It’s Easter Sunday, and my mouth is watering at the aroma of ham warming in the oven, cheesy potato casserole resting on the counter, and raspberry tart showcased on the silver platter.
My brain begins the digestive process long before the actual eating takes place. The sight, sound, odor, taste, or texture associated with food triggers the digestive system to kick into high gear (Lipski, 2012, p.11). Salivation is controlled by the autonomic nervous system, (Tortora & Derrickson, 2015, p.466), and no amount of willpower can stem the flood. Release of saliva from my parotid, submandibular, and sublingual glands prepare my mouth for the feast ahead (Tortora & Derrickson, 2015, p.444). My heart rate and blood flow to my digestive organs increase in anticipation (Lipski, 2012, p.12).
I load my plate up with a little of everything. I work hard on my nutrition and fitness the rest of the year so I can indulge on days like this, and I look forward to enjoying every mouthful. My food consists of carbohydrates, fats, and proteins, macronutrients crucial to every bodily function. With the first bite, salivary amylase begins the digestion of starches and lingual lipase begins the digestion of fats in my mouth (Tortora & Derrickson, 2015, p.444, 456). My teeth and tongue work in concert to masticate, or chew, and mix the food to a manageable mixture (Tortora & Derrickson, 2015, p.446). I swallow that first bite, passing the bolus through the esophagus toward the stomach. Well-chewed food talks only about six seconds to travel, with peristalsis moving the food through the lower esophageal sphincter into the body of the stomach (Lipski, 2012, p.13), (Tortora & Derrickson, 2015, p.447, 448). I take my next bite.
My stomach readily receives this feast, as it is extremely flexible and can accommodate up to about 6 quarts of food at once (Tortora & Derrickson, 2015, p.447). I don’t really plan on eating that much, but we’re celebrating, so anything goes! The folded rugae of the stomach stretch as the food makes its way, revealing surface mucous cells, gastric glands, and gastric pits (Tortora & Derrickson, 2015, p.449). These function together to further the digestion process.
Chief cells in gastric glands secrete the inactive enzyme pepsinogen, while parietal cells produce hydrochloric acid to kill microbes and help convert pepsinogen into its active form, pepsin, beginning the digestion of proteins into peptides (Tortora & Derrickson, 2015, p.449, 450). The thick coating of mucous keeps the hydrochloric acid from burning through my stomach lining (Lipski, 2012, p.14). Parietal cells also secrete intrinsic factor, crucial in the absorption of vitamin B12 in my food (Tortora & Derrickson, 2015, p.449). These enzymes work with mucous released from surface mucous cells and mucous neck cells and are often lumped together and labeled gastric juices (Tortora & Derrickson, 2015, p.449). Add to the mix G cells, also found in gastric glands, which secrete the hormone gastrin into the blood stream, also aiding the digestion of proteins (Tortora & Derrickson, 2015, p.449). A small amount of the enzyme lipase is released and begins the digestion of the fats in my food, and alcohol, water, and certain salts are absorbed directly from my stomach into the bloodstream (Lipski, 2012, p. 14-15).
The stomach acts as the body’s blender, mixing, churning, and liquefying food into a soupy liquid called chyme (Lipski, 2012, p. 14). My food stays in the upper part of my stomach for about an hour, as the salivary amylase continues to break down starches (Lipski, 2012, p. 14). The chyme then moves down towards the lower part of the stomach, called the pyloris, and the pyloric sphincter, which will regulate its passage into the duodenum, the uppermost part of the small intestine. This takes from two to four hours depending on the fat content of my food and my stress level (Lipski, 2012, p.14). I’m eating lots of delicious fat today but my stress level is low, so my digestion should be moderately timed.
