The Digestive Systems of Complex Organisms

When we discuss complex digestive systems we are typically referring to organisms that have several specialized digestive organs that perform specific tasks. As such, worms would be considered complex organisms because they have the following digestive organs.


The anatomy and physiology of the organs of the digestive system will be covered shortly. Note that different species can have different organs. For example, humans do not have a crop or gizzard, while worms and birds do.

Human Digestive System


This process involves the placing food in an external cavity where the following processes can occur


Digestion begins in the mouth. There are two types of digestion, mechanical and chemical. Mechanical Digestion is the breaking down of food particles into smaller particles, but DOES NOT involve breaking bonds. Chemical Digestion is the process of breaking down molecules via bond cleavage(s).

In the mouth, the teeth are used to mechanically digest food. The tongue, a muscle, is used to move the food around as well as for taste.


Saliva is secreted by the salivary glands into the mouth where it coats the food. Saliva contains amylases, which are enzymes that breakdown starches (polysaccharides). Because the enzymes are breaking bonds in the starch molecules, it is considered chemical digestion. As such, both chemical and mechanical digestion occur in the mouth.

Food that has been chewed and mixed with saliva is referred to as a bolus. The bolus is swallowed when the tongue muscle pushes the bolus into the pharynx, down the esophagus, and into the stomach.

The pharynx is a muscular tube that both air and bolus’ pass through. A small flap of cartilage called the epiglottis, is pushed over top of the opening to the trachea (wind-pipe), when the tongue performs swallowing.


As stated above, from the pharynx the bolus moves down the esophagus, a muscular tube connecting the pharynx and the stomach. Food DOES NOT move down the esophagus due to gravity. Rather, peristalsis, the sequential contraction and relaxation of muscle tissue, that pushes the food down the esophagus.


The bolus must then pass through the cardiac sphincter, into the stomach. The cardiac sphincter is a valve that opens to allow food into the stomach, but also prevents stomach acid from entering the esophagus.

The stomach is a muscular j-shaped organ in which food is both chemically and mechanically digested. The stomach contains folds (referred to as rugal folds) that allow the stomach to expand. The muscular walls of the stomach are used to churn the food (mechanical digestion). The stomach contains hydrochloric acid which makes the gastric juices very acidic (pH ~ 2). The stomach lining is protected with a thick layer of mucous that prevents the acidic stomach contents from coming in contact with the stomach lining. The mucous is produced by cells in the stomach lining. Gastric juices also contain an enzyme called pepsin. Pepsin chemically digests proteins into polypeptides. Pepsin can also be present in an inactive form called pepsinogen. The low pH of the stomach causes structural changes to pepsinogen that change in into the active enzyme pepsin. Once the food has been digested it is referred to as chyme.


The chyme is squirted out of the stomach, through the pyloric sphincter, into the small intestine. The small intestine is categorized into three parts, the duodenum, jejunum, and ileum. It is in the duodenum where the final majority of digestion occurs. Three organs, the liver, gallbladder, and pancreas, secrete digestive fluids into the duodenum.

The liver produces a substance called bile that is stored in the gallbladder, which releases it into the duodenum. Bile is a substance that contains bile salts, which emulsify or mechanically digest fat globules into smaller globules. Because fat is insoluble in water the fat content from food stays in large globules, much like oil in water. The bile salts surround fat globules and prevent them from rejoining with others.

The pancreas, a finger-like organ that projects from the duodenum, produces and secretes enzymes and bicarbonate ions into the duodenum. As in the stomach, the enzymes secreted by the pancreas are only active at a specific pH. The bicarbonate ions produce a pH close to 7 by neutralizing the acidic chyme, which allows the pancreatic enzymes to function and prevents the duodenum from being burned by acidic chyme. In the duodenum, these digestive enzymes chemically digest lipids, proteins, and carbohydrates.

Liver gallbladder


Absorption of nutrients occurs primarily in the small intestine. Specifically the digested nutrients are absorbed into the blood stream or lymphatic system via diffusion or active transport. Diffusion refers to the movement of particles from high to low areas of concentration. Active transport refers to the movement of particles through a cell membrane via membrane proteins. Water soluble nutrients are absorbed into the blood stream and fat soluble nutrients are absorbed into the lymphatic system. Note that absorption refers to the crossing of a cell membrane, from into the internal environment.

The lining of the small intestine is folded and is lined with finger-like projections called villi on which there are more, smaller projections called microvilli. This serves to increase the surface area for absorption. Inside the lining of the small intestine (referred to as intestinal mucosa) there are capillaries and lacteals (smallest vessels of the lymphatic system). It is into the capillaries and lacteals that nutrients enter.


As nutrients are removed from the mass of chyme it becomes feces. The feces passes from the small intestine through the cecum, into the large intestine. The large intestine (aka the colon) is a muscular tube that has folds but DOES NOT have villi or microvilli, and is the primary site of water absorption.



Feces is transported from the large intestine into the rectum, where it is stored until it is egested through the anus.

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