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Digestive System – The Kick start function for Energy

By A K Shyam
A Single cell called Zygote – after a union of the sperm and ovum, has the transcript of the ‘human’ we are all transformed into after trillions of multiplication. 

The Zygote replicates itself into various organs of the body right from the Heart to toe based on the genetic contributions of the parents.  Since they are all from the same origin, each cell of our body is believed to exhibit the same potential as the Zygote. Understandably, progenies are the new combinations of both the positive and the negative elements of the genetic material of the parents.  This has been the natural phenomenon over decades and we see better progenies from generation to generation.  Let me cite an example, son/daughter is always better than the individual parents (at any given age) and they progress in a new but also, a better environment available to them unlike their parents.

It is also true that this basic difference of the ‘Genetic Pool’ makes every individual different in their physiology as well apart from the gender difference.  Human physiology right from the infant stage till the adult form is subjected to various ‘ingredients’ (variety of food) depending upon the generation – at this stage, I can only relate it to my previous generation (my parents) and the next (younger) generation (technology bearers) after me.

How do we derive the energy for the day?

A broad comparison tells me that the kind of food that our parents enjoyed continued through our generation to some extent.  However, the choice for the younger generation today,  is entirely different.  It is this basic input into our system that makes what we are as the popular saying, “You are what you eat”.

I have further added another phrase, “You are what you think” – good thoughts always present positive energy in us and therefore, it is better to be positive in life. 

What I had exposed to you in my previous attempt was just a broad outline where I selectively dwelt with the three principal systems – Digestive/Circulatory and Nervous. 
Read here – Interrelationship Between Various Systems of Body

Let me now attempt to highlight finer details of the Kick start system (Digestive) and educate you on the benefits.  In this first part of the series, I will restrict it to the entry point of food – i.e. Mouth and take you through the rest, later.

We derive energy through the food we eat – stomach digests food.  Carbohydrates (starch and sugars) in the food get broken down into another type of sugar – glucose which is the principal source of energy.  Glucose can be immediately burnt as fuel or ends up in the liver and muscles as glycogen.  During cellular respiration, glucose and oxygen are converted into water and carbon dioxide and the energy is transferred to ATP.

Stomach is elastic and often is problematic as well – I am sure you have heard the saying, “Stomach is full but, mind craves for more” (pet bara hein lekin man nahin).  I am just cautioning you at this stage as the tendency to eat more than required lands us into many complications at a later stage.

Let us now examine the finer details of this digestive system starting from the infant stage.

Digestive System: Infants

We have heard Breakfast, Lunch and dinner the timing of which may change from person to person, family to family.  One may ask why three of them, why can’t we have just one.  Although there are different types of cries, mother does recognize the baby’s cry for milk which is the starter for the human life.  Why does the child require milk?  Milk is the energy source for the baby and as the child cannot express in words, cry is its indication.

In fact, every one of us has gone through this phase in our life.  Breast feeding a newborn initially may make you anxious and that is when either the grandmother or midwife guides you.  ‘Colostrum’ which contains more protein and less fat compared to mature milk is the name for the first milk which is the most suitable food for the baby’s immature stomach and intestine.  This power packed milk is just enough for the baby at this stage.  It is also a laxative that helps baby clearing meconium (the first tarry stool).  This is where the first chemistry creeps in – Colostrum is packed with antibodies that are crucial for the initial immunity development 1.  This is also considered as the first dose of vaccination. This is how everyone’s journey starts BUT, it manifests differently as one matures over a period of time into an adult.

Although it looks quite a natural phenomenon, mother-baby relationship of this phase has much more than we normally visualize.  Gradually as the baby continues to suckle, the reflex stimulation leads to increase in prolactin hormone responsible for milk production of mothers.  ‘Milk-ejection reflex’ also known as let-down reflex is a function of oxytocin hormone released in the brain that facilitates milk flow through suckling impact.  The suckling becomes stronger as days go by and oxytocin is a happy hormone for the mother which makes her relax and de-stress.  A calm and playful baby indicates that he is well fed.

The new born’s stomach barely 5-7 ml increases to 80-150 ml of milk in the first month. Understandably, there could be variation depending upon the birth weight and weight gain over a period. 

