Contents
Learning Objectives
By the end of this section, you should be able to:
- Describe the digestive processes.
- Describe the digestive system.
- Describe the components of the digestive system and function of each component.
Introduction
Food is essential for the survival of every living organism. Body cells require energy to function, and this energy is derived from food. Food is transformed into energy in cells of every animal's body with the aid of oxygen which we have discussed in a previous topic. Food is primarily one of three basic needs of every living thing. How an animal obtains its food and the body organs involved is what we will be discussing in this section.
Food and Digestion
Food is any material that can be broken down and utilised by the body. Many limit food to rice, beans, vegetables etc. If our bodies were able to break down plastic and convert it to materials needed by our body cells, plastic would have definitely been a food; plastic cannot be broken down by the body however, so it is not a food. There are many things around you that can be broken down by the body, but the society we are now is so advanced that we do not need to eat just anything anymore.
Digestion on the other hand involves the break down of large food molecules into materials that can be used by the body. Without digestion, many of the foods that we know would be very useless.
Digestive system
In higher organisms, there is no one organ that can perform the whole processes of digestion. Several organs that work together primarily for the ingestion, digestion and absorption of food, make up the Digestive system.
Functions of the Digestive system
The digestive system carries out 6 functions. These functions are:
- Ingestion
- Mechanical processing
- Digestion
- Secretion
- Absorption
- Elimination
Ingestion
Ingestion defines the taking in of food by an organism, usually by using it's mouth.
Mechanical processing
Mechanical processing involves the crushing of food. This action is carried out by higher animals, and it makes food to be able to move along the digestive canal (or digestive tract, to be discussed below) easily.
Digestion
Digestion is the chemical breakdown of food into smaller organic materials that can be absorbed by body cells.
Secretion
Secretion is the release of water, acids, enzymes, buffers, and salts that act on food.
Absorption
Absorption involves the movement of digested food, from the digestive canal, into the bloodstream.
Elimination
Elimination is the removal of wastes and undigested food materials from inside the body.
Digestion in the different Phyla of the animal kingdom
Different organisms in the animal kingdom have different ways through which they get their food and digest them.
Protozoans
Protozoans are majorly unicellular (one celled) organisms, and their digestive process involve the simplest mechanism. Some protozoans (e.g paramecium) have cilia (small hair-like structures) which create disturbance in water to sweep particles into their mouth opening through which digestion occur and their wastes, eliminated. This type of feeding is referred to as filter feeding. Some other protozoans do not have mouth, they engulf around food materials (e.g amoeba) this is called phagocytosis. Once the particle is within the protozoan, it is acted upon (or digested) by enzymes, until the amoeba can make effective use of it, wastes are then left in vacuoles (spaces) which exits the protozoan via exocytosis (=removal of substances from a cell).
Porifera (Sponges)
Sponges are simple multicellular organisms with holes. They drive in water into their structures via their holes (called ostia). While water moves through a sponge in a unidirectional orientation, particles and other small organisms are trapped, then water exits via a larger opening (called osculum). The trapped particles are digested and circulated within the mesohyl (internal structures) of the sponge while wastes exit via the osculum.
Cnidarians
Cnidarians like jelly fish, hydra are simple organisms with tentacles which trap other organisms and deliver them into the mouth. Digestion occur inside the structures of the cnidarian while wastes exit via the mouth.
Flat worms
Parasitic flatworms (e.g tapeworm) do not have mouth to ingest food. Instead, they absorb food which have already been digested in the guts of their hosts. This type of feeding is known as absorptive feeding. Wastes leave parasitic flatworms via specialised glands.
Round worms, non-parasitic flat worms
Nematodes and non-parasitic flat worms ingest food using their mouth, and wastes exit via anus opening. Food is digested and absorbed in their digestive canal.
Annelids
Annelids like earthworms burrow into soil, which serve as their food. This is called Substrate Feeding. Annelids ingest food using mouth, digest food in the digestive canal, and eliminate waste product via anus.
Mollusca, arthropoda
Molluscs like snails, and all arthropods have internal tracts in which digestion and absorption occur. Some arthropods (e.g butterfly, mosquito) draw fluids from the body of other organisms and they are called fluid feeders.
Chordates
Chordates like fishes, birds and mammals have the most complex digestive tract in which digestion occur, and wastes are eliminated via anus
Intracellular and extracellular digestion
Many people often mince these words so read carefully.
Intra- within, extra- outside.
In Intracellular digestion, digestion occur within a cell. A typical example is digestion in protozoans, they ingest and digest their foods in themselves (cells).
