DIGESTIVE SYSTEM

DIGESTIVE SYSTEM

The digestive system includes the alimentary tract and all glands and organs associated with digestion and assimilation of food in animals.

Digestion is the breakdown of large molecules of food into simple and absorbable form for use by animals. Ingestion is the taking in of food from outside through the mouth. Egestion is the discharge of undigested and unabsorbed food from the alimentary canal. Absorption is the passage of food through the wall of the alimentary canal. Assimilation is the manufacture of the body’s own materials from the absorbed substances and their use in metabolism.

TYPES OF DIGESTIVE TRACTS

There are two types of digestive tracts:

A.    Incomplete digestive tract: has only one opening- the mouth which serve for ingestion and egestion e.g Hydra, Planaria

B.     Complete digestive tract: has two openings to the outside world- mouth (for ingestion) and anus (for egestion) e.g. earthworm, goat

Complete digestive system has an advantage that incoming food does not mix with outgoing undigested food materials and each part is specialized to perform different functions.

Alimentary Canal of invertebrates

Animals have different alimentary tracts which vary in size, complexity, mode of feeding or the type of diet.

Alimentary canal of Planaria

Planaria is a free-living flatworm that feeds on small aquatic animals- zooplankton. It has a simple digestive tract with one opening- the mouth. The digestive tract consist of a ventrally placed mouth, muscular pharynx and intestine. The pharynx can be protruded when attacking a prey. Digestion is intracellular and undigested food is egested through the mouth. Digested food is distributed through the body by diffusion.

Alimentary canal of Earthworm

The earthworm has a complete digestive tract.

The mouth has a lip-like structure called prostomium which leads into the buccal cavity. The pharynx secrete mucus and protein digesting enzymes into the pharyngeal cavity. The oesophagus secrete CaCO3 [calcium trioxocarbonate(IV)] which removes excess calcium from ingested food. The crop acts as temporary storage chamber. Food is ground into small particles by the churning action of the gizzard. Digestion of food takes place in the intestine. Indigestible materials are removes from the body through the anus as casts.

Alimentary canal of Cockroach

The digestive tract of Cockroach consist of mouth (mandibles) which is modified for cutting and chewing. Oesophagus is short, narrow and tubular and connects the mouth with the crop. The crop acts as temporary storage. Most digestion occur in the crop. The gizzard is muscular and grinding of food takes place here. Digestion and absorption take place in the midgut. The rectum absorbs water and faeces is passed out through the anus.

Alimentary canal of birds

The mouth parts have been modified to beak with no teeth. Food is passed from the mouth through the oesophagus into the crop where it is stored temporarily. The food then passes to the stomach consisting of the true stomach (proventriculus) and muscular gizzard. The proventriculus also known as glandular stomach has glands which secrete digestive enzymes. The food then proceeds into the gizzard (grindular stomach) where grinding takes place. Bile and pancreatic juice is secreted in the first part of the intestine (duodenum) while digestion is completed and absorption takes place in the lower intestine. Undigested food passes into the rectum and anus (cloaca) to be discharged as faeces.

Similarities in the alimentary canal of Birds and Cockroach

1.         Possession of narrow oesophagus;
2.         Possession of crop;
3.         Possession of caecum;
4.         Possession of muscular gizzard;
5.         Possession of mid-gut/intestine;
6.          Possession of rectum.

Differences between the alimentary Canal of Birds and Cockroach

Bird	Cockroach
-mouth is modified into beak for pecking	mouth modified for chewing and biting;
- tongue is present in the mouth	tongue is absent in the mouth;
- duodenum is present	duodenum is absent;
- alimentary canal is long	alimentary canal is relatively short;
- hindgut terminates into cloaca	hindgut terminates into anus;
- malpighian tubules are not attached to the alimentary canal	malpighian tubules are attached to to the alimentary canal
- liver/pancreas present in the alimentary canal	liver/pancreas absent in the alimentary canal;
- salivary gland absent	salivary gland present.

Alimentary canal of Rabbit

The rabbit is an herbivore that feeds mainly on grasses and leaves. It digestive system consist of mouth, oesophagus, stomach, small intestine, large intestine, caecum, rectum and anus.

Food is physically broken-down in the mouth by the teeth. The food then passes into the stomach where gastric juice breaks down the food into simpler substances. Digested food is absorbed into the bloodstream through the walls of the small intestine. The rabbit has a very large caecum which contains a large number of beneficial microorganisms. These microorganisms digest fine food materials and undigested food from the caecum form soft moist pellets which are excreted through the anus.

Alimentary canal in Man

The mouth contains teeth, tongue and salivary glands. The teeth are used to cut, grind or chew (masticate) the food into tiny particles. The tongue rolls the food into bolus, allows mixing of the food with ptyalin and aids swallowing the food. Saliva contains the enzyme ptyalin, serve as lubricant for the mouth and solvent for the food. Food passes through the oesophagus through peristaltic movement into the stomach.

