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Blue is blood that has given up its oxygen, red is blood that has just collected some. Click any stage to read what happens there.
The circulatory loop runs on its own; click any stage.
Biology asks you to remember a lot and to understand how several systems fit together. Those need different treatment. Systems you can poke at until their shape is obvious. Facts you need to see again before they fade, and anything you get wrong comes back later.
You describe the goal in your own words and the plan asks questions back until it is one you would follow. Goals usually look like these.
There is no fixed syllabus. The plan is built around your goal and drops what it does not need. These are the topics it draws from.
Each one has its own lesson block to try.
See how it works and what it costs.