CoachThe small intestine folds its lining into circular folds, then into villi a millimetre high, each covered by a single layer of enterocytes whose apical membranes bear microvilli — the brush border — so that a tube of offers some of membrane (Chapter 4). Inside each villus run a capillary network, which carries away sugars and amino acids to the liver by the portal vein, and a central lacteal, a lymphatic vessel that carries away fat. The enterocytes live four days and are replaced from the crypts between the villi.
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Example 22.9 (The water budget)
Of the nine litres that enter a human gut each day, the small intestine absorbs eight, the colon a further one, and leaves in the faeces. The small intestine’s sodium-coupled uptake of glucose drags water with it — which is why a solution of glucose and salt, taken by mouth, rehydrates a child with cholera when the toxin has turned the crypts into secretors: the symporter is untouched and absorbs water faster than the toxin loses it (Chapter 7).
Example 22.4 (A meal’s chemistry)
A slice of bread with butter and cheese: the starch is attacked by salivary amylase in the mouth, stopped by the acid of the stomach, resumed by pancreatic amylase in the duodenum and finished by the brush-border maltase on the enterocytes themselves; the cheese protein is unfolded by the acid, cut by pepsin, cut further by trypsin and chymotrypsin, and finished to amino acids and dipeptides at the brush border; the butter is melted, emulsified by bile, split by lipase into fatty acids and monoglycerides, and ferried in micelles to the membrane. Three hours from mouth to blood.