Before we begin to examine the physiology of the digestive tract we need to be familiar with the structure of its core elements. For many of you this will be revision and shouldn't take very long at all. However this information will be assumed knowledge for the remainder of the lesson so it is probably worth spending a little time on if your anatomy is a little hazy.
MACROSCOPIC STRUCTURE
The digestive system consists of two main structural elements:
Digestive Tract
The digestive tract is a single long tube along which food is transported. Food enters the digestive tract at the mouth (a process referred to as ingestion) and leaves it through the anus in a process known as defecation.
The digestive tract is sub-divided into five major compartments that vary significantly in their diameter and functions.
Movement of food along the digestive tract is controlled by valve-like structures known as sphincters that open and close and regulate the flow between various compartments. The products of digestion (and other small molecule dietary elements) traverse the wall of the digestive tract and flow into the blood in a process referred to as absorption.
Accessory Organs
The accessory organs are exocrine glands that are responsible for the secretion of substances into the digestive tract. These exocrine secretions mix with the food as it moves along the digestive tract and include substances that contribute to the processes of digestion and absorption (as well as metabolic waste products destined to be eliminated in faeces).
The diagram on the right shows the relationship between the various compartments that form the digestive tract and the accessory organs. Click on any of the labels to learn a little more about the named structural element. Note: it would probably be useful to review the section on microscopic structure (below) before following these links.
MICROSCOPIC STRUCTURE
As we have seen above, the digestive tract is a single long tube that is split up into five major compartments. Although there are subtle differences in the microscopic structure of the digestive tract between these compartments they are all composed of a hollow core (lumen) that the food moves through and a wall made up of four distinct layers.
The inner surface of the digestive tract is often highly folded. In some cases these folds are formed by finger-like projections of the wall of the tract into the lumen. In other parts of the tract these folds are a consequence of glands that form pits in its wall.
The ducts of the accessory glands have to penetrate the walls of the digestive tract so that their secretions can reach its lumen.
The general characteristics of these four layers are summarized in the diagram on the left. Click on the label of each layer to learn a little more about it.
BLOOD SUPPLY OF DIGESTIVE TRACT
Given the importance of the cardiovascular system in transporting nutrients from the digestive tract to the rest of the body, it will probably come as no surprise to hear that it has a large blood supply.
Even at rest the digestive tract receives around about 20% of the cardiac output and this increases quite substantially following a meal.
As can be seen in the diagram on the right, blood reaches the digestive tract through the celiac artery and the superior and inferior mesenteric arteries.
This blood feeds the capillaries of the mucosal layer and subsequently drains in to a number of veins that eventually form the portal vein.
This vein passes the nutrient rich blood draining from the digestive tract directly to the liver. As we will see later in this lesson, this unusual arrangement has some very important functional consequences.
In addition to its blood supply the digestive tract also contains lymphatic vessels that are responsible for draining extracellular fluid and returning it to the circulation. Lymphatic capillaries form extensive networks in the mucosa layer and play an important roles in protecting the body from pathogens present in food and (as we will see) in the absorption of fats from the small intestine.