Dominic Areago

Digestion

Digestion is an extremely vital process required in everyday life. It is beyond the actual ingestion of food material into the mouth. Digestion is defined as a process that takes place in the digestive tract, where food is broken down mechanically and chemically into a substance that is readily absorbed and assimilated in the body1. Below is a diagram of the digestive tract, otherwise known as the alimentary canal. Every part of it plays a vital role in the break down of food material, whether the contribution is mechanical breakdown as in the churning of the stomach or chemical, in the case of enzyme activity.

The first part of the digestive tract is the mouth; this is comprised of parts A, B, J and D on the diagram. The mouth is made up of various parts, these all contribute towards digestion. The teeth help tear food up, into smaller pieces so the body can deal with it much easier.  The salivary glands lubricate food by secreting saliva, which contain amylases, which work chemically to break down starch. The other parts of the mouth support digestion by ensuring the food do not enter the trachea. The most important is the epiglottis, which is a flap of cartilage that covers the trachea when food is entering the oesophagus.   The oesophagus is a passageway that carries food from the mouth to the stomach. Food moves along the tube by muscle contractions called peristalsis.  The stomach is seen as the core of digestion where, physical and chemical digestion both take place; gastric juices containing enzymes and HCl work on the food material whilst muscles churn the food. The stomach has sphincters at both the entrance and exit of the stomach (cardiac and pyloric respectively) to contain food and release it in small amounts to the rest of the digestive tract. The small intestine compromise of two main parts, the duodenum and ileum, the duodenum is where the remainder of digestion takes place and the ileum is where absorption takes place. The small intestine itself does not secret enzymes or digestive juices; rather it receives its digestive fluids from other parts of the digestive tract. Pancreatic juice and bile from the liver are poured into the duodenum. The pancreas produces a digestive juice that contains several enzymes that work on proteins, amino acids and fatty acids. The pancreatic juice is contains sodium hydrogen carbonate to neutralize the acid liquid from the stomach2. The liver is responsible for production of bile, which is stored in the gall bladder. Bile enters the duodenum through the bile duct. Its job is to emulsify fats. The ileum’s major role is absorption of the remains of digestion. Its structure is specially adapted for this. Folds and finger-like projections called villi are present to increase the surface area for absorption. The thin epithelial lining and dense capillary network make it the necessary place for absorption to take place. Remnants of digestion such as glucose, fats and amino acids are absorbed here. The large intestine also known as the colon is segmented into parts, (G, Q, R and N on the diagram). The colon’s function is to retain as much fluid in undigested matter as possible. No enzymes are present in the colon; bacteria reside there to digest parts of fibre, which form into fatty acids that the colon can absorb. After absorption of fluids take place the remains of a semi solid called faeces are passed through the rectum and expelled at regular intervals by peristalsis through the anus, this is known as defecation. The caecum and appendix are responsible for digestion of plant cell wall material. Because humans have a hard time digesting plant material these organs remain quite small and useless in humans.

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Digestion of a meal such as potato chips and fish coated with batter, fried in peanut oil would begin right at the mouth, because of the chemical composition of the food; that being made of carbohydrates, protein and fats; the foundation of all food material.

As the mixture of food enters the mouth this is where food processing begins, this is known as ingestion. Once the fish and chips enter the mouth, it is chewed and mixed with saliva. Chewing breaks the food down into smaller manageable bits; it also increases the surface area for enzymes to work on. ...

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