9.1 – Anatomy & Physiology of the Digestive System

Let’s begin by understanding what this title means.

  • Anatomy refers to the structure — what the digestive system is made of.
  • Physiology refers to the function — how the digestive system works.

What is the Digestive System?

The digestive system is like a big machine inside our body that takes in food, breaks it down into smaller parts (like nutrients), and absorbs these useful parts into our blood. The waste that’s not useful is removed from our body.

The digestive system has two main parts:

1) Gastrointestinal (GI) Tract

This is a long tube that runs from your mouth to your anus (the opening where poop exits). This tube is responsible for carrying and breaking down food. It includes:

  • Oral cavity (mouth)
  • Pharynx (throat)
  • Oesophagus (food pipe)
  • Stomach
  • Small intestine
  • Large intestine

You can imagine it like a tunnel that food travels through — each part has a different job.

2) Accessory Digestive Organs

These are helper organs that don’t form the tube but assist in digestion by making digestive juices or helping store and transport them. These include:

  • Salivary glands (make saliva)
  • Liver (makes bile)
  • Gallbladder (stores bile)
  • Pancreas (makes digestive enzymes)

These helper organs are like the support team of the digestive system.

🟦 ORAL CAVITY (Mouth)

This is the starting point of digestion. When you put food in your mouth, digestion starts immediately.

Structure: Lips and Tongue

  • The lips are soft, flexible structures that help keep the food inside your mouth. They are full of blood vessels and nerves, which is why they are sensitive and can feel touch and temperature.
  • The tongue is a muscular organ that moves food around and helps you taste and swallow it.

✳️ FUNCTIONS OF THE ORAL CAVITY

1. Selection of Food

This means we choose what to eat based on taste, smell, and appearance.

  • The tongue has taste buds that detect flavors like sweet, sour, salty, bitter, and umami (savory).
  • If something tastes bad, smells off, or feels wrong (like a stone or bone), we spit it out. This is the body’s way of avoiding harmful things.
  • Even the smell (from the nose) and sight (from the eyes) help in food selection. That’s why something that smells bad or looks weird might make you lose your appetite.

2. Mechanical Digestion

This means physically breaking the food down into smaller pieces.

  • The teeth do this job by chewing (also called mastication).
  • Chewing breaks food into small pieces so enzymes can work more efficiently. Think of it like cutting vegetables before cooking to help them cook faster.

3. Chemical Digestion

Now we go beyond just chewing.

  • The salivary glands make saliva, which is released into the mouth.
  • As we chew, saliva mixes with food.

➤ Types of salivary glands:

  1. Sublingual glands – under the tongue
  2. Submaxillary (or submandibular) glands – under the lower jaw
  3. Parotid glands – in front of the ears

➤ What does saliva contain?

  • Water and mucus – moistens food and makes it easy to swallow
  • Bicarbonate ions – act like a cleaner that balances the acid/base (pH)
  • Thiocyanate ions – help kill bacteria (natural disinfectant)
  • Salivary amylase – an enzyme that starts breaking down starch (a carbohydrate) into maltose (a simpler sugar)

💡 Think of starch as long chains of sugar beads. Amylase begins cutting those chains into smaller parts.

  • The chewed and moistened food becomes a soft lump called a bolus, which is ready to be swallowed.

🟦 PHARYNX

  • The pharynx is the back part of your throat, a common passage for both food (digestive tract) and air (respiratory tract).
  • When we swallow, the bolus moves to the back of the mouth and into the pharynx.

✳️ SWALLOWING OF FOOD

Swallowing is a complex but smooth process. Here’s what happens:

  1. Tongue pushes the bolus toward the back of the mouth.
  2. Soft palate lifts to close the nasal passage, so food doesn’t go up the nose.
  3. Larynx (voice box) moves up.
  4. Epiglottis (a small flap of cartilage) folds over the trachea (windpipe) to prevent food from entering the lungs.
  5. The food is directed into the oesophagus.

🔒 The body has built-in “safety doors” to make sure food goes to the stomach, not the lungs!

🟦 OESOPHAGUS (Food Pipe)

  • The oesophagus is a 25 cm long tube that connects the pharynx to the stomach.
  • Its main job is to carry the bolus down to the stomach.

✳️ MOVEMENT OF FOOD – Peristalsis

  • Food is pushed down by a special movement called peristalsis.
  • Peristalsis is a wave-like motion — the muscles behind the food contract, and the muscles in front of the food relax, pushing it forward.

💡 Imagine squeezing toothpaste from the end of the tube — that’s peristalsis!

The Cardiac Sphincter

  • At the bottom of the oesophagus is a ring-like muscle called the cardiac (or lower oesophageal) sphincter.
  • This opens to let food into the stomach and closes afterward to prevent acid from flowing back up (acid reflux).

🟦 MOTILITY OF THE ALIMENTARY CANAL

“Motility” means the movement of muscles in the digestive tract.

There are different types of movements:

1. Peristalsis

As explained, this is the forward wave movement that pushes food along.

2. Segmentation

  • Found mainly in the small and large intestines.
  • Instead of pushing food forward, the muscles contract and relax in different segments, which moves the food back and forth.
  • This helps mix food with enzymes and increases absorption of nutrients.

💡 Think of washing clothes in a machine that swirls water back and forth for better cleaning — that’s like segmentation.

3. Antiperistalsis

  • This is the reverse movement — food moves backward.
  • This usually happens when you’re about to vomit.
  • It can also occur when your stomach is irritated.

Hunger Contractions

  • When your blood sugar drops (like when you haven’t eaten in hours), the digestive muscles start moving, creating a rumbling sound in your stomach.

These are called hunger pangs or hunger contractions — your body’s way of telling you to eat