You may have noticed that the emphasis even at this stage has been on the initial immune system.  Let us now look at how this emphasis gets diluted during the progress towards adult form depending upon the choice of food that we tend to make.

It is quite normal to avoid solid food in the initial six months and experienced mothers and midwife are aware of this. They suggest that you start feeding solid food gradually in order to ensure that the baby relishes it.  Once the cycle of solid food gets initiated, the breast feeding frequency automatically gets reduced.  Squash, sweet potato, carrots and beans seem to appeal to babies.  What you need to mark here is that the entry point – mouth, facilitates the sense of taste to the baby which over a period of time could undergo a change. Some babies like rice cereal.  Mothers determine these preferences and feed the babies accordingly.

The food preference of for example, a three year old differs from country to country depending upon the cultural habits. Some babies develop a taste for vegetarian food while the others may choose non vegetarian food.  The question here is what kind of initial preferences you develop with the choice before you and whether you hold on to it during the later stages or not – AND why?

What makes your preferences change?

Much more happens inside the entry point – mouth,  than we can visualize – the very first activity that permits you to accept or throw out food is its taste and taste buds play a major role here as it not only allows you to consume food but also indicates the nature of food-sweet, sour, bitter and so many.  Imagine your quick reaction if the food is not to your taste – you throw it out but, the reflex action happens in a split second.

Childhood experience, gender and age are some of the major factors that influence the taste-cells that detect taste are housed in the little bumps that we see on our tongue and they are specialized as well and even babies have taste preferences. As I said earlier, some mothers experiment with new foods and others provide restricted foods – food preferences get differentiated right here.  I am certain that you have seen those who love to experiment new items of food while others feel comfortable with the better known ones and refrain from a try.  In addition to our own preferences, food choice also depends on the environment, influence of our own people and even friends, as we grow up.

It is documented that every cell changes once in seven years.  But, surprisingly taste buds change every two weeks – so, it is possible that the food that we did not like initially may taste good after this period of two weeks.  Brain of course also a role in this change of mind.

What are the other aspects that we need to consider at this entry point (Mouth)

‘Oral Health’ is very important as it is the entry to many vital ingredients that we feed in order to maintain our energy level through the day.  It is therefore pretty important that we pay careful attention to oral health – every one of us is aware of the precautions that we need to take. There are clearly five advantages of a good oral health – reduces risk of heart disease; maintains our memory; improves fertility; lowers chances of diabetes and finally and most importantly decreases the risk of cancer.

Healthy mouth – healthy gum (preferably pink); firm teeth; a pleasant breath; rosy tongue and above all, a healthy smile are a few of them.  We do honestly take routine care through regular brushing, avoiding bad breath and in a few cases consulting dentist regularly.

Lips or labia are vascular with a thin layer of keratin and have the huge representation on the cerebral cortex and they cover orbicularis oris muscle which regulates entry and exit. The labial frenulum is a middle fold of mucous membrane that attaches the inner surface of each lip to the gum.  Cheeks make up the oral cavity’s side walls and their inner covering is the mucous membrane.  Between the skin and mucous membrane are connective tissue and buccinators muscles.  The latter and lip muscle help keep the food falling out of the mouth. Interestingly, they are different while you speak.

As we move on, the opening between oral cavity and throat (oropharynx) is called ‘Fauces’. The main open area of the mouth (oral cavity proper) runs from gums, teeth to fauces. 


Tongue has a multiple role – facilitates ingestion, mechanical/chemical digestion, sensation (taste, texture and temperature of food) swallowing and vocalization.

The top and sides of the tongue are studded with papillae – Fungiform papillae, larger towards the rear and smaller on the tip and sides containing taste buds; Filiform papillae create an abrasive surface.  Lingual glands secrete mucus and watery serous fluid that contains ‘lingual lipase’ which breaks down triglycerides BUT, needs stomach activation prior to its functioning.  Lingual frenulum tethers the tongue to the floor

Salivary Glands

Salivary glands, housed within the mucous membrane and the tongue secrete saliva – an average 1-1.5 liters of it each day.  It plays an important role as it moistens food and initiates breakdown of carbohydrates.  Practically, every part of the mouth ensure adequate saliva.

