Digestion And Absorption

Food is one of the basic requirements of all living organisms. The major components of our food are carbohydrates, proteins and fats. Vitamins and minerals are also required in small quantities. Food provides energy and organic materials for growth and repair of tissues. The water we take in, plays an important role in metabolic processes and also prevents dehydration of the body. Bio macro molecules in food cannot be utilised by our body in their original form. They have to be broken down and converted into simple substances in the digestive system. This process of conversion of complex food substances to simple absorbable forms is called digestion and is carried out by our digestive system by mechanical and biochemical methods.

1.0Anatomy of Alimentary Canal

The human digestive system consists of the alimentary canal and the associated glands.

2.0Alimentary Canal

The alimentary canal begins with an anterior opening – the mouth, and it opens out posteriorly through the anus.

Mouth and Buccal Cavity

• Mouth is a horizontal transverse slit like aperture which is surrounded by upper and lower
• lip. 
• Lips are movable. 
• The mouth leads to the buccal cavity.

Buccal vestibule 

• The space between the gums and cheeks where the food is stored temporarily.

Oral cavity 

• The oral cavity has a number of teeth, a muscular tongue and palate. 
• It is the inner & central part which is surrounded by upper and lower jaw, lined by non keratinized stratified squamous epithelium. 
• The upper jaw is fixed and the lower jaw is movable.

Palate- The roof of the oral cavity is called Palate. Palate is differentiated into two parts:

Hard Palate 

• It is the anterior part of the palate. It is made up of maxilla and palatine bones in humans.
• On the ventral surface of hard palate, some projections or transverse ridges are present which are called palatine rugae.

Soft Palate 

• It is the posterior part of the palate.

• The posterior outgrowth of the soft palate which hangs down in the form of a finger-like process called as Uvula or Velum palati. 
• On the upper side of the uvula one pair internal nostrils are present.
• Uvula or Velum palati covers the opening of internal nostrils during ingestion of food, so food particles can not move inside the nasal chamber.
• One pair of large lymph nodes is present on the posterolateral surface of the soft palate, called Palatine tonsil or Tonsils.

Tongue 

• The tongue is a freely movable muscular organ attached to the floor of the oral cavity by the frenulum. 
• The upper surface of the tongue has small projections called papillae, some of which bear taste buds.

• The anterior part of the tongue is free while the posterior part of tongue is connected to the Hyoid bone.
• On the dorsal surface of the tongue, it is divided into two unequal parts by a V shaped sulcus, called sulcus terminalis.
• The two limbs of the 'V' meet at a median pit named Foramen caecum.
• Tongue is divided into two parts –
• Pharyngeal part – It is the posterior 1/3 part of the tongue. Many small lymph nodes are present in this part which are called Lingual tonsil.
• Oral or papillary part – It is the anterior 2/3 part of the tongue. Papillae are found in this part in which gustatory or taste receptors are present in the form of taste buds on a few papillae.

Fungiform Papillae 

• It is pink coloured, small & spherical in shape. It is found on the entire surface of the tongue but mostly present at the anterior part of the tongue. It is attached to the tongue with the help of a small pedicle. It provides a pink colour to the tongue.

Filiform papillae (Conical papillae) 

• They are thread like, white coloured & conical in shape. They are also found on the entire surface of the tongue. They are most numerous, but devoid of taste buds.

Circumvallate papillae 

• These are the largest and least existed papillae (8 to 12). They are large spherical shape papillae which are found near sulcus terminalis.

Teeth

• Teeth are ecto mesodermal in origin.
• In human beings three basic features of teeth are Thecodont, diphyodont, heterodont
• Each tooth is embedded in a socket of jaw bone. 
• This type of attachment is called the codont.
• The majority of mammals including human beings form two sets of teeth during their life, a set of temporary milk or deciduous teeth replaced by a set of permanent or adult teeth. 
• This type of dentition is called diphyodont. 
• An adult human has 32 permanent teeth which are of four different types (Heterodont dentition), namely, incisors (I), canine (C), premolars (PM) and molars (M). 
• Arrangement of teeth in each half of the upper and lower jaw in the order I, C, PM, M is represented by a dental formula which in human is
• 2123
• 2123
• The hard chewing surface of teeth, made up of enamel, helps in the mastication of food.

Dental formula 

• (a) Incisor- These are long, chisel like teeth for gnawing the food. They are more developed in gnawing animals e.g. rodents. 
• (b) Canines- These are sharp pointed teeth meant for tearing and shredding the food. Canines are more developed in carnivorous animals.Canines are absent in herbivorous animals. In herbivores, the space of canine in gums is empty and this empty space is called diastema.
• (c) Pre molars - These teeth are meant for chewing and crushing food. They are triangular in shape.
• (d) Molars – These also meant for chewing & crushing of food. They are rectangular in shape.
• In humans, except Premolar and Last molar, all type of teeth appear twice in life. Teeth which appear during childhood are called milk teeth/temporary teeth/lacteal teeth/deciduous teeth/primary teeth. Due to the activity of Osteoclast cells milk teeth are shed, then permanent teeth appear.
• Cheek teeth - Premolars and molars. In humans, premolar teeth appear in the alveoli of molar teeth while permanent molar teeth are developed in new alveoli.
• Human Dentition :- Arrangement of teeth on jaws is dentition. Human dentition has the following features.
• Monophyodont :- The teeth which appear only once in life. eg. Premolars and last molars of humans. Diphyodont :- The teeth which appear twice in life. eg. Incisors, Canines, 1st and 2nd molars.
• Thecodont :- The teeth which are present in the bony socket of Jaw. eg. Mammals and Crocodiles.
• Heterodont :- When the teeth are of four different type in mammals on the basis of structure and function.

