Homeostasis and body fluids
Introduced| English | Meaning |
|---|---|
| homeostasis/ˌhəʊmiːəˈstɑːsiz/ | Keeping the internal environment stable by feedback control. |
| set point/set pɔɪnt/ | A reference level that a regulatory system acts to maintain. |
| negative feedback/ˈneɡətɪv ˈfiːdbæk/ | A control process in which a change triggers responses that oppose that change. |
| positive feedback/ˈpɒzɪtɪv ˈfiːdbæk/ | positive feedback |
| intracellular fluid | The fluid inside cells, about two-thirds of body water, rich in K⁺. |
| extracellular fluid | The fluid outside cells, plasma and interstitial fluid, rich in Na⁺. |
Constant inside, changing outside
- The body is a controlled system. Its whole design goal is keeping the inside steady while the outside swings.
- Learn the control frame once, and every later system in this course is one more example of it.
The feedback loop
- Homeostasis 内环境稳态 runs on loops: a sensor detects a change, an integrator compares it with a set point 设定点, and an effector pushes back.
- In negative feedback 负反馈 the response opposes the change. Baroreceptors sense pressure, the brainstem integrates, and heart and vessels respond.
Blood pressure falls when a person stands up. Put the reflex loop in order.
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Sensor, integrator, effector, correction: the response opposes the change, which is negative feedback.
Positive feedback
- In positive feedback 正反馈 the response adds to the change: clotting and labour are the classic examples.
- It does not hold a steady state. It drives an event to completion, then stops when the event is done.
Positive feedback is how the body keeps blood glucose steady.
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Steady control uses negative feedback. Positive feedback drives events to completion, as in clotting or labour.
Three tanks of water
- A 70 kg adult holds about 42 L of water: 28 L of intracellular fluid 细胞内液 (two-thirds) and 14 L of extracellular fluid 细胞外液 (one-third).
- The extracellular part is 3 L of plasma plus 11 L of interstitial fluid between the cells.
Match each compartment to its volume in a 70 kg adult.
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28 + 11 + 3 = 42 L; plasma and interstitial fluid together make the 14 L of extracellular fluid.
Worked example: where does a drink go?
- Drink 2 L of water. Water crosses cell membranes freely, so it spreads in proportion to each tank: one-third (0.67 L) stays outside cells and two-thirds (1.33 L) goes inside.
- Give 1 L of isotonic saline instead: its sodium stays outside cells, so all of it stays in the extracellular fluid, and plasma gains $1 \times 3/14 \approx 0.21$ L.
A patient drinks 3 L of pure water. How many litres end up outside the cells?
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Water spreads in proportion to the tanks, one-third outside cells, so 3 × 1/3 = 1 L; the other 2 L goes inside cells.
A patient receives 2 L of isotonic saline. How many litres stay in the plasma?
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Saline stays in the 14 L extracellular space, of which plasma is 3/14: 2 × 3/14 ≈ 0.43 L.
Worked example: the loop that holds the wrong target
- (Fictional patient) A steadily low plasma sodium, yet the urine dilutes and concentrates normally around that lower level. The osmostat's set point has been reset low.
- The loop works perfectly; it just holds the wrong target. Loops fail in three ways: sensor, effector or set point, and diagnosis is deciding which.
Plasma sodium is steadily low, but the urine dilutes and concentrates normally around that level. Which part of the loop has failed?
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The loop holds a steady value perfectly, so sensor and effector work; the target itself has been reset low.
Three common slips
- Forgetting plasma is extracellular. Plasma is part of the 14 L.
- "Feedback means negative." Positive feedback exists; it completes events rather than holding a steady state.
- Missing the hidden water. Metabolism makes about 0.3 L a day, and skin and breath lose about 0.9 L a day.
Recap
- 42 L = 28 L inside cells + 14 L outside (3 L plasma + 11 L interstitial).
- Sensor → integrator → effector around a set point; negative feedback holds, positive feedback completes.
- Water spreads across all tanks; saline stays outside cells. Loops fail at sensor, effector or set point.