Human Digestive System Physiology Notes

Digestive System Introduction

• 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. All living organisms require nutritive substances and water for survival and growth. Most of the substances in the diet cannot be utilized as such.
• These substances must be broken into smaller particles. Then only these substances can be absorbed into the blood and distributed to various parts of the body for utilization.

Read And Learn More: Medical Physiology Notes

• The digestive process is accomplished by the mechanical and enzymatic breakdown of foods into simpler chemical compounds. A normally young healthy adult consumes about one kg of solid diet and about 1–2 liters of liquid diet every day.
• All these food materials are subjected to a digestive process before being absorbed into the 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:
  • Ingestion or consumption of food substances
  • Breaking them into small particles
  • Transport of the small particles to different areas of the digestive tract
  • Sycrsifon of necessary enzymes and other substances for digestion
  • Digestion of the food particles
  • Absorption of the digestive products (nutrients)
  • Removal of unwanted substances from the body.

Functional Anatomy Of The Digestive System

The digestive system is made up of the gastrointestinal tract (Gl tract) or alimentary canal and accessory organs, which help in the process of digestion and absorption. The Gl tract is a tubular structure extending from the mouth up to the anus with a length of about 30 feet. It opens to the external environment on both ends. Gl 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:

• Mouth
• Pharynx
• Esophagus
• Stomach
• Small intestine
• Large intestine.

2. Accessory Digestive Organs: Accessory digestive organs help the primary digestive organs in the process of digestion. The accessory digestive organs are:

• Teeth
• Tongue
• Salivary glands
• Exocrine part of the pancreas
• Liver
• Gallbladder.

Wall Of Gastrointestinal Tract

In general, the wall of the Gl 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 innermost layer of the wall of the Gl tract is known as the mucus layer. It is also called gastrointestinal mucosa or mucous membrane. It faces the cavity of the Gl tract.

• The mucosa has three layers of structures:

1. 1. Epithelial lining
   2. Lamina propria
   3. Muscularis mucosa.

1. Epithelial Lining: Epithelial lining is in contact with the contents of the Gl tract. The type of cells in this layer varies in different parts of Gl tract. The inner surface of the mouth, the surface of. Tongue, inner surface of the pharynx and esophagus have stratified squamous epithelial cells. However, the mucous membrane lining the other parts such as the stomach, small intestine, and large intestine has columnar epithelial cells.
2. Lamina Propria: It is formed by connective tissues, which contain fibroblasts, macrophages, lymphocytes, and eosinophils.
3. Muscularis Mucosa: This layer consists of a thin layer of smooth muscle fibers. it is absent in the mouth and pharynx. It is present from the esophagus onwards.

2. Submucus Layer: This is also present from the esophagus onwards. And, it is absent in the mouth and pharynx. This layer contains loose collagen fibers, elastic fibers, reticular fibers, and a few cells of connective tissue. Blood vessels, lymphatic vessels, and nerve plexus are present in this layer.

3. Muscular Layer: This layer in lips, cheeks and wall of the pharynx have skeletal muscle fibers. The esophagus has both skeletal and smooth muscle fibers. The wall of the stomach and intestine is formed by smooth muscle fibers.

The smooth muscle fibers in the stomach are arranged in three layers:

1. Inner oblique layer
2. Middle circular layer
3. Outer longitudinal layer.

The smooth muscle fibers in the intestine are arranged in two layers:

1. Inner circular layer
2. Outer longitudinal layer.

Auerbach’s nerve plexus is present in between the circular and longitudinal muscle fibers. The smooth muscle fibers present in the inner circular layer of the anal canal constitute the internal anal sphincter. The external anal sphincter is formed by skeletal muscle fibers.

4. Serous Or Fibrous Layer:

• The outermost layer of the wall of the Gl tract is either serous or fibrous in nature. The serous layer is formed by connective tissue and mesothelial cells. It is also called serosa or serous membrane.
• It covers the stomach, small intestine, and large intestine. The fibrous layer is otherwise called fibrosa. It is formed by connective tissue. It covers the pharynx and esophagus.

Nerve Supply To Gastrointestinal Tract

Gl tract has two types of nerve supply:

1. Intrinsic nerve supply
2. Extrinsic nerve supply.

1. Intrinsic Nerve Supply – Enteric- Nervous System

The intrinsic nerves to the GI tract form the enteric nervous system that controls all the secretions and movements of the gastrointestinal tract. The enteric nervous system is present within the wall of Gl tract from the esophagus to the anus. The nerve fibers of this system are interconnected and form two major networks called

1. Auerbach’s plexus
2. Meissner’s plexus.

These nerve plexus contain nerve cell bodies, processes of nerve cells, and receptors. The receptors in the Gl tract are stretch receptors and chemoreceptors. The enteric nervous system is controlled by extrinsic nerves.

1. Auerbach’s Plexus: It is also known as my enteric nerve plexus. It is present in between the inner circular muscle layer and the outer longitudinal muscle layer.

• Functions of Auerbach’s plexus: The major function of this plexus is to regulate the movements of the Gl tract. Some nerve fibers of this plexus accelerate the movements by secreting the excitatory neurotransmitter substances like acetylcholine, serotonin,, and substance P. Other fibers of this plexus inhibit the Gl motility by secreting inhibitory neurotransmitters such as vasoactive intestinal polypeptide (VIP), neurotoxin, and enkephalin.

2. Meissner’s Nerve Plexus: Meissner’s plexus is otherwise called submucous nerve plexus, it is situated in between the muscular layer and submucosal layer of the Gl tract.

• Functions of Meissner’s plexus: The function of Meissner’s plexus is the regulation of secretory functions of Gl tract. And these nerve fibers cause constriction of blood vessels of the Gl tract.

2. Extrinsic Nerve Supply: The extrinsic nerves that control the enteric nervous system are from the autonomic nervous system. Both sympathetic and parasympathetic divisions of the autonomic nervous system innervate the Gl tract.

• Sympathetic Nerve Fibers: Preganglionic sympathetic nerve fibers to the Gl tract arise from lateral horns of the spinal cord between the fifth thoracic and second lumbar segments. From here, the fibers 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 fibers from these ganglia are distributed throughout the Gl tract.
  • Functions of sympathetic nerve fibers: Sympathetic nerve fibers inhibit the movements and decrease the secretions of the Gl tract by secreting the neurotransmitter noradrenaline. It also causes constriction of sphincters.

• Parasympathetic Nerve Fibers: Parasympathetic nerve fibers to the Gl tract pass through some of the cranial nerves and sacral nerves. The pre-ganglionic and postganglionic parasympathetic nerve fibers to the mouth and salivary glands pass through facial and glossopharyngeal nerves.
  • The preganglionic parasympathetic nerve fibers to the esophagus, stomach, small intestine,, and upper part of the large intestine pass through the vagus nerve. The preganglionic nerve fibers to the lower part of the large intestine arise from the second, third, and fourth sacral segments of the spinal cord and pass through a pelvic nerve.
  • All these preganglionic parasympathetic nerve fibers synapse with the postganglionic nerve cells in the myenteric and submucus plexus.
  • Functions of parasympathetic nerve fibers: Parasympathetic nerve fibers accelerate movements and increase the secretions of the Gl tract. The neurotransmitter secreted by the parasympathetic nerve fibers is acetylcholine.