The Human Digestive System
The human digestive system consists of two parts; alimentary canal and associated digestive glands. The human alimentary canal is a continuous muscular digestive tube that runs through the body. Alimentary canal is 9 meters long tube extending from mouth to anus. Parts of Alimentary Canal;
Mouth: open into a large buccal cavity. Main function is to receive food and start mechanical digestion by mastication of the food. Mouth have parts; checks, lips, tongue and teeth.
Pharynx: oral cavity leads into the pharynx. The pharynx serves as a common passage for food and air.
Oesophagus: long tube which connects the buccal cavity to the stomach. A muscular sphincter; called gastroesophageal sphincter regulates the opening of oesophagus into the stomach.
Stomach: J-shaped bag-like structure. It is situated in the upper left portion of the abdominal cavity.
Small intestine: longest tube (7 meters long) of alimentary canal, lies coiled and folded in the abdomen. It is divisible into duodenum (U-shaped, short upper part), jejunum (coiled, slightly longer part, about 2 meters long) and ileum (highly coiled, longest, 4 meters long, twisted).
Large intestine: absorbs water and electrolytes and forms and stores faeces and has three parts;
The caecum is a small blind pouch at the junction of the small and large intestine,
The colon is the next part which is about 1.0 m long and much broader than ileum. Absorption of water takes place in colon and
The rectum is the last part of the large intestine, about 15cm long, open outside through anus.
Digestive Glands (Sources of Digestive Enzymes)
There are two sources of digestive enzymes: (i) the glandular cells of the lining of stomach and intestine, which directly pour their secretion into the lumen of the gut or the alimentary canal. (ii) Special glands such as the salivary glands, the liver and the pancreas which pour their secretions into the gut through their ducts.
Saliva is mainly produced by three pairs of salivary glands, the parotids (cheek), the sub-maxillary/sub-mandibular (lower jaw) and the sublingual (below the tongue). These glands situated just outside the buccal cavity secrete salivary juice into the buccal cavity.
Functions of Saliva
Chemical digestion: breaks down starch by the function of salivary amylase.
Helps chewing and swallowing,
Lubricating effect: moisturizes the inside of the mouth and creates smoother speech,
Solvent effect: dissolves food and allows the tongue to taste food,
Cleaning effect: washes away food debris and bacteria remaining in the mouth,
Antibacterial effect: Lysozyme, peroxidase and lactoferrin fight against pathogenic microorganisms,
Saliva helps in the digestion of food as it contains an enzyme salivary amylase which digests starch converting it into sucrose. That is why starch when chewed leaves a sweet taste in the mouth.
Liver is the largest gland in the human body and weight about 1.5kg in an adult human being. It is located in the upper right side of the abdominal cavity. The liver is divided into right and leaf lobes, and is enclosed by a fibrous capsule. It secretes bile, which gets collected in gall bladder and is finally poured into the duodenum through the common bile duct. The bile secreted by the hepatic cells passes through the hepatic ducts and is stored and concentrated in a thin muscular sac called the gall bladder. The duct of gall bladder (cystic duct) along with the hepatic duct from the liver forms the common bile duct.
Pancreas is a reddish brown gland located in the bend of the duodenum. Pancreas has an exocrine function of producing pancreatic juice that aids in digestion. Pancreas is closely associated with the small intestine.