Contrast is the case of extracellular digestion, here, food is digested not in the cells, but in a digestive tract. The digested food is then absorbed into the blood stream where it is finally assimilated by cells. An example is the digestion in mammals.
Herbivores, carnivores, Omnivores
The different animals in the animal kingdom are known as heterotrophs, that is, they depend on other things for their food.
Herbivores are animals that feed on plants (or leaves, grasses).
Carnivores are animals that feed on other animals (flesh) while;
Omnivores feed on both plants and flesh.
Digestive Tract
The digestive tract is also called the gastrointestinal (GI) tract, or alimentary canal. It is a muscular tube that extends from the mouth, to the anus. The digestive tract passes through the pharynx, oesophagus, stomach, small and large intestines.
Components of the digestive system
Mouth
Mouth is the opening through which food is ingested by an organism. Mouth is home to teeth, tongue and salivary glands. Mechanical processing such as chewing take place in the mouth by the use of structures known as teeth (absent in birds and some insects). Teeth is food grinder, mainly to aid easy passage of food, down the tract. Also in the mouth are the salivary glands. Salivary glands secrete Ptyalin or salivary amylase which hydrolyses starch to maltose. Saliva is about 99% water, and it does not only moistens food but cleanses the mouth, dissolves food chemicals so that they can be tasted. There are three pairs of salivary glands: parotid, sublingual, and submandibular (also called the submaxillary gland).
Pharynx
Pharynx is a structure located just behind the mouth. Food that has been grinded and mixed with saliva, when found in the pharynx, is called Bolus. Reflex actions in the pharynx lead to the swallowing of food.
Oesophagus
Oesophagus is a tube–like structure which connects the pharynx (or mouth) to the stomach. Epiglottis is a cartilage that is located at the anterior end of the oesophagus. It closes when food is coming in from the mouth, making food enter only the oesophagus (and not the trachea or windpipe).
Food moves through the oesophagus by peristalsis.
Peristalsis refers to the wave of involuntary contraction and relaxation of the muscles which line the oesophagus.
Food from the oesophagus enter the stomach via gastro-oesophageal sphincter (junction at the beginning of the stomach).
Stomach
The stomach is an elastic, muscular sac, and the widest section in the digestive tract. Food is temporarily stored here, churned by the muscular contractions of the stomach, and is also acted upon by enzymes.
The food in the stomach is viscous, highly acidic, and a soupy mixture formed from the combination of food, saliva, and gastric gland secretions, this is called chyme–semiliquid form of food.
Stomach has 4 major sections:
- Fundus
- Cardia
- Body
- Pylorus
Glands in the stomach release chemicals called Gastric juice which act on food. The major glands in the stomach are parietal cells, G cells and Chief cells. Glands in the pylorus secrete mucus and hormones that coordinate and control digestion.
Parietal cells secrete:
- Intrinsic factor: Glycoprotein that aids in vitamin B12 absorption.
- Hydrochloric acid (HCl): Activates pepsinogen, keeps stomach at pH 1.5 – 2 (acidic) and kills microorganisms.
G cells (enteroendocrine cells) produce variety of hormones.
Chief cells secrete:
- Pepsinogen: activated by HCl to become pepsin (protein-digesting) enzyme.
- Newborns produce rennin and gastric lipase: these enzymes are important for the digestion of milk and fat respectively.
Small intestine
Food, from the stomach, enters the small intestine via pyloric sphincter. The small intestine is a long, coiled tube, where final digestion and absorption occur. The small intestine has three major regions:
- Duodenum
- Jejunum
- Ileum
The small intestines | photo credit: Medical gallery of Blausen Medical 2014
Duodenum
Duodenum is the upper, most active part of the small intestine, where chyme enters.
Duodenum acts as a “mixing bowl” because it receives chyme from stomach, and digestive secretions from liver, gallbladder, and pancreas. Duodenum has few circular folds and small villi, and its main function is to neutralize acidic chyme.
Jejunum
Jejunum is the region of the small intestine between the duodenum and ileum. It has numerous circular folds and abundant, long villi. Majority of chemical digestion and nutrient absorption occur in jejunum.
Ileum
Ileum is the final segment of small intestine; it connects the small intestine to the large intestine.
Absorption of food molecules into the blood is aided by tiny finger-like structures called villi (singular: villus) which are found in the walls of the small intestine. They also increase the surface area of the small intestine.
Large intestine
Large intestine is also known as large bowel.
Its major functions are:
- Reabsorbing water and compacting intestinal contents into faeces.
