After reviewing and studying this module’s content, answer the following questions. Be sure to complete all lab activities and attend/watch all live lectures before completing this assignment. All of your answers should be written in your own words, using full sentences, correct terminology, and proper spelling and grammar.
Explain the anatomical concepts associated with the digestive system. Summarize this module’s key points in 5-6 sentences.
Explain the physiological concepts associated with the digestive system. Summarize this module’s key points in 5-6 sentences.
How will you apply the concepts you have learned about the digestive system in real life and in your future career?
Which topic within this module has been the most valuable to your learning experience and why?
Which topic(s) within this module did you struggle to understand and why?
The digestive system can be visualized as a 30-foot tube that runs from mouth to anus. This tube is called the alimentary canal, and it consists of the oral cavity, pharynx, esophagus, stomach, and intestines. This system works together closely with other organs, including the spleen, adrenal gland, pancreas, and kidneys. The term “gastrointestinal tract,” is not interchangeable with “digestive system,” as the gastrointestinal tract refers to only the stomach and intestines.
Digestive system
Digestive system (Betts et al., 2013)
The Oral Cavity
The oral cavity includes the mouth, lips, cheeks, palate, tongue, salivary glands, and teeth. Following the oral cavity, food passes into the oropharynx and laryngopharynx, then the esophagus. The epiglottis is responsible for blocking the larynx to direct food away from the airway (trachea) and into the esophagus. The esophagus crosses thru the diaphragm via the esophageal hiatus then connects to the stomach at the gastroesophageal (cardiac) sphincter.
The Stomach
The stomach is located in the upper left quadrant of the abdominal cavity. It is divided into four regions: cardia, fundus, body, and pylorus regions. The empty stomach has a volume of approximately 50 mL, but it can expand up to 1.5 liters after eating.
Stomach
Stomach anatomy (Betts et al., 2013)
Alimentary Canal
Like many other organs, the alimentary canal has layers:
Serosa: The serosa is the outer layer, and it provides anchoring to other digestive organs, as the alimentary canal is not free-floating.
Muscularis externa: This is a smooth layer of muscle that provides contraction throughout the alimentary canal.
Submucosa: Submucosa lies inside the smooth muscle layer, and contains nerves, blood vessels, and lymphatic vessels.
Mucosa: This inner-most layer has a diverse composition, depending on its region within the alimentary canal. Notably, the stomach mucosa contains the gastric glands, which include the mucous cells, parietal cells, chief cells, and enteroendocrine cells. The secretions from all these cells collectively is called gastric juice.
Mucous cells produce the mucous layer that coats the stomach.
Parietal cells produce hydrochloric acid and intrinsic factors, which mechanically break food into chime, and aid in vitamin B12 absorption. B12 is critical to red blood cell function.
Chief cells produce pepsinogen, lipases, and rennin. Lipases are essential for fat digestion. Rennin is produced in infants to help with milk digestion.
Enteroendocrine cells secrete hormones, of which gastrin is the most well-known; this helps regulate stomach digestive motility.
alimentary canal
Layers of the alimentary canal (Betts et al., 2013)
Peritoneum
Moving outward from the alimentary canal, the digestive system is surrounded by connective tissue called peritoneum. The peritoneum lines the entire abdominal cavity, and is comprised of two layers. Parietal peritoneum attaches to the abdominal wall and pelvic floor. Visceral peritoneum lies deeper, as it encases the digestive organs. The term “mesentery” refers to the double layer of peritoneum that attaches digestive organs to the posterior abdominal wall. It is rich in blood vessels, nerves, and lymphatic vessels. Omentums are specific kinds of mesenteries. The lesser omentum connects the liver to the stomach. The greater omentum connects the inferior aspect of the stomach to the intestines.
The Small Intestine
Continuing inferiorly, the stomach is followed by the small intestine portion of the alimentary canal. The small intestine completes chemical digestion, leading to nutrient absorption. The small intestine begins at the pyloric sphincter and ends at the ileocecal sphincter. The small intestine has three regions: duodenum, jejunum, and ileum.
Small intestine
Regions of the small intestine (Betts et al., 2013)
The Large Intestine
Following the small intestine, indigestible food/material enters into the large intestine, which reaches from the ileocecal valve to the anus for fecal elimination. The large intestine has three zones: ascending, transverse, and descending.
