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Gastrointestinal physiology

The gastrointestinal (GI) tract includes the stomach and the intestines. To avoid confusion, the digestive tract refers to the esophagus, stomach, and intestines. The alimentary tract is synonymous with the term digestive tract. The term alimentary in itself refers to the organs of digestion. Since alimentary includes the digestive tract and the peripheral organs of digestion, we will review the digestive system in this fashion, considering it paints a more complete picture. Keep in mind that the digestive tract is basically a tubelike structure (for schematic purposes). An additional understanding of the digestive organs enables us to envision the remarkable way, these organs interact to absorb and assimilate electrolytes, water and nutrients.

The alimentary tract provides us with electrolytes, water and nutrients. To ensure that these goals are met, the alimentary tract must be able to:

1) Move foodstuffs through the alimentary tract

2)secrete digestive juices (enzymes, acids, solvents, etc.) and digest the foodstuffs.

3)absorb the products of digestion, electrolytes and water.

4)effectively circulate blood through the organs of the alimentary tract to transport the absorbed substances, and

5)control all of these functions through nerve and hormonal stimulation.



By examining the figure above, we can see the basic outline of the alimentary tract. Each component of the alimentary tract is tailored to a function. The mouth chews or palatizes food, while saliva from the salivary and parotid glands mixes with the food, providing water and the enzyme, amylase, to help initiate carbohydrate digestion. The esophagus transfers foodstuffs from the mouth to the stomach. The stomach temporarily stores the food. The pancreas and gallbladder emit substances to aid in food digestion and absorption. Food is absorbed in the small intestine. Waste products are kept in transit until they reach the end of the alimentary tract and are expelled. As we continue, we shall explore the entire system in much more detail.


The figure below illustrates a cross section of the intestinal wall.

Keep in mind that the intestine is a long tube. The section in the figure above is cut similar to the hose below, to reveal the inside. In anatomy, this type of section is called a cross section. This particular cross section of the intestine demonstrates the different layers of tissue.

As we can see, the intestine is a complex set of tissues. Each tissue has a specific function. The different layers of intestinal smooth muscle (eg., circular muscle, longitudinal muscle) are responsible for the motor functions (moving of food, etc., through the gut, peristalsis) of the gut.

By examining the intestinal cross-section, we can see that the two layers of smooth muscle are separated by connective tissue. This connectve tissue, not only provides strength, but also separates the two types of muscle since they are aligned in different directions. The longitudinal muscle runs longitudinally through the intestine, while the circular smooth muscle encircles the intestine. The combined action of these two muscle arrangements aids in peristalsis, moving material along the gut, similar in fashion to working toothpaste through a toothpaste tube. There are also nerve connections between the circular and longitudinal muscle layer. These interconnections ensure stimulation of both muscle layers, even if only of the layers is initially stimulated. This cooperation between muscle layers maintains the fidelity of peristalsis.

The muscle layers of the GI tract experience slow, continual electrical activity. This elecrical activity has two types of waves: 1)slow waves and 2) spikes. Slow undulating waves may be set off by the periodic activity of sodium-potassium pumps inside the cell. Keep in mind that sodium, potassium and other ions (you get these ions from salt, etc.) possess an electrical charge. When they migrate, the net charge changes and simply causes the muscle cell to fire, or contract. When this electrical signal spreads through the intestine, it causes the intestine to become active. We can sometimes hear this activity when the intestine churns and moves air and fluids. This causes the characteristic gurgles that we can hear through the stomach wall. The stomach contraction rhythm is around 3 per minute. The duodenum (first portion of the small intestine) has a contraction rhythm of around 10-14 per minute, and the ileum (terminal portion of the small intestine) around 9 per minute.

Aside from the normal auto-excitation giving rise to the rhythmic contractions of the GI system, there are other factors that can cause contraction of the smooth muscle.


1)Stretching of the muscle: This happens when material moves through the intestine. Non-starch polysaccharides (fiber) can attract water and swell, thereby, stretching the muscle and causing peristalsis. This is one reason why fiber has been declared beneficial. If food if propelled through the gut at a faster rate, the theoretical risk of cancer and other illnesses decrease.

2)Stimulation from certain GI hormones, and other hormones.

3)Stimulation by acetylcholine (ACh), a neurotransmitter.



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