The system that we use to digest food is called the digestive system. Organs making up this digestive system include the gastrointestinal tract (mouth, esophagus, stomach, small intestine and large intestine), liver, gallbladder and pancreas.
We eat when we get hungry or when we have an appetite. Hunger is a response to a lack of nutrients that is detected by the hypothalamus in the brain. Appetite, on the other hand, is a desire to eat certain foods. It is not a response to hunger but is influenced by various stimuli such as social gathering with friends, holiday traditions, emotional state, or just being around food that we like.
The amount of food that we eat at any given sitting may be affected by the food composition. For example, meals containing high proteins or high fiber content will make us feel full. This fullness is called satiety.
So how does the digestive system work? The process of digestion actually starts before we start eating. From the very moment we think about food, and especially when we see or begin to smell food, our brain sends signals to the stomach resulting in secretion of gastric juices. This is called the cephalic phase of digestion.
When we start eating, the food is mixed with saliva. Saliva contains amylase enzymes that converts complex carbohydrates like starch, to less complex carbohydrates such as maltose. Saliva also contains lingual lipase that starts the breakdown of lipids. Other components of saliva include mucus for lubricating food, bicarbonates to neutralize acids, and antibodies to kill bacteria.
We experience taste in our mouth as we eat. This helps us to enjoy the experience. There are only five tastes. These are bitter, sweet, sour, salty, and umami (savory). We experience these tastes as food molecules interact with taste receptors on our tongue. But, in addition to taste, we also experience flavor as we eat. For example we can differentiate a banana flavor from an orange flavor. This is due to volatile flavor compounds that interact with olfactory receptors in our nose. Therefore we taste with our tongue and detect flavor with our nose. That is why when we are stuffy, it’s hard to pickup on flavors and we do not enjoy the food we are eating that much.
After the partial digestion and lubrication of food in our mouth, the salivary mass is swallowed into our esophagus. Notice that when we swallow, we can’t breath at the same time since a valve called the epiglottis shuts off the respiratory tract to prevent food from getting into our breathing passage. At the same time the nasal cavity is shut off to prevent food from getting in the sinuses. Therefore you may also notice that you can’t swallow and inhale at the same time.
Once the bolus enters the esophagus, it makes its way quickly to the stomach in about 5-8 seconds. This is possible due to powerful muscles that contract and relax, pushing down the food in a process called peristalsis. Before entering the stomach a valve called the esophageal sphincter at the entrance of the stomach relaxes to allow the bolus in. Once in, the valve closes again to prevent gastric juice from escaping.
The stomach can hold about a gallon of food at a time, and will generally hold food for about 2 hours. Its main role is to denature protein to make them more easily digestible in the small intestine. It does this by secrete extremely acidic hydrochloric acid (pH = 1) from gastric gland cells into the stomach, in addition to pepsinogen. Pepsinogen is a precurser to pepsin which is the enzyme used to breakdown proteins. Pepsinogen is converted to pepsin when it is exposed to acid in the stomach. The membrane lining the stomach wall produces a film of mucous to protect the stomach and also secretes bicarbonates to neutralize the acid that is in direct contact with the stomach wall. The stomach churns the food into a soupy mass called chyme and then deposit it into the first part of the small intestine called the duodenum. It is able to do this when a valve at the base of the stomach called the pyloric sphincter opens up.
The small intestine is divided into 3 parts, that is, the duodenum, the jejunum and ileum. The small intestine is the longest part of the gastrointestinal tract, making up about ⅔ its length. This is where the majority of digestion and absorption of nutrients take place. Digestion and absorption occurs with the help of enzymes from the pancreas including proteases, amylases, and lipases. In addition, bile salts from the liver is secreted to emulsifies fats, making them more easily accessible to lipases.
So, how does absorption occur in the small intestine? The small intestine consists of a rough surface with a large surface area made up of tiny projections called villi. Each villi contain smaller projections called microvilli which helps to maximize nutrient absorption. Each villi is connected with blood vessels called capillaries to absorb amino acids and glucose. Villis also contain another type of vessel called lacteals which are used to absorb fats.
The capillaries are connected to the circulatory system which takes nutrients to the liver for processing. Lacteals on the other hand empties into the lympatic system. This system bypasses the liver and takes fat directly to the heart which then distributes fat to the tissues where it is needed.
I mentioned earlier that bile salts are secreted from the liver to emulsify fats. The liver has various other functions in digestion. It is also used to convert glucose to glycogen when there is excess glucose in the blood. This process is called glycogenesis. The liver also converts amino acids to glucose if blood glucose is too low. This process is called gluconeogenesis. It may also use amino acids to make proteins, and is also responsible for the manufacture cholesterol. Cholesterol is an important ingredient used to make vitamin D and hormones in the body. The liver also converts harmful substances to less harmful ones such as converting ammonia that is made in protein fermentation to urea which is then excreted in the urine. The liver detoxifies the body of other substances such as alcohol and drugs.
Jumping back to my discussion on absorption in the small intestine, it is important to note that absorption occurs in four ways.
- Passive diffusion – movement of nutrients from high concentration to low concentration. No energy is expended
- Facilitated diffusion – similar to passive diffusion, but require a protein channels to allow the passage of nutrients
- Active transport – transportation of nutrients with the use of energy. Energy is needed because nutrients are moving against the concentration gradient (low to high concentration)
- Endocytosis – absorption of bulk nutrients. This occurs when cell membranes engulf nutrients to form a vesicle. The vesicle breaks off and migrates into the cell where it opens up and releases the nutrients
After digestion and absorption in the small intestine, the remaining food goes to the large intestine. This part of the GI tract consist of the ascending colon, transverse colon, descending colon, sigmoid colon, rectum and anus. Functions of the large intestine include water absorption, bacterial fermentation of undigested materials, and production of vitamins (K, B12, thiamin, and riboflavin). Most of the bacteria that live in the GI tract, are in the large intestine. They are known as probiotics and play an important role in promoting digestive and immune health. These probiotic bacteria are enhanced when we eat foods that are rich in soluble fiber. Fiber-rich foods are called prebiotic foods. Fruits, vegetables and grains are a rich source of prebiotic nutrients.
Reference: Thompson, J. L., Manore, M. M. & Vaughan, L. A. (2017). The Science of Nutrition (4th ed.). San Francisco, CA: Pearson Education.
