Digestion and Absorption — Notes

This chapter is CBSE reading material for formative (school-based) assessment only; it is not examined in a board paper.

Food (carbohydrates, proteins, fats) must be broken into simple, absorbable substances by digestion, using mechanical and biochemical methods. The digestive system consists of the alimentary canal and the associated glands.

Mouth and teeth. The mouth leads to the buccal (oral) cavity. Each tooth is embedded in a socket of the jaw bone (thecodont). Humans form two sets of teeth — milk (deciduous) teeth replaced by permanent teeth (diphyodont). An adult has 32 permanent teeth of four types (heterodont): incisors (I), canines (C), premolars (PM) and molars (M). Their arrangement in each half of the upper and lower jaw, in the order I, C, PM, M, is the dental formula, 2123/2123 (8 per half jaw × 4 = 32). The hard chewing surface is enamel. The tongue is a freely movable muscular organ attached to the floor of the oral cavity by the frenulum; its upper surface has papillae, some bearing taste buds.

Pharynx and oesophagus. The short pharynx is a common passage for food and air; the oesophagus and trachea open into it. The cartilaginous epiglottis prevents food entering the glottis during swallowing. The thin, long oesophagus passes through the neck, thorax and diaphragm into the stomach; the gastro-oesophageal sphincter regulates its opening.

Stomach. A J-shaped bag in the upper left abdomen with a cardiac portion (where the oesophagus opens), a fundic region and a pyloric portion that opens into the small intestine, guarded by the pyloric sphincter.

Small intestine. C-shaped duodenum, long coiled jejunum and highly coiled ileum, which opens into the large intestine.

Large intestine. The caecum, a small blind sac with some symbiotic micro-organisms, from which the vestigial vermiform appendix arises; the colon — ascending, transverse, descending and sigmoid; and the rectum, opening out through the anus.

From the oesophagus to the rectum the wall has four layers:

• Serosa — outermost, a thin mesothelium (epithelium of visceral organs) with some connective tissue.
• Muscularis — smooth muscle, usually an inner circular and an outer longitudinal layer; an oblique layer in some regions.
• Sub-mucosa — loose connective tissue with nerves, blood and lymph vessels; in the duodenum it also has glands (Brunner’s glands).
• Mucosa — lines the lumen; forms irregular folds (rugae) in the stomach and finger-like villi in the small intestine. Cells lining the villi have microvilli, giving a brush border; these modifications increase the surface area enormously. Each villus has a network of capillaries and a large lymph vessel, the lacteal. Goblet cells secrete mucus for lubrication. The mucosa forms gastric glands in the stomach and crypts (crypts of Lieberkuhn) between the bases of villi in the intestine.

All four layers show modifications in different parts of the canal. (A simple model in this chapter multiplies the surface by 3 for folds, 10 for villi and 20 for microvilli, giving 600 times; published estimates vary.)

Salivary glands. Saliva is mainly produced by three pairs: the parotids (cheek), the sub-maxillary or sub-mandibular glands (lower jaw) and the sub-linguals (below the tongue).

Liver. The largest gland of the body, about 1.2 to 1.5 kg in an adult human, situated just below the diaphragm, with two lobes. Hepatic lobules, with hepatic cells arranged in cords, are its structural and functional units; each lobule is covered by a thin connective tissue sheath, Glisson’s capsule. Bile secreted by hepatic cells passes through the hepatic ducts and is stored and concentrated in the gall bladder. The cystic duct of the gall bladder and the hepatic duct form the common bile duct.

Pancreas. A compound (exocrine and endocrine) elongated organ between the limbs of the C-shaped duodenum. The exocrine part secretes alkaline pancreatic juice with enzymes; the endocrine part secretes the hormones insulin and glucagon.

The bile duct and the pancreatic duct open together into the duodenum as the common hepato-pancreatic duct, guarded by the sphincter of Oddi.

Mouth. Teeth and tongue, with saliva, masticate and mix food; mucus helps form a bolus, which is swallowed (deglutition) and moved down the oesophagus by peristalsis; the gastro-oesophageal sphincter controls its entry into the stomach. Saliva contains electrolytes (sodium, potassium, chloride and bicarbonate ions), salivary amylase and lysozyme. About 30 percent of starch is hydrolysed in the mouth by salivary amylase (optimum pH 6.8) into the disaccharide maltose. Lysozyme is an antibacterial agent.