Large intestine
Large intestine (Betts et al., 2013)
The Nervous System’s Role
Like other organs and body systems, the nervous system coordinates function. The enteric nervous system is the part of the autonomic nervous system designated for the digestive system. It is known for parasympathetic “rest and digest function,” which stimulates digestion, and sympathetic “fight or flight” function which inhibits digestion.
Now that we have completed an overview of digestive system anatomy, we will move on to the physiological function.
The digestive system’s primary function is nutrient extraction from food and elimination of waste.
4-Step Process of Nutrient Extraction
This process occurs in four steps:
Ingestion: Ingestion pertains to taking food in.
Digestion: Breaks food down both chemically and mechanically. Mechanical digestion occurs via teeth and the stomach, as they mechanically shred food. Chemical digestion relies on enzymes to break food into chemical components that the body can process in the following absorption phase.
Absorption: Absorption refers to our body’s utilization of the nutrient components of what is ingested.
Defecation: Defecation is the process of fecal elimination of by-products.
Deglutition
Ingestion of food requires deglutition, which means swallowing. Swallowing occurs in two phases. The first phase is called the buccal phase. In this phase, mastication (chewing) occurs to form a bolus. This first phase is under conscious control, while the second phase is unconscious. The second phase is called the pharyngeal-esophageal phase; this phase coordinates smooth muscle of the pharynx and esophagus via higher control centers in the medulla and pons.
Breakdown in the Stomach
From here, food passes through the esophageal (cardiac) sphincter to enter the stomach. In the stomach, the mucous from the mucous cells protects the stomach lining from its acidic pH. Without this mucous, ulcers form. 90% of the time, the mucosal barrier breakdown is caused by excessive bacteria, H.Pylori. HCl from the parietal cells has a pH of 1.0-3.0, which mechanically breaks food into chyme.
Next, all nutrients must be converted into individual molecules for absorption. Pepsinogen from the chief cells is converted to pepsin by its exposure to the acidic environment. The pepsin breaks amino acid chains from ingested protein into individual amino acids. The lipases from the chief cells break fats into individual units for absorption. The gastrin hormone is also released by the chief cells, and it regulates the speed of motility.
Gastric Juice Secretion
Gastric juice secretion consists of three phases, all of which occur simultaneously. Their release is controlled by hormones and nerves in the brain, stomach, and small intestines.
The cephalic phase is triggered by the sight, taste, smell, or thought of food; this stimulates the vagus nerve component of the parasympathetic nervous system to stimulate the enteric nervous system. This phase is only a few minutes in length.
The gastric phase begins when food enters the stomach. This phase is initiated by the enteric nervous system and gastrin release from cells in the stomach. The gastrin release further promotes HCl release for mechanical digestion within the stomach. This phase lasts 3-4 hours.
The intestinal phase occurs when partially digested food enters the small intestine, activating the stimulatory phase.
The system is maintained in homeostasis as the enterogastric reflex quickly inhibits too much activity by tightening the pyloric sphincter as the duodenum begins to distend.
Primary Function of the Small Intestine
The small intestine’s primary function is chemical digestion and nutrient absorption. Movement of chyme within the small intestine involves two processes: peristalsis and segmentation. Peristalsis is like a giant blender, mixing chyme. Segmentation is like the blender’s slow speed, as the process allows more time for digestive enzymes to chemically break down nutrients.
Passing to the Large intestine
Following passage through the small intestine, contents pass to the large intestine where the major functions of absorbing water, briefly storing indigestible materials, and passing these materials on to the anus for defecation to occur. The large intestine also contains over 1000 different bacterial flora which serve to ferment some indigestible carbohydrates, synthesize vitamin B, and synthesize vitamin K.
Chemical vs Mechanical Digestion
While the above content reviews the overall digestive processes, digestion varies slightly based on which macronutrient (carbohydrate, protein, or lipid) we ingest. All macronutrients have a mechanical digestive component beginning in the mouth and continuing into the stomach. All macronutrients are chemically digested and absorbed in the small intestine.
The chemical-digestion starting location varies by macronutrient, as carbohydrate chemical digestion occurs in the mouth via the amylase in saliva. The chemical digestion for proteins and lipids begins in the stomach via pepsinogen conversion into pepsin and lipases, respectively.
Digestion by macronutrient
Table: Digestion by macronutrient (Betts et al., 2013)
Mechanical and chemical digestion by anatomical region.png
Mechanical and chemical digestion by anatomical region (Betts et al., 2013)