Digestive System

Digestion is the physico-chemical process of splitting complex ingested food into the smallest, water soluble, absorbable units. The organs that take part in digestion are called digestive organs, and these organs are continuously connected to form a structure called the digestive system.

Types of Digestion (Based on Site)

Based on the site where digestion occurs, digestion is of 2 types:

1. Intracellular digestion: digestion that takes place within the cell. It is a feature of protozoa.
2. Extracellular digestion: digestion that takes place outside the cell, in a lumen. It is a feature of higher animals like humans.

Steps in Digestion

In humans, digestion involves the following events:

• Ingestion: the process of taking food into the oral cavity.
• Mastication: the ingested food is chewed, ground, and smashed with the help of different kinds of teeth. It makes the food into a final, swallowable form.
• Deglutition (swallowing): the ground food is swallowed and passed behind. This process is called deglutition.
• Peristalsis: the wave-like movement in the walls of the alimentary canal. It pushes the food behind, to different regions, for digestion.
• Secretion: at different regions of the alimentary canal, digestive enzymes are released; these enzymes help in digestion.

Human Digestive System

The human digestive system is well developed and is represented by the alimentary canal and its associated glands. These associated glands are called digestive glands.

Alimentary Canal

A continuous tube of varying diameter that extends between the oral cavity and the anus is called the alimentary canal. It is about 9 meters in length (as stated in the source) and has the following roles:

• It gives a pathway for the ingested food to pass through.
• It receives the secretions of several digestive glands.
• It provides space for the chemical reactions on ingested food.
• It allows absorption of digested food.

The human alimentary canal has the following parts.

1) Oral Cavity

The alimentary canal starts with a cavity called the oral cavity, which is bounded above by the palate, below by the floor, and on the lateral sides by the cheek walls. The roof of the oral cavity is the palate, which separates the oral cavity from the nasal cavity. The floor of the oral cavity carries the tongue, on which taste buds are available.

a) Palate

The roof of the oral cavity is called the palate, which has 2 parts. The anterior part is the hard palate, where many transverse ridges help in cutting and grinding food. The posterior part is smooth and slippery; this region is the soft palate, which is raised during swallowing and prevents food from passing into the nasal cavity.

b) Teeth

The upper and lower jaws carry teeth of different kinds, adapted for different foods. In humans, 4 kinds of teeth are present; this condition is called heterodont.

In humans, 2 sets of teeth are found: milk teeth and permanent teeth. Milk teeth are replaced by permanent teeth, and the permanent teeth are never replaced again. This form of dentition is called diphyodont.

The teeth remain embedded in the jaw bones; this kind of teeth attachment is stronger and is called thecodont.

Structure of a tooth

A tooth in transverse section shows an outer, non-living part called enamel, followed by the dentine and the pulp cavity. Based on location, a tooth has the following parts:

• Crown: the exposed part of the tooth, externally lined with the hardest layer, the enamel. Enamel is composed of salts of calcium and phosphorus. It makes the tooth stronger.
• Inner to the enamel is a living layer called dentine, composed of cells called odontoblasts. The dentine layer encloses a cavity called the pulp cavity, where an artery, a vein, and nerves are located.
• Neck: the short segment of the tooth that remains embedded in the gum.
• Root: the part of the tooth that remains embedded in the socket of the jaw bone.

c) Tongue

The floor of the oral cavity carries a skeletal muscle called the tongue, which is posteriorly fixed. Its dorsal surface has elevations called papillae, which may or may not carry taste buds. Based on structure, papillae are of the following types:

1. Filiform papillae: the smallest, most numerous papillae; they do not have taste buds and are sensitive to touch.
2. Fungiform papillae: found among the filiform papillae; they carry taste buds for sweet, salt, and sour.
3. Circumvallate papillae: the larger, posterior papillae on the tongue, also called vallate papillae; they carry numerous taste buds.

2) Pharynx

Posterior to the oral cavity is a tubular chamber called the pharynx, which is a crossroad for both food and air. Functionally, the pharynx has 3 regions:

• Nasopharynx: the upper region of the pharynx where the nose opens internally; it has two openings called the internal nostrils.
• Oropharynx: the region posterior to the oral cavity; it receives food.
• Laryngopharynx: the base of the pharynx, where two openings are available. The ventral opening is the glottis, guarded by the epiglottis, which runs behind as the trachea. Dorsal to the glottis is the gullet, which passes behind into a tube called the oesophagus.