Explanation:
Table 1. Functions of the Digestive Organs
OrganMajor functionsOther functions
Mouth
Ingests food
Chews and mixes food
Begins chemical breakdown of carbohydrates
Moves food into the pharynx
Begins breakdown of lipids via lingual lipase
Moistens and dissolves food, allowing you to taste it
Cleans and lubricates the teeth and oral cavity
Has some antimicrobial activity
Pharynx
Propels food from the oral cavity to the esophagus
Lubricates food and passageways
Esophagus
Propels food to the stomach
Lubricates food and passageways
Stomach
Mixes and churns food with gastric juices to form chyme
Begins chemical breakdown of proteins
Releases food into the duodenum as chyme
Absorbs some fat-soluble substances (for example, alcohol, aspirin)
Possesses antimicrobial functions
Stimulates protein-digesting enzymes
Secretes intrinsic factor required for vitamin B12 absorption in small intestine
Small intestine
Mixes chyme with digestive juices
Propels food at a rate slow enough for digestion and absorption
Absorbs breakdown products of carbohydrates, proteins, lipids, and nucleic acids, along with vitamins, minerals, and water
Performs physical digestion via segmentation
Provides optimal medium for enzymatic activity
Accessory organs
Liver: produces bile salts, which emulsify lipids, aiding their digestion and absorption
Gallbladder: stores, concentrates, and releases bile
Pancreas: produces digestive enzymes and bicarbonate
Bicarbonate-rich pancreatic juices help neutralize acidic chyme and provide optimal environment for enzymatic activity
Large intestine
Further breaks down food residues
Absorbs most residual water, electrolytes, and vitamins produced by enteric bacteria
Propels feces toward rectum
Eliminates feces
Food residue is concentrated and temporarily stored prior to defecation
Mucus eases passage of feces through colon
Digestive Processes
The processes of digestion include six activities: ingestion, propulsion, mechanical or physical digestion, chemical digestion, absorption, and defecation.
The first of these processes, ingestion, refers to the entry of food into the alimentary canal through the mouth. There, the food is chewed and mixed with saliva, which contains enzymes that begin breaking down the carbohydrates in the food plus some lipid digestion via lingual lipase. Chewing increases the surface area of the food and allows an appropriately sized bolus to be produced.
This image shows the peristaltic movement of food. In the left image, the food bolus is towards the top of the esophagus and arrows pointing downward show the direction of movement of the peristaltic wave. In the center image, the food bolus and the wave movement are closer to the center of the esophagus and in the right image, the bolus and the wave are close to the bottom end of the esophagus.
Figure 1. Peristalsis moves food through the digestive tract with alternating waves of muscle contraction and relaxation.
Food leaves the mouth when the tongue and pharyngeal muscles propel it into the esophagus. This act of swallowing, the last voluntary act until defecation, is an example of propulsion, which refers to the movement of food through the digestive tract. It includes both the voluntary process of swallowing and the involuntary process of peristalsis. Peristalsis consists of sequential, alternating waves of contraction and relaxation of alimentary wall smooth muscles, which act to propel food along (Figure 1). These waves also play a role in mixing food with digestive juices. Peristalsis is so powerful that foods and liquids you swallow enter your stomach even if you are standing on your head.
Digestion includes both mechanical and chemical processes. Mechanical digestion is a purely physical process that does not change the chemical nature of the food. Instead, it makes the food smaller to increase both surface area and mobility. It includes mastication, or chewing, as well as tongue movements that help break food into smaller bits and mix food with saliva. Although there may be a tendency to think that mechanical digestion is limited to the first steps of the digestive process, it occurs after the food leaves the mouth, as well. The mechanical churning of food in the stomach serves to further break it apart and expose more of its surface area to digestive juices, creating an acidic “soup” called chyme. Segmentation, which occurs mainly in the small intestine, consists of localized contractions of circular muscle of the muscularis layer of the alimentary canal. These contractions isolate small sections of the intestine, moving their contents back and forth while continuously subdividing, breaking up, and mixing the contents. By moving food back and forth in the intestinal lumen, segmentation mixes food with digestive juices and facilitates absorption.
