Human Digestive System and Nutrient Absorption in the Digestive Tract

“I eat enough calories but still feel terribly tired”

“I have weak digestion”

“We are 90% diet and 10% exercise”

“Need some quick remedies for indigestion” 

You surely might have come through these statements at least once in your life. 

It is because our guts rule our lives. If the digestion is good, we feel fantastic and life feels easy. If it is bad, the beauty fades away and sickness arrives. And to stop it from happening, we must first know how digestion works. 

We feel tired and hungry; we eat food; the food goes into the digestive tract; nutrient absorption takes place; we feel good and energetic. 

Is this all? 

No! There is much more to our digestion and nutrition than we see, quite literally. So, let’s absorb some digestive knowledge so we can make better decisions regarding our diet. 

What organs are involved in digestion?

We generally believe that food means stomach and stomach means digestion. Nothing more or less. But the case is different. Although the whole body is involved in digestion, 7 organs remain at the core. Here are the organs that are primarily involved in the human digestive system:

• Mouth and salivary glands
• Oesophagus 
• Liver
• Stomach
• Pancreas
• Intestines – Large and Small
• Rectum

Let’s see the whole digestive process, as it happens, from food ingestion to nutrient absorption. As we proceed, we’ll focus on how nutrition intake and absorption can be enhanced. 

The Digestive Process – As It Happens

As told earlier, the whole body is involved in the digestive system. Be it the brain, tongue, or even eyes, every part leads to one thing or another, directly or indirectly. But, primarily, it all happens when we feel those rats running in our stomachs.

1. Why do we feel hungry?

When your stomach stays empty, it is time for a new digestive cycle. The stomach lining releases a hormone called ‘Ghrelin’. It is a hunger hormone that creates cravings within you, making you imagine delicacies and walk down the kitchen lanes.

Apart from an empty stomach, low blood sugar levels also induce hunger. The body thinks that it is low on energy and needs to replenish itself as soon as possible. Evolutionary buildup, you know.

Is this all? No. As we said earlier, the whole body is involved. Many other factors also make us eat more and more. Here are a few:

• Stress
• Boredom
• Anxiety
• Psychological disorder

All these things lead to emotional eating, even when your body doesn’t need that pasta sitting on the table. 

Stress management exercises such as yoga, meditation, and mindfulness can enhance your feeling of hunger.

2. The role of saliva in digestion

Once we realise that we are hungry, it is time to seek food. Even a slight smell or sight of food can act as a huge motivator there. 

When that juicy food enters your mouth, the primary digestive process begins. We chew food, and as we chew it, the salivary glands start secreting saliva. By getting mixed into the food, this saliva serves many purposes.

• It contains digestive enzymes such as amylase that start breaking down carbs into smaller units.
• The food gets softened and lubricated. This eases down the swallowing process.
• Saliva also makes us perceive the taste of food in a better way.

Saliva production is crucial to digestion and nutrient absorption. One way to enhance it is to stay hydrated.

Chew it down: The food must be chewed well before swallowing. A good number is to chew it 32-40 times, even when you feel extremely hungry and want to hog on it. Eat slowly and never overeat.

3. The passage – How food enter your stomach?

Once the food is swallowed, it passes down through your oesophagus (food pipe) to your stomach. The food that is chewed well is called ‘bolus’. This bolus is pushed down to your stomach by a muscular contraction called peristalsis. When food touches the end of your tongue, it triggers this action. You might even sense it by putting your finger at the end of your tongue – that sudden reaction. 

This contraction called peristalsis is so strong that you can ingest food even while standing on your head. Just don’t do it unless you want to invite some serious trouble. 

When the food enters your stomach through the oesophagus, the oesophageal sphincter (a rubberband-like structure) tightens and closes the stomach. This prevents food and acid from going back into your oesophagus. 

Avoid eating in stressful or uncomfortable situations. This can affect the passage of food into the stomach and may lead to digestive issues.

4. What happens to food inside your stomach?

The food in your stomach triggers an action – numerous gastric glands on your inner stomach wall release mucous. This mucous prevents your delicate stomach wall from the strong gastric acid. Surely, when food enters your stomach, it triggers the release of gastric acid. However, many other factors also lead to its secretion. Here are a few:

• Seeing the food (eyes, as told before)
• Smelling some delicacy (you surely know this feeling)
• Spices (certain spices such as fennel seeds enhance digestion)

This gastric juice contains hydrochloric acid, pepsin, and lipase (for fat digestion). You surely might relate this to the soft drink brand ‘Pepsi’.

Pepsi got its name after the digestive enzyme called pepsin because its inventor, Caleb Bradham, thought that his drink enhanced digestion.

The churning in the stomach starts – food gets mixed into the digestive juices. And do you know how much gastric juice is produced per day? Around 1 to 2 litres. This is huge. 

