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Digestive System - Questions and Answers
1. What is digestion?
Digestion is the breaking down of larger organic molecules obtained from the diet, e.g. carbohydrates, fats, proteins, into smaller ones, like glucose, fatty acids, glycerol and amino acids.
2. How different are intracellular and extracellular digestion? What is the evolutionary advantage of extracellular digestion?
Intracellular digestion is that in which the breaking down of macromolecules takes place within the cell. Extracellular digestion is that in which macromolecules are broken down in places outside the cell (in the extracellular space, in the surrounds, in the lumen of digestive tubes, etc.)
The advent of extracellular digestion in evolution allowed organisms to benefit from a greater variety of food. The breaking down of larger molecules into smaller ones outside the cell permitted the use of other foods than those that, due the size of their molecules, could not be interiorized by diffusion, phagocytosis or pinocytosis.
3. How is extracellular digestion related to cellular and tissue specialization?
A variety of specialized cells and tissues appeared with extracellular digestion to provide enzymes and special structures for the breaking down of dietary macromolecules.
This phenomenon allowed other cells to be liberated for other tasks and differentiations while benefiting from nutrients distributed through the circulation.
4. What is the difference between a complete digestive system and an incomplete digestive system? How are these types of digestive tubes associated or not to extracellular digestion?
Animals with an incomplete digestive system are those in which the digestive tube has only one opening (cnidarians, platyhelminthes). Animals with a complete digestive system are those in which the digestive tube has two openings, mouth and anus (all other animal phyla, with the exception of poriferans, that do not have any digestive tube).
In animals with incomplete digestive tubes the digestion is mixed, it begins in the extracellular space and finishes in the intracellular space. In animals with complete digestive systems extracellular digestion within the digestive tube predominates.
5. What are some evolutionary advantages of animals with complete digestive tube?
The complete digestive tube allows animals to continuously feed themselves without waiting for residuals to be eliminated before beginning the digestion of new foods. In this way the absorption of larger amount of nutrients is possible and therefore bigger and more complex species can develop. Digestive tubes with two openings also make digestion more efficient since they provide different sites with different physical and chemical conditions (mouth, stomach, bowels) for the action of different complementary digestive enzymatic systems.
6. What is mechanical digestion? In molluscs, arthropods, earthworms, birds and vertebrates, in general, which organs respectively participate in this type of digestion?
Mechanical digestion is the fragmentation of food aided by specialized physical structures, such as teeth, previous to extracellullar digestion. The mechanical fragmentation of food helps digestive enzymatic reactions because it provides a larger total area for the contact between enzymes and their substrates.
In some molluscs, the mechanical fragmentation is done by the radula (a teeth-like structure). Some arthropods, like lobsters and dragonflies, have mouthparts that make mechanical digestion of food. In earthworms and birds, the mechanical digestion is made by an internal muscular organ. In mandibulate vertebrates there are mandibles and chewing muscles to triturate food previous to the chemical digestion.
7. Concerning extracellular digestion what is meant by chemical digestion?
Chemical digestion is the series of enzymatic reactions to break macromolecules into smaller ones.
8. Which type of chemical reaction is the breaking of macromolecules into smaller ones that occurs in digestion? What are the enzymes that participate in this process called?
The reactions of the extracellular digestion are hydrolysis reactions, i.e., breaking of molecules with the help of water. The enzymes that participate in digestion are hydrolytic enzymes.
9. Which organs of the body are part of the human digestive system?
The digestive system, also known as “systema digestorium”, or gastrointestinal system, is composed of the digestive tube organs plus the digestive adnexal glands. The digestive tube is composed of mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (caecum, colon, rectum) and anus.
Digestion System - Image Diversity: human digestive system
10. What are peristaltic movements? What is their role in human digestion?
Peristalsis is the process of synchronized contractions of the muscular wall of the digestive tube. Peristaltic movements may occur from the esophagus until and including the bowels.