- Absorbing important vitamins generated by bacterial action.
- Storing faecal material prior to defaecation.
Three segments of the large intestine are: caecum, colon, rectum.
Caecum
Caecum is an expanded pouch connected to the end of the ileum of small intestine.
Caecum collects and stores materials from the small intestine.
It begins the process of compaction (compression into faeces). The opening between caecum and ileum is called ileocecal valve.
Caecum also connects to another structure called appendix, which acts as a lymphatic organ and contains many lymphatic nodules. Read more in Lymphatic system.
Colon
Colon is the longest portion of the large intestine. It has a larger diameter and thinner wall than the small intestine.
Colon is subdivided into four regions:
- Ascending colon
- Transverse colon
- Descending colon
- Sigmoid colon
Rectum
Rectum marks the end of the large intestine and also the digestive tract. It is expandable for temporary faeces storage. Movement of faecal materials into the rectum triggers the urge to defecate.
Major characteristics of the Large intestine wall
- Lacks villi.
- Contains several intestinal glands, Dominated by mucin-secreting goblet cells.
- Mucus lubricates faeces as it becomes drier and more compact.
- Internal lining does not produce enzymes.
- The rectum terminates into the Anus.
Absorption in the large intestine
This accounts for less than 10 percent of all nutrient absorption in the digestive tract.
Vitamin absorption like Biotin, vitamin K, vitamin B5 which are produced by normal bacteria in the colon, occurs in the large intestine.
Water reabsorption prevents dehydration.
Approximately 1500 mL of material enters colon, and over 1000 mL of water is reabsorbed through osmosis, approximately 200 mL of faeces are finally ejected out into the rectum.
Faeces
About 200 mL of faeces are ejected each day.
Faeces is a mixture of water, indigestible materials, inorganic matter, remains of epithelial cells, bacteria and bacterial compounds.
Bacterial compounds contribute to odour of faeces (which the main contributors are):
- Ammonia.
- Indole and skatole (Nitrogen-containing compounds).
- Hydrogen Sulphide (produces a “rotten-egg” odour).
Accessory Digestive organs
There are four accessory digestive organs:
- Salivary glands
- Pancreas
- Liver
- Gall Bladder
Salivary glands
Salivary glands comprise of 3 pairs of glands which secrete saliva into the oral cavity through ducts. Saliva from each pair has slightly different properties.
Sublingual salivary glands
These glands lie under either side of the tongue, and they secrete into numerous sublingual ducts which open in the sublingual frenulum.
Saliva from the sublingual salivary glands act as a buffer and lubricant, and make up about 5 percent of total saliva secretion.
Submandibular salivary glands
These glands lie along the inner surface of the mandible and secrete saliva into a submandibular duct.
Saliva from submandibular glands contain a mixture of buffers, mucins, salivary amylase and provide about 70% of total saliva secretion.
Amylase is an enzyme that breaks down starch into maltose.
Parotid salivary glands
Parotid salivary glands produce secretion containing large amounts of salivary amylase and provides about 25% of total saliva secretion.
Pancreas
Pancreas are usually located posterior to (or at the back of) the stomach.
A pancreas is divided into head, body and tail.
It secretes pancreatic juice (mixture of water, ions, buffers and enzymes) which is carried via ducts, into the duodenum of the small intestine, and endocrine hormones (produced in the pancreatic islets) which is delivered into the bloodstream.
Major enzymes of the pancreatic juice
Pancreatic alpha-amylase: almost identical to salivary amylase, it breaks down starch.
Pancreatic lipase: breaks down certain lipids or fats into absorbable forms.
Nucleases: breaks down RNA or DNA.
Proteolytic enzymes: breaks down protein complexes into a mixture of dipeptides, tripeptides and amino acids. These enzymes are released from the pancreas in their inactive forms, they become activated in the duodenum. Active forms include trypsin, chymotrypsin, carboxypeptidase, elastase.
Digestive Hormones secreted by the pancreas include insulin and glucagon (explained in endocrine system).
Liver
Liver is popularly known as the largest internal organ in animals.
It is wrapped in tough fibrous capsule and composed of 4 lobes: right, left, caudate and quadrate.
The Liver performs so many functions which includes some digestive functions like production of bile, storage of excess glucose in the form of glycogen.
Liver cells (or Hepatocytes) secrete bile into a network of narrow channels called bile canaliculi. Bile canaliculi merge to form bile ductules.
Bile ductules carry bile to bile ducts in nearest portal area.
