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Biophysics: The Physics of Living Bodies

Biophysics uses physics to understand the body. The heart is a pump that makes pressure; blood flows through vessels, and narrowing a vessel cuts the flow a lot because flow depends on radius to the power 4. Bones and muscles work as levers. We use units, graphs and safety limits to study and protect the body.

🎬 Step-by-step story

  1. A pump (the heart) and a tube (a blood vessel). Biophysics asks how physics rules explain what the body does.
  2. The heart squeezes and makes pressure. Pressure is a push on the blood.
  3. The push moves the blood cells along the vessel. Watch the red cells travel.
  4. Make the vessel a little narrower. The cells now move more slowly: a narrow tube resists the flow more.
  5. Cut the radius to half. The flow does not halve. It falls to 1/16, because flow depends on r to the power 4.
  6. Free play: slide the radius and read the flow. This is why a small blockage in an artery is a big danger.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Why does blood move at all?

It moves because the heart makes a higher pressure at one end than at the other. A pressure difference pushes fluid, just like a pump pushes water.

Does flow halve when the radius halves?

No. It becomes 1/16, because flow is proportional to r⁴. Set the slider to 50 percent to see 6.25 percent.

Why do narrow vessels raise blood pressure?

The narrow tube resists more, so the heart must push harder to keep the flow.

Can I try other radii?

Yes. In the last step move the slider and watch the red cells slow down or speed up.

Physics in living systems

Biophysics uses physics ideas (force, pressure, energy, waves, electricity) for living things. The body is a machine: the heart is a pump, lungs move air by pressure changes, bones and muscles are levers, nerves carry tiny electric signals, eyes and ears work with light and sound.

Blood flow: pressure, flow and radius

Blood moves because of a pressure difference between the heart and the rest of the body. Flow is the volume per second: Q = ΔP ÷ R, where R is the resistance of the vessel (like Ohm's law).

For a long thin tube, resistance depends strongly on the radius: R ∝ 1/r⁴. So at the same pressure, Q ∝ r⁴. Half the radius gives only 1/16 of the flow. To keep the flow the same through a narrower vessel, the pressure must go up.

Blood pressure is written as systolic/diastolic, for example 120/80 mmHg. 1 mmHg ≈ 133 Pa.

Body mechanics and health and safety

The forearm is a lever. The elbow is the pivot, the biceps pulls close to the pivot and the load is far away. So the muscle must pull with a much bigger force than the load weighs: F × dmuscle = W × dload (turning effects balance).

Pressure p = F ÷ A: a thin heel under a body gives high pressure; wide shoes lower it. Safety ideas: lift with legs, keep loads close to the body (short load arm), do not stay in loud noise (sound intensity harms the ear), and limit radiation doses (X-rays) to what is needed.

Analysing biological phenomena

To study the body with physics:

  1. Pick the physical quantity (pressure, force, speed, power).
  2. Draw a simple model (a pipe for a vessel, a lever for an arm).
  3. Use the right unit (Pa or mmHg, N, m/s, W).
  4. Estimate first: heart pumps about 70 mL per beat at 70 beats/min, about 5 L per minute.
  5. Check if the answer makes sense for a real body.

Try it

Sip water through a thin cocktail straw and then a wide straw (or two thin ones together). Predict which is easier. Then cut the radius in your head to half. How many times harder should it be?

Key formulas and definitions

Worked examples

1. A vessel's radius becomes 0.8 of its old value. What fraction of the old flow remains (same pressure)?

Q ∝ r⁴ = 0.8⁴ = 0.41, so about 41 percent.

2. A person of weight 600 N stands on one foot of area 0.02 m². Find the pressure.

p = F ÷ A = 600 ÷ 0.02 = 30 000 Pa = 30 kPa.

3. The biceps pulls at 4 cm from the elbow and a 5 kg bag (49 N) is held 32 cm from the elbow. Find the muscle force.

F × 0.04 = 49 × 0.32, so F = 15.68 ÷ 0.04 = 392 N.

Common mistakes

Practice quiz

1. Blood flows because of:
2. If a vessel's radius halves, flow becomes:
3. In the forearm lever, the pivot is the:
4. 1 mmHg is about:
5. Cardiac output equals:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What is biophysics?

Biophysics is the use of physics to understand how living things work, for example blood flow, breathing, muscles and nerves.

Why is a narrow artery so dangerous?

Flow depends on radius to the power 4, so a small narrowing cuts the flow a lot and raises the pressure the heart must make.

Why does lifting a heavy bag with a bent arm feel so hard?

The bag is far from the elbow and the biceps pulls close to it, so the muscle must pull with several times the weight of the bag.

Where this is taught

NetherlandsHAVO 4 (bovenbouw, 2e fase)Optional domains (choose two)
NetherlandsVWO 5Optional subdomains (choose two)

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