Digestion is defined as the process by which food is broken down into simple chemical substances that can be absorbed and used as nutrients by the body. Most of the substances in the diet cannot be utilized as such. These substances must be broken into smaller particles so that they can be absorbed into the blood and distributed to various parts of the body for utilization. The digestive system is responsible for these functions.
The digestive process is accomplished by the mechanical and enzymatic breakdown of food into simpler chemical compounds. A normally young healthy adult consumes about 1 kg of solid diet and about 1 to 2 liter of liquid diet every day. All these food materials are subjected to the digestive process, before being absorbed into blood and distributed to the tissues of the body. The digestive system plays a major role in the digestion and absorption of food substances.
Thus, the functions of the digestive system include:
1. Ingestion or consumption of food substances
2. Breaking them into small particles
3. Transport of small particles to different areas of the digestive tract
4. Secretion of necessary enzymes and other substances for digestion.
5. Digestion of the food particles
6. Absorption of the digestive products (nutrients)
7. Removal of unwanted substances from the body.
FUNCTIONAL ANATOMY OF DIGESTIVE SYSTEM
The digestive system is made up of the gastrointestinal tract (GI tract) or alimentary canal and accessory organs, which help in the process of digestion and absorption. GI tract is a tubular structure extending from the mouth up to anus, with a length of about 30 feet. It opens to the external environment on both ends.
GI tract is formed by two types of organs:
1. Primary digestive organs.
2. Accessory digestive organs.
1. Primary Digestive Organs
Primary digestive organs are the organs where actual digestion takes place.
Primary digestive organs are:
v. Small intestine
vi. Large intestine.
2. Accessory Digestive Organs
Accessory digestive organs are those which help primary digestive organs in the process of digestion.
Accessory digestive organs are:
iii. Salivary glands
iv. Exocrine part of pancreas
WALL OF GASTROINTESTINAL TRACT
In general, wall of the GI tract is formed by four layers
which are from inside out:
1. Mucus layer
2. Submucous layer
3. Muscular layer
4. Serous or fibrous layer.
1. MUCUS LAYER
The mucus layer is the innermost layer of the wall of GI tract. It is also called gastrointestinal mucosa or mucus membrane. It faces the cavity of GI tract.
Mucosa has three layer of structures:
i. Epithelial lining
ii. Lamina propria
iii. Muscularis mucosa.
Epithelial lining is in contact with the contents of GI tract. The type of cells in this layer varies in different parts of GI tract. The inner surface of the mouth, surface of the tongue, the inner surface of the pharynx and oesophagus have stratified squamous epithelial cells. However, mucus membrane lining the other parts such as stomach, small intestine and large intestine has columnar epithelial cells.
Lamina propria is formed by connective tissues, which contain fibroblasts, macrophages, lymphocytes and eosinophils.
The muscularis mucosa layer consists of a thin layer of smooth muscle fibres. It is absent in the mouth and pharynx. It is present from oesophagus onwards.
2. SUBMUCOUS LAYER
The submucous layer is also present in all parts of GI tract, except the mouth and pharynx. It contains loose collagen fibres, elastic fibres, reticular fibres and few cells of connective tissue. Blood vessels, lymphatic vessels and nerve plexus are present in this layer.
3. MUSCULAR LAYER
The muscular layer in lips, cheeks and wall of the pharynx contains skeletal muscle fibres. The oesophagus has both skeletal and smooth muscle fibres. The Wall of the stomach and intestine is formed by smooth muscle fibres.
Smooth muscle fibres in the stomach are arranged in three layers:
i. Inner oblique layer
ii. Middle circular layer
iii. Outer longitudinal layer.
Smooth muscle fibres in the intestine are arranged in two layers:
i. Inner circular layer
ii. Outer longitudinal layer.
Auerbach nerve plexus is present in between the circular and longitudinal muscle fibres. The smooth muscle fibres present in the inner circular layer of the anal canal constitute the internal anal sphincter. The external anal sphincter is formed by skeletal muscle fibres.
