Whenever we have a meal, it travels through our digestive system a journey of 8 meters. The meal undergoes a process by which it is crushed, nutrients are broken down, chemical reactions take place, and energy and building material are released. To summarize all this process by one word, we can say it gets “digested”.
The pathway of this process of digestion:
1- The mouth: It begins in the mouth, where the food is chewed to become accessible to digestive enzymes. Salivary glands found there secrete saliva that is used both to lubricate food making it easier to swallow and also begins breakdown of starch by a certain enzyme found in the saliva called Amylase.
2- Esophagus: After swallowing, by contractions called peristalsis, food is swallowed down the esophagus bypassing the trachea by a certain mechanism done by a flap of tissue acting as a “trapdoor”.
3- Stomach: food spends 4 hours in the stomach where they are reduced into liquid. The stomach is protected by a thick coating of mucus that protect it against the highly acidic pH (between 1.5 and 2.5) that is caused by hydrochloric acid (HCl) and pepsin that are responsible for the breaking down of proteins to small chains of amino acids. Lipase is also present, and thus a small amount of lipid digestion takes place in the stomach.
4- Small intestine: tightly coiled within abdomen and is six meters long. The remaining carbohydrates and proteins are digested here, and almost all lipids are too. The critical function of the small intestine is that it absorbs the nutrients released by digestion and then transfers them to the blood.
5- Pancreas and Liver: these two organs secrete chemicals and enzymes that cause additional food breakdown.
Pancreas has 3 roles:
*regulates blood sugar through the secretion of insulin and glucagon
*Secretes a variety of digestive enzymes
* Secretes bicarbonate which helps neutralize stomach acids so that lipid digestion can proceed and in order for the acids not to digest the walls of small intestine.
Liver on the other hand:
*Secretes bile that emulsifies fat globules in order to be broken down and absorbed
*Removes excess glucose from the blood, storing it as glycogen
*Detoxifies many substances, such as producing enzymes that break down alcohol and drugs.
6- Large intestine: stores the indigestible material and eliminates them with the help of water and fibers from the diet. Some vitamins such as B-vitamins and vitamin K are produced here by beneficial bacteria that play a role in nourishing the host and resisting disease.
7- Rectum and anus: The rest of the food, especially undigested food such as bacteria and cellulose, then pass out of the body in the form of feces through the rectum and anus.
Questions:
a) Define Digestion
b) What is the critical role of the small intestine?
c) What is the role of each of the pancreas and liver in the process of digestion?
