PHYSIOLOGY

© 2006 Elsevier Ltd59ANAESTHESIA AND INTENSIVE CARE MEDICINE 7:2

Most foods cannot be absorbed in the form in which they are present but require digestion into smaller compounds that can be more easily absorbed. The basic process of digestion for the three major foods (carbohydrates, proteins, fats) is hydrolysis. Most absorption takes place in the small intestine and the absorptive surface is increased about 1000-fold by: • valvulae conniventes (mucosal folds)• villi (finger-like projections composed of epithelial cells, blood

supply and a central lacteal)• microvilli (finger-like projections on the surface of epithelial

cells).

Absorption of ions and waterSodium is actively transported from inside the epithelial cell through the basal and lateral cell walls into the paracellular spaces. Intracellular sodium is replaced by sodium from the chyme moving down the electrochemical gradient. In addition, some potassium and hydrogen ions are transported in the opposite direction in exchange for sodium ions, and some chloride ions are transported in the same direction with the sodium ions. Osmotic equilibrium is maintained by water following sodium through the epithelial mem-brane. In the colon, the junctions between the epithelial cells are much tighter than those in the small intestine. This prevents back diffusion of ions and water, allowing more complete absorption of sodium, especially when under the influence of aldosterone. A typical adult ingests 2000 ml/day of fluid. In addition, about 7000 ml/day of fluid is secreted into the gastrointestinal tract. Of this 9000 ml, only about 1500 ml enters the colon and only about 200 ml is lost in the stool. Similarly 5–8 g/day of sodium is ingested and 20–30 g/day is secreted into the gastrointestinal tract, but less than 0.5% is lost in the faeces. If the absorptive mechanisms are overwhelmed (e.g. in severe diarrhoea) dehydration and electrolyte depletion can occur rapidly. The epithelial cells in the ileum and large intestine can absorb chloride ions in exchange for bicarbonate ions, which are secreted. Calcium, iron, magnesium, potassium, phosphate and other ions are all actively absorbed from the small intestine.

Adrian Thomas is Consultant Paediatric Gastroenterologist at Booth Hall

Children’s Hospital, Manchester. He qualified from Manchester where he

also completed an honours BSc in physiology. His training in paediatric

gastroenterology was completed in Manchester, London, and Sydney,

Australia. His research interests include inflammatory bowel disease and

nutritional support.

Digestive functionsAdrian Thomas

CarbohydratesThe main carbohydrates in the diet are sucrose, lactose, starch and cellulose (Figure 1). However, cellulose cannot be digested by humans. Starch is digested by salivary amylase (ptyalin) in the mouth and upper stomach. Ptyalin is inhibited by the low gastric pH but carbohydate digestion is continued in the small intestine by pancre-atic amylase, which is more powerful than salivary amylase. Starch digestion produces maltose and short chains of glucose molecules. These, together with ingested sucrose and lactose, are broken into monosaccharides by luminal enzymes (lactase, sucrase, maltase, α-dextrinase). Fructose crosses the epithelium by facilitated dif-fusion, while glucose and galactose undergo secondary active transport coupled to sodium. Thus, the final products of carbo-hydrate digestion are all monosaccharides, which are absorbed immediately into the portal blood. Most ingested carbohydrate is digested and absorbed in the first 20% of the small intestine.

ProteinsPepsin precursors (pepsinogen I and II) are activated by gastric acid and inactivated by alkali in the duodenum. Proteins are broken down to peptides in the stomach and by the major proteo-lytic pancreatic enzymes (trypsin, chymotrypsin, proelastase) in the small intestine. The peptides are further digested to free amino acids by carboxypeptidase (pancreatic), aminopeptidase and sev-eral dipeptidases (in the small intestinal brush border).D amino acids are absorbed by passive diffusion. L amino acids are actively absorbed by basic, neutral and amino acid transporters (the latter two are coupled with sodium). Dipeptides and tripeptides are absorbed by secondary active transport coupled with hydrogen ions, are hydrolysed to amino acids in the epithelial cell, then enter the blood by facilitated diffusion. Occasionally, large pep-tides and whole proteins are absorbed intact by endocytosis and subsequent exocytosis. This can result in severe allergic reactions and declines with age. Overall, about 50% of digested protein originates from food, 25% from digestive juices and 25% from desquamated cells.

FatsTriglycerides are the main dietary fat. They are emulsified by bile salts, lecithin and monoglycerides and the process is assisted by

Carbohydrates in food

Monosaccharides Disaccharides Polysaccharides

Glucose Sucrose Starch

(glucose + fructose)

Fructose Lactose Cellulose

(glucose + galactose)

Galactose Maltose Glycogen

(glucose + glucose)

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© 2006 Elsevier Ltd60ANAESTHESIA AND INTENSIVE CARE MEDICINE 7:2

PHARMACOLOGY

agitation in the small intestine. Procolipase is secreted by the pan-creas and is activated by trypsin. It displaces emulsifying agents and binds lipase to the fat droplets. Lipase (lingual, gastric, pan-creatic) produces free fatty acids, glycerols, monoglycerides and diglycerides. Cholesterol esters and phospholipids are hydrolysed by pancreatic cholesterol ester hydrolase and phospholipase A2

to release cholesterol, monoglycerides and fatty acids. Lipids and bile salts interact to produce micelles, which contain fatty acids, monoglycerides and cholesterol. Micelles facilitate fat digestion in three ways:• they lower the concentration of monoglycerides and free fatty

acids thereby allowing triglyceride hydrolysis to continue• they move down the concentration gradient to the brush border

where the lipids diffuse into the epithelial cells and are esteri-fied

• they are essential for cholesterol absorption. The bile salts are released back into the chyme and are re-absorbed in the terminal ileum, to be re-used to form micelles. Medium chain fatty acids (10–12 carbon atoms long) enter the portal blood directly. Longer fatty acids are re-esterified to triglyc-erides and together with cholesterol esters are coated with protein, cholesterol and phospholipid to form chylomicrons, which enter the lymphatics within the villi. Fat absorption occurs mainly in the proximal small intestine but also, to some extent, in the ileum. Overall, about 95% of ingested fat is digested and absorbed.

MalabsorptionThe causes of malabsorption are summarized in Figure 2.

Reasons for malabsorption

Loss of absorptive surface• Surgical resection of small intestine (short bowel syndrome)

• Villous atrophy (e.g. coeliac disease)

Specific enzyme defects• Lactase deficiency

Motility disorders• Intestinal pseudo-obstruction

Inflammatory disorders• Crohn’s disease

Pancreatic insufficiency• Cystic fibrosis

Hepatobiliary dysfunction• Cirrhosis

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