My Biomedical Notebook: Digestive System

An Overview

Why do we eat?

Consider for a moment a Big Mac. The purpose in your eating a Big Mac, other than simple hedonism, is to assimilate the nutrients it represents and make them available to build, repair and maintain your own tissues, as well as provide energy for studying and occasional other pursuits.

You may have asked yourself - "Exactly what nutrients are present in a Big Mac that I can assimilate?" MacDonald's comes close to full disclosure in this regard, but what they don't tell you is that in order to take advantage of these nutrients, you have to provide the means to carefully break them down into much smaller molecules that can be imported into blood. Luckily, your digestive system takes care of this very complex process so efficiently that most of the time you don't even need to think about it.

At its simplest, the digestive system it is a tube running from mouth to anus. This tube is like an assembly line, or more properly, a disassembly line. Its chief goal is to break down huge macromolecules (proteins, fats and starch), which cannot be absorbed intact, into smaller molecules (amino acids, fatty acids and glucose) that can be absorbed across the wall of the tube, and into the circulatory system for dissemination around your body.

The breakdown of foodstuffs like a Big Mac is accomplished through a combination of mechanical and enzymatic processes. To accomplish this breakdown, the digestive tube requires considerable assistance from accessory digestive organs such as the salivary glands, liver and pancreas, which dump their secretions into the tube. The name "accessory" should not be taken to mean dispensable; indeed, without pancreatic enzymes you would starve to death in short order.

In many ways, the digestive system can be thought of as a well-run factory in which a large number of complex tasks are performed. The three fundamental processes that take place are:

Secretion: Delivery of enzymes, mucus, ions and the like into the lumen, and hormones into blood.

Absorption: Transport of water, ions and nutrients from the lumen, across the epithelium and into blood.

Motility: Contractions of smooth muscle in the wall of the tube that crush, mix and propel its contents.

Each part of the digestive tube performs at least some of these tasks, and different regions of the tube have unique and important specializations.

Like any well-run factory, proper function of the digestive system requires robust control systems. Control systems must facilitate communication among different sections of the digestive tract (i.e. control on the factory floor), and between the digestive tract and the brain (i.e. between workers and management). Control of digestive function is achieved through a combination of electrical and hormonal messages which originate either within the digestive system's own nervous and endocrine systems, as well as from the central nervous system and from endocrine organs such as the adrenal gland. Different parts of these systems are constantly talking to one another. The basic messages are along the lines of "I just received an extraordinary load of food, so I suggest you get prepared" (stomach to large intestine) or "For goodness sake, please slow down until I can catch up with what you've already given me" (small intestine to stomach).

Finally, a note about differences in digestive anatomy and physiology among animals. The digestive systems of humans, dogs, mice, horses, kangaroos and great white sharks are, to a first approximation, virtually identical. If you look more carefully however, it becomes apparent that each of these species has evolved certain digestive specializations that have allowed it to adapt to a particular diet.

These differences become particularly apparent when you compare a carnivore like a dog with a herbivore like a goat or a horse. Goats and horses evolved from ancestors that subsisted on plants and adapted parts of their digestive tracts into massive fermentation vats which enabled them efficiently utilize cellulose, the major carbohydrate of plants.

In contrast, dogs evolved from animals that lived on the carcasses of other animals, and have digestive systems that reflect this history - extremely small fermentation vats and essentially no ability to utilize cellulose. Bridging the gap between carnivores and herbivores are omnivores like humans and pigs, whose digestive tracts attest to a historical diet that included both plants and animals.

Digestive Tube Anatomy

Remarkably diverse and specialized processes take place in different sections of the digestive tract, but there is a fundamental consistency in the architecture of the tubular digestive tract. From the mouth to the anus, the wall of the digestive tube is composed of four basic layers or tunics.

Tunica serosa is the outermost covering of the digestive tube. In most of the digestive tract (stomach and intestines) it consists of a thin layer of loose connective tissue covered by mesothelium (a type of squamous epithelium that lines body cavities); within the peritoneal cavity, this structure is also referred to as visceral peritoneum.

In the abdominal cavity, the serosa on each side of the tube fuses together to form a suspensory structure called mesentery, which houses vascular and nervous supplies to the digestive tract and is continuous with the lining of the cavity. In regions outside of the abdominal cavity where the digestive tube is essentially affixed to adjacent structures via its outer layer of connective tissue (esophagus and rectum), this tunic is referred to as tunica adventitia instead of tunica serosa.

Tunica muscularis endows the digestive tube with an ability to be motile. In most of the digestive tube, this tunic consists of two thick layers of smooth muscle. Muscle fibers in the inner layer are aligned circularly, whereas those in the outer layer have a longitudinal orientation.

This combination of circular and longitudinal smooth muscle gives the tube an ability to perform complex movements that squeeze and propel ingesta in the lumen. Between the inner circular and outer longitudinal layers of smooth muscle is another critical component of the digestive tract's nervous system - the myenteric plexus.

Tunica submucosa, immediately beneath the mucosa, is a layer of loose to dense connective tissue containing blood and lymphatic vessels. The submucosa also contains the submucous plexus, a critical component of the digestive tract's nervous system which provides nervous control to the mucosa.

Tunica mucosa is the innermost layer of the digestive tube and lines the lumen. Among the four tunics, the mucosa is most variable in structure and function, endowing the tube with an ability to perform diverse and specialized digestive tasks along its length. Of critical importance in this regard are the epithelial cells that cover the mucosa and are thus in direct contact with the lumen.

This epithelial cell sheet (lamina epithelialis) is distinctly different in different regions of the tract. Indeed, in most of the tract, several different cell types contribute to the epithelium, including cells dedicated to secretion, absorption or production of hormones.

These distinctive differences in architecture of the epithelium can be seen below in the micrographs of mouse digestive tube. The magnification of all four images is identical and the epithelial layer is oriented toward the top.

Beneath the epithelium, but still within the tunica mucosa is a layer - the lamina propria - of loose connective tissue through which course blood vessels and lymphatics that supply the epithelium. This layer also contains lymphatic nodules important to immune functions of the digestive tract. Finally, beneath the lamina propropria is a thin layer of smooth muscle ( lamina muscularis mucosae) which permits the mucosa to dynamically move and fold.