The Alimentary System

The alimentary or digestive system is a long tube that consists of the mouth, esophagus, stomach, small intestine, colon, rectum, and anus. Some important accessory organs connected to the digestive tract are the salivary glands, gallbladder, pancreas, and liver. Along this tract, foods are broken down into smaller units, both physically and chemically, and then absorbed for use by the body. Figure 1 shows the general outline of the entire human digestive system.

The Food Path

The food placed in the mouth is chewed, softened, and swallowed. In the stomach, it is churned and then propelled into the small intestine where it is mixed with the bile from the gallbladder and digestive enzymes from the intestinal walls. The products of this digestion are partly or completely absorbed into either the portal vein or the lacteal system.

In the mouth, chewing (mastication) reduces large food lumps into smaller pieces and mixes them with saliva. This wetting and homogenizing facilitates later digestion. In clinical dietetics, edentulous (toothless) patients or those with reduced saliva secretion have trouble eating dry foods and require a soft, moist diet. Saliva facilitates swallowing and movements of the tongue and lips, keeps the mouth moist and clean, serves as a solvent for taste bud stimulants, acts as an oral bugger, provides some antibiotic activity, and inhibits loss of calcium from the teeth by maintaining a neutral pH. Saliva contains ptyalin (salivary amylase, a digestive enzyme) and mucin (a glycoprotein). Mucin lubricates food and ptyalin digests carbohydrates to a small extent. Each day the salivary gland makes about 1,500 mL of saliva (ca 5.5-6 cups).

The bolus of food is propelled forward by rhythmic contractions of the entire intestinal system. These peristaltic waves move the food from the mouth, through the esophagus, and into the stomach. Certain individuals, especially nervous people, tend to swallow air when eating. When part of the air is expelled through the mouth, belching results; the remaining air is expelled as flatus. If too much air is swallowed, there will be abdominal discomfort.

From the mouth, food travels through the esophagus, the stomach cardia, the stomach body, the greater curvature, the pylorus, and the duodenum. These are all parts of the stomach, where food is well mixed. Figure 2 shows the general structure of this organ and the site of specific secretions. The acid, mucus, and pepsin cause partial digestion, and peristalsis mixes up the food. The food is then released gradually through the pylorus into the duodenum.

The gastrointestinal (GI) system, the stomach and intestines, breaks down complex carbohydrates, proteins, and fats into absorbable units, mainly in the small intestine. Vitamins, minerals, fluids, and most nonessential nutrients are also digested and absorbed to varying degrees. Foods are digested by enzymes secreted by different parts of the GI system. Table 1 summarizes the major digestive enzymes and their actions. After the digestive process is complete, nutrients are ready for absorption, which occurs mainly at the small intestine. The absorption of each nutrient is discussed later.

Cardiac

Pylorus

Cardiac

Pylorus

Fundus

Greater curvature

Antrum

Duodenum

Figure 2. General structure of the stomach.

Fundus

Greater curvature

Antrum

Duodenum

Liver Duodenum

(duodenoie)unal (tenure not shown) Transverse colon

Ascending colon Small intestine

Oral cavity

Esophagus

Stomach Pancreas

Descending colon

Sigmoid colon

Rectum

Figure 1. The digestive system of the human body.

Oral cavity

Esophagus

Rectum

Liver Duodenum

(duodenoie)unal (tenure not shown) Transverse colon

Ascending colon Small intestine

Stomach Pancreas

Descending colon

Sigmoid colon

Figure 1. The digestive system of the human body.

Figure 2. General structure of the stomach.

After the nutrients have been absorbed, they enter the circulation in two ways. Most fat-soluble nutrients enter the lacteal or lymphatic system, which eventually joins the systemic blood circulation at the thoracic duct. Other nutrients enter the hepatic portal vein and are received by the liver, which eventually releases them to the bloodstream.

Enzymes and Coenzymes

After digestion and absorption, the nutrients exist as hex-oses (mainly glucose and fructose), fatty acids, glycerols, and amino acids and are then metabolized in various fashions. Many of the metabolic processes require the presence of a catalyst, a substance that can facilitate a chemical reaction. Although participating in the process, it may or may not undergo physical, chemical, or other modification itself. Nonetheless, the catalyst usually returns to its original form after the reaction.

In the body, most biological reactions require a special class of catalysts: the protein catalysts or enzymes. Each enzyme catalyzes only one or a small number of reactions. There are many enzymes, each with a specific responsibility. Without enzymes, most biological reactions would proceed at a very slow speed. Coenzymes are accessory substances that facilitate the working of an enzyme, mainly by acting as carriers for products of the reaction. In this case, the enzyme is composed of two parts: a protein (apoenzyme) and a nonprotein (cofactor or coenzyme). Many coenzymes contain vitamins or slightly modified vitamins as the major ingredient. A coenzyme can catalyze many types of reaction. Some coenzymes transfer hydrogens; others transfer groups other than hydrogens. Table 2 describes the characteristics of the former; Table 3, those of the latter. Because most of the metabolic reactions discussed in this article involve coenzymes, a knowledge of the biochemical role of vitamins is important.

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