The pancreas plays a supporting, but important role in the digestion of my Easter meal and regulation of my blood sugar (Lipski, 2012, p.18). A fluid called pancreatic juice, composed of mostly water, salts, sodium bicarbonate (to inactivate the pepsin from the stomach), and enzymes enters the duodenum via the hepatopancreatic duct (Tortora & Derrickson, 2015, p.450). Starch-digesting pancreatic amylase, protein-digesting trypsin, chymotrypsin, and carboxypeptidase, and triglyceride-digesting pancreatic lipase, along with nucleic acid-digesting ribonuclease and deoxyribonuclease, are all enzymes produced, regulated, and released by the pancreas (Tortora & Derrickson, 2015, p.450). Each has its own job to break down the chyme into smaller molecules to prepare for absorption. Once it reaches the small intestine, the inactive form of trypsin is activated by an enzyme called enterokinase, which facilitates the activation of other protein-digesting pancreatic enzymes (Tortora & Derrickson, 2015, p.450). In short, the fats I eat are broken down into triglycerides with pancreatic lipase; the carbohydrates I eat are broken down into starches with pancreatic amylase, maltase, sucrase, and lactase; and the proteins I eat are broken down into fatty acids and monoglycerides with pancreatic lipase (Tortora & Derrickson, 2015, p.456). Still too large to pass into my bloodstream, these substrates move on to my small intestine.
The pancreas also helps regulate my blood sugar, secreting insulin to manage the rise in blood glucose due to my ingestion of the potatoes and raspberry tart (Lipski, 2012, p.19). If my blood sugar were to dip too low, my liver would secrete glucagon to bring its levels back to normal (Lipski, 2012, p.20).
The liver also plays an integral role in the digestion of my Easter dinner. After the skin, it’s the second largest organ of the body, and facilitates the regulation of hormones, including cholesterol, testosterone, and estrogen; it regulates the metabolism of carbohydrates, fats, and proteins; it regulates blood sugar levels; it filters, or processes, all materials passing through the digestive tract and lets them pass, breaks them down, or stores them (Lipski, 2012, p.20). The liver holds on to glycogen, fats, vitamin B12, vitamins A, D, E, and K, and zinc, iron, copper, and magnesium until they are needed (Lipski, 2012, p.20). The liver also breaks down any toxins in my food, protecting my body from harmful substances (Lipski, 2012, p.20). Just about every vitamin and mineral I consume in my dinner are enzymatically processed in the liver so they can be absorbed into the blood stream (Lipski, 2012, p.20).
The vast majority of digestion and absorption of my food happens in the small intestine (Lipski, 2012, p.17). Hardly small, this organ would stretch out to 15 to 20 feet long; if spread flat, it would cover the surface of a tennis court (Lipski, 2012, p. 17). The intestinal wall acts as a filter, allowing nutrients to pass into the bloodstream while blocking the absorption of larger molecules such as chemicals and bacteria (Lipski, 2012, p.17). Fingerlike folds called villi in the epithelium of the small intestine, numbered by the hundreds, allow this absorption through simple diffusion, facilitated diffusion, active transport, and secondary active transport, depending on the nutrient (Tortora & Derrickson, 2015, p.457). Microvilli coat each villi like fuzz, called a brush border, and aid in producing digestive enzymes, absorbing nutrients, and blocking absorption of unwanted products (Tortora & Derrickson, 2015, p.454), (Lipski, 2012, p.17). Lacteals, or lymphatic capillary, found within the villi, allow nutrients absorbed by the epithelial cells covering the villus to travel to the lymphatic system (Tortora & Derrickson, 2015, p.454).
The small intestine is composed of three parts: the duodenum, the jejunum, and the ileum (Lipski, 2012, p.18). As my digestion progresses, chyme passes from my stomach through the pyloric sphincter, a process called gastric emptying (Tortora & Derrickson, 2015, p.450). This happens slowly to avoid overwhelming my duodenum with too much material. Carbohydrates spend the least amount of time in my stomach, with high protein foods staying longer and high fat foods spending the most time in my stomach before passing through to the small intestine (Tortora & Derrickson, 2015, p.450).
My gallbladder springs into action at this point, responding to the release of cholecystokinin from the duodenum, compelling it to release bile into the hepatopancreatic duct to help emulsify and break down the large lipid globules into smaller lipids (Tortora & Derrickson, 2015, p.452). Bile also emulsifies fat-soluble vitamins and cholesterol (Tortora & Derrickson, 2015, p.452). This prepares the lipids for further breakdown by the enzyme lipase in the duodenum (Tortora & Derrickson, 2015, p.456).
My chyme is churned with further waves of peristalsis and segmentation in the duodenum (Tortora & Derrickson, 2015, p.467). Enzymes from pancreatic juice and bile continue to break down disaccharides to monosaccharides; protein digestion is completed with peptidases; triglycerides are broken down monoglycerides by pancreatic lipase; and nucleases break down nucleic acids to pentoses and nitrogenous bases (Tortora & Derrickson, 2015, p.467).
