Introduction to GIT & Saliva

Gastrointestinal tract or digestive system consists of a tube like structure starting from mouth and ending in anus.

It helps in ingestion of food and supplying nutrients to the body.

The structures from mouth to anus are as follows

• Mouth (Oral cavity)
• Pharynx
• Oesophagus
• Stomach
• Small intestine
• Large intestine
• Anus

 Apart from these, there are accessory structures of digestive system, which are: Teeth, Salivary Glands, Liver, Pancreas etc.

As food enters the mouth, it is broken down into smaller molecules by the chewing mechanism.

The broken smaller molecules are mixed with saliva to form a bolus which is then swallowed.

The swallowed bolus enters the esophagus.

Contraction of muscles of oesophagus pushes the bolus into stomach after opening of lower esophageal sphincter.

The bolus in the stomach is mixed with the HCl.

HCl further breaks down the bolus into chyme.

Chyme enters the duodenum which is the first part of Small Intestine.

Chyme undergoes enzymatic breakdown in the duodenum and enters the Jejunum and Ileum, the 2nd and 3rd part of Small Intestine.

The absorption of nutrients takes place in the Small Intestine.

Small intestine continues as Large Intestine which has and ascending, transverse and descending parts.

Only absorption of water takes place in Large Intestine

The terminal part of Large Intestine is Sigmoid colon after which the undigested food products are stored in the Rectum and evacuated when suitable time arises.

Functions of Digestive System

Digestive system provides nutrition to the body by breaking down complex food molecules to simpler ones and then absorbing them into the blood stream.

It does this by following process:

Ingestion

• Food particles in the mouth gets chewed and moistened with the saliva and swallowed

Digestion:

• Large food molecules are broken down into simples one by the enzymatic action

Absroption

• The simpler molecules are absorbed in the Small intestine into the blood stream

Egestion:

• The undigested food products are ejected from the body by defecation

Organisation of Structures of Digestive System

The wall of the digestive tract has the following layers

1. Serous layer
2. Muscular layer
3. Submucus layer
4. Mucus layer

Serous layer

• This helps in the attachment of digestive tract to the surrounding structures.

Muscular layer

• It consists of longitudinal and circular smooth muscle layer.

These muscular layers help in local mixing and forward movement of contents of digestive tract.

Submucus layer

• It consists of loose connective tissue, blood vessels and lymphatics.

Mucus layer

• It is lined by epithelial cell and contains glands, sooth muscle and loose connective tissue.

Innervation of Digestive System

The enteric nervous system has two networks

1. Intrinsic
2. Extrinsic

Intrinsic innervation

It has two plexus

1. Myentreric plesus (Auerbach’s plexus)
2. Submucous plexus (Meissner’s plexus)

Myentreric plexus (Auerbach’s plexus)

• Lies between longitudinal and circular smooth muscle layers.
• Has motor function.
• Controls tone and intensity of rhythmic contraction of gut wall.

Submucus plexus (Meissner’s plexus)

• Lies between submucous layer and inner ciruclar smooth muscle layer.
• Sensory in function.
• Concerned with control of exocrine and endocrine secretion of cells of gut.

 

Extrinsic innervation

It is under autonomic control that is Parasympathetic and Sympathetic nerve fibers

Stimulation of Parasympathetic nerves

• Increase in motility and tone
• Relaxation of sphincters
• Increased secretions

Stimulation of Sympathetic nerves

• Decrease in motility and tone
• Contraction of sphincters
• Inhibitions of secretions

Small Intestine

The mucosal surface of Small Intestine is adapted to provide a huge area for absorption.

Mucous shows finder-like projections called villi

They are covered by a layer of columnar cells which possess a brush border consisting of microvilli

Each villus in its core contains

• A lymph vessel
• Smooth muscle fibers
• An arteriole and venule
• A nerve net

Between villi are intestinal glands known as crypts of Lieberkuhn

They divide rapidly and replace the cell shed from the tip of the villi

They produce variety of enzymes capable of digesting proteins, carbohydrates, fats and nucleic acids.

 

Large Intestine

Mucosal surface smooth: no villi

Simple tubular glands are abundant which secrete mucus

The walls are folded into sacs by contraction of circular smooth muscle producing sacculations.

The longitudinal smooth muscle layer is collected into the three distinct bands called taenia coli.

Application and implication in Nursing

Q. What happens when the digestive tract is not working  properly?

A. An unhealthy digestive tract can impair the body’s ability to absorb nutrients, store fat and regulate blood sugar. An urge to over eat due to decreased nutrient absorption might cause weight gain. On the other hand, weight loss may be a result of bacterial overgrowth in the small intestine.

Saliva

Saliva is secreted by salivary glands which are

1. Parotid glands
2. Submandibular (or submaxillary) glands
3. Sublingual glands

These glands have two types of cells

Mucus cells which form a secretion containing mucin.

Serous cells which form a thin watery secretion containing enzyme ptyalin.

Composition of Saliva

• Daily secretion: 1500 mL/day
• Digestive enzymes
• Ptyalin or salivary amylase
• Lysosome (bactericidal)
• Kallikerin (proteolytic enzyme
• Lipase (lipolytic enzyme)
• Mucin
• Cations: Na, K, Ca
• Antions: Cl, HCO3
• Ph: <7.0

Functions of Saliva

1. Ptyalin (salivary amylase)

• Aid in digestion of starch to maltose.

2. Mucin

• Lubricates the food this assisting in mastication and swallowing
• Protecting oral mucosa
• Aid in speech

3. Keeps mouth moist

4. Reduce the risk of buccal infection as it contains lysosome.

5. Buffer in saliva help to maintain the oral pH at 7.

Control of Salivary secretion

• Stimulation of parasympathetic nerves causes profuse secretion of watery saliva and low content of organic material and protein.
• Stimulation of sympathetic nerves causes secretion of small amounts of saliva rich in organic constituents and mucus.
• Salivary secretion increases either by taste of food or smell, sight or thought of food.

Application and implication in Nursing

Q. What is dry mouth?

A. Dry mouth or Xerostomia is a condition in which the salivary glands do not make enough saliva to keep the mouth wet. Dry mouth is often due to the side effect of certain medications or the ageing process or as a result of radiation therapy for cancer.