Bile play a role in the digestion of fats. Bile contain bile salts which break lipid droplet apart into tiny particles in a process called emulsification. Bile salts also surround fats to form micelles. Pancreatic lipase then acts on these small micelles to aid absorption.
Glucagon which is produced in the pancreas, also enter the liver from the blood to stimulate the break down of stored, inactive, excess glucose (glycogen) which has been stored in the liver. This process takes place when the body cells require more glucose and blood-glucose level is low.
Gall bladder
Gall bladder is a hollow, pear–shaped organ which is located behind the right lobe of the liver.
It is divided into three regions: the fundus, the body, and the neck.
It stores and concentrates bile which has been secreted from the liver.
Path of Bile
Right and left hepatic ducts collect bile from the liver bile ducts.
Hepatic ducts unite to form the common hepatic duct.
Bile flows from the common hepatic duct into:
- The bile duct (to the duodenum).
- The cystic duct (to the gallbladder for storage).
When needed, contraction of the gallbladder forces bile back along the cystic duct to the common bile duct.
The common bile duct penetrates the wall of the duodenum and meets the pancreatic duct at the duodenal ampulla (in the duodenal papilla).
The hepatopancreatic sphincter encircles the lumens (hole or pathway) of these areas, where they enter the duodenum. This prevents flow of bile into the duodenum except at mealtime.
Digestion in birds and poultry animals
Birds do not chew or digest food the same way as humans or other mammals do.
The process of birds' digestion is as follows:
Birds pick up their food (feed or worms or grains) with their beak. Birds cannot chew food because of the absence of teeth.
Food mix with saliva in their bill and roll down the tongue where it is swallowed into the oesophagus. Food then move down into the crop.
The crop is where food is temporarily stored and released bit by bit for slow digestion.
From the crop, food move into the proventriculus which is where food is mixed with mucus and several digestive juices.
From the proventriculus, food move into the gizzard (also called ventriculus) where mechanical processing takes place. This is where the grinding of food into smaller pieces takes place (since there's absence of teeth). If the food is tough, it may move between the proventriculus and the gizzard several times for efficient breakdown and digestion.
From the gizzard, food move into the small intestine where final digestion and absorption take place.
Waste products move into the short, large intestine. Large intestine of birds is joined with the urinary system and terminates in the cloaca through which wastes are eliminated.
Digestion of the several classes of food
Digestion of carbohydrates
Carbohydrates (example cassava grain, corn meal, rice, yam etc) digestion starts from the mouth, with the action of salivary amylase (from saliva) which hydrolyses starch into maltose. Carbohydrate move through the oesophagus into the stomach. There's no significant action on carbohydrate here, until food move into the duodenum. In duodenum, Amylase from the pancreas properly break down starch into maltose, which move into the small intestine where it as acted upon by maltase. Maltase break down maltose into sucrose. Sucrose is then broken down by sucrase into glucose. Glucose is the absorbable form of carbohydrates, and it is assimilated by body cells where it is used to produce energy in the mitochondria.
Digestion of Proteins
Proteins (example legumes, bean meal, meat etc) digestion starts in the stomach. An enzyme called pepsin breaks down complex proteins into simpler forms called peptones. Peptones enter the duodenum in the small intestine where it is acted upon by trypsin and is converted into polypeptides. Polypeptides is then broken down by the action of erepsin which converts polypeptides into dipeptides, tripeptides and amino acids. Amino acids is the absorbable/usable form of proteins.
Digestion of Fats and Oil
Fats and Oil digestion starts in the duodenum, where fat is acted upon by bile salts which break fat down into tiny droplets (by emulsification), and with the help of lipase, fat is converted into fatty acids or glycerol which can be utilised by the body.
Digestion of vegetables
Vegetables are made up of highly insoluble fibre which cannot be broken down by animals' digestive systems. These fibres can only be broken down by an enzyme called cellulase which is not produced in the bodies of animals. However, bacteria in the gut help digest vegetables and fibres. In herbivorous ruminants, vegetables are fermented to produce important nutrients for the body.
Diseases of the Digestive system
Gingivitis: inflammation of the teeth gums, lead to tooth decay amongst other teeth diseases.
Mumps: infection of the salivary glands caused by mumps virus, mostly affect the parotid salivary gland.
Esophagitis: inflammation of the oesophagus. Caused by acidic contents from the stomach reaching the esophagus via leaking gastro-esophageal sphincter.
Gastro-esophageal reflux: results in a symptom commonly called heartburn. Caused by backflow of acidic contents from the stomach into the oesophagus.
