Digestion in Humans - Nutrition in Plants and Animals

- The process through which complex food substances is broken down physically and chemically into simpler food substances that can be absorbed by body cells.
- However, small molecules like those of vitamins, mineral salts and water are directly absorbed into the bloodstream without undergoing digestion.
- Digestion occurs in the mouth, stomach, duodenum and ileum.
- There are glands also associated to the digestive system. These include the pancreas, gall bladder, salivary glands.

- At the mouth both physical and chemical digestion takes place.
- The food is mechanically broken down by the teeth through grinding and chewing.
- This process is called mastication.
- Mastication reduces the food into small size to increase the surface area for enzymasticaction.
- The tongue helps in manipulation of the food as it mixes the food with the saliva secreted from the salivary glands. The salivary glands are:

• Sublingual salivary gland; beneath the tongue
• Sub mandibular gland: under the jaw
• Parotid gland: Found in the cheeks in front of the ears.

- All the glands have ducts through which saliva is directed to the mouth.
- The tongue also rolls the food into small round masses called boluses.
- The boluses are then pushed to the back of the mouth to initiate the swallowing process.
- The boluses are then moved to the stomach via oesophagus.
- Movement is facilitated by a wave of muscular contractions of longitudinal and circular muscles of the oesophagus known as peristalsis.
- There is a flap of cartilage, epiglottis which closes the wind pipe (trachea) during swallowing.

- Upon swallowing, the boluses move down the gullet to the stomach.
- The boluses enter the stomach via the cardiac sphincter (a muscular valve).
- The stomach has thick circular and longitudinal muscle layers which contract and relax to produce movements that mix the contents of the stomach.
- The mixing process is known as churning and results in formation of a fluid called chyme.
- Arrival of food in the stomach stimulates secretion of the hormone gastrin which stimulates the gastric glands in the stomach walls to secrete gastric juice which contains:

• Pepsinogen- This is activated to pepsin which breaks down proteins to peptides.
• Rennin- Digests caseinogens protein in milk to casein (curd).
• Hydrochloric acid- This:
• Activates pepsinogen to pepsin
• Provides a favorable medium for action of the enzymes rennin and pepsin
• Kills some bacteria ingested with food.
• Mucus- Forms a protective barrier to the stomach wall against corrosion by the HCl. - Mucus is secreted by goblet cells in the epithelial membrane of the alimentary canal.

- The chyme then passes down to the duodenum through the pyloric sphincter.
- Duodenum is the first section of the small intestine.
- In humans it measures about 25-38 cm.
- The chyme is let down into the duodenum in small quantities.
- Secretions that contribute to digestion at the duodenum are received from:

• Gall bladder in the liver- Secretes bile.
• Pancreas- Secrete hormones and digestive enzymes.

- Arrival of food in the duodenum stimulates secretion of;

• Secretin hormone from the pancreas: Secretin stimulates secretion of pancreatic juice into the duodenum.
• Cholecystokinin from the duodenal wall: This stimulates secretion of bile from the gall bladder.

- Pancreatic juice contains:

• Pancreatic amylase- This facilitates breakdown of the remaining starch into maltose
• Trypsin- Digests proteins into peptides.
• Pancreatic juice- Digests lipids into fatty acids and glycerol
• Sodium hydrogen carbonate- It provides alkaline medium for activity of the duodenum enzymes. It also neutralizes the acidic chyme.

- The bile juice contains bile salts that include sodium glycocholate and sodium taurocholate. These salts:

• Aid in emulsification (breakdown of fat molecules into tiny fat droplets to increase surface area for digestion).
• The salts also provide a suitable alkaline medium for action of the duodenal enzymes.
• In addition they neutralize the acidic chyme.

- Ileum is the final part of the small intestine.
- The inner cells contain secretory cells some of which secrete mucus while some secrete an alkaline fluid known as succus entericus (intestinal juice).
- The arrival of chyme in ileum stimulates secretion of intestinal juice which contains:

• Maltase: speeds up breakdown of maltose to glucose
• Sucrase: speeds breakdown of sucrose to glucose and fructose
• Peptidase: speeds breakdown of peptides to amino acids
• Lipase: speeds breakdown of lipids to fatty acids and glycerol.
• Lactase: speeds breakdown of lactose to glucose and galactose.
• Polypeptidase: speeds breakdown of plypeptides into amino acids

NOTES
- The mucus secreted by the goblet cells lubricates food along the alimentary canal and also protect the canal from being digested by enzymes.
- At the end of digestion in the ileum, the resulting watery emulsion is called chyle; it contains soluble end products of digestion ready to be absorbed.