4. SEROUS OR FIBROUS LAYER
The outermost layer of the wall of the GI tract is either serous or fibrous in nature. The serous layer is also called serosa or serous membrane and it is formed by connective tissue and myoepithelial cells. It covers the stomach, small intestine and large intestine.
The fibrous layer is otherwise called fibrosa and it is formed by connective tissue. It covers pharynx and oesophagus.
NERVE SUPPLY TO GASTROINTESTINAL TRACT
GI tract has two types of nerve supply:
I. Intrinsic nerve supply
II. Extrinsic nerve supply.
INTRINSIC NERVE SUPPLY –ENTERIC NERVOUS SYSTEM
Intrinsic nerves to the GI tract form the enteric nervous system that controls all the secretions and movements of GI tract. The enteric nervous system is present within the wall of GI tract from the oesophagus to anus. Nerve fibres of this system are interconnected and form two major networks called
1. Auerbach plexus
2. Meissner plexus.
These nerve plexus contain nerve cell bodies, processes of nerve cells and receptors. The receptors in the GI tract are stretch receptors and chemoreceptors. The enteric nervous system is controlled by extrinsic nerves.
1. Auerbach Plexus
Auerbach plexus is also known as myenteric nerve plexus. It is present in between the inner circular muscle layer and the outer longitudinal muscle layer.
Functions of Auerbach plexus
The major function of this plexus is to regulate the movements of GI tract. Some nerve fibres of this plexus accelerate the movements by secreting the excitatory neurotransmitter substances like acetylcholine, serotonin and substance P. Other fibres of this plexus inhibit the GI motility by secreting the inhibitory neurotransmitters such as vasoactive intestinal polypeptide (VIP), neurotensin and enkephalin.
2. Meissner Nerve Plexus
Meissner plexus is otherwise called submucous nerve plexus. It is situated in between the muscular layer and submucosal layer of GI tract.
Functions of Meissner plexus
The function of the Meissner plexus is the regulation of secretory functions of GI tract. These nerve fibres cause constriction of blood vessels of GI tract.
EXTRINSIC NERVE SUPPLY
Extrinsic nerves that control the enteric nervous system are from autonomic nervous system. Both sympathetic and parasympathetic divisions of autonomic nervous system innervate the GI tract.
Sympathetic Nerve Fibers
Preganglionic sympathetic nerve fibres to GI tract arise from lateral horns of spinal cord between fifth thoracic and second lumbar segments (T5 to L2). From here, the fibres leave the spinal cord, pass through the ganglia of the sympathetic chain without having any synapse and then terminate in the celiac and mesenteric ganglia. The postganglionic fibres from these ganglia are distributed throughout the GI tract.
Functions of sympathetic nerve fibres
Sympathetic nerve fibres inhibit the movements and decrease the secretions of GI tract by secreting the neurotransmitter noradrenaline. It also causes constriction of sphincters.
Parasympathetic Nerve Fibers
Parasympathetic nerve fibres to GI tract pass through some of the cranial nerves and sacral nerves. The preganglionic and postganglionic parasympathetic nerve fibres to the mouth and salivary glands pass through facial and glossopharyngeal nerves
Preganglionic parasympathetic nerve fibres to oesophagus, stomach, small intestine and upper part of large intestine pass through the vagus nerve. Preganglionic nerve fibres to lower part of large intestine arise from second, third and fourth sacral segments (S2, S3 and S4) of spinal cord and pass through pelvic nerve. All these preganglionic parasympathetic nerve fibres synapse with the postganglionic nerve cells in the myenteric and submucus plexus.
Functions of parasympathetic nerve fibres
Parasympathetic nerve fibres accelerate the movements and increase the secretions of GI tract. The neurotransmitter secreted by the parasympathetic nerve fibres is acetylcholine (Ach).