Hepatitis: inflammation of the liver. Caused by abuse of alcohol, drugs or infection.
Cirrhosis: degeneration of hepatocytes (or liver cells) and replacement with scar tissue. Also caused by alcohol abuse, drugs or infection.
Viral hepatitis A, B and C: Virus destroys liver cells. Leads to inflamed, tender liver and high fever.
Jaundice: excessive bilirubin, causes yellowing of skin and eyes.
Gall stones: crystals of insoluble minerals and salts forming in the bile ducts and cystic ducts when bile becomes too concentrated.
Cholecystitis (chole - bile, kystis - bladder, itis - inflammation): Irritation and damage to the gallbladder wall caused by gallstones too large to pass.
Gastritis: inflammation of the mucous membrane lining the stomach, caused by ingesting drugs (e.g., aspirin, alcohol), severe stress, bacterial infection, ingestion of strong chemicals.
Peptic ulcer: forms when gastric enzymes and acids erode through the stomach or duodenal lining. Gastric ulcer is a peptic ulcer in the stomach wall. Duodenal ulcer is a peptic ulcer in the duodenum wall. Bacterial infection by Helicobacter pylori responsible for over 80 percent of peptic ulcers.
Pancreatitis: inflammation of the pancreas. It can be due to duct blockage, viral infection, or toxic drugs (including alcohol).
Enteritis: inflammation of the small intestine. Leads to watery bowel movements (diarrhea).
Dysentery: inflammation of small and large intestine, producing diarrhea containing blood and mucus.
Colitis: inflammation of the colon.
Colorectal cancer: third most common cancer in the world. Most common in those over age 50. Begin as small, localized tumors (polyps) which metastasize to give rise to colorectal cancer.
Summary
🍛Food is required by every living organism to grow and live. Animals obtain their food, ingest and digest it, food is then converted into energy for cells.
🍛Several organs and tissues that aim at digestion are collectively referred to as digestive system.
🍛Food is ingested and swallowed. Enzymes act on it to break it down to the most absorbable forms.
🍛Intracellular digestion is a digestion that occurs insidd the cell of an organism, especially unicellular organisms, while extracellular digestion occurs in a digestive tract, and then simple molecules are passed down to cells.
🍛Filter feeders create disturbance in water and then sweep particles into their mouth. Absorptive feeders absorb already digested food into their bodies. Fluid feeders feed on fluids. Substrate Feeders feed on things that they live on.
🍛The components of the digestive system comprises the mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus.
🍛Ruminants have 4 stomachs while non-ruminants have one.
🍛Birds have crop, proventriculus and gizzard.
Key Terms
By understanding this topic, you should be able to explain all these Key Terms, if you can't, watch the videos below and read the post again.
Food • Digestion • Ingestion • Secretion • Absorption • Elimination • Phagocytosis • Exocytosis • Filter feeding • Intracellular digestion • Extracellular digestion • Herbivore • Carnivore • Omnivore • Gastrointestinal tract • Ptyalin • Salivary amylase • Sublingual salivary gland • Submandibular salivary gland • Parotid salivary gland • Bolus • Pharynx • Oesophagus • Epiglottis • Peristalsis • Gastro-esophageal sphincter • Stomach • Chyme • Gastric Juice • HCl • Pepsinogen • Pepsin • Rennin • Pyloric sphincter • Duodenum • Jejunum • Ileum • Villi • Caecum • Colon • Rectum • Indole • Skatole • Amylase • Maltose • Maltase • Hepatocyte • Bile • Glycogen • Micelles • Gall Bladder • Lipase • Hepatopancreatic sphincter • Crop • Proventriculus • Gizzard • Trypsin • Erepsin • Fermentation
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Discuss and Explain
1. Explain why and how contents from the stomach have to be neutralised before digestive process continues in the small intestine.
2. Explain why the stomach has to be acidic and what will happen if it were alkaline.
3. Briefly explain the peristaltic movement of the oesophagus. What would be the effect if thickened muscles line the walls of the esophagus.
4. During test of a patient animal that was reported sick, fat build up was discovered in the duodenum of the animal. List the components of the Animal's digestive system you would deduce have been damaged and why?
5. The stomach of an Animal has permanently lost its acidity, explain which digestive functions would be altered.
6. Damage to the parotid salivary glands would affect the digestion of which nutrient? Explain why?
7. Explain the function of the gall bladder in the digestive process.
8. Describe the enzymes that must be present for proteins to be digested.
9. Explain the effects of drug and alcohol in the digestive system of humans.
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