Submandibular, sublingual and parotid glands positioned differently secrete majority of saliva – parotic secretes watery solution that contains amylase; submandibular also contains amylase in liquid thickened with mucous; Sublingual glands contain mostly mucous cells.  While 95.5% of saliva is water, the remaining 4.5% is a complex mixture of ions, glycoproteins, enzymes, growth factors and waste products.  Of these, salivary amylase breakdown carbohydrates though it is not complete as the food stays for a short time until it gets inactivated by stomach acids. The pH of saliva is maintained at 6.35-6.85 by bicarbonate and phosphate ions.  Interestingly, immunoglobin A which prevents microbes from penetrating the epithelium and lysozyme makes saliva antimicrobial.

As I mentioned in my earlier write up, autonomic nervous system regulates salivation – When you are not eating food, saliva’s flow is just enough for comfort as you speak, swallow, sleep and go about life. Smell of food or when you are under stress, sympathetic stimulation reduces salivation resulting in dry mouth – you may quench with water.  Salivation is pretty sensitive to mere sight, smell and taste of food – you can feel right away when you are reading about food, as well.

Food that we eat is not only just for energy but also supplementing the essential nutrients to various parts of the body.  That means, the food that we eat may contain either all or some of them which stimulates taste receptors of the tongue.  Increase salivation even after swallowing food is just to cleanse the mouth, neutralize any irritating chemical remnants. 


We are all familiar that the initial 20 deciduous teeth or baby teeth between the ages of 6 and 12 are replaced by 32 permanent teeth –  8 Incisors four each at the top and bottom, sharp are used for biting into food; 4 Cupids (canines) flank incisors and have a pointed edge to tear up food; 8 premolars posterior to the cuspids mashes food; most posterior and largest 12 molars crush food to make it easy to swallow.  The third member of each set of three molars (top and bottom) is wisdom teeth which often may fail to erupt.

Digestive functions of the mouth



Throat (Pharynx) is involved both in digestion and respiration and when food enters, involuntary muscle contraction closes air passage.

Pharynx is short tube lined with mucous membrane and is connect between nasal cavities and esophagus.   While nasopharynx is involved in breathing only, oropharynx and laryngopharynx do both breathing and digestion.

Variety of food

Depending upon the culture, our body receives a variety of food which is different in nature – cereals, vegetables and fruits are all, different.  I save the other options like bread, egg and corn flakes at this juncture.  Every mineral is essential to either one or many parts of our body to accomplish holistic development. 

We will look at them in detail in the ensuing write ups of this series, please.

The simple mechanism of hunger and contentment are triggered by the in-built software as I mentioned earlier – Imagine for a while IF we are deprived of this trivial sensation.


Every cell of this human body including the taste bud has been rendering its service for our well-being.  But then, why is it that we don’t recognize their role a little more seriously than we have been doing so far.

Apart from the initiation in the mouth, the food passes through many phases before a nutrient is absorbed into the blood or lymph.  Similar to the mouth, every organ of the digestive system has a vital contribution.

The following diagrams explain the co-ordination of system better:

It is very clear from the above diagrams that even a single system like the Digestive, attracts co-ordination of other players and perhaps this may be true even for the other 10 systems that we talked about in the earlier write up, right.

Alimentary canal (aliment – to Nourish) is about 25 feet (7.62 m) in length during life.  However, this reaches to 35 feet (10.67 m) when measured after death – perhaps because smooth muscle tone is lost.  The length is in fact from mouth to anus between which are, Pharynx, Esophagus, Stomach, small and large intestine for the functional needs of the body.  Since both the openings of the alimentary system are considered outside the body, process of absorption of nutrients nourishes body’s inner space.

Teeth and tongue begin mechanical digestion followed by chemical digestion by salivary glands.  The gallbladder, liver, pancreas (bile and enzymes) referred to as accessory organs; take over the digestion process once the food reaches the stomach.  Malfunction of any of these organs results in complications.


Ingestion, propulsion, mechanical/physical digestion, chemical digestion, ABSORPTION and DEFECTION are involved in the nutritive material that we feed through the mouth. The entire process of digestive process presents elements of physics, chemistry and even engineering if ONE wishes to probe a little deeply.

First of all,  let us look at the major and additional functions of the various organs involved in the process of digestion and absorption of essential nutrients:

Every function is very important in fulfilling the ultimate objective of supplying the nutrients to the bloodstream.  The entire process is so very well programed that nothing goes wrong (not even the reversal of steps involved) and each organ is so very well equipped to complete its task efficiently.

