Digestion and Absorption of Food- Part 1

In my past few posts regarding human bio, we've looked at different kinds of nutrients and why they are essential for our bodies. Today I'm going to talk about how these nutrients are actually broken up and absorbed into our bodies so that we can benefit from them. Since there is a lot to talk about here, I'm going to break this up into two posts. This one will cover the mouth down to the stomach.

Digestion involves both mechanical and chemical processes- that is, there are ways that food is torn apart by some physical means (e.g. chewing), and there are also chemical reactions that occur that break down the food molecules into smaller ones that can be absorbed by the body. Digestion is actually kinda complex which is why loads of organs are involved in the process. Most of these organs lie in the alimentary canal- the tube running from the mouth to the anus- including the pharynx, oesophagus, stomach and intestines. There are other organs that don't lie in this canal, but are considered to be part of the digestive system all the same, most notably the liver and the pancreas.

A basic run-down of how the digestive system works is this: you eat food, your body breaks it down through various processes across various organs, the food is then absorbed into your body, and then anything that can't be absorbed becomes bodily waste. Let's look at all of the various organs involved in order to get a closer look at how this works:

The Mouth

The mouth is where it all begins, because that's where the food generally goes in the first place (unless of course you have some serious illness that prevents you from doing so and you have to get nutrients via IV or something instead). Both physical and chemical digestion occurs here: physical via chomping on stuff with your teeth, and chemical via the chemicals in your saliva.

We have four kinds of teeth: incisors, canines, premolars and molars. They all serve different purposes- chisel-shaped incisors are for biting or cutting, conical canines are used for tearing and ripping into your food, and premolars and molars are used for crushing and grinding due to the shape of their crowns. The cusps on the crowns of molars fit into the depressions of the crowns of the molars on the opposite side creating sets of mini mortars and pestles.

Teeth have several different parts to them. The three main parts of teeth are the crown, the neck and the roots. The crown is the bit that's visible, the roots are the one or two long thin bits that extend into the sockets in the bones of your jaws, and the neck is between them. The outer layer of the tooth on the crown side is enamel. It's the hardest substance in the body and it's used to protect your teeth. My friend had a tooth that naturally didn't have any enamel, and she had to have it extracted because it was just too hard for her to eat anything without pain. The outer layer of the tooth on the root side is not enamel, but a different substance called cementum, which, true to its name (which looks like "cement" with two extra letters on the end), it helps to anchor the roots of the teeth to the jaw bone. Underneath the enamel and cementum is a thick layer called dentine, which comprises much of the tooth and is hard and bone-like. Underneath all of these layers of protection is a pulp cavity which contains nerves, blood vessels and a soft tissue called pulp (hence "pulp cavity").

Oh and a fun fact for you: baby teeth are also called deciduous teeth. Kinda like deciduous trees, except while a deciduous tree only loses its leaves for a few months before growing them all back, deciduous teeth don't grow back.

Now that I've rambled on about the technicalities of the tools needed for physical digestion, let's look at the tool required for chemical digestion: saliva! It's secreted into the mouth by three pairs of salivary glands. (I would normally make some crude joke about the unimaginative name here, but considering that there is so much stuff to just remember in human bio, I'm going to just be grateful that it's easy to remember.) It contains mucus, which not only lubricates the mouth but also helps to hold the food together in a lump, or bolus, for swallowing; a digestive enzyme called salivary amylase which breaks down starch (a carbohydrate, remember?) into smaller units (I don't think they get broken down fully into monosaccharides here, just into shorter chains than what they originally were); and antibodies, which kill many of the bacteria in your food. When food is dissolved, the taste receptors can be stimulated so you can taste just how nice- or not- your food is!

Once the food has been turned into a bolus by your saliva and your tongue, your tongue then pushes it into...

The Pharynx

The pharynx is a cavity in the back of the mouth. The pharynx doesn't really do much in the digestive process, but the food has to pass through here before it gets to the next part of the alimentary canal- the oesophagus!

Oh and one thing awesome about the human body is that it has a safety plan neatly worked out so that food won't get into your lungs or come out through your nose (unless you start laughing, of course. So try and keep a straight face when eating, even if people are telling jokes that are so bad that they're funny). Basically, when food is in the pharynx, a flap of tissue closes off the opening to the trachea (windpipe), and the soft palate (the back of the roof of the mouth) closes off the opening to the nasal cavity. It's not foolproof, but it does the job most of the time.

