 | U14 | Inactive |
| Assigned amplification step 2 | U15 | Inactive |
| Assigned amplification step 2 | U16 | Inactive |
| Assigned amplification step 3 | U1 | Active |
| Assigned amplification step 3 | U2 | Active |
| Assigned amplification step 3 | U3 | Active |
| Assigned amplification step 3 | U4 | Active |
| Assigned amplification step 3 | U5 | Active |
| Assigned amplification step 3 | U6 | Active |
| Assigned amplification step 3 | U7 | Active |
| Assigned amplification step 3 | U8 | Active |
| Assigned amplification step 3 | U9 | Inactive |
| Assigned amplification step 3 | U10 | Inactive |
| Assigned amplification step 3 | U11 | Inactive |
| Assigned amplification step 3 | U12 | Inactive |
| Assigned amplification step 3 | U13 | Inactive |
| Assigned amplification step 3 | U14 | Inactive |
| Assigned amplification step 3 | U15 | Inactive |
| Assigned amplification step 3 | U16 | Inactive |
| Inactive pool exhausted | U1 | Active |
| Inactive pool exhausted | U2 | Active |
| Inactive pool exhausted | U3 | Active |
| Inactive pool exhausted | U4 | Active |
| Inactive pool exhausted | U5 | Active |
| Inactive pool exhausted | U6 | Active |
| Inactive pool exhausted | U7 | Active |
| Inactive pool exhausted | U8 | Active |
| Inactive pool exhausted | U9 | Active |
| Inactive pool exhausted | U10 | Active |
| Inactive pool exhausted | U11 | Active |
| Inactive pool exhausted | U12 | Active |
| Inactive pool exhausted | U13 | Active |
| Inactive pool exhausted | U14 | Active |
| Inactive pool exhausted | U15 | Active |
| Inactive pool exhausted | U16 | Active |
| Separate reset and closed gate | U1 | Inactive |
| Separate reset and closed gate | U2 | Inactive |
| Separate reset and closed gate | U3 | Inactive |
| Separate reset and closed gate | U4 | Inactive |
| Separate reset and closed gate | U5 | Inactive |
| Separate reset and closed gate | U6 | Inactive |
| Separate reset and closed gate | U7 | Inactive |
| Separate reset and closed gate | U8 | Inactive |
| Separate reset and closed gate | U9 | Inactive |
| Separate reset and closed gate | U10 | Inactive |
| Separate reset and closed gate | U11 | Inactive |
| Separate reset and closed gate | U12 | Inactive |
| Separate reset and closed gate | U13 | Inactive |
| Separate reset and closed gate | U14 | Inactive |
| Separate reset and closed gate | U15 | Inactive |
| Separate reset and closed gate | U16 | Inactive |
| From identifier | From structure or process | To identifier | To structure or process | Relationship code | Meaning |
|---|---|---|---|---|---|
| S0 | Initial activation | S1 | Assigned amplification step 1 | state-activation | Recruit inactive units |
| S1 | Assigned amplification step 1 | S2 | Assigned amplification step 2 | state-activation | Recruit inactive units |
| S2 | Assigned amplification step 2 | S3 | Assigned amplification step 3 | state-activation | Recruit inactive units |
| S3 | Assigned amplification step 3 | S4 | Inactive pool exhausted | state-activation | Recruit inactive units |
| S4 | Inactive pool exhausted | S5 | Separate reset and closed gate | independent-reset | Assigned reset; gate closes |
| Quantity | Assigned value | Unit or counting convention |
|---|---|---|
| Grid rows | 4 | Grid rows |
| Grid columns | 4 | Grid columns |
| Total units: 16 | 16 | Count |
| Meaning | Representation |
|---|---|
| Active | Filled circle |
| Inactive | Hollow circle |
| Identifier | Meaning |
|---|---|
| unit | Unit |
| active | Active units |
| inactive | Inactive units |
| total | Total units: 16 |
| gate | Amplification gate |
| open | Open |
| closed | Closed |
| reinforcement | Activity promotes further activation |
| noCreation | No new units created |
| reset | Reset is a separate mechanism |
Abstract activation states of sixteen conserved units, not a clotting pathway or a measured biological recruitment rate.
Worked example
Reconstruct the return toward the reference
Begin at amount 5. During the first interval, input 3 and removal 1 give 5+3−1=7. With basal input restored, the next removal is 1+0.5×(7−5)=2, giving 7+1−2=6.
At 6, removal 1.5 gives 5.5 next; at 5.5, removal 1.25 gives 5.25. Successive excesses above five are 2, 1, 0.5 and 0.25. The finite record approaches the reference without exactly reaching it, and material continues to flow.
Check your understanding
Does an unchanged concentration prove that no material moves?
Show reasoning
No. Equal inflow and outflow can maintain a steady concentration with continuous turnover. The pool model begins with one unit entering and one leaving each interval.
At amount 7, why does removal of 2 units give a later amount of 6 rather than 5?
Show reasoning
One input unit also enters during that interval: 7+1−2=6. The next value depends on both flows, not removal alone.
Why can the sixteen-unit activation model not keep doubling indefinitely?
Show reasoning
Its finite inactive pool is exhausted at sixteen active units. The subsequent reset is a separate assigned mechanism; amplification itself does not create extra units or explain the reset.
What to carry forward
Identify the sensed variable, the direction of the response and the material account; a feedback label alone does not establish a stable outcome.
References
Resources supporting this lesson’s explanations and examples.
- A physiologist's view of homeostasisHarold Modell, William Cliff, Joel Michael, Jenny McFarland, Mary Pat Wenderoth and Ann WrightPhysiology education review · 2015
- Biology 2eMary Ann Clark, Matthew Douglas and Jung ChoiOpen textbook · 2018Biology 2e33.1 Animal Form and Function33.2 Animal Primary Tissues33.3 Homeostasis34.1 Digestive Systems34.2 Nutrition and Energy Production34.3 Digestive System Processes
- Signaling During Platelet Adhesion and ActivationZhenyu Li, M. Keegan Delaney, Kelly A. O’Brien and Xiaoping DuMechanistic review · 2010