Life Sciences Glossary: Digestion in Mouth Cavity and Stomach

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Digestion in Mouth Cavity

In man, the digestion starts from mouth. Masticated food in mouth is mixed with saliva secreted by salivary glands (3 pairs) . Daily secretion of saliva is about 1 - 1.5 litter. pH of sliver is about 6.8 (slightly acidic, almost neutral) . Saliva contains a starch splitting enzyme ptyalin (salivary amylase) acts on cooked starch changing them into a sugar maltose, isomaltose and ‘limit’ dextrin՚s.

Digestion in Mouth Cavity

Saliva contains an antibacterial enzyme lysozyme. It dissolves the cell wall of Gram-positive bacteria and kills them. There is no digestion in oesophagus. It only conducts the food from mouth into stomach.

Digestion in Stomach

  • Stomach secretes gastric juice which is acidic in nature (pH 1 - 3.5) . Protein digestion starts in the stomach. Food mixed with gastric juice in the stomach is called ‘chyme’ . The columnar epithelium of stomach form many gastric pits with gastric glands. Gastric glands are located on mucosa of stomach.
  • Gastric giants are lined with three kinds of secreting cells: Zymogenic (main, peptic or chief) cells, parietal (oxyntic) cells and mucus cells. The main, peptic or zymogen cells secrete digestive proenzymes namely pepsinogen and prorenin. Oxyntic cells secrete HCI. Pepsinogen is activated into pepsin by HCI. It is a protein splitting endoptidase.
Digestion in Stomach

Pepsin can digest even collagen of connective tissue fibers, but not keratin of nail or hair. The mucus secreted by stomach protects its wall from the action of pepsin. Rennin is found in calf gastric of nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles and alveoli.

Organs of Respiratory System Nasal Cavity

Nose performs the function of olfaction and in addition it conditions the inhaled air by,

  • Filtering the air for dust particles and microbes.
  • Moistening the air.
  • Increasing its temperature.


Pharynx is the common passage for food and inhaled air. Glottis is an opening in the floor of pharynx leading to trachea. Epiglottis is the structure that prevents the entry of food into respiratory tract during swallowing.


  • Larynx is the first part of windpipe present in the neck. Larynx is supported by four cartilages: thyroid, cored and a pair of arytenoids.
  • In man, the midventral portion of the thyroid cartilage forms a prominent protuberance called ‘Adam՚s apple’ . A small nodule-like cartilage of Santorini is attached to the anterior tip of each arytenoid cartilage.
  • Larynx contains vocal cords, the sound producing elastic fibers. Air forced out of lings may vibrate vocal cords and column of air are set in to produce sounds. Ian women and children, the vocal cords are usually short, and the voice is high-pitched. In men, the vocal cords are usually longer (brought about by the male sex hormone) 0, and voice is correspondingly lower.


  • The trachea is a tube about 10 cm long in man, supported, by C-shaped rings of cartilage in its walls. It branches into two bronchi, one to each lung and these branch within the lungs into many smaller bronchioles. Even when there is no air in it, trachea does not collapse due to the presence of C-shaped narrow cartilaginous rings (discs) .
  • Traches is lined with a pseudo stratified ciliated epithelium. This helps in pushing mucus out.


  • Lungs are the organs involved in gaseous exchange. These are solid, but highly spongy structures. They are without central cavities but are divided into lobes. Covering of the lungs are called pleural membranes. Each lung is enclosed in two membranes called pleurae. The outer covering is adhered to chest wall and diaphragm and is called parietal pleura. The inner covering membrane which closely covers the lung is called visceral pleura. Accumulation of fluid between the pleura is called pleurisy.
  • In man, the left lung has two lobes, superior lobe and inferior lobe, the right lung has three lobes, superior lobe, middle lobe and inferior lobe. The terminal bronchioles are sub-divided into a number of respiratory bronchioles further giving rise to alveolar ducts. Alveolar duct divides into atria which expand into a number of alveolar sacs (air sacs) . The alveoli of lungs (like villi of intestine in mammals) provide a large surface for gaseous exchange. There number of alveoli in the human lungs has been estimated to be approximately 750 million, exposing a surface area of nearly 100 . In adult man, the surface area of skin is around 1.6 only. So, the surface area of lungs is more than 50 times of the skin.
  • Besides lungs, the term ‘alveolus’ is associated with bony socket for tooth.


Diaphragm is characteristic of mammals. It is highly muscular and fibrous portion elevated towards thorax like a dome. It is a partition which separates the thoracic and abdominal cavities. The most important function of diaphragm of mammals is to aid in respiration. Puncturing of diaphragm results in stoppage of breathing and is fatal. The term ‘Phrenic’ is associated with diaphragm.

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