Pepsin, on the other hand, breaks down proteins in your stomach. 

All this churning and burning turns this ‘bolus’ into ‘chyme’, a semi-liquid mixture ready to enter your small intestine. 

5. The role of small intestine in digestion

Now, the chyme is ready for nutrient absorption. At this point, another rubber-band-like muscle called the pyloric sphincter muscle starts its work. It stays at the stomach end and opens only a few millimetres to allow well-chewed and broken food to enter the small intestine. The bigger pieces do not get their pass to make their way through. This is why not chewing food leads to delayed digestion or even indigestion. So, pay respect to the chewing process. 

At this point in time, the pancreas comes into the picture (or the digestive system).

The pancreas now releases more digestive enzymes such as lipase, amylase, and proteases. These enzymes further break down fats, carbs, and proteins. And what about your liver? Surely, it has a function too.

At this point of time in our digestive process, the liver secretes bile, as we know it. The gallbladder stores this bile juice and releases it once it is triggered. When the food reaches the small intestine, especially food with fat content, a hormone called cholecystokinin gets into action. This makes the liver secrete our beloved bile juice. This bile juice has a work – to emulsify fat and break it down into smaller droplets. 

And now, the process of nutrient absorption. 

6. How does nutrient absorption happen?

All the energy that we get, each calorie to hop or argue around, gets extracted mainly in the smaller intestine. The small intestine, if we expand it lengthwise, will reach up to 22 feet or 7 metres. It usually remains 3 times our body length. 

When food enters the small intestine, small feather-like or finger-like projections in its inner surface start their work. These are called villi and microvilli. And their purpose? To increase the surface area for enhanced nutrient absorption. These projections have small blood capillaries inside them that absorb nutrients from there to carry to other areas through the bloodstream.

Here, nutrient absorption takes place like this –

• Carbs are broken down into sugars (glucose, fructose, and galactose)
• Proteins are broken down into amino acids
• Fats are broken down into fatty acids and glycerol
• Minerals and vitamins are absorbed through a specialized transport mechanism
• Water and electrolytes (sodium, potassium, chloride, magnesium, etc.) are absorbed as well

Consume a balanced diet rich in carbohydrates, proteins, fats, vitamins, and minerals. Keep a healthy and colourful pantry that includes lots of fruits, veggies, and fibre.

Some nutrients such as glucose do not require any cab service (or a carrier mechanism). They get freely transported into the bloodstream. On the other hand, other nutrients such as iron require carriers such as transferrin. Different cells and tissues take up the nutrients they need for various functions, such as energy production, growth, and repair.

And now, the larger intestine.

7. The exit

Once the nutrients are absorbed through the small intestine, it passes down the remaining food into the large intestine. Although the large intestine doesn’t have millions of villi to absorb nutrients, it sure does have billions of bacteria. 

The large intestine contains an estimated 100 billion bacteria

The purpose of these bacteria? Many. 

• Production of vitamins
• Decomposition of fibre and use it for energy
• Killing harmful germs to strengthen immunity

And as the chyme (or churned food) is moved through the large intestine, water is removed from it. Furthermore, it adds mucous to make its journey easier. Unutilized bile juice is carried back to the liver from hereon. 

The unabsorbed food is thus moved to the rectum. There, we feel the call of nature and go to the washroom to get some relief. 

Stay hydrated to support the removal of waste and maintain proper bowel function. However, avoid drinking water during or immediately after your meal.

When the energy (in the form of blood sugar) is moved into the bloodstream, the pancreas releases insulin. These hormones allow the blood sugar to enter cells and be used as energy. As a result, the blood sugar is lowered again. But sometimes, we suffer from insulin resistance. Poor days indeed. It must be treated well in time.

So, here you have it, the complete digestion process.

Conclusion 

Eating is easier, but digestion? Never! Human digestion is a complex process. It starts with a simple craving but ends up taking numerous passages with one purpose – nutrient absorption. But not all people get the same amount of energy from the same food. It all depends on how strong the digestive system is. 

When we eat food, the saliva gets mixed in it and starts breaking down carbs right there. So, chew your food much, around 32-40 times. 

After chewing, as we swallow, it goes through the food pipe, into the stomach, and gets mixed with the digestive juices. Once done, it is moved into the smaller intestine where the nutrient absorption takes place. From there, this food (called chyme) is moved into the large intestine where billions of bacteria await it. And then we feel the call of nature. 

From what we have seen so far, one thing is sure – the stronger our digestive system, the better our lives. And to enhance it, we must work on it. Here is a small takeout –

• Chew your food thoughtfully
• Don’t overeat
• Stay hydrated but don’t drink during or immediately after your meal
• Keep a healthy pantry
• Eat with time
• Exercise
• And practice yoga, mindfulness, and meditation

For it’s not just about what you eat, but how well you digest it that truly nourishes your existence. Bene digesta!

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