Digestion and Absorption of Food

Introduction

Digestion and absorption of food are two essential processes for the survival of any living organism. There are mainly two types of organisms that can be classified on the basis of nutrition and these are autotrophs and heterotrophs.

1. Autotrophs – The ones that can make their food with the help of solar energy such as plants and blue-green algae.
2. Heterotrophs- The ones that are dependent on autotrophs for their food such as human beings.

Digestion and absorption of food are two important aspects of nutrition because without both of these processes eating food is of no use.

Digestion

Digestion is the process by which complex food substances are broken down into simpler substances which are easily absorbed and thereafter utilised by the organism.

Human Digestive System

The system that helps in completing the process of digestion is called the digestive system. Digestion is done by using various mechanical and biochemical methods. 

The human digestive system has several parts such as the alimentary canal, various glands, and other parts.

Alimentary Canal

It is a long tube from the mouth to the anus. It is about 6 to 9m in length. It is lined by glandular epithelium. It includes various organs such as the mouth, buccal cavity, pharynx, oesophagus, stomach, small intestine, large intestine and anus. There are several important glands associated with the alimentary canal such as salivary glands, gastric glands, liver,gallbladder, pancreas and intestinal glands.

Mouth and Oral cavity

The mouth is a transverse slit-like opening that is surrounded by lips. The upper maxilla i.e.upper jaw and lower mandible i.e. lower jaw form the oral cavity. The uvula is a tiny finger-shaped extension of the roof. It helps in secreting mucus and lubricating the cavity. The stratified squamous epithelium lines the buccal cavity that helps in secreting mucus. The cheeks house the lateral walls of the oral cavity.

The tongue is a muscular organ at the bottom of the oral cavity which is joined by the frenulum (small fold of mucous membrane). It conducts many activities including absorption, swallowing, and chewing of food materials, and it also includes taste buds.

Teeth

Dentition is made up of the number of teeth present, their order and the shapes they form. In humans the dentition is of following types :

1. Thecodont
2. Heterodont
3. Diphyodont

The presence of teeth in sockets (cavities) in the upper and lower jawbones is known as the thecodont dentition.

Heterodont dentition is a condition characterised by the occurrence of multiple kinds of teeth like incisors, canines, premolars, and molars are the four categories of teeth found in human beings.

•       Incisors- Responsible for cutting food into small pieces
•       Canines- Responsible for tearing food and are dagger-shaped
•       Premolars and Molars- Responsible for crushing and grinding the food.

Occurence of teeth twice in lifetime is termed as  diphyodont dentition.Teeth appears twice in human life. The first set of teeth are called milk teeth which are replaced by another set of teeth called permanent teeth.. Milk teeth are 20 in number while an adult human’s mouth contains 32 permanent teeth.

Pharynx

Pharynx is the part which starts from the hind part of both the buccal cavity and the nasal passage.It connects mouth to oesophagus and nose to trachea via larynx .Pharynx allows air passage to trachea through glottis which is an aperture in the larynx. During breathing, the glottis remains constantly open. The glottis is covered by the epiglottis which is a flap-like structure. Epiglottis covers glottis and blocks food from going into the windpipe while swallowing of food.Thus, pharynx gives passage to food and air. 

Oesophagus (Oesophagus)

The oesophagus is a lengthy tube made up of muscles. It connects the throat to the stomach. This tube enters the body through the neck, thorax, and diaphragm. Peristalsis is the wave-like contraction and relaxation of the oesophagus. This helps in the passage of food to the stomach.

Stomach

It is the biggest part of the alimentary canal. It is a C-shaped structure. It is present in the upper region. It is divided into four parts:

1.       Cardiac – It is where the oesophagus opens.
2.       Fundic- Helps in taking air into the stomach while swallowing food.
3.       Body- It is the central region.
4.       Pyloric- It opens in the small intestine.

Small intestine

It is the longest part of the alimentary canal. The stomach and small intestine are connected by the pyloric sphincter. Small intestine is divided into three sections:

1.       Duodenum – It is a U shaped region 
2.       Jejunum – It is longer than duodenum.
3.       Ileum- It is a highly coiled and longest region of the small intestine.

Large intestine:

The large intestine is about 2 metres in length. It is shorter but broader than the small intestine.  It has three parts: caecum, colon, and rectum.

Caecum is a tiny pouch which attaches the ileum (last part of the small intestine) to the large intestine and is the first compartment of the colon. 