The peristaltic movements are involuntary and they have the function of moving and mixing food along the digestive tube. Peristaltic movement deficiency, for example, in case of injuries of the innervation of the muscular wall of the digestive tube caused by Chagas’ disease, can lead to the interruption of the food traffic inside the bowels and to severe clinical consequences like megacolon (abnormal enlargement of the colon) and megaesophagus (enlargement of the esophagus).
Digestion System - Image Diversity: peristaltic movements
11. From the lumen to the external surface what are the tissues that form the digestive tube wall?
From the internal surface to the external surface, the digestive tube wall is made of mucosa (epithelial tissue responsible for the intestinal absorption), submucosa (connective tissue beneath the mucous membrane and where blood and lymphatic vessels and neural fibers are located), muscle layers (smooth muscle tissue, two layers, one interior circular and other exterior longitudinal, structures responsible for the peristaltic movement), serous membrane (associated epithelial and connective tissue forming the external surface of the organ). In the bowels the serous membrane prolongs to form the mesentery, a serosa that encloses blood vessels and supports the bowels within the abdominal cavity.
Digestion System - Image Diversity: histology of the digestive tube
12. What is the location of the salivary glands in humans?
There are 6 major salivary glands and they are located one in each parotid gland, two beneath the mandibles (submandibular) and two in the base of the tongue (sublingual). More than 700 other minor salivary glands exist dispersed on the lip mucosa, gingiva, palate and pharynx.
13. What is the approximate pH of the salivary secretion? Is it an acid or basic fluid? What are the main functions of saliva?
The saliva pH is approximately 6.8. It is thus a slightly acid pH.
Saliva lubricates the food bolus and initiates the enzymatic extracellular digestion of food. It also works as a buffer for the mouth pH and it has an important role of having IgA antibodies (also present in tears, colostrum, mother’s milk and in the mucosae of the intestine and airways) that protect the organism against pathogens.
14. What is the salivary digestive enzyme? Which type of food does it digest and into which smaller molecules does it transform the food?
The salivary hydrolase is known as salivary amylase, or ptyalin. Ptyalin digests carbohydrates breaking starch and glycogen, glucose polymers, into maltose (a glucose disaccharide) and dextrin.
15. Why doesn't the food enter the trachea instead of going to the esophagus?
When food is swallowed the swallow reflex is activated and the larynx elevates and closes to avoid portions of the food bolus entering the trachea causing aspiration of strange material to the bronchi.
Digestion System - Image Diversity: esophagus
16. Is the esophagus a muscular organ? Why even in a patient lying totally flat on a hospital bed can the swallowed food reach the stomach?
The esophagus is a predominantly muscular organ so the assertion is correct. The esophagus is a muscular tube formed in its superior third of striated muscle tissue, in its middle third of mixed muscle tissue (striated and smooth) and in its lower third of smooth muscle tissue. The peristalsis of the esophagus provides the movement of the food towards the stomach even without gravitational help.
17. What is the route of the ingested food from swallowing until the duodenum?
Until reaching the duodenum the food enters the mouth, passes the pharynx, goes down the esophagus and passes the stomach.
18. what is the valve that separates the stomach from the esophagus called? What is its function?
The valve that separates the stomach from the esophagus is the cardia. It has the function of preventing acid gastric content from entering back into the esophagus. Insufficiency of this valve causes gastroesophageal reflux, a disease in which patients complain of bloating and heartburn (retrosternal burning).
Digestion System - Image Diversity: stomach
19. What is the valve that separates the duodenum from the stomach called? What is its function?
The valve that separates the stomach from the duodenum is the pylorus. It has the function of keeping the food bolus within the gastric cavity for enough time to allow the gastric digestion to take place. It also has the function of preventing the intestinal content from going back into the stomach.
20. What is the pH inside the stomach? Why is there a need to keep that pH level? How is it maintained? Which are the cells that produce that pH?
The normal pH of the gastric juice is around 2. So it is an acid pH.
