A Trip Through the Digestive System
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. This is intended for weaker readers or those needing a quick introduction.
Let's imagine that you are riding a molecular-sized submarine through the digestive system of someone who has just eaten a cheese sandwich with lettuce and tomato on whole wheat. The food has examples of most of the micromolecules a human needs. The cheese contains both proteins and fats, along with the sugar lactose, the mineral calcium, and fat-soluble vitamins A and D. The bread has starch, and it and the lettuce contain some water-soluble B-vitamins, iron, and cellulose (fibre). The butter is mostly fat, with some dissolved cholesterol, and the tomato contains some sucrose (sugar) and vitamin C.
The sandwich is first bitten into smaller pieces by the teeth, so that it will fit into the mouth. Co-ordinated actions of the lips, tongue, and teeth break the bite down into particles only a few millimetres across. Your submarine will have a wild ride here, and may be lucky to escape being crushed.
The salivary glands pump saliva into the mouth when they are stimulated by the sight, smell or taste of food. This saliva, consisting of water, dissolved mucus proteins, and the enzyme amylase, mixes with the food, moistening and lubricating it. The amylase begins nibbling at the ends of exposed starch molecules, breaking off disaccharides (double sugars) of maltose. Tiny amounts of maltose are absorbed through the lining of the mouth. It is unlikely that this is significant as a nutrient, but it may serve as a "wake-up call" for organs downstream.
After a few second of chewing, the mouth has turned the bite of sandwich into a moist, slippery pulp called a bolus. When the tongue pushes the bolus to the pharynx at the back of the mouth, it triggers the swallowing reflex, and swallowing then becomes automatic. The epiglottis is pulled down over the opening to the lungs, and the food is pushed down the oesophagus by waves of muscular contraction called peristalsis. Your ride here will be smooth.
For a more detailed description of peristalsis, click on the icon At the bottom of the oesophagus, the bolus pushes on the cardiac, sphincter which opens, admitting the bolus into the stomach. Reverse pressure on the cardiac sphincter has the opposite effect, causing it to clamp shut. Thus the food cannot normally get back into the oesophagus once it has passed through. This is strictly a one way journey unless something goes seriously wrong.
In the stomach, the food is churned fairly violently by muscular contractions of the three muscle layers in the stomach wall. At the same time, large quantities of hydrochloric acid (HCl) and the enzyme pepsin are pumped into the lumen (hollow) of the stomach. The HCl helps to break down cell membranes and causes large molecules to unravel, exposing their bonds to enzyme action. The starches of the bread and the proteins of the cheese would have been wrapped into tight globular shapes or twisted into helices when they were eaten. Now they will be stretched out. Your submarine is going to need strong defences against chemical attack in the stomach and your ride is going to be extremely turbulent.
The HCl also provides the highly acidic environment in which pepsin works best. Pepsin works by chopping long protein molecules into polypeptides of moderate length. It finds a particular amino acid in a cheese protein and breaks the bond between it and the adjacent amino acid. Of course, it does the same thing to any other proteins present. This way, it can protect the body against incoming bacteria or other microorganisms. It also breaks down most of the salivary amylase, which thus stops digesting starch. By the time the stomach has finished churning and chemically attacking the food, very little solid material is left. The stomach contents consist of a very acidic liquid referred to as chyme.
Meanwhile, the fats that were in the cheese and butter find themselves released into the lumen, which is full of watery medium. The fats, being repelled by water, tend to clump together as tiny droplets, much as you might find in a salad dressing just after shaking. These droplets make the chyme opaque, so that it looks somewhat like milk.
The bottom end of the stomach is kept closed by the very powerful pyloric sphincter, a circular muscle. When food has been in the stomach long enough for the pepsin to have done its job, the pyloric sphincter begins releasing it one squirt at a time into the duodenum.
