Villus absorption sim

What the sim shows

Absorption is the movement of digested food from the small intestine into the blood or the lymph. It happens mainly in the ileum, whose inner wall is covered in millions of villi. Each villus is covered by a wall of epithelial cells one cell thick, so the distance to the blood is short. Each epithelial cell has about 3000 microvilli on its free surface: fingers about 1 µm long and 0.1 µm wide, about 60 on each square micrometre, which multiply that surface by 1 + π × 0.1 × 1 × 60 = 19.85, the textbook "about 20 times". Glucose and amino acids pass into the blood capillaries, which drain into a venule and on to the hepatic portal vein and the liver. (The lacteal is named in the O-Level and IGCSE courses; AQA's course asks about villi as an exchange surface and does not name it.) Fatty acids and glycerol pass into the cells, join back into fat droplets that are too big to enter the capillaries, and go into the lacteal, a lymph vessel. The model: the wall lets food through in proportion to its area and inversely to its thickness (Fick's law); blood arrives with 5 mmol/dm³ of glucose and carries away what crosses, so a faster flow keeps the difference across the wall large. Active transport, using energy from the cells' mitochondria, takes glucose and amino acids in even when the gut has less than the blood. Rates are in units a minute, where a normal villus just after a meal takes in 100 of each; with no microvilli it takes in 9.1 units of glucose and amino acids: the surface is about 20 times smaller, but uptake falls only about 11 times, because the blood flow also limits it. Time moves only when you press Run 1 minute. Model limits: the units are relative, not measured; the gut's glucose (50 mmol/dm³ after a meal, 3 later) is illustrative; diffusion and active transport are separate simple terms; amino acids follow the same rule as glucose; a wall more than one cell thick never happens in a healthy gut, the knob shows why. Scales: the block is about 70 times life size, the board about 220 times in height (drawn wider than scale, with a thicker epithelium and longer microvilli so they show), and the round plaque 4500 times, at true proportions. The colours of the food molecules are a diagram key; real ones are invisible.

• Round plaque: epithelial cells, their microvilli and mitochondria
• Board: one villus cut open; blood (red) and lacteal (milky white)
• Food (a diagram key): glucose orange, amino acids violet, fatty acids and glycerol small yellow dots, fat droplets yellow

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