The colon is the biggest section of the large intestine. It is divided into three parts:

• Ascending colon – It runs upwards
• Transverse colon- It runs across the belly
• Descending colon – It runs downwards

Rectum – Last part of the large intestine.It receives  aids in excrement storage. It is subsequently followed by the anus. The anus is bordered by the voluntary sphincter muscle i.e.,anal sphincter.

Histology of Human Alimentary Canal

Following are the four layers present in the wall of the alimentary canal-

1.       Serosa- Outermost layer made up of mesothelium and connective tissues.
2.       Muscularis- It is made up of smooth muscles, outer longitudinal and inner circular        

            muscles.

3.       Submucosa- It contains lymph, blood, and nerves. It contains glands only in the duodenum part. It is made up of loose connective tissues.
4.       Mucosa- It contains irregular folds and gastric glands in the stomach area. It is the        

           innermost layer.

Digestive glands

Following digestive glands play role in the human digestive system-

Salivary glands

These glands open in the oral cavity and secrete saliva.Salivary glands are three in pairs:

(i) Parotid glands

(ii) The Sublingual gland

(iii) Submandibular or submaxillary glands.

Liver

It is found in the abdominal region and is the largest gland. The liver is divided into two lobes. Bile juice is secreted by the hepatic cells and stored in the gallbladder. Hepatic lobules are the structural and functional unit of the liver. Glisson’s capsule protects the hepatic lobules which is a sheath of connective tissue The bile duct is made up of the cystic duct (which comes from the gallbladder) and the hepatic duct.

Pancreas

It performs as exocrine gland as well as endocrine gland. The exocrine part of pancreas secretes digestive juice known as pancreatic juice. Pancreatic juice is alkaline in nature.

Sphincter of Oddi

 It is present at the opening of the hepato-pancreatic duct (bile duct + pancreatic duct) into the duodenum. It regulates the passage of bile and pancreatic juice into the duodenum.

Digestion of Food

There are various types of enzymes that help in process of digestion through various actions which are mentioned as-

The different chemical reactions that occur are summarised below:

Pancreatic juices:

Juice that converts – Conversion that takes place

•       Amylase           – Starch into Maltose
•       Enterokinase     – Trypsinogen (inactive) into Trypsin (active)
•       Trypsin               – Proteins into Dipeptides
•       Trypsin               – Chymotrypsinogen (inactive) to Chymotrypsin (active)
•       Chymotrypsin     – Peptones into Dipeptides
•       Trypsin               – Procarboxypeptidase into Carboxypeptidase
•       Carboxypeptidase – Proteoses into Dipeptides
•       Trypsin                – Proelastase into Elastase
•       Elastase           –  Elastin into Dipeptides
•       Pancreatic amylase    – Polysaccharides (Starch) into Disaccharides
•       Nucleases                   -Nucleic acids to nucleotides and nucleosides.

Intestinal juices:

•       Maltase – Maltose into glucose
•       Sucrase- Sucrose into glucose & fructose
•       Lactase – Lactose into glucose & galactose
•       Aminopeptidases – Peptides into amino acids
•       Dipeptidases- Dipeptides into amino acids
•       Bile – Fat globules into fat droplets  (Process is called emulsification)

Pancreatic lipase – Triglycerides into fatty acids & glycerol

Absorption and Assimilation of Food

The process of breaking down bio-macromolecules starts from the stomach and continues till the food reaches the duodenum. Digested food components are absorbed in the jejunum and ileum of the gut. The undigested food travels to the large intestine. In the large intestine absorption of  some water and minerals take place. Food is absorbed by diffusion owing to a concentration gradient or through aided transport by a carrier protein. The osmotic gradient transports water. Active transport against the gradient is used to absorb amino acids, glucose,other monosaccharides,Na+ in the blood.

Digested fats are absorbed in the form of micelles that create chylomicrons.

Chylomicrons are protein-coated fat globules that are carried into the villi’s lacteals (lymph vessels) and discharged into the circulation.

Absorption of food takes place in different parts such as the mouth, stomach, etc. The maximum absorption takes place in the small intestine.

By assimilation, the absorbed nutrient reaches the different parts of the body through the bloodstream and lymph for utilisation.

Conclusion

Digestion and food is essential for effective absorption and thereafter utilisation of nutrients.Both digestion and absorption are complex processes and include multiple organs as well as steps. Both digestion and absorption of digested food and expelling of undigested food from the body play an important role in living a healthy life.