It is necessary for the gastric pH to be kept acid for the activation of pepsinogen (a proenzyme secreted by the gastric chief cells) into pepsin, the digestive enzyme that acts only under low pH. This pH level is attained by the secretion of hydrochloric acid (HCl) by the parietal cells.
Digestion System - Image Diversity: parietal cells
21. Besides being fundamental for the activation of the main gastric digestive enzyme how does HCl also directly participate in digestion?
With its corrosive effect, HCl also helps the rupture of the adhesion between food particles, facilitating the digestive process.
22. How is the gastric mucosa protected from the acid pH of the stomach?
The gastric epithelium is mucus secretory, i.e., it produces mucus. The mucus covers the stomach wall preventing corrosion by the gastric juice.
23. What is the digestive enzyme that acts within the stomach? Which type of food does it digest? What are the cells that produce that enzyme?
The digestive enzyme that acts in the stomach is pepsin. Pepsin has the function of breaking proteins into smaller peptides. The gastric cells that produce pepsinogen (the zymogen precursor of pepsin) are the chief cells.
Digestion System - Image Diversity: chief cells
24. What name does the food bolus that passes from the stomach to the duodenum get?
The partially digested and semifluid food bolus that leaves the stomach and enters the duodenum is called chyme.
25. Which are the three parts of the small intestine?
The small intestine is divided into three portions: duodenum, jejunum and ileum.
Digestion System - Image Diversity: small intestine
26. By generally dividing food into carbohydrates, fats and proteins and considering the digestive process until the pylorus (exit of stomach), which of these mentioned types of food have already undergone chemical digestion?
Until the exit of the stomach, carbohydrates, in the mouth, and proteins, in the stomach, have already undergone chemical breaking by digestive enzymes. Carbohydrates have suffered action of the salivary amylase (ptyalin) and proteins have suffered action of the enzyme pepsin of the gastric juice. Fats, until reaching the duodenum, do not undergo chemical digestion.
27. What is the substance produced in the liver that acts in the small intestine during digestion? How does that substance act in the digestive process?
Bile, an emulsifier liquid, is made by the liver and later stored within the gallbladder and released in the duodenum.
Bile is composed of bile salts, cholesterol and bile pigments. Bile salts are detergents, amphiphilic molecules, i.e., molecules with a polar water-soluble portion and a non-polar fat-soluble portion. This feature allows bile salts to enclose fats inside water-soluble micelles in a process called emulsification for them to be in contact with intestinal lipases, enzymes that break fats into simpler fatty acids and glycerol.
Digestion System - Image Diversity: gallbladder
28. What is the adnexal organ of the digestive system in which bile is stored? How does this organ react to the ingestion of fat rich food?
Bile is concentrated and stored in the gallbladder.
When fat rich foods are ingested the gallbladder contracts to release bile inside the duodenum. (This is the reason why patients with gallstones must not ingest fatty food, the reactive contraction of the gallbladder may move some of the stones to the point of blocking the duct that drains bile into the duodenum, causing pain and possible severe complications.)
29. What are the digestive functions of the liver?
Besides making bile for release in the duodenum, the liver has other digestive functions.
The venous network that absorbs nutrients from the guts, called mesenteric circulation, drains its blood content almost entirely to the hepatic portal vein. This vein irrigates the liver with absorbed material from the digestion. So the liver has the functions of storing, processing and inactivating nutrients.
Glucose is polymerized into glycogen in the liver; this organ also stores many vitamins and the iron absorbed in the intestine. Some important metabolic molecules, like albumin and clotting factors, are made in the liver from amino acids of the diet. In the liver ingested toxic substances, like alcohol and drugs, are inactivated too.
Digestion System - Image Diversity: liver
30. Besides the liver which is the other adnexal gland of the digestive system that releases substances in the duodenum participating in extracellular digestion?
The other adnexal gland of the digestive system is the pancreas. This organ makes digestive enzymes that digest proteins (proteases), lipids (lipases) and carbohydrates (pancreatic amylases). Other digestive enzymes, like gelatinase, elastase, carboxipeptidase, ribonuclease and deoxyribonuclease are also secreted by the pancreas.