Given the choice, everyone including me prefers flow diagrams or simple tables for easy grasping.  With this in mind, I have tried to explain the entire process of digestion through this flow diagram below.  I have further included appropriate scientific discipline that is vital for the process to be completed.


The first step – Entry of food. Food is chewed, mixed with saliva containing enzymes that breakdown carbohydrates; In addition, lipid digestion
Both Physics & Chemistry –Helps you understand the intricacy prior to actual absorption into the system


Tongue and pharyngeal muscles propel food from mouth into esophagus; Swallowing thus is the last voluntary act until defection; peristalsis is sequential alternate wave of contraction and relaxation; Digestive juices get mixed up during this process; Swallowing into stomach.IMAGINE, this is believed to happen even if you are in shirshashan.
Engineering and chemistry – swallowing followed by digestive fluids


Food is broken into smaller size through mastication (chewing as well as tongue movement) ; mechanical churning continues in the stomach later which creates an acidic soup – Chyme. Segmentation in the small intestine involves back and forth movement allowing a good mix with digestive juices facilitating absorption.
Engineering – breaking down, churning


Digestive secretions starting from mouth break down food molecules into chemical blocks – proteins into amino acid for example. Although these secretions vary in composition, they are typically composed of water, enzymes, acids and salts. Digestion process is completed in small intestine.

Chemistry. What is important here is the combination of food that we take – every nutrient is essential to some or the other part of our body.


Broken down food is of no value unless the nutrients enter blood stream and the nutrients are made to work.  This primarily occurs in small intestine – nutrients are absorbed through epithelial cells that make up mucosa; lipids are absorbed into lacteals and are transported via lymphatic vessels to the bloodstream.
Biology and chemistry. This is the ultimate objective of the digestive system


Undigested materials are removed as feces.
You do get a signal to empty the undigested material. Software part that indicates the last step of the process that started in the mouth BUT, a crucial one at that.

Neural Co-ordination:

Mechanoreceptors, chemoreceptors and osmo-receptors along the alimentary canal facilitate digestive functions smoothly.  Minor details like, whether food particles have been broken down sufficiently, food presence causing stomach to expand, quantity of liquid, type of nutrients (lipids, carbohydrates and proteins).  Stimulation of these receptors trigger appropriate reflex for further action of the digestive process.  Glandular secretion into lumen or muscle stimulation within alimentary canal thereby activating peristalsis and segmentation are also the result of this sensory stimulation.

In addition, nerve plexuses of the alimentary canal interact with nervous system which is responsible for certain reflexes.  For example, sight, smell and taste of food initiate long reflexes that begin with sensory neuron delivering signal to medulla oblongata.  This response stimulates stomach in secreting digestive juices.

Hormonal control

Gastrin of the stomach stimulates secretion of gastric acid and other GI hormones produced act upon the gut and accessory organs.  Duodenum produces secretin prompting pancreas to water bicarbonate secretion, Cholecystokinin (CCK) stimulates pancreatic enzymes and bile from liver and release bile from gallbladder.  Epithelium of the stomach and small intestine secrete GI hormones which enter bloodstream through which they reach target organs.

Appetite suppression to Constipation:

Age related influences hit even the digestive system – starting from mouth (taste buds, teeth loss, diseased gums, in-efficient salivary glands, swallowing).  In addition to these, ingested food moves slowly, neurosensory feedback weakens are added age related influences.

Pathologies – hiatal hernia, gastritis and peptic ulcers occur at greater frequencies as we age.  While Small intestine would throw up duodenal ulcers, mal-digestion and mal-absorption large intestine presents hemorrhoids, diverticular disease and constipation. Pancreatic enzyme and bile to small intestine also get weakened presenting jaundice, pancreatitis, cirrhosis and gallstones.

Since the food passes through various organs of the digestive system, there is a wonderful co-ordination among them.  Weakness in the starting point (mouth) may hamper the subsequent sequence as well and the end result would be that we would not have derived the good benefit of the nutrients.

A K Shyam, 373, I Main Road, 1st Cross, Vidyaranyapura Post, Bengaluru-560 097.

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