Digestion and Absorption
Digestion begins in the mouth, where food is chewed by teeth and moved around by the tongue and muscles of the cheeks. It is mixed with saliva to form a soft mass or bolus, ready for swallowing. Saliva contains the enzyme ptyalin, which converts starches into maltose, and also moistens and lubricates the food, cleans the mouth, and keeps oral structures soft and pliable.
The bolus is pushed into the pharynx by the tongue. Pharyngeal muscles propel it into the oesophagus, where peristalsis moves it toward the stomach. The oesophageal walls are lubricated by mucus to facilitate this movement.
Stomach Digestion
The stomach acts as a temporary reservoir, allowing digestive juices to act on the food. Gastric juice contains water, mineral salts, mucus, hydrochloric acid, and enzymes such as pepsinogen, renin, and the intrinsic factor. Key functions include:
- Water liquefies the food for easier digestion.
- Hydrochloric acid acidifies food, stops salivary ptyalin, activates pepsinogen to pepsin, and kills harmful microbes.
- Pepsin converts proteins into peptones.
- Renin (in infants) converts soluble caseinogens into insoluble casein, which is further digested by pepsin.
- Intrinsic factor is essential for vitamin B12 absorption.
- Mucus protects the stomach wall mechanically and chemically.
Limited absorption occurs in the stomach, including water, glucose, alcohol, and some drugs. Muscular actions mix the food with gastric juice and propel it toward the small intestine.
Small Intestine Digestion and Absorption
The small intestine progresses food through peristaltic, segmental, and pendulum movements while secreting intestinal juice, which is alkaline and contains water, mucus, and enzymes. Key enzymatic actions include:
- Enteropeptidase: Converts inactive pancreatic trypsinogen and chymotrypsinogen to active trypsin and chymotrypsin, further converting peptones into peptides and polypeptides.
- Pancreatic and intestinal amylase: Convert polysaccharides into disaccharides.
- Peptidases: Convert peptides and polypeptides into amino acids.
- Lipase: Converts fats into fatty acids and glycerol.
- Sucrose, maltase, lactase: Convert corresponding disaccharides into monosaccharides.
The small intestine contains lymph follicles in the villi for protection against microbes. Nutrient absorption occurs as follows:
- Monosaccharides (mainly glucose) and amino acids are absorbed into capillaries of villi and transported to the liver via the portal vein.
- Fatty acids and glycerol are absorbed into lacteals and transported via the thoracic duct to the left subclavian vein, then to the liver for recombination.
Large Intestine Absorption
The large intestine absorbs water until semi-solid feces are formed. Mineral salts and some drugs are also absorbed into the blood. Strong peristaltic waves occur at long intervals, moving contents into the descending colon, pelvic colon, and finally to the anus for excretion.
Detailed Notes
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