Stomach. Gastric glands have mucus neck cells (mucus), peptic or chief cells (pepsinogen) and parietal or oxyntic cells (HCl and intrinsic factor, needed for absorption of vitamin B12). Food is stored for 4 to 5 hours and, churned with acidic gastric juice, becomes chyme. HCl converts pepsinogen into pepsin, which converts proteins into proteoses and peptones; HCl provides the optimal pH 1.8. Mucus and bicarbonates protect the mucosa from excoriation by the acid. Rennin in the gastric juice of infants helps digest milk proteins; small amounts of lipases are also secreted.

Small intestine. Muscular movements mix food with bile, pancreatic juice and intestinal juice.

• Bile has bile pigments (bilirubin, biliverdin), bile salts, cholesterol and phospholipids but no enzymes; it emulsifies fats into very small micelles and activates lipases.
• Pancreatic juice has inactive trypsinogen, chymotrypsinogen, procarboxypeptidases, and amylases, lipases and nucleases. Enterokinase from the intestinal mucosa activates trypsinogen to trypsin, which activates the other enzymes.
• Succus entericus (secretions of the brush border cells and goblet cells) contains disaccharidases (e.g. maltase), dipeptidases, lipases and nucleosidases. Mucus and bicarbonates from the pancreas give an alkaline medium (pH 7.8); Brunner’s glands of the sub-mucosa help.

Breakdown of biomacromolecules occurs mainly in the duodenum; the simple products are absorbed in the jejunum and ileum. No significant digestion occurs in the large intestine. Gut activities are under neural and hormonal control: the sight, smell or presence of food stimulates saliva, and local hormones from the gastric and intestinal mucosa control the secretion of digestive juices.

Absorption is the process by which end products of digestion pass through the intestinal mucosa into the blood or lymph, by passive, active or facilitated transport.

• Simple diffusion: small amounts of glucose, amino acids and chloride ions, down a concentration gradient.
• Facilitated transport: fructose and some amino acids, with the help of carrier ions such as sodium ions.
• Osmosis: water, along the osmotic gradient.
• Active transport: against the concentration gradient, using energy — amino acids, glucose, sodium ions.
• Fats: fatty acids and glycerol are insoluble, so they form micelles, move into the mucosa, are re-formed into protein-coated chylomicrons, and pass into the lacteals, which release them into the blood.

Where: mouth (certain drugs through the mucosa and under the tongue); stomach (water, simple sugars, alcohol); small intestine (principal organ: glucose, fructose, fatty acids, glycerol, amino acids); large intestine (water, some minerals and drugs).

Assimilation is the use of absorbed substances by the tissues. The large intestine also secretes mucus, which sticks waste particles together and lubricates them. Undigested material enters the caecum through the ileo-caecal valve (preventing back flow) and is stored in the rectum. Faeces there start a neural reflex causing an urge to remove them; defaecation (egestion) is a voluntary process carried out by a mass peristaltic movement.

Calorific value of food. Energy in food is measured in kilocalories (1 kcal = 4.184 kJ). Gross calorific values measured in a bomb calorimeter are about 4.1 (carbohydrate), 5.65 (protein) and 9.45 (fat) kcal per g; the energy the body actually obtains, the physiological fuel values, are about 4, 4 and 9 kcal per g. (Values are rounded and vary slightly between sources.)

Protein–energy malnutrition (PEM) affects infants and children whose diets lack proteins and total calories:

• Kwashiorkor — protein deficiency with adequate calories, often when mother’s milk is replaced too early by a high-calorie, low-protein diet: wasting of muscles, thinning of limbs, failure of growth and brain development, extensive oedema and swelling.
• Marasmus — simultaneous deficiency of proteins and calories, usually in infants under a year when mother’s milk is replaced too early by food poor in both: impaired growth, extreme emaciation, thin limbs, dry, thin and wrinkled skin; growth and brain development impaired.

Digestive disorders. Inflammation of the intestinal tract from bacterial or viral infections is the most common ailment; intestinal parasites (tapeworm, roundworm, threadworm, hookworm, pinworm) also cause infection.

• Jaundice: the liver is affected; skin and eyes turn yellow from deposited bile pigments.
• Vomiting: ejection of stomach contents through the mouth; a reflex controlled by the vomit centre in the medulla; nausea usually precedes it.
• Diarrhoea: abnormal frequency of bowel movement and increased liquidity of faeces; reduces absorption of food.
• Constipation: faeces retained in the rectum as bowel movements occur irregularly.
• Indigestion: food not properly digested, a feeling of fullness; caused by inadequate enzyme secretion, anxiety, food poisoning, over-eating and spicy food.