Human Physiology: Why Your Heart Beats Without Your Brain
A tour of how the body really works, from the charge across every cell membrane to the hormone loops that keep you in balance.
Try 3 questions
1. A neuron's resting membrane potential is negative inside. Which ion movement is mainly responsible?
- Sodium leaking into the cell
- Chloride leaking into the cell
- Potassium leaking out of the cell
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Answer: Potassium leaking out of the cell
At rest the membrane is far more permeable to potassium, which leaks out down its gradient and leaves the inside negative, close to potassium's equilibrium potential.
2. If a cell's sodium-potassium pumps suddenly stopped, what would happen to its resting potential in the first few seconds?
- It would barely change at first
- It would instantly reverse its sign
- It would instantly drop to zero
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Answer: It would barely change at first
The pump contributes only a few millivolts directly; the potential depends on ion gradients, which take minutes to hours to run down once pumping stops.
3. Kidney cells move water across their membranes far faster than a pure lipid bilayer allows. How?
- ATP-driven pumps push water molecules across
- Protein water channels called aquaporins
- Water dissolves easily into the fatty layer
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Answer: Protein water channels called aquaporins
Aquaporins are channels that let water pass in single file at high speed; the kidney inserts more of them when the body needs to conserve water.
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