Lab Report #5: The Human Digestion System

Food begins its journey through the digestive system in the mouth. Here, chewing begins to mechanically digest and break down food. Salivary glands secrete saliva which moistens food and lubricates it for its passage through the esophagus. Saliva also contains amylase, an enzyme which begins to break down starches and glycogen. When food is swallowed, it triggers a reflex of peristaltic waves. These are a wave of muscle contractions which push chewed food (bolus) down into the stomach, much like you push toothpaste down it’s tube. The esophagus also secretes mild amounts of mucus which further aids in easing food into the stomach as well as protect the esophagus wall.

The next step in the digestive process occurs in the stomach. It’s primary function is to store, mix, and most importantly, dissolve food. Only minimal amounts of it are absorbed. Digestion is partly performed using muscle contractions, but mostly using three key substances: Hydrochloric acid, pepsin, and mucin.

HCl acid is the primary of the three. It is an acid with a pH of 3 which chemically digests and breaks down food as well as kill many potentially damaging microbes and bacteria. Pepsin is an enzyme which breaks down the protein which HCl acid cannot. When it mixes with HCl acid, however, it becomes pepsinogen, which is more active than pepsin, and digests proteins more efficiently as well. And finally, mucin itself does not play a role in the digestion of food, but rather protects the stomach walls from damage. It also aids in churning food around when muscle contractions occur.

Food remains in the stomach for 2 to 3 hours. After it has been thoroughly dissolved and digested, it begins the next step in the digestive process by exiting the stomach through the Pyloric Sphincter valve. From here it enters the small intestine.

The small intestine is 3.25m in length in a grown adult. This is where the bulk of food digestion and absorption occurs. Several gastrointestinal organs take over the process of further dissolving food (chyme), by secreting various enzymes and other chemicals into a 30cm section of the small intestine called the duodenum.

Pancreas
The first of these organs is the pancreas. It firstly excretes bicarbonate (NaHCO3), a mild base, to aid in neutralizing the Hydrochloric acid from the stomach. It also releases proteinases to digest proteins and lipases to digest fats.

Liver
The liver, one of the larger organs in the body, secretes bile salts to dissolve water-insoluble fats. More bicarbonate is released to further neutralize stomach acids, and also secretes other body wastes, such as filtered blood waste (i.e.: alcohol) to be eliminated from the body.

Gallbladder
The gallbladder’s function is simply to store and concentrate bile salts for meals and then release them during digestion.

After chyme has passed through the duodenum section of the small intestine, it enters the jejunum and ileum which altogether are nearly 3m long. Here, water and mucus are released to aid chyme pass through smoothly. Most of the body’s nutrients and minerals are absorbed through little buds called villi. Altogether the chyme has spent 4-6 hours in the small intestine, and it now nears the end of it’s journey in the large intestine.

The large intestine is about 1.5 m long in a fully grown adult. It concentrates waste products by re-absorbing the water released by the small intestines. It also contains friendly bacteria which help digest the cellulose found in plants, and produce vitamins B and K. After all these processes are complete, waste products are expelled from the body in the familiar manner. Thus completes the digestive process.