The Digestive System: Organs, Tissues and Absorption

From poutine to protein

You eat a plate of poutine: potatoes, cheese curds and gravy. A few hours later, the starch from the potatoes is glucose in your blood, the protein in the curds is amino acids building new muscle, and the fat is being stored or burned for energy. Your cells can't use a french fry. They can only use small molecules. The digestive system's job is to break food down into those molecules, absorb them, and get rid of what's left.

Four stages

1. Ingestion: taking food in.
2. Digestion: breaking food down. Mechanical digestion (chewing, churning) makes smaller pieces with more surface area. Chemical digestion uses enzymes to split large molecules (see below).
3. Absorption: small molecules pass into the blood (or lymph).
4. Egestion (elimination): undigested material leaves the body as feces.

The three main groups of digestive enzymes:

• starch (a carbohydrate) → sugars, by amylase,
• proteins → amino acids, by proteases such as pepsin,
• fats → fatty acids and glycerol, by lipase.

Organ by organ

• Mouth: teeth start mechanical digestion. Salivary glands release saliva, which moistens food and contains amylase to begin breaking down starch. The tongue shapes food into a ball (bolus) and pushes it back to swallow.
• Pharynx and epiglottis: when you swallow, a flap called the epiglottis covers the opening to the windpipe so food goes down the esophagus.
• Esophagus: a muscular tube about 25 cm long. Peristalsis (waves of smooth muscle contraction) pushes food down, even if you're upside down.
• Stomach: a stretchy, muscular bag. Its smooth muscle churns food. Its lining secretes hydrochloric acid (which kills many microbes and creates the acidic conditions pepsin needs) and pepsin, which starts protein digestion. A thick layer of mucus protects the stomach lining from its own acid. Food leaves as a thick liquid called chyme.
• Small intestine: about 6 m long in an adult. In the first part (the duodenum), bile from the liver and pancreatic juice (amylase, proteases, lipase) finish digestion. The rest of the small intestine absorbs nutrients through its huge surface area.
• Liver: makes bile, which emulsifies fat (breaks it into tiny droplets so lipase can work faster). The liver also processes absorbed nutrients and removes toxins from the blood.
• Gallbladder: stores bile and releases it after a fatty meal.
• Pancreas: makes pancreatic juice with enzymes for all three nutrient types and a base that neutralizes stomach acid. (It also makes the hormone insulin.)
• Large intestine: about 1.5 m long. It absorbs water and some minerals and is home to trillions of helpful bacteria, which make some vitamins. What's left becomes feces.
• Rectum and anus: store feces and release them from the body.

From tissues to an organ: the gut wall

The digestive tract from the esophagus to the large intestine is built from the same four layers of tissue, adapted in each organ. This is a great example of how tissues form organs (expectation B3.3).

Villi: maximum surface area

Nutrients are absorbed by diffusion and active transport across the lining of the small intestine. The more surface there is, the faster absorption happens. The small intestine increases its surface area in three ways:

1. It is long (about 6 m) and its inner wall has large circular folds.
2. The folds are covered in millions of villi, tiny finger-like projections about 0.5 to 1 mm tall.
3. Each epithelial cell on a villus has its own microscopic projections called microvilli.

Each villus contains a network of capillaries, which absorb sugars and amino acids, and a lacteal, a lymph vessel that absorbs fats. The lining is only one cell thick, so nutrients have a very short distance to travel.