Plicae, villi and microvilli multiply the area 600-fold
Expansion
Three levels of amplification
- Plicae circulares (valvulae conniventes) - permanent circular mucosal folds, visible to the eye. Increase area about 3 times. Tallest in the jejunum
- Villi - finger like mucosal projections, about 1 mm. Increase area about 10 times
- Microvilli - the brush border on each enterocyte, about 1 micrometre. Increase area about 20 times
3 x 10 x 20 gives roughly 600 times, producing a surface area of about 200 square metres.
Structure of a villus: a core of lamina propria containing a central lacteal (the lymphatic that absorbs chylomicrons), a capillary network draining to the portal vein, and smooth muscle, covered by enterocytes and goblet cells.
Crypts of Lieberkuhn lie between the villi and contain the stem cells that renew the epithelium every 3 to 5 days, plus Paneth cells secreting defensins and lysozyme.
“Fat goes to the lacteal, everything else to the portal vein.” That is why chylomicrons bypass the liver initially and why lymphatic obstruction causes a protein losing enteropathy and steatorrhoea.
Villous atrophy in coeliac disease removes the second level of amplification, which is why malabsorption is so pronounced.