Digestion System - Image Diversity: pancreas
31. How does the pancreatic juice participate in the digestion of proteins? What are the involved enzymes?
The pancreas secretes trypsinogen that, undergoing action of the enzyme enterokinase secreted by the duodenum, is transformed into trypsin. Trypsin in its turn catalyzes the activation of pancreatic chymotrypsinogen into chymotrypsin. Trypsin and chymotrypsin are proteases that break proteins into smaller peptides. The smaller peptides are then broken into amino acids by the enzyme carboxipeptidase (also secreted by the pancreas in a zymogen form and activated by trypsin) helped by the enzyme aminopeptidase made in the intestinal mucous membrane.
32. How does the pancreatic juice resume the digestion of carbohydrates? What is the involved enzyme?
Carbohydrate digestion begins with the action of the salivary amylase (ptyalin) in the mouth and it continues in the duodenum by the action of the pancreatic juice. This juice contains the enzyme pancreatic amylase, or amylopsin, that breaks starch (amylum) into maltose (a disaccharide made of two glucose molecules).
33. How does the pancreatic juice help the digestion of lipids? What is the involved enzyme?
The enzyme pancreatic lipase is present in the pancreatic juice. This enzyme breaks triacylglycerol (triglyceride) into fatty acids and glycerol.
34. Besides the pancreatic juice in the intestine there is the releasing of the enteric juice that contains digestive enzymes too. What are these enzymes and which type of molecule do each of these enzymes break?
The enteric juice is secreted by the small intestine mucosa. The enzymes of the enteric juice and their respective functions are described as follows:
Enterokinase: enzyme that activates trypsinogen into trypsin. Saccharase: enzyme that breaks sucrose (saccharose) into glucose and fructose. Maltase: enzyme that breaks maltose into two glucose molecules. Lactase: enzyme that breaks lactose into glucose and galactose. Peptidases: enzymes that break oligopeptides into amino acids. Nucleotidases: Enzymes that break nucleotides into its components (nitrogen-containing bases, phosphates and pentoses).
35. Coming from the acid pH of the stomach which pH level does the chyme find when it enters the duodenum? Why is it necessary to maintain that pH level in the small intestine? What are the organs responsible for that pH level and how is it kept?
Entering the duodenum the chyme meets the pancreatic juice under a pH of approximately 8.5. The neutralization of the chyme acidity is necessary to keep adequate pH level for the functioning of the digestive enzymes that act in the duodenum. Without the neutralization of the chyme acidity the mucous membrane of the intestine would be injured.
When stimulated by the chyme acidity the duodenum makes a hormone called secretin. Secretin stimulates the pancreas to release the pancreatic juice and also the gallbladder to expel bile in the duodenum. The pancreatic secretion, rich in bicarbonate ions, is released in the duodenum and neutralizes the chyme acidity; this acidity is also neutralized by the secretion of bile in the duodenal lumen.
36. What are the five human digestive secretions? Which of them is the only one that does not contain digestive enzymes?
The human digestive secretions are: saliva, gastric juice, bile, pancreatic juice and enteric juice. Among these secretions only the bile does not contain digestive enzymes.
37. Why do protease-supplying cells of the stomach and of the pancreas make only precursors of the active proteolytic enzymes?
The stomach and the pancreas make zymogens of the proteases pepsin, chymotrypsin and trypsin and these zymogens are released into the gastric or duodenal lumen for activation. This happens to prevent the digestion of these organs' (stomach and pancreas) own cells and tissues by the active form of the enzymes. So the production of zymogens is a protective strategy against the natural effects of the proteolytic enzymes.
38. After digestion the next step is absorption done by cells of the mucous membrane of the intestine. For this task a large absorption surface is an advantage. How is it possible in the small internal space of the body of a pluricellular organism to present a large intestinal surface?
