Introduction

Food is the basic necessity of our lives. We cannot imagine our life without food, there is a very famous saying in English “Some people eat to live whether some live to eat “. All of us eat without properly knowing how is food ingested, digested and assimilated and how small changes in our eating habits can make us healthier?

So to understand the mechanism of digestion we should try and understand the basic physiology of the process. The digestion process of the food starts as soon as food enters mouth. For better understanding of the digestion let’s know the physiology of digestion of food. It has been divided into following parts-

• Ingestion
• Digestion
• Absorption
• Excretion

Ingestion

The process of taking of food in mouth is ingestion. Digestion of food starts as soon as we take food in mouth due to presence of saliva which has an enzyme called salivary amylase. Salivary amylase acts on the starch component of the food and starch can only be digested in mouth. All of us have heard our parents and grandparents teaching us right from the childhood that food must be properly and slowly chewed. It has a scientific reason behind it i.e. salivary amylase that is secreted in saliva and helps in breakdown of starch works at pH-6.9 which is alkaline but as food move downs the oesophagus into the stomach pH is as low as 3.9 which is acidic. In this low pH salivary amylase becomes inactive and hence starch is left undigested.

So for the digestion of starch to take place food must be slowly chewed. Chewing (mastication) breaks up large food particles and mixes the food with the secretions of the salivary glands.

Once the food is properly chewed, a bolus of food is formed with saliva. This bolus is swallowed in the oesophagus by the action of tongue. The bolus is pushed backward by the tongue which is taken into the oesophagus by the wave like motion or peristaltic motion of the pharyngeal muscles. The epiglottis closes and respiration is stopped momentarily as a reflex action. Solid foods move down by peristaltic action whereas in upright position liquid and semisolid foods move down into the lower oesophagus under gravity before the peristaltic motion takes place.

Oesophageal motor dysfunction can cause Gastro oesophageal reflux disease or GERD and achalasia.

Digestion

Down the oesophagus food reaches the stomach where it gets stored. When food reaches stomach the pH of stomach is highly acidic which is around 3.9. The food which is in form of bolus is mixed with acid, mucus and pepsin. The salivary amylase that was acting on starch in mouth gets inactivated in acidic pH of stomach. The gastric juices in the stomach are secreted by the wall of the stomach. The food that reaches the stomach is stored in form of concentric rings with food eaten last near the esophageal sphincter and the food eaten earlier near the outer wall of the stomach.

Now the when the gastric secretions get mixed with the food in stomach it is termed as chyme. The chyme is a greyish or dark semifluid paste. The consistency of the chyme depends upon the amount of food ingested, the amount of water present, the amount of gastric secretions and to what amount the digestion has already occurred. Hydrolysis of fats is maximum in stomach.

The major functions of stomach are storage of food and controlled release of food into duodenum. Stomach also has castle’s intrinsic factor and vitamin B12 also known as cyanocobalamin, the deficiency of this vitamin causes pernicious anaemia. The food is passed from stomach to duodenum by the peristaltic contraction wave. The peristaltic contraction waves occur in the presence of food. But when the stomach is empty the contractions can also occur at that time due to high gastro-intestinal tonus. These are called hunger contractions. The most common GASTRIC MOTOR DYSFUNCTION is gastroparesis, which is delayed gastric emptying in the absence of mechanical obstruction.

Further digestion and breakdown of food occurs when food reaches small intestine. The food is mixed with bile which is secreted from liver and pancreatic juice which is secreted by pancreas. Pancreatic juice has trypsin and chymotrypsin enzymes which help in breakdown of proteins, and mucosal surface and brush border cells of small intestine secretes enzymes that help in digestion of sugars. About 90% of the food is digested in duodenum and is broken down into simpler forms.

Absorption

Small intestine is densely coiled to increase its surface area for absorption of nutrients, further it has specialized cells with brush borders which increases its surface area. Maximum absorption of nutrients occurs in small intestine. Small intestine is divided into three parts duodenum, jejunum and ileum. Absorption of iron and calcium occurs in duodenum. Absorption of most of nutrients including amino acids, vitamins and minerals occurs actively in jejunum. Absorption of fat, fat soluble vitamins and cholesterol occurs in duodenum and jejunum by the action of bile.

Most of the nutrients are absorbed in duodenum and jejunum, reabsorption of bile takes place in ileum. Absorption of vitamin B12 also occurs in ileum.

Absorption of water and nutrients occurs in large intestine. INTESTINAL MOTOR DYSFUNCTION present with symptoms such as nausea, vomiting, abdominal distension, colic, diarrhoea, and constipation may result from abnormalities in moving luminal contents through the small intestine.

Excretion

Large intestine consists of colon and rectum. As the food reaches the large intestine most of the digestion and absorption has already taken place. The left over chyme which is devoid of nutrients and any vitamins has now become faeces. When the faeces reach the rectum, the distension of rectum occurs. It causes the contraction of rectus muscles which causes the desire defecate. Defecation is reflex mechanism but can be voluntarily controlled by relaxing the sphincter and contracting abdominal muscles.

Some interesting knowledge

Why do we fart?

All of us in our life have experienced gas in the stomach. Farting may be an unpleasant uncultured thing publically but farting is a normal physiological process. Let’s know why do we fart and why does fart smell so bad? When we eat food some of the air is unavoidably swallowed in the process of eating and drinking (aerophagia). The swallowed air is regurgitated which is also known as belching. This sound can be heard with the stethoscope. Since air is a mixture of gases when it is regurgitated some of the gases dissolved in it are absorbed while the remaining gas is passed as such into the colon. In colon again there is absorption of certain amount of oxygen and hydrogen. To this absorbed gas hydrogen sulphide, carbon-dioxide and methane produced by bacteria present in the colon is added. Now this is the gas that is expelled as flatus.

The foul smell of the flatus can be attributed to the presence of sulfides and methane. The presence of the gas in intestine can cause abdominal discomfort, rumbling noises and cramps.

What are hunger pangs or hunger contractions?

Sometimes when you haven’t eaten anything for a long time you feel contractions of the stomach. These are called hunger contractions or hunger pangs. They may sometimes last for 2 to 3 minutes. Hunger contractions are most intense in young, healthy people who have high degrees of gastrointestinal tonus; they are also greatly increased by the person’s having lower than normal levels of blood sugar. When hunger contractions occur in the stomach, the person sometimes experiences mild pain in the pit of the stomach, called hunger pangs. Hunger pangs usually do not begin until 12 to 24 hours after the last ingestion of food; in starvation, they reach their greatest intensity in 3 to 4 days and gradually weaken in succeeding days.