Digestion in the mouth,stomach,duodenum and ileum
Digestion in the mouth
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 enzymaticaction.
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:
a) Sublingual salivary gland: beneath the tongue
b) Sub mandibular gland: under the jaw
c) 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.
Digestion in the stomach
Upon swallowing, the boluses move down the gullet to the stomach. The boluses enter the stomach via the lower esophageal/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:
a) Pepsinogen-This is activated to pepsin which breaks down proteins to peptides.
b) Rennin- Digests caseinogens protein in milk to casein (curd).
c) 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.
d) Mucus- Forms a protective barrier to the stomach wall against corrosion by theHCl. Mucus is secreted by goblet cells in the epithelial membrane of the alimentary canal.
Digestion in Duodenum
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:
a) Gall bladder in the liver- Secretes bile.
b) Pancreas- Secrete hormones and digestive enzymes.
Arrival of food in the duodenum stimulates secretion of:
i. Secretin hormone from the pancreas: Secretin stimulates secretion of pancreatic juice into the duodenum
ii. Cholecystokinin from the duodenal wall: This stimulates secretion of bile from the gall bladder.
Pancreatic juice contains:
a) Pancreatic amylase- This facilitates breakdown of the remaining starch into maltose.
b) Trypsin- Digests proteins into peptides.
c) Pancreatic juice-Digests lipids into fatty acids and glycerol.
d) Sodium hydrogen carbonate- This:
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 sodiumtaurocholate. These salts:
i. Aid in emulsification (breakdown of fat molecules into tiny fat droplets to increase surface area for digestion).
ii. The salts also provide a suitable alkaline medium for action of the duodenal enzymes.
iii. In addition they neutralize the acidic chyme.
Digestion in the ileum
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:
a) Maltase: speeds up breakdown of maltose to glucose.
b) Sucrase: speeds breakdown of sucrose to glucose and fructose.
c) Peptidase: speeds breakdown of peptides to amino acids.
d) Lipase: speeds breakdown of lipids to fatty acids and glycerol.
e) Lactase: speeds breakdown of lactose to glucose and galactose.
f) Polypeptidase: speeds breakdown of plypeptides into amino acids.Note:
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.