Evolution tried to solve this problem in two ways. The simplest is the long and tubular shape of the bowels (approximately eight meters in extension), making possible that numerous small intestine loops fold closely. More efficient solutions are the intestinal villi and the microvilli of the mucosal membrane cells.
The intestinal wall is not smooth. The mucous membrane, together with its submucosa, projects inside the gut lumen like glove fingers forming invaginations and villi that multiply the available surface for absorption. In addition the epithelial cells that cover these villi have themselves numerous hairlike projections called microvilli on the external face (lumen face) of their plasma membrane. The absorptive area of the intestines is thus increased hundreds of times with these solutions.
In the jejunum and ileum there are folds that have the function of increasing the absorption surface too.
Digestion System - Image Diversity: intestinal villi microvilli
39. In which part of the digestive tube is water is chiefly absorbed? What about the mineral ions and vitamins?
Most part of water, vitamins and mineral ions are absorbed by the small intestine. The large intestine, however, is responsible for the reabsorption of nearly 10% of the ingested water, an important amount that gives consistency to feces (colon diseases can cause diarrhea).
Digestion System - Image Diversity: colon
40. From the intestinal lumen through to the tissues - what is the route of nutrients after digestion?
Monosaccharides, amino acids, mineral salts and water are absorbed by the intestinal epithelium and collected by capillary vessels of the intestinal villi. From the capillaries, nutrients go to the mesenteric circulation, a system of vessels that drains the intestinal loops. The blood of the mesenteric circulation is drained to the portal hepatic vein and some nutrients are processed by the liver. From the liver, nutrients are gathered by the hepatic veins that discharge its blood content into the inferior vena cava. Blood from the inferior vena cava then gains the right chambers of the heart and is pumped to the lungs for oxygenation. From the lungs the blood then returns to the heart where it is pumped to the tissues distributing nutrients and oxygen.
41. What is the special route that lipids follow during digestion? What are chylomicrons?
Triglycerides emulsified by the bile within micelles suffer the action of lipases that break them into fatty acids and glycerol. Fatty acids, glycerol and cholesterol are absorbed by the intestinal mucosa. In the interior of the mucosal cells fatty acids and glycerol form again triglycerides that together with cholesterol and phospholipids are packed in small vesicles covered by proteins and called chylomicrons. The chylomicrons are released in minuscule lymphatic vessels not in blood vessels and they gain the lymphatic circulation. So the lymphatic system plays an important role in the absorption of lipids.
The lymphatic circulation drains its content to the venous blood circulation. In that manner chylomicrons reach the liver where their lipid content is processed and released in the blood under the form of protein-containing complexes called lipoproteins, like HDL, VLDL and LDL.
Digestion System - Image Diversity: chylomicrons lymphatic circulation
42. What are the so-called “good” and “bad” cholesterol?
Lipoproteins are complexes made of lipids (triglycerides and cholesterol) and proteins. The lipoproteins present different densities according to the relationship between their protein and lipid quantities since lipids are less dense than proteins. Low-density lipoproteins (LDL) are those with a low protein/lipid relation; high-density lipoproteins (HDL) have a high protein/lipid relation; another group is the very low-density lipoproteins (VLDL) with very low protein/lipid relation.
LDL is known as “bad cholesterol” because it transports cholesterol from the liver to the tissues and so induces the formation of atheroma plaques inside blood vessels, a condition called atherosclerosis (do not confuse with arteriosclerosis) that can lead to severe circulatory obstructions like acute myocardial infarction, cerebrovascular accidents and thrombosis. HDL is known as “good cholesterol” since it transports cholesterol from the tissues to the liver (to be eliminated with the bile) and elevation of the HDL blood level reduces the risk of atherosclerosis. (VLDL transforms into LDL after losing triglycerides in the blood).
43. Why does the ingestion of vegetable fibers improve the bowel habit in people that suffer from hard stools?
Some types of plant fibers are not absorbed by the intestine but play an important role in the functioning of the organ. They retain water inside the bowels and thus contribute to the softening of the fecal bolus. A softer fecal bolus is easier to be eliminated during defecation. People that eat less dietary fiber may suffer from hard stools and constipation.