The Oesophagus

The oesophagus is about 23-25cm long, and has four layers (actually, the rest of the alimentary canal also has four layers which are the same or similar to those of the oesophagus).

The innermost layer is called the inner mucosa. It secretes mucus which lubricates the oesophagus and makes it easier for the food to move through.

The next layer is called the submucosa, which contains glands and connective tissue. Within the connective tissue are other things that connect different parts of the body- blood vessels, lymph vessels and nerves.

The third layer is consists of several different muscles. In the oesophagus (and presumably most other parts of the alimentary canal), there are only two: circular muscle (muscle fibres are arranged in a circle around the alimentary canal) and longitudinal muscle (muscle fibres are arranged along the length of the alimentary canal). When the bolus enters the oesophagus, the circular muscle contracts behind it. Then the next band of circular muscle contracts, pushing the bolus along, and so on and so forth. This "wave of constriction" which causes the food to move down the alimentary is also known as peristalsis. (If that explanation doesn't make much sense to you, imagine that you're trying to squeeze the last little bit of toothpaste out from the bottom of the tube. To do so, you would squeeze the end tightly, and then gradually move your fingers towards the opening at the end, squeezing as you go. The constricting action caused by your fingers causes the toothpaste to move towards the opening. Peristalsis is kinda the same, except muscles are causing constriction, forcing a bolus to move along.)

The outermost layer is called the serosa. It's made out of connective tissue.

The oesophagus passes through the diaphragm before opening out into...

The Stomach

The stomach has four layers, just like the oesophagus, but there are some differences:

1. The mucosa here specialises in secreting gastric juice. The mucosa has narrow "pits" known as gastric pits, which contain gastric glands (which are the glands actually responsible for secreting the gastric juice). Gastric juice contains HCl (hydrochloric acid), mucus and digestive enzymes.
2. Unlike the rest of the alimentary canal, the muscle layer contains a third kind of muscle: an oblique muscle layer. Having more layers means that the stomach has more different ways in which it can churn the food, mixing it with the gastric juice and turning it into a soupy liquid called chyme.

Since I'm in the mood for making lists at the moment, here are some more. First up is a list of two fun facts about the mucosa that didn't fit above:

1. When the stomach is empty, it's all folded up in "large, longitudinal folds"; when the stomach is full with food, the folds smooth out.
2. The mucosa can cover itself in a protective layer of mucus to stop the stomach from digesting itself.

Second is a list of enzymes and other important chemicals in the stomach:

1. Pepsin (gastric protease)- breaks proteins and other long chains of amino acids down into shorter chains (a.k.a. polypeptides). It's secreted in an inactive form called pepsinogen so that the protein won't be digested in the lining of the stomach. It is inactive in alkaline surroundings and requires contact with HCl to activate.
2. Hydrochloric acid kills many of the bacteria that enter the stomach.
3. Rennin is an enzyme only found in the gastric juice of infants. It makes milk proteins coagulate (i.e. makes something turn from liquid to something solid or semi-solid), allowing the stomach more time to break down the milk proteins- necessary for infants as that's all they eat. Rennin isn't in the gastric juice of adults, so don't try living off milk and milk alone.

Third is a quick run-down of what's happening in the stomach when you're digesting stuff. Or at least, this is what I think the process is. Please let me know if anything's in the wrong order, because something probably is:

1. Food enters stomach from oesophagus.
2. Folds of the mucosa smooth out as the stomach fills with food.
3. Gastric glands secrete gastric juice.
4. Stomach muscles move in various ways, churning around the food and gastric juice and turning it all into chyme; the stomach mucosa becomes protected by a layer of mucus.
5. Pepsinogen in the gastric juice is activated by the HCl to become pepsin.
6. Enzymes get to work- pepsin breaks down amino acids, HCl kills bacteria, other enzymes do whatever it is that they're doing
7. 2-8 hours later, the stomach contents are pushed out into the next part of the alimentary canal- the duodenum (basically the top bit of the small intestine). First to leave are the high-carb foods, followed by high-protein foods, and last but not least are the foods higher in fat.

Oh and one last bit- through all of that churning, you might expect something to leak out of the stomach early, but that doesn't generally happen. This is because the lower end of the stomach has a thick circular muscle called the pyloric sphincter which stops the contents of your stomach from leaving prematurely (not to be confused with the anal sphincter, which stops the contents of your rectum from leaving prematurely).

Next up: small and large intestines!