Interactive physiology module · 12–15 min
Journey of a Red Blood Cell Inside a Human
Follow one erythrocyte through pulmonary and systemic circulation. Predict what happens next, track oxygen saturation, and connect anatomy to function.
01 · Guided journey
Trace the circulation loop
Arteries and veins are named by direction of flow—not by oxygen content.
02 · Mechanisms
What changes along the way?
Loading in the lungs
Alveolar PO₂ exceeds pulmonary-capillary PO₂. Oxygen diffuses into blood and binds haemoglobin.
Unloading in tissues
Low tissue PO₂ plus higher CO₂, H⁺ and temperature favour oxygen release—the Bohr effect.
Pressure drives flow
Blood flows down a pressure gradient. Arteriolar radius strongly influences resistance and regional perfusion.
Flow = ΔP / Resistance03 · Mental model
Five linked ideas
Open each idea to connect anatomy, gradients, haemoglobin and whole-body oxygen delivery.
04 · Compare the circuits
Same flow, different pressures
At steady state, the same volume passes through both circuits. Their resistance—and the pressure required—are very different.
- PurposeGas exchange
- Typical arterial pressure25 / 10 mmHg
- Vessel characterThin, compliant
- PurposeOrgan perfusion
- Typical arterial pressure120 / 80 mmHg
- Vessel characterThicker arterial walls
Key inference The left ventricle does not pump more blood. It produces more pressure to move the same flow through the higher-resistance systemic circuit.
05 · Thought experiment
Make the tissue work harder
Change local metabolic activity and observe the predicted direction of physiological change.
06 · Extend the model
When saturation is not the whole story
Anaemia
Pulse oximetry may read 98%, yet fewer haemoglobin molecules mean lower arterial oxygen content. The percentage is normal; carrying capacity is not.
Carbon monoxide
CO occupies haemoglobin sites and increases affinity at remaining sites. Standard pulse oximetry can be misleading while tissue delivery is impaired.
Exercise
Cardiac output rises, active muscle receives more flow, and local conditions favour unloading. Venous saturation can fall as extraction increases.
Right-to-left shunt
Some venous blood bypasses ventilated alveoli and mixes with oxygenated blood, lowering arterial oxygenation.
07 · Knowledge check
Test the circuit
SCORE 0/3
Prediction checkpoint