The digestive and circulatory systems

Why food must be digested

Food molecules such as starch and protein are far too large to pass through the wall of the small intestine. Digestion breaks them into small, soluble molecules — starch into glucose, protein into amino acids — which can be absorbed into the blood. That sentence answers most digestion questions.

The journey

Mouth (teeth and saliva begin starch digestion) $\to$ oesophagus $\to$ stomach (acid and protein digestion) $\to$ small intestine (digestion finishes, absorption happens) $\to$ large intestine (water absorbed) $\to$ rectum. Enzymes speed every chemical step.

Built for absorption

The small intestine is long, folded and covered in villi with a very thin wall and a rich blood supply. Every one of those features increases surface area or shortens the diffusion distance — link the feature to the function and the mark is yours.

The circulatory system

The heart pumps blood; arteries carry blood away from the heart at high pressure with thick walls; veins return it at low pressure and have valves; capillaries are one cell thick so substances can pass in and out.