Coupon Accepted Successfully!


Structure and Function of the Digestive System

Since humans and all vertebrates are heterotrophs, we must obtain food from the environment by eating plant or animal tissue. This food contains nutrients, which are needed to supply energy, raw materials for building new molecules, and other substances the body requires but cannot synthesize. Since the material we must consume is often in complex form, it must be chemically broken down into its simpler components before it can enter the body and be used by cells. Thus the digestive system’s primary functions are the ingestion (taking in), digestion (breaking down), and absorption (actual entry into the bloodstream) of the nutrients found in food.
While the digestive system consists of many organs, each performing a different function, it in essence is a long tube through which food moves in one direction (see Figure 16.1). The following discussion refers to the human digestive system; while fundamentally similar in all vertebrates, keep in mind that specific differences may exist in different types of organisms. 
  • Mouth: Food initially enters the body through the mouth, the cavity that begins the digestive tract. The actions of the teeth in the process of chewing (mastication) begin the physical breakdown of food, while the salivary glands, located near the base of the tongue, secrete saliva into the mouth. This saliva contains salivary amylase, an enzyme which breaks down starch into maltose (a disaccharide), and causes the food to become more liquid in consistency. Thus food is both physically and chemically altered while in the mouth.
  • Pharynx, the esophagus, and swallowing: When food has been sufficiently chewed and mixed with saliva, it is now referred to as a bolus and can be swallowed. Swallowing is a muscular reflex action that causes the rapid movement of the bolus through the pharynx, or throat, and into the esophagus. Since the paths of food and air cross in the pharynx, the epiglottis, a flap of tissue, moves during the process of swallowing so that it covers the glottis, or entrance to the trachea (windpipe), to prevent choking. The esophagus carries food from the pharynx to the stomach. Rhythmic contractions of the smooth muscle that surrounds the esophagus result in the unidirectional movement of food towards the stomach, a process called peristalsis.
  • Stomach: The stomach serves several functions. First, it stores food until it can be processed further by the small intestine. Epithelial cells of the stomach wall secrete gastric juice, a combination of hydrochloric acid (HCl), mucus, and pepsinogen (an enzyme precursor). The epithelium of the stomach also secretes a hormone, gastrin, which controls the secretion of gastric juice. The HCl causes the pH of the stomach to be extremely low, usually with a value of about 2, and these acidic conditions kill many ingested bacterial cells and help to physically dissolve the food further. 

The human digestive system


In addition, the acidic conditions cause pepsinogen to be converted into pepsin, an enzyme that begins to chemically break down proteins. Contractions of the muscular stomach walls cause a churning action which contributes to the continuing physical breakdown of the bolus, until it exists in a homogeneous, semi-liquid state. This partially digested food, referred to as chyme, is now ready for further processing. The pyloric sphincter is a strong muscle that guards the entrance to the small intestine, and allows chyme to slowly enter.
  • Small intestine - digestion: Most of the “action” of digestion occurs in the small intestine. It is here that the chemical breakdown of complex carbohydrates, proteins, and fats is completed. The small intestine is also the site of absorption of the products of digestion into the bloodstream. Chemical digestion is completed in the first portion of the small intestine, the duodenum. Enzymes manufactured by the pancreas attack undigested nutrients, and bile manufactured by the liver aids in the process. Specifically, the pancreas produces pancreatic amylase, which breaks down starch into maltose, trypsin and other proteolytic enzymes, which break down proteins into amino acids, and lipases, which hydrolyze fats, yielding glycerol and fatty acids. These enzymes are dissolved in pancreatic juice and enter the duodenum via the pancreatic duct. These juices also have the ability to neutralize the acid entering the duodenum from the stomach. The liver produces bile, which is stored in the gall bladder; it travels to the duodenum via the bile duct and participates in digestion by emulsifying fats, which otherwise would not be appreciably soluble and would not be fully exposed to the action of lipases. Finally, the epithelium of the small intestine itself also produces enzymes that break disaccharides into monosaccharides. As you might expect, hormones regulate the production and secretion of digestive enzymes and bile. Secretin and cholecystokinin are secreted by the duodenal epithelium in response to acid and the presence of nutrients, and stimulate the pancreas and gall bladder to release their products. When digestion is complete, monosaccharides, amino acids, fatty acids, and glycerol, along with various vitamins and minerals, are ready to be absorbed into the blood.
  • Small intestine - absorption: Absorption of nutrients occurs in the remainder of the small intestine, which is anatomically broken into two parts, the jejunum and the ileum. These portions are adapted in several ways to facilitate absorption; many of these adaptations exist to increase the surface area of the intestinal wall, which will allow nutrient absorption to take place at a faster rate and to increase total absorption. Firstly, the length of the small intestine is great, approximately six meters in humans. In order to fit in the abdominal cavity, it must be greatly coiled. Additionally, the walls of the small intestine project fingerlike folds, known as villi, whose surface area is increased even further by the presence of tiny cellular projections known as microvilli. All in all, the total absorptive surface area of the small intestine is over three hundred square meters! Monosaccharides, amino acids, and small fatty acids enter capillaries in the villi by both facilitated diffusion and active transport. Mineral electrolytes also enter the bloodstream directly, usually by active transport, and water follows by osmosis. Larger fatty acids, glycerol, and cholesterol do not enter the capillaries of the villi, but instead are combined and packaged into lipoprotein particles called chylomicrons, which enter a lacteal. The lacteal is a small lymphatic vessel, and the chylomicrons ultimately enter the blood by way of the lymphatic system.
  • Large intestine: Following the small intestine is the large intestine, so named because its diameter is greater than that of the small intestine. Its surface area is comparatively small, however, and by the time material reaches the large intestine, most of the nutrients have already been absorbed. The large intestine is anatomically divided into two parts, the pouchlike cecum, which represents the beginning of the organ, and the much longer colon, which has the shape of an inverted “U”. (The appendix, a small pouch which apparently plays no role in digestion, is also located near the cecum.) There is a large population of symbiotic mutualistic bacteria, mostly Escherichia coli, which reside in the colon of humans and many other vertebrates. They are sustained by particles of food which could not be digested by their host, and find the warm temperature of the colon ideal for their growth. In return, they synthesize certain vitamins and amino acids, most notably vitamin K, which is involved in proper blood clotting. The major digestive role of the large intestine is to reabsorb water that is still in the digestive tract before the remaining waste material is eliminated. To facilitate this, any remaining salts are also absorbed here. The resulting waste, or feces, should be relatively solid, and consists of indigestible materials including cellulose fibers, some water, and many bacterial cells. It is stored in the rectum until it is eliminated by defecation through the anus. (You should note that this elimination is not properly referred to as excretion, as the materials present in the feces have simply passed through, but never entered, the body.)
  • Pancreas and liver - accessory organs: We noted above that the pancreas and liver are important in the overall process of digestion. They are referred to as accessory organs, however, because food never actually passes through them. Instead, they function by producing necessary substances and secreting them into the digestive tract. To reiterate, the pancreas produces digestive enzymes dissolved in an alkaline fluid which is emptied into the duodenum. The liver produces bile, which is stored in the gall bladder and enters the duodenum to help with the digestion of fats. While both organs have other functions, their roles in digestion cannot be overstated. 

The pancreas also plays a major role, along with the liver, in controlling the level of blood glucose.

Test Your Skills Now!
Take a Quiz now
Reviewer Name