Digestion and Absorption of Food- Part 2

In my last post I said that Part 2 would cover the intestines, which is true, but first I'm going to quickly talk about the liver and the pancreas.

The Pancreas

The pancreas is just beneath the stomach, in the curve of the duodenum (the duodenum is the first C-shaped curve of the small intestine. If you don't understand, just Google it so I don't have to plagiarise someone else's picture, or *shudder* go through the effort of drawing my own diagram :P). The pancreas contains cells that secrete pancreatic juice as well as other cells which secrete the hormones insulin and glucagon, which regulate sugar levels in the blood.

Pancreatic juice is important for digestion as it contains digestive enzymes, including pancreatic amylase which breaks down starch into disaccharides, particularly maltose; trypsin (a.k.a. pancreatic protease- one way to remember this one is that "protease" sounds kinda like "protein") which breaks down proteins into smaller peptide chains; ribonuclease and deoxyribonuclease, which break down RNA and DNA, respectively; and pancreatic lipases (remember, fatty acids are a kind of lipid), which break down fats into fatty acids and glycerol.

One other important attribute of pancreatic juice is that it is slightly basic with a pH of 8. This helps neutralise any HCl that might still be in the chyme from the stomach (as I said in my last post, the chyme is basically the mixture that you get after everything's been digested in the stomach).

Pancreatic juice enters the aforementioned duodenum (beginning of small intestine) via two ducts: first it goes through the pancreatic duct, which is then joined by the common bile duct, which then enters the duodenum.

The Liver

The liver is a rather large organ. It lies just below the diaphragm on the right side of the body. One thing unique about the liver is that it receives blood from two different sources: through a hepatic artery and a hepatic portal vein. (FYI, "hepa-" is a prefix that generally refers to anything to do with the liver.) The hepatic artery, which branches from the aorta (the main artery in the body), carries highly oxygenated blood to the liver. The hepatic portal vein, which passes through the capillaries of the stomach and intestines before going through the liver, takes nutrients from the alimentary canal to the liver.

The liver has a lot of functions- in fact, if my quick Google search is anything to go by, it has over 500 known functions! Fortunately, you don't have to learn about all of them for now. Here's a list of just a few of the functions of the liver:

• Blood glucose regulation: The hepatic portal vein carries glucose to the liver, which is then either used to provide energy to the liver, converted into glycogen, or converted into fat. Glycogen is basically the form in which carbohydrates are stored as an energy reserve for the body. Glycogen can be converted back into glucose and back again to maintain healthy blood glucose levels.
• Deamination: Converts excess amino acids to carbohydrates (since excess amino acids can't be stored). In the deamination process, the -NH2 amine group is removed and converted to ammonia (NH3) which is then converted to urea, which is removed by the blood by the kidneys before being excreted in urine.
• Fat conversion: Fat produced from excess glucose can be transported by the blood to fat storage tissues. Between meals, fat storage tissues release fatty acids into the blood, which are converted by the liver into substances to be used as energy sources for tissues.
• Plasma protein production: The liver produces proteins that can be found in the liquid portion of the blood, otherwise known as plasma.
• Production of blood-clotting factors: Stuff that's required to clot blood is also produced in the liver.
• Storage: The liver stores a variety of different substances, including but not limited to glycogen, iron and vitamins A and D.
• Toxin breakdown: Toxic substances, both those naturally produced in the body and those from external sources, are broken down into harmless substances in the liver.
• Hormone inactivation: Pretty self-explanatory. Some hormones can be inactivated in the liver.
• Heat production: The liver does a helluva lot of stuff, and many of those chemical processes produce heat. Thus the liver also helps to maintain constant body temperature.

Now aside from the above, why else is the liver important to the digestive system? That's because...

• the liver produces bile, which contains both bile salts and bile pigments, among other things. Bile salts are necessary for the mechanical digestion of fat, as they emulsify the fat (i.e. turn it into separate components that can't be mixed back together). This increases the surface area on which enzymes can break them down further. Bile pigments, however, aren't so useful. They're just the end result of red blood cells breaking down, and have no digestive function.
• Bile first travels from the liver to the gallbladder, a sac on the outside of the liver, where it is stored and concentrated before travelling to the duodenum via the common bile duct (the same duct that pancreatic juice travels through right before it gets to the duodenum). After being used in the intestines, the bile salts are nearly all reabsorbed into the blood to be reused, while the bile pigments just get pooped out later on.

Now that the liver and the pancreas are out of the way, it's now time to talk about...

The Small Intestine

The small intestine is only "small" in terms of diameter as compared to the large intestine. In reality, it's pretty long- around 6m. The first part, the aforementioned duodenum, is about 25cm long on its own- and that's just the top bit that goes from the stomach and circles around the pancreas in a sort of C shape before you get to the main part of the small intestine! It's the longest part of the alimentary canal, and it is where both digestion and absorption of food molecules takes place.

As if it wasn't already long enough, the inside of the small intestine has several modifications to increase its surface area and make it even better at digestion. (See Reaction Rates for a tiny bit more info on why increased surface area should make reactions more efficient.) In the small intestine, the mucosa and submucosa are not smooth, but rather have many small folds that extend into the interior. Additionally, the mucosa also has small bristle-like projections called villi (singular: villus), which are each about 1mm long, and these villi in turn have small projections called microvilli (and the whole idea of small bristles having even smaller bristles attached to them just reminds me of a certain other picture that I saw on the Internet). All these projections further increase the surface area of the insides of the small intestine.