The duodenum is the digestive system's great mixing pot. Secretions pour into it from the pancreas, liver, and gall bladder via the common bile duct. The liver secretions, which have usually been stored temporarily in the gall bladder, constitute bile. Bile includes amphiphilic molecules called bile salts. These salts are slightly basic and help neutralise the stomach acid, but their main job is to emulsify the fat droplets. The hydrophobic ends of the salts dissolve in the fats, and the hydrophilic ends dissolve in surrounding water, thus dispersing the fats throughout the intestinal fluid. The fats end up in tiny microdroplets with a very high surface area to volume ratio, making it easier for enzymes to attack them.
The bile also includes bile pigments, which are wastes left over when the liver breaks down haemoglobin from expired red blood cells. This is simply an efficient way of piggy-backing one required function on another. The liver has a waste product that it must eliminate, and it already has an outlet to the intestine, so it is efficient just to add the bile pigments to the faeces. It is these pigments that colour faeces.
The inflow from the pancreas is far more complex. The pancreatic fluid is rich in the buffered base NaHCO3 (sodium bicarbonate) which neutralises the HCl from the stomach and makes the duodenal fluid slightly basic. The pancreas also secretes a cocktail of enzymes that are responsible for most of the rest of the chemical digestion.
Pancreatic amylase takes up where salivary amylase left off, nibbling maltose molecules off the ends of starch. This is a relatively simple chemical reaction, and so the bread will be almost entirely digested to disaccharides within about half an hour of its release from the stomach.
Lipase begins to work on the microdroplets of fat, slowly breaking them down into glycerol and fatty acids. This is extremely slow, and would take hours to complete. Usually the body is unable to digest all the fat in a meal in the time available. The milk fats from the cheese and butter will be partly digested and partly still in droplets when it is time to absorb the nutrients.
Nucleases break DNA and RNA down into their basic molecules for absorption.
A host of protease enzymesare released by the pancreas. Two of these, trypsin and chymotrypsin, are choppers, which find specific amino acids in the polypeptide chains and snip them into short peptides. Two others (aminopeptidase and carboxypeptidase) are nibblers, which begin snipping off individual amino acids from these peptides, one working from either end. By the time the cheese proteins are ready to be absorbed, they will consist of dissolved individual amino acids and very short chains of perhaps two to four amino acids. Your submarine's sensors and analysers could be kept busy for days just trying to figure out what was pouring into the duodenum and how it was working on the food.
The mixture of chyme, bile, sodium bicarbonate, and enzymes from the duodenum is then passed along to the next part of the small intestine, the jejunum. From here on, your ride will be smooth and you can concentrate on what is happening around you.
As the intestinal fluid flows slowly along the three metres of jejunum, the enzymes have time to work on the food. The indigestible fibre from the lettuce and whole wheat bread now plays an important role. As a bulking agent, it helps to keep the insoluble large molecules separated so the enzymes can work on them, and it also increases the rate of peristalsis in the intestine, preventing stagnation (see more below).
The main function of the jejunum is to permit enzyme digestion. The jejunum grades into the last part of the small intestine, the ileum. The distinction between the two is mainly quantitative; both do the same jobs, but while chemical breakdown is dominant in the jejunum, absorption is dominant in the ileum. The surface of small intestine is covered with villi and microvilli, giving it a huge surface area, and it is well supplied with blood from the anterior mesenteric artery. This blood carries food away to the hepatic portal vein efficiently.
Embedded in the membranes of the intestinal epithelial cells are the last group of digestive enzymes. These consist of specialised disaccharide splitters - maltase, sucrase and lactase- and short peptide splitters called dipeptidases. Maltase splits the maltose from the bread starch to glucose, sucrase splits the tomato's sucrose to glucose and fructose, and lactase splits the lactose from the cheese into glucose and galactose. These simple sugars ( monosaccharides) can be absorbed by facilitated diffusion through protein channels into the cells of the intestinal epithelium. The dipeptidases do much the same thing with the short chain peptides left over from the actions of the protease enzymes. All the amino acids can then be absorbed into the epithelial cells.
