Fantastic Voyage: Digestion and Excretion in the Human Body

Introduction

Today I will take you on a trip inside the gastrointestinal tract — and beyond — to observe the processes of digestion and excretion in the human body. As simple as it may seem, this process is far more than just moving contents down a hollow tube. To understand this more deeply, let us begin our journey.

Oral Cavity and Swallowing

Being reduced to eight microns in a hamburger holds the unsettling possibility of being chewed, ground, and dissolved in gastric acid. Even with special shield defenses, that slight possibility remains a daunting thought. Despite the risk, curiosity and wonder at the human body proved motivation enough.

In a moment, I found myself in the mouth of a 55-year-old man. Mixed in with me were fries, meat, lettuce, cheese, and a burger washed down with beer. Staying away from the teeth was a difficult task, since the tongue was constantly working to bring me and the other contents toward them. The teeth — mainly the molars — were busy grinding and crushing the food, breaking it into smaller pieces. With every bite, a gush of secretions was produced from minute pores at various points in the oral cavity. These secretions enveloped the food, making it smoother and helping to break it into smaller fragments. The major enzyme produced by the salivary glands is amylase, which helps break down carbohydrates such as the starch in the bread component of this man's meal (Barrett et al., 2010).

Eventually, the food and I were rolled into a ball — a bolus — and pushed to the back of the mouth by the tongue, then squeezed against the oral part of the pharynx. At this moment, I experienced an important reflex: swallowing. This process is mediated by the coordinated involvement of 55 muscles of the oropharyngeal, laryngeal, and esophageal regions, 6 cranial nerves, and 2 cervical nerve roots. Two sets of muscles worked simultaneously to allow the man to swallow his bite: one set kept the mouth closed, while the other created positive pressure in the oral cavity (Leonard & Kendall, 2008).

Once in the pharynx, the remaining stages of swallowing proceeded quickly. As one set of muscles contracted, the other relaxed, propelling us further down into the esophagus. These muscles include the longitudinal group — the stylopharyngeus, salpingopharyngeus, palatopharyngeus, and palatothyroideus — and the circular group (Hansen, Koeppen & Netter, 2002). Contraction of the longitudinal muscles squeezed the pharynx laterally, shortening it and assisting the opening of the upper esophageal sphincter (Barrett et al., 2010).

At this point, the opening to the trachea was sealed by the epiglottis — a structure that acts like a gateway, directing food into the esophagus while protecting the airway from aspiration. As the man sipped his beer, the liquid trickled past us far more quickly, making me envious of its faster transit time (Barrett et al., 2010).

Esophagus and Stomach

The ride through the esophagus was nothing like a water slide. Each muscular contraction squeezed us against its walls and forcefully pushed us downward. This squeeze-push cycle was strongest at the lower esophageal sphincter, after which we dropped into the stomach. Arriving there felt like being an unwelcome guest. The squeeze-push cycle resumed immediately, but this time it did not push us further down — the stomach was sealed at its lower end by the pyloric sphincter. A wave of acidic secretion bathed us and nearly completely dissolved the food particles, particularly the protein molecules. Some particles — such as starch molecules from the bread — were larger than others. By the end of the gastric phase, the food had been reduced to a paste-like slurry, and I was thoroughly mixed within it (Barrett et al., 2010).

It was fascinating to observe the activity within the stomach. Certain glands secreted acidic fluid containing the enzyme pepsin, responsible for protein digestion. Other glands secreted a thicker fluid containing mucus and bicarbonate ions, which protected the stomach wall from ulceration caused by the gastric contents (Barrett et al., 2010).

After approximately 30 minutes, the pyloric sphincter opened and we were propelled into the duodenum — a C-shaped tube where most chemical digestion takes place. Descending into the second part of the duodenum, I noticed two ducts opening into it: the pancreatic duct and the bile duct, joining through a common opening. As we arrived, the sphincter of Oddi opened to release secretions that broke the remaining molecules into smaller particles. The fat molecules were emulsified and, along with everything else, were pushed further down into the jejunum (Barrett et al., 2010).