4.3 The Function of Digestive Organs

Normal Function of the Digestive Organs

Before food ever enters the body, the sensory system starts the digestive process. The sight, smell, feel, and taste of food influences the desire to eat. These senses trigger the salivary glands to begin releasing saliva in the anticipation of eating, which helps break down food once it enters the oral cavity. When food enters the mouth, chewing, also known as mastication, starts the breakdown of food. The tongue secretes mucins and lingual lipase to help break down lipids. At the same time, the parotid, sublingual, and submandibular salivary glands release a mixture of buffers, glycoproteins known as mucins, and salivary amylase to lubricate the food for swallowing and to begin the digestion of complex carbohydrates and starches (National Institute of Diabetes and Digestive and Kidney Diseases, 2022; Ogobuiro, Gonzales, & Tuma, 2022).

Food then passes to the pharynx. To swallow food into the esophagus and not the trachea, the swallowing center in the brain stem of the central nervous system triggers certain actions:

• The nasopharynx is closed off by the rise of the soft palate when the tongue initiates swallowing by pushing food to the back of the mouth.
• The larynx is closed in the pharynx to prevent food from entering the trachea (Mealie, Ali, & Manthey, 2022; Ogobuiro, Gonzales, & Tuma, 2022).
• Once this swallowing reflex is initiated, it cannot be stopped until completed.

Once food is swallowed, it enters the esophagus by passing through the upper esophageal sphincter. The food travels down the esophagus body involuntarily by way of a series of muscular contractions known as peristalsis. It then exits the esophagus through the lower esophageal sphincter into the stomach (National Institute of Diabetes and Digestive and Kidney Diseases, 2022).

While in the stomach, food is temporarily stored while mechanical and chemical processes break it down further into chyme. The stomach uses its three muscular layers (the inner oblique, middle circular, and external longitudinal layers) to contract and relax in a mixing and churning action to help mechanically break down food. To assist with the chemical breakdown of food, the stomach releases hydrochloric acid secreted by its parietal cells, enzymes such as pepsin, and hormones such as somatostatin and gastrin (Ogobuiro, Gonzales, & Tuma, 2022). The stomach is also responsible for secreting an intrinsic factor (a glycoprotein that is necessary for the absorption of vitamin B₁₂) from the parietal cells.

The food, in the form of chyme, exits the stomach through the pyloric sphincter into the first of the three segments of the small intestine, the duodenum. While in the duodenum, the pancreas (by way of the pancreatic duct) and liver (by way of the gallbladder and biliary tree) empty enzymes into the duodenum through the sphincter of Oddi. The liver secretes bile that is stored and concentrated in the gallbladder before it is released to aid in digestion (National Institute of Diabetes and Digestive and Kidney Diseases, 2022; Ogobuiro, Gonzales, & Tuma, 2022). The pancreas’s exocrine glands generate various enzyme precursors and enzymes that play a crucial role in the process of digestion (Ogobuiro, Gonzales, & Tuma, 2022).

The food continues to the second segment of the small intestine, the jejunum, by way of peristalsis. This is where the bulk of chemical digestion and nutrient absorption will occur before it moves into the last segment, the ileum, which has some digestive and absorptive duties as well (Ogobuiro, Gonzales, & Tuma, 2022). Absorption in the small intestine is done through nutrient exchange in extensive capillary networks that are located on the walls of the small intestine. The surface area of the small intestine is increased to promote increased absorption through the use of villi and microvilli that cover the walls and are home to the extensive capillary networks. There are also multiple exocrine and endocrine glands that assist in the absorptive process by producing and releasing hormones, alkaline mucinous materials, and enzymes.

Undigested food materials that pass through to the end of the small intestine through the ileocecal valve end up as feces in the cecum, which is the first of the five segments of the large intestine (Ogobuiro, Gonzales, & Tuma, 2022). While peristalsis moves the food through the large intestine, water and electrolytes are absorbed from the feces. Final digestion of undigested food particles in the feces is completed by microbes that live in the large intestine. Nutrients like vitamin K are produced and absorbed in the large intestine as well (Cleveland Clinic, 2022a; Ogobuiro, Gonzales, & Tuma, 2022). The feces travel through the cecum, ascending colon, transverse colon, descending colon, and sigmoid colon before moving into the rectum, where stretch receptors signal the brain to defecate.

From the rectum, the stool exits the body through the anal canal by passing through the internal and then external anal sphincters. Relaxation of the internal anal sphincter is a reflexive action in response to the activation of the stretch receptors, and the relaxation of the external anal sphincter is a conscious process that allows for stool to exit the anus (Ogobuiro, Gonzales, & Tuma, 2022).

Pathophysiology Associated with Digestive Organs and Function

Many causes of diseases and disorders that impact the gastrointestinal system are linked with nutrition, as are many of their treatments. Table 4.2 lists several organs impacted by nutritional deficit and the corresponding disease(s).

Aging of the Digestive System

The nurse should be aware of considerations for aging clients in connection with the digestive system and the role of nutrition. Older adults tend to have a decrease in appetite over younger adults due to comorbidities and/or medications that decrease the desire to eat, cause nausea, or create dietary food restrictions due to drug interactions. Older adults may experience the following issues:

• Appetite can decrease due to a loss in smell or taste or changes in the release of the hormones that control appetite (e.g., ghrelin, peptide tyrosine, tyrosine, cholecystokinin, insulin, and leptin) (Nigam & Knight, 2017).
• Food may appear less attractive or appetizing due to changes made to food consistency for safety reasons (Bartel, 2022; Nigam & Knight, 2017).
• The feeling of being full may happen faster or last longer due to the stomach wall losing elasticity, resulting in a loss of hydrochloric acid secretion into the stomach, making food accommodation more difficult and gastric emptying slower.
• Esophageal reflux may occur, and food may backflow more easily into the esophagus (Austin Gastroenterology, 2019).
• Hyposecretion of hydrochloric acid and protective mucus that occurs with age can result in chronic atrophic gastritis, increased peptic ulcer disease, and decreased vitamin B₁₂ absorption.
• An increase in the bacterial populations in the small intestine may lead to increased bloating and abdominal pain as well as decreased absorption of calcium, folic acid, and iron (Bartel, 2022; Nigam & Knight, 2017).
• Decreases in digestive enzymes from the pancreas result in a reduction in the digestion of proteins and fats.
• The liver decreases bile production, rate of protein synthesis and metabolism, and its ability to detoxify substances (Nigam & Knight, 2017).
• Risk for constipation increases because in the large intestine, there are reductions in neurotransmitters and neuroreceptors, atrophies of mucosa and muscle layers, formations of diverticula, declines in the rate of cell division in the digestive epithelium, and increases in inflammatory conditions due to the increase in facultative anaerobes. The increase in constipation increases the risk for hemorrhoid development.

Biological changes due to age make it important for an older client to be educated so that they eat nutrient-dense options for adequate nutritional intake. With a lower caloric intake demand due to a slower metabolic rate from aging, older adults’ need for calories will likely decrease, so the nutritional value of the foods they ingest is important.