The stomach secretes renin (curdles milk) and pepsin (breaks down proteins into peptones). It also secretes HCl an acid which helps to kill some bacteria in the stomach. Churning movement of the stomach convert the food into semi-liquid called chyme.

Duodenum secrete amylase (convert starch to maltose), lipase (convert protein into peptones) and bile. The food is now in liquid form- chyle. Absorption and digestion takes place in the ileum. Enzymes secreted in the ileum include lipase, erepsin, maltase, sucrase and lactase.

Water is absorbed in the large intestine while undigested food is converted to faeces and egested through the anus.

The liver secretes bile which is temporarily stored in the gall bladder. Bile aid digestion of fats. The pancreas produce the pancreatic juice which contains digestive enzymes and produces the hormone – insulin which regulates the amount of sugar in the blood.

Digestion in Man

Enzyme	Secretion	Source	Site of action	Substrate	Product
Ptyalin	Saliva	Salivary glands	Mouth	Cooked starch	maltose
Pepsin	Gastric juice	Gastric glands	Stomach	Protein	Polypeptides (Peptone)
Renin	Gastric juice	Gastric glands	Stomach	Soluble casein (milk protein)	Insoluble casein
Amylase	Pancreatic juice	Pancreas	Small intestine (Duodenum)	Starch	Maltose
Trypsin	Pancreatic juice	Pancreas	Small intestine (Duodenum)	Protein	Polypeptides (peptones)
Lipase	Pancreatic juice	Pancreas	Small intestine (Duodenum)	Fats	Fatty acids and glycerol
Erepsin	Intestinal juice	Intestinal glands	Small intestine (Ileum)	Polypeptides (peptones)	Amino acids
Maltase	Intestinal juice	Intestinal glands	Small intestine (Ileum)	Maltose	Glucose
Lactase	Intestinal juice	Intestinal glands	Small intestine (Ileum)	Lactose	Glucose and galactose
Sucrose	Intestinal juice	Intestinal glands	Small intestine (Ileum)	Sucrose	Glucose and fructose
Lipase	Intestinal juice	Intestinal glands	Small intestine (Ileum)	Fats	Fatty acids and glycerol

Similarities in Alimentary Canal of Birds and Grasshopper

1. Both have narrow oesophagus
2. Both have crop for temporary storage of food
3. Both have muscular gizzard
4. Both have midgut

Differences in Alimentary Canal of Birds and Grasshopper

	Birds	Grasshopper
1	Mouth is modified into beaks	Mouth is modified into mandible and maxillae for biting and chewing
2	Duodenum is present	Duodenum is absent
3	The alimentary canal is relatively longer	The alimentary canal is relatively shorter
4	Alimentary canal ends in cloaca	Alimentary canal ends in anus
5	Malpighian tubules are absent	Malpighian tubules are present
6	Pancreas is present	Pancreas is absent
7	Tongue is present in the mouth	Tongue is absent in the mouth

FEEDING MECHANISMS IN SOME ANIMALS

A.          Absorbing mechanism: e.g. Tapeworm. They do not have alimentary canal, they possess a scolex or head with a rostellum surrounded by hooks and four suckers for attachment; they possess a flat body for large surface area for absorption; entire body surface adapted for absorption and thick cuticle to resist digestion by digestive enzymes.

B.           Biting and chewing mechanism: e.g. grasshopper, cockroach. They have four different mouth parts:

• -          Labrum (upper lip): this prevents food from falling off the mouth
• -    Mandibles: these are heavy toothed and law-like structure for cutting and chewing food materials
• -     Maxillae: a biting blade which breaks down the food chewed by mandibles into smaller particles
• -          Labium (lower lip): this prevent wastage of food from the mouth

C.     Sucking mechanism: e.g. mosquito, butterfly and housefly. The mosquito has piercing mouth parts called proboscis, which is used for sucking blood of animals. The mouth produce saliva to prevent clotting of the blood.

The butterfly has long coiled proboscis used for sucking nectars of flowers. The proboscis can be coiled or extended when in use.

The housefly has enlarged labella for sucking liquid food. It uses a sponging mechanism to absorb the food.

D.    Grinding mechanism: this is common among mammals. The grinding is aided by hard, strong teeth made of enamel and dentine. The teeth can withstand biting, chewing, grinding or cracking of food. There are different sets of teeth for special purposes- incissors for cutting bits of food, canine for tearing food, pre-molars and molars with wide surfaces for grinding.

E.     Trapping and absorbing mechanism: this is common in carnivorous (insectivorous) plants e.g. bladderwort, sundew. The plants have structures for trapping small insects. Sundew has long hairs which carry digestive glands which digest insects and digested food is absorbed into the plant.