Human digestive system

Basics of Digestion

• What is Digestion? The body breaking down food so it can absorb nutrients and use them for energy.

• Where it Starts: Digestion begins in the mouth with both mechanical and chemical breakdown.

• Mechanical Digestion: Physically chewing or mashing food into smaller pieces to make chemical digestion easier.

• Chemical Digestion: Using special chemicals called enzymes to break food down into tiny nutrients that can pass into the blood. This starts in the mouth, where saliva begins breaking down carbohydrates.

The Path of Food

• Mouth: Chews food and mixes it with saliva to start breaking down carbohydrates.

• Oesophagus: A muscular tube that pushes food down into the stomach using wave-like squeezes called peristalsis.

• Stomach: Churns food with stomach gastric acids and enzymes, turning solid food into a thick liquid called chyme.

• Small Intestine: Finishes digestion and absorbs most nutrients directly into the blood.

• Large Intestine (Colon): Absorbs extra water and minerals back into the body. Helpful bacteria here help break down leftovers and make vitamins, turning waste into solid faeces.

• Rectum: Holds waste (faeces) until the body is ready to push it out.

• Anus: The exit opening where waste leaves the body.

Moving Food: Peristalsis

• What It Is: Coordinated, wave-like muscle contractions that push food and liquid through the digestive tract.

• Works Without Gravity: Because muscles actively squeeze the food along, you can still swallow and drink even when hanging completely upside down.

Nutrient Absorption in the Small Intestine

• Circular Folds: Ridges inside the intestinal lining that increase surface area by about $3\text{-fold}$.

• Villi: Tiny, finger-like bumps lined with blood vessels that absorb nutrients straight into the blood.

• Microvilli: Microscopic hairs covering each villus that increase surface area even more to capture as many nutrients as possible.

Damaged Villi

• The Problem: If villi become flat or damaged, the small intestine loses absorptive surface area and cannot take in enough nutrients, even if plenty of food is eaten.

• Common Symptoms:

  • Low energy and feeling tired.

  • Weight loss.

  • Poor growth and development.

  • Diarrhoea.

Digestion Time: Infants vs. Adults

• Infants ($12\,\text{hours}$):

  • Have a much shorter digestive tract.

  • Drink milk, which is liquid and very easy to digest.

• Adults ($24\text{ to }72\,\text{hours}$):

  • Have a much longer digestive tract.

  • Eat solid foods containing fats, proteins, and fibres that take longer to break down.