Soluble fatty acids and glycerol from the cheese and butter are absorbed in the same way. By the time the fluid reaches the ileum, however, only a portion of the available lipids have been absorbed. The rest remain in tiny droplets. Rather than waste this great potential source of energy, the epithelial cells use a new trick. Proteins in the cell membranes are stimulated by fat droplets, and cause the membrane to fold inwards and pinch off, forming a tiny food vacuole (vesicle) by endocytosis. In this way, undigested fats are absorbed and can later be broken down by intracellular enzymes as needed. A secondary advantage to this system is the fact that substances dissolved in the lipids can be absorbed at the same time. Fat soluble vitamins and cholesterol from the cheese and butter enter the body this way.
Water soluble minerals and vitamins may also be absorbed in either the small or the large intestine, but one, iron, requires special conditions. Alone it cannot easily pass through the cell membrane. The small amount of iron in the lettuce and whole wheat (note that this meal is NOT a good source of iron) will combine with small organic acids, such as the ascorbic acid (vitamin C) from the tomato. Only in this form will it easily enter the cells of the epithelium.
As absorption proceeds, the remaining value of the intestinal fluid drops, but we have not extracted all the useful material yet. The small intestine provides an excellent environment for a wide variety of bacteria (Enterobacteriaceae) and other microorganisms. These specialised creatures like an environment that is warm, wet, nutrient-rich and low in oxygen, and many of them have become essential to us as symbiotic helpers. While feeding themselves, these microbes release additional nutrients from the food, and frequently synthesise substances that were not originally in the food itself. Some of our essential vitamins are produced this way. We in turn provide the bacteria with a near-perfect environment. This sort of mutual cooperation among organisms is far more common than one might imagine in the natural world.
Of course, non-cooperative bacteria may also occupy the intestine, and some may produce by-products that are useless or even harmful. For this reason, it is important that intestinal fluid be moved steadily through the lumen, and eliminated before too much complex fermentation takes place. Pockets of stagnation, or a general slowing of the entire system may have long-term effects on health.
As your submarine passes out of the small intestine into the colon, you will pass a small chamber, the caecum, below you (if you're still right side up) with a narrow tunnel leading off the opposite side. If you follow the tunnel, you will come to a dead end after a couple of centimetres. You have entered the human appendix, a degenerate part of the caecum. In humans the caecum and appendix have no important function, but many herbivores have much larger caeca that house cellulose digesting bacteria. Since there is nothing very interesting happening here, you will return to the main digestive tract.
By the end of the ileum, most of the useful nutrients will have been removed from the food, and waste material will be left. This consists primarily of undigested fibre from the food, mucus from various digestive secretions, dead cells and by-products from the intestinal bacteria, and bile pigments, but it also contains a large amount of water. Not only is there water from the food itself, but also from the huge quantities of fluid that have been added to the food in the form of saliva, gastric juice, pancreatic juice, intestinal secretions, and all the water in the lubricating mucus that has protected the epithelium throughout the system. Several litres of water are dumped into the digestive tract every day. Without an efficient recovery system, the body would rapidly dehydrate.
The colon is specialised to recover water. Its surface is a honeycomb of ridges, providing a large surface area, and its epithelial cells actively transport ions from the lumen of the intestine to increase the rate of osmosis from the intestinal fluid. In the end, about 90% of all the water released into the intestine is recovered. Your submarine will now be immobilised, and you can only wait.
The remaining semi-solid waste material is combined with a thick binding mucus from the epithelium of the colon. The faeces are then passed on to the rectum, where they are stored until they are eliminated. Some reabsorption of water continues here, but the quantity is small.
The rectum ends in the very strong anal sphincter muscle, which is under voluntary control. This muscle is considerably more powerful than the circular muscles moving the faeces forward by peristalsis, so that, except under extreme circumstances, we can decide consciously when to eliminate the faeces. This probably had significant survival value for our ancestors, allowing them to defecate in places away from other routine activities, thus reducing both the threats to health and the danger of attracting predators that hunt by scent.
Your journey is now complete, but the potential value of the original cheese sandwich has not been completely exhausted. Human faeces still contain substantial energy, and large amounts of useful raw materials that may be exploited by other organisms with different digestive strategies. You should don your anti-contamination suit and escape your now stranded vehicle before the attacks by the dung beetles, fly maggots, bacteria and fungi begin!