Pharynx

• The oral cavity leads into a short pharynx which serves as a common passage for food and air. The oesophagus and the trachea (wind pipe) open into the pharynx. 
• A cartilaginous flap called epiglottis prevents the entry of food into the glottis – opening of the wind pipe – during swallowing. 
• The upper part of the pharynx is called Nasopharynx which is related to the nasal chamber. 
• The lower part of the pharynx is called oropharynx which is related to the oral cavity. 
• One pair of openings of the Eustachian tube is present in the nasopharynx. This Eustachian tube is related to the middle ear: It maintains air pressure.

Oesophagus

• Two apertures are found in pharynx.
• Ventral aperture is called Glottis which is related to the larynx which is guarded by a cartilaginous flap epiglottis.
• The dorsal aperture is called a gullet which opens into the oesophagus.
• The oesophagus is a thin, long tube which extends posteriorly passing through the neck, thorax and diaphragm and leads to a ‘J’ shaped bag like structure called stomach. 
• A muscular sphincter (gastro-oesophageal) regulates the opening of oesophagus into the stomach. 
• The oesophageal hiatus is an opening in the diaphragm through which the oesophagus and the vagus nerve pass. 
• In oesophagus digestive glands are absent, only mucous glands are present in mucosa. 
• The length of the oesophagus depends on the length of the neck so the longest Oesophagus is present in Giraffe.

Stomach

• It is situated on the left side of the abdominal cavity. 
• It is the widest part of the alimentary canal. It is a J shaped bag with a muscular structure. 
• The stomach contains four parts (Cardiac, Fundic, Body, Pyloric) - A cardiac portion into which the oesophagus opens, a fundus region, body main central region and a pyloric portion which opens into the first part of the small intestine. 
• It has two orifices (opening) Cardiac orifice is joined by the lower end of the oesophagus. Pyloric orifice opens into the duodenum. 
• The stomach is covered by a layer of peritoneum, fat tissues and lymph tissue deposits on the peritoneum. Such types of peritoneum are called Ommentum.

• The mucous membrane of the stomach is thick. 
• In an empty stomach numerous temporary longitudinal folds are found in the mucosa of the stomach called rugae. 
• They disappear when the stomach is distended.
• Gastric Glands :- These are numerous microscopic, simple branched tubular glands
• formed by the invagination of epithelium (mucosa) in the stomach. The following
• types of cells are present in the epithelium of the gastric glands.

Mucous neck cells

• Enteroendocrine cells
• Oxyntic cells or parietal cells
• Chief cells or peptic cells or Zymogen cells

 Intestine - It is divided into two part

Small intestine

• The small intestine is distinguishable into three regions, a ‘C’ shaped duodenum, a long coiled middle portion jejunum and a highly coiled ileum. 
• The opening of the stomach into the duodenum is guarded by the pyloric sphincter. For absorption of digested food a very large surface area is required. 
• Therefore some adaptations are present here.
• Great length of the intestine.
• Villi and microvilli (Brush border).

3.0Large intestine – Large intestine is differentiated into three parts: caecum, colon and rectum.

Ceacum

• Ileum opens into the large intestine.
• The lower end of the ileum opens into the caecum at the Ileo-caecal junction. 
• The Ileocaecal opening is guarded by an Ileocaecal valve. 
• Caecum is a small blind sac. About 2 cm below the ileocaecal orifice, a narrow finger-like tubular projection arises from the caecum called the vermiform appendix. It is a vestigial organ. 
• Caecum is well developed in rabbit and other herbivore mammals but is vestigial in humans.

Colon

• Caecum opens into the colon, which is the middle part of the large intestine.
• The longitudinal muscle coat forms three ribbon-like bands called Taeniae coli. 
• Due to the presence of taeniae, pouch like structure develops in Lumen of colon called as Haustra. 
• Colon has following parts in humans – an ascending, a transverse, descending part and a sigmoid or pelvic colon.

Rectum

• This colon then continues in a uniform tube called Rectum. (Storage chamber for faeces)
• Rectum opens into a small bag-like structure called anal-canal.
• Anal canal opens outside by anus. Anus is controlled by anal sphincter.
• Two types of anal sphincter are found at the opening of anus.
• Internal Anal sphincter Involuntary
• External Anal sphincter Voluntary