44. What are the main functions of the bacterial flora within the human gut?
Bacteria that live inside the gut have great importance in digestion. Some polysaccharides like cellulose, hemicellulose and pectin are not digested by the digestive enzymes secreted by the body, instead, they are broken by enzymes released by bacteria of the gastrointestinal tract. The intestinal bacterial flora also make vital substances for the functioning of the bowels facilitating or blocking the absorption of nutrients and stimulating or reducing peristalsis. Some gut bacteria are the main source of vitamin K for the body and so they are essential for the blood clotting process.
In the intestinal flora there are utile but also potentially harmful bacteria. It is estimated that more than 100 trillion bacteria live in a human gut. Some bacteria are useful too because they compete with other species preventing excessive proliferation of these bacteria.
45. The releasing of digestive secretions is controlled by hormones. What are the hormones that participate in this regulation?
The hormones that participate in the regulation of digestion are gastrin, secretin, cholecystokin and enterogastrone.
46. How is it produced and what is the function of gastrin in the digestive process?
The presence of food in the stomach stimulates the secretion of gastrin that in its turn triggers the releasing of the gastric juice.
47. Where is it produced and what is the function of secretin in the digestive process?
Secretin is made in the duodenum. The chyme acidity causes the duodenum to release this hormone that in its turn stimulates the secretion of the pancreatic juice.
48. How is it produced and what is the function of cholecystokin in the digestive process?
The fat level of the chyme detected in the duodenum stimulates the secretion of cholecystokin (CCK). CCK acts by stimulating the secretion of the pancreatic juice also, and the releasing of bile by the gallbladder.
49. Where is it produced and what is the function of enterogastrone in the digestive process?
When the chyme is too fatty there is a secretion of enterogastrone by the duodenum. This hormone reduces the peristalsis of the stomach thus slowing the entrance of food into the duodenum (as the digestion of fats takes more time).
50. What are the special structures of the avian digestive tube and their respective functions?
The digestive tube of birds has special structures, in this sequential order: the crop, the proventriculus and the gizzard.
The crop has the function of temporary storage of ingested food and it is a more dilated area of the avian esophagus. The proventriculus is the chemical stomach of the birds where food is mixed with digestive enzymes. The gizzard is a muscular pouch that serves as a mechanical stomach where the food is ground to increase the exposure area of the food particles for the digestive enzymes to act.
Digestion System - Image Diversity: avian digestive system
51. Compared to mammals do birds absorb more or less water in their digestive system? Why is this phenomenon an adaptation to flight?
Bird feces are more liquid than mammal feces, i.e., less water is absorbed in the avian digestive system. The more frequent elimination of feces in birds due to their less solid feces is an adaptation to flight since their body weight is kept lower.
52. What is meant by “mutualist exploration of cellulose digestion”, a phenomenon that occurs in some mammals and insects?
Herbivorous animals eat great amounts of cellulose, a substance not digested by their digestive enzymes. In these animals regions of the digestive tube are colonized by microorganisms that digest cellulose. This mutualist ecological interaction between animals and microorganisms occurs, e.g., in horses, cows, rabbits and in some insects such as termites.
53. Cows swallow their food once and then this food goes back to the mouth to be chewed again. How can this phenomenon be explained?
The food ingested by cows and other ruminant animals passes first within two compartments of the digestive tube called the rumen and the reticulum. Within them the food suffers the action of digestive enzymes released by microorganisms that live there in mutualist ecological interaction. In the reticulum the food is divided in some food bolus too. After passing the reticulum the food (cud) is regurgitated to the mouth to be again chewed and swallowed in a process called rumination. The digesting food then enters the omasum where it is mechanically mixed. After that the food goes to the abomasum, the organ where the chemical digestion takes place. After leaving the abomasum (the true stomach) the food bolus gains the intestine.
Digestion System - Image Diversity: ruminant digestive system
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