14. Human Nutrition - part 01 - Human digestive system

 


14. Human Nutrition - part 01 - Human digestive system

Nutrition :

  • Nutrition  is the sum of the processes by which an organism consumes and utilises food substances. 
  •  Nutrition as the intake of food, considered in relation to the body's dietary needs -[WHO] 
  • Nutrition includes the processes like-
  1. Ingestion
  2. Digestion, 
  3. Absorption 
  4. Assimilation  
  5. Egestion. 

Digestion : 

  • Digestion is defined as the process by which the complex, non-diffusible and nonabsorbable food substances are converted into simple, diffusible and assimilable substances.

Human Digestive System :


  • Digestive system of man consists of-
  1. Alimentary canal 
  2. Associated digestive glands.

Alimentary canal : 

  • It is a long tubular structure starting from mouth and ending with anus.
  • It is about 8-10 meters long and consists of following organs :

Mouth :

  •  Also called as oral or buccal cavity is bounded by fleshy lips. 
  • It’s side walls are formed of cheeks, roof is formed by palate and floor by tongue. 
  • It is internally lined by a mucous membrane. 
  • Salivary glands open into the buccal cavity.

Teeth :

  • 32 teeth are present in the buccal cavity of an adult human being. 
  • Human dentition is described as thecodont, diphyodont and heterodont. It is called thecodont type because each tooth is fixed in a separate socket present in jaw bones by gomphosis type of joint. 
  • In our life time, we get only two sets of teeth, milk teeth and permanent teeth. This is called diphyodont dentition. 
  • We have four different type of teeth hence we are heterodont. 
  • Types of teeth are
  1.  incisors (I) 
  2. canines (C) 
  3. premolars (PM) and 
  4. molar (M). 
  • Each half of each jaw has two incisors, one canine, two premolars and three molars.
  • Thus, dental formula of adult human can be represented as.
  • A tooth consists of the portion that projects above the gum called crown and the root that is made up of two or three projections which are embeded in gum. 
  • A short neck connects the crown with the root. 
  • The crown is covered by the hardest substance of the body called enamel. 
  • Enamel is made up of calcium phosphate and calcium carbonate. 
  • Basic shape of tooth is derived from dentin, a calcified connective tissue. 
  • The dentin encloses a cavity called pulp cavity. 
  • It is filled with connectivetissue pulp. 
  • Pulp cavity contains blood vessels and nerves. 
  • Pulp cavity has extension in the root of the tooth called root canal. 
  • The dentin of the root of tooth is covered by cementum, a bone like substance that attaches the root to the surrounding socket in the gum.
  • The study of teeth with respect to their number, arrangement, development etc is known as dentition.

Tongue : 

  • It is a muscular, fleshy organ and roughly triangular in shape. 
  • It lies along the floor of the buccal cavity. 
  • The upper surface of the tongue bears numerous projections called papillae. 
  • Some papillae bear sensory receptorscalled taste buds.



Pharynx :

  • The buccal cavity leads to a short pharynx. 
  • Pharynx is a common passage for food and air. 
  • The upper region of pharynx is called trachea. 
  • The pharynx opens into trachea through an opening called glottis. 
  • The glottis is guarded by a cartilaginous flap called epiglottis.
  • The epiglottis closes during the swallowing(deglutition) action and prevents entry of food into the trachea. 
  • The lower region of pharynxis called oropharynx. 
  • Oropharynx opens into oesophagus through gullet.
Oesophagus : 

  • The oesophagus is a thin, muscular tube. 
  • It lies behind the trachea.
  • This ≈25cm long tube passes through theneck, central aspect of rib cage, pierces the diaphragm and joins the stomach. 
  • It is lined by mucus cells. 
  • Mucus lubricates the passageway of food. 
  • Histologically, oesophagus is made up of longitudinal and circular muscles. 
  • The rhythmic wave of contraction and relaxation of these muscles is called peristalsis that helps in passage of food through oesophagus.

Stomach : 


  • The stomach is located in the upper left portion of the abdominal cavity. 
  • It is a muscular sac-like 'J' shaped organ, around 25 to 30cm in length. 
  • It is divided into upper cardiac region and lower pyloric region.



Cardia or Cardiac :

  • It is first part in which oesophagus opens. 
  • The cardia surrounds the band of circular muscles present at the junction of oesophagus and stomach called cardiac sphincter. 
  • The cardiac sphincter prevents back flow or regurgitation of food from stomach to oesophagus.

Fundus :

  •  It is the dome shaped region above and left of cardia.

Body : 

  • It forms the large central portion of stomach that stores the food.

Pylorus : 

  • It is a narrow posterior region of stomach. 
  • It opens into duodenum, the initial part of small intestine. 
  • This opening is guarded by a set of sphincter muscles called pyloric sphincter. 
  • It regulates the flow of food from stomach to small intestine.
  • The stomach temporarily stores the food and gives the feeling of satiety. 
  • It churns the food and helps in mixing the food with gastric juice.

Small Intestine :

  • In human, it is about 6 meters long and 2.5 cms broad tube coiled within abdominal cavity. 
  • The coils are held together by mesenteries, supporting the blood vessels, lymph vessels and nerves. 
  • It is divided into three parts.
  1. Duodenum 
  2. Jejunum
  3. Ileum 

1. Duodenum : 

  • It is about 26 cm long ‘U’ shaped structure. 
  • The duodenum turns towards left side of abdominal cavity below the stomach.


2. Jejunum : 

  • It is about 2.5 meters long, coiled middle portion of small intestine. 
  • It is narrower than the duodenum.

3. Ileum :

  •  It is about 3.5 meters long. 
  • It is highly coiled and little broader than jejunum. 
  • The ileum opens into the caecum of large intestine at ileocaecal junction.

Large Intestine : 

  • Ileum opens into large intestine. 
  • It is 1.5 meters in length. 
  • It is wider in diameter and shorter than small intestine. 
  • It consists of - 
  1. caecum
  2. colon and 
  3. rectum.

1. Caecum :

  • Caecum is a small, blind sac present at the junction of ileum and colon. 
  • It is 6cm in length. 
  • It hosts some symbiotic microorganisms. 
  • An elongated worm like vermiform appendix arises from the caecum. 
  • Appendix is vestigial organ in human beings and functional in herbivorous animals for the digestion of cellulose.

2. Colon : 

  • Caecum opens into colon. 
  • Colon is tube like-organ consist of three parts, ascending colon, transverse colon and descending colon.
  • The colon is internally lined by mucosal cells.

3. Rectum : 

  • It is posterior region of large intestine. 
  • It temporarily stores the undigested waste material called faeces till it is egested out through anus.

Anus : 

  • Anus is the terminal opening of alimentary canal. 
  • It is guarded by sphincter.
  • It expels faecal matter by a process called egestion or defaecation.

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