Some mineral and vitamins are absorbed in the duodenum, including chlorine, sulfur, calcium, copper, iron, thiamin, manganese, zinc, vitamins A, D, E, B1, B2, B6, C, and folic acid (Lipski, 2012, p.18). My dinner then moves into remaining portions of the small intestine.
Digestion of nutrients proceeds to the jejunum, then the ileum, as peristalsis and segmentations continue. Sugars, proteins, and amino acids are the primary nutrients absorbed in the jejunum, while cholesterol, B12, and bile salts are absorbed in the ileum (Lipski, 2012, p.18).
As mentioned earlier, absorption of nutrients in the gastrointestinal tract happens through simple diffusion, osmosis, facilitated diffusion, and active transport (Smolin & Grosvenor, 2013, p.86-88). Substances such as vitamin E and fatty acids are absorbed by simple diffusion because their fatty nature more closely resembles the lipid environment of cell walls (Smolin & Grosvenor, 2013, p.86). Water is absorbed by osmosis, balancing the concentration of dissolved substances such as sugar and sodium (Smolin & Grosvenor, 2013, p.86-87). Some nutrients need help to pass across cell membranes, so utilize facilitated diffusion, using carrier molecules, moving from an area of high concentration to an area of low concentration (Smolin & Grosvenor, 2013, p.87). Fructose in fruit is absorbed by facilitated diffusion (Smolin & Grosvenor, 2013, p.88). Yet other substances need a process called active transport to reach the cells. Both a carrier molecule and the expenditure of energy (ATP) are needed to complete the process (Smolin & Grosvenor, 2013, p.87). Glucose and amino acids are absorbed via active transport (Smolin & Grosvenor, 2013, p.88).
About three to five hours after my first bite of food, what remains of my dinner passes through the ileocecal valve at the end of the ileum into the colon, or large intestine (Tortora & Derrickson, 2015, p. 458). Peristalsis here is slower than in the small intestine, allowing for water absorption, as well as the absorption of potassium, salt, vitamin K, and short-chain fatty acids (Lipski, 2012, p.18). It takes up to 24 hours for the remaining matter to make its way through the colon and out the rectum, even though the colon is short, measuring about three to five feet long (Lipski, 2012, p.21), (Smolin & Grosvenor, 2013, p.88). Up to 500 species of bacteria reside in the colon, which benefit from the slow movement of matter (Smolin & Grosvenor, 2013, p.88). This microflora feeds on unabsorbed portions of my dinner, such as fiber, producing nutrients that nourish the beneficial gut bacteria (Smolin & Grosvenor, 2013, p.88). Water absorption is tremendous, up to two and a half gallons each day, which is absorbed in to the bloodstream (Lipski, 2012, p.22).
The large intestine is divided into four portions: the ascending, transverse, descending, and sigmoid segments, with the rectum ending at the sigmoid colon (Tortora & Derrickson, 2015, p.458-459). The last inch of the rectum is called the anal canal, with the opening called the anus (Tortora & Derrickson, 2015, p.459). Two sphincters, an internal (involuntary) and external (voluntary) control bowel movements (Tortora & Derrickson, 2015, p.459).
If the chyme passes through my colon too quickly, water isn’t absorbed, resulting in diarrhea (Lipski, 2012, p.22). If chyme sits too long in my colon, too much water is absorbed, resulting in a dry, hard stool, or constipation (Lipski, 2012, p.22).
My bowel transit time is the average 12- 24 hours, so I’m able to have a normal bowel movement at lunchtime the day after Easter dinner (Lipski, 2012, p.23). I’m ready to start the whole process over again as I break my fast with a delicious, healthy lunch.
Always yours in health,
Lipski, E. (2012). Digestive wellness: Strengthen the immune system and prevent disease through healthy digestion. New York, NY: McGraw-Hill.
Smolin, L. A., & Grosvenor, M. B. (2015). Nutrition: Science and applications(3rd ed.). Hoboken, NJ: Wiley.
Tortora, G. J., & Derrickson, B. (2015). Introduction to the human body essentials of anatomy and physiology. New York: Wiley.