The mucosa of the intestines, just like the stomach, contains glands that produce useful digestive juices. This time, it's not HCl-containing gastric juice, but rather intestinal juice. Other juices used in the small intestine are those produced in the liver and pancreas (namely bile and pancreatic juice, respectively). The intestinal juice, like pancreatic juice, contains many enzymes. After the pancreatic amylase in the pancreatic juice breaks down starch into disaccharides, the enzymes in the intestinal juice break it down further into monosaccharides. Each disaccharide requires a specific enzyme. To work out which enzyme you need, just take the name of the disaccharide and substitute an "a" in for the "o"- e.g. lactase breaks down lactose and sucrase breaks down sucrose. Enzymes in the intestinal juice can also further break down proteins and lipids. Proteins are broken down via peptidases (it breaks down the small peptides formed after the pancreatic protease has broken down longer chains) and lipids are broken down via lipases.

Aside from chucking a whole lot of digestive juices in, how else does the small intestine digest food? Well, you see, the circular muscle fibres alternately contract and relax in a movement called segmentation, which moves food back and forth, allowing it all to mix well. Let me explain:

Basically, the small intestine contracts at evenly spaced intervals along its length, creating small compartments. Then the middle of each "compartment" contracts, and each originally contracted muscle relaxes. This creates compartments in different places, and the food in the original compartments is sloshed around into the new compartments. Here is a terrible Paint diagram that might help you understand:

Once food has been digested enough, the small intestine begins its next job: absorbing the food! Each villus only has one layer of cells on its surface, allowing the digested food to diffuse into the blood capillaries in the villus, which surround a lymph capillary, called a lacteal. (According to InnerBody.com, a lymph capillary's primary function is to drain fluids from the tissues around it.) Some foods enter the blood capillaries, while others enter the lacteals. Aside from using diffusion to absorb food, another method of absorption is active transport, where the villi use energy to forcibly absorb nutrients against a concentration gradient (i.e. bring them from a place with a lower concentration of those nutrients to a place with a higher concentration).

Here's a quick run-down on how foods are absorbed:

• Monosaccharides- active transport. Enter blood capillaries
• Amino acids- active transport. Enter blood capillaries
• Fatty acids and glycerol- diffusion. Once in the villi, fatty acids and glycerol recombine to form triglycerides (each glycerol molecule is combined with three fatty acid molecules), before being coated with protein and entering lacteals as tiny droplets called chylomicrons.
• Fat-soluble vitamins- absorbed with the fatty acids and glycerol. Water-soluble vitamins- absorbed via diffusion into blood capillaries.
• Water- osmosis into the cells of the villi.

Food absorbed in blood capillaries then goes to the liver via the hepatic portal vein, where it's either removed for processing or retained in the blood to be carried to other body cells. Food absorbed into the lacteals are eventually emptied into the blood through veins in the upper part of the chest after being carried around by the lymph system.

Food left unabsorbed, on the other hand, continues on to...

The Large Intestine

The large intestine is only a quarter of the length of the small intestine, but it's thicker in diameter. It begins with a 6cm pouch called the caecum, which ends in the appendix. The other end of the caecum joins the colon, which has 3 parts: first is the ascending colon (which goes up), then the transverse colon (which goes from the person's right to their left), and finally the descending colon (which goes down towards the rectum and anus). Surrounding the anal opening is a circular muscle known as the anal sphincter which basically controls when you do a number 2.

The large intestine has no villi and secretes no enzymes, though it does contain bacteria, some of which break down many of the remaining organic compounds into simpler substances, releasing CO2 (carbon dioxide), methane (CH4) and hydrogen sulfide (H2S). (The book doesn't say anything but I wouldn't be surprised if this is where farts come from.) Some other bacteria produce vitamins, which are then absorbed through the walls of the large intestine into the blood. Other things that are absorbed into the blood from the large intestine include water and minerals. The absorption of water makes your poo more solid. If not enough water is absorbed, you'll end up with diarrhoea- more on this later.

The lining of the large intestine contains glandular cells which secrete a large amount of mucus. Despite this, stuff in the large intestine moves fairly slowly, taking 18 to 24 hours to pass through the colon (and remember, it's only covering a roughly 1.5m distance as opposed to the roughly 6m distance in the small intestines). Eventually peristalsis pushes it all the way to the rectum. When the outside anal sphincter relaxes and the rectum contracts (the former is voluntary, the second is automatic when the rectum is full), the body finally gets to eliminate waste in a process known as defecation. The result of this, as you well know, is a nice little pile of faeces containing all the crap (if you'll pardon the pun) that didn't get digested, such as cellulose, bacteria, bile pigments etc.

What Can Go Wrong?

Now we've covered the basics of the whole digestive system, we're going to look at a few common disorders of the alimentary canal. Make sure you're sitting down, and don't read this when eating!

Vomiting- The diaphragm and abdomen contract, forcing the stomach contents into the oesophagus and out through the mouth. Has many causes, from dizziness to sickness that causes irritation of the stomach.

Ulcers- Can occur in the walls of the stomach, duodenum or oesophagus. Usually result from pepsin and acid eroding part of the mucosa. If severe, can bleed or even make a hole right through the wall of the alimentary canal. Caused by the bacterium helicobacter pylori.

Indigestion- Has many causes, including eating too much or excessive production of HCl.

Constipation- Dry, hard faeces which are difficult to eliminate. This happens when the stuff in the large intestine moves slower than usual. Could be caused by a lack of roughage in the diet (roughage is the stuff that can't be digested but promotes the movement of food), lack of exercise or emotional problems.

Diarrhoea- An irritation in one or both of the intestines increase peristalsis, making the food move through so quickly that not enough water is absorbed, resulting in watery faeces. Can be caused by bacterial or viral infections.

Appendicitis- Inflammation of the appendix. One possible cause is blockage of the appendix by faecal matter or a foreign body, but there could be other causes.

Whew. That was a lot. I'm going to take a break now! TTFN!