Structure and Function of the Human Digestive System

Structure and Function of the Human Digestive System
By Caitlin Ong

Organs of the Digestive System

Mouth

Structure: The mouth consists of the upper and lower jaw, that contain gums which hold teeth, the roof of the mouth is the palate and the tongue occupies the floor of the mouth. The salivary glands have ducts that open in the mouth.

Function: The mouth is where the first step of digestion begins. Known as mechanical digestion, this involves chewing with your teeth to break the food into smaller pieces. The tongue moves the food around to assist chewing and together with saliva, the food is lubricated for ease of swallowing.

Salivary glands

Structure: There are three main parts of salivary glands: the parotid, the submandibular and the sublingual glands.

Function: Salivary glands produce saliva which keeps parts of the digestive system moist. Saliva contains the enzyme amylase which helps chemically break down starch into glucose. Food and saliva form a smooth lump called a bolus, water and mucus help lubricate it from the mouth as it moves through the oesophagus and down into the stomach.

Oesophagus

Structure: The oesophagus is a muscular tube, about 25cm long and extends from the mouth to the stomach.

Function: By contracting using circular muscles around your oesophagus when you swallow, a movement called peristalsis, it pushes the bolus along the oesophagus to the stomach. There is a valve just before the oesophagus connects to the stomach, where there is a “zone of high pressure,” called the lower oesophageal sphincter, this valve is meant to keep food from passing backwards from the stomach to the oesophagus.

Liver

Structure: The liver is on the upper right side of the abdomen, below the diaphragm. It weighs around 1.5kg in the average adult human body and is the largest internal organ.

Function: The main function of the liver within the digestive system, is to process nutrients absorbed from the small intestine. The liver converts glucose into glycogen for storage in the liver and muscles, it can be converted back into glucose when needed. Bile is produced from the liver and helps break down fats.

Stomach

Structure: The stomach is a J-shaped organ that contains food while it is being mixed with enzymes.There are two sphincters in the stomach, one is located at the top of the stomach and stops acid and partly digested food from rising up into the oesophagus, the other is at the bottom of the stomach and protects the lower digestive tract from acid.

Function: Cells in the lining of the stomach, secrete two litres of gastric juice a day which contain the enzyme pepsin and hydrochloric acid that continue the process of breaking down food.

Gall bladder

Structure: The gall bladder is a pear-shaped pouch that sits under the liver and stores up to 50mL of bile produced by the liver.

Function: The bile travels through a channel called the cystic duct into the gall bladder. When eating, the gall bladder contracts, releasing bile to the small intestine.

Pancreas

Structure: The pancreas is a gland organ shaped like a flat pear that is located in the abdomen, behind the stomach and small intestine. It is about 15cm long.

Function: Once food enters the stomach, it is pushed into the duodenum and the pancreas produces digestive juices and enzymes that pass through a small duct in to the duodenum. These enzymes break down protein, fats and carbohydrates. The pancreas also produces the hormone insulin, which controls blood sugar levels.

Small Intestine

Structure: The small intestine is around 5m long, making it the longest section of the digestive tract and is only 3-4cm wide - hence its name. It is made up of three segments, duodenum, jejunum and ileum.

Function: Its job is to help digest carbohydrates, fats and proteins using enzymes from the pancreas and bile from the liver. Peristalsis is used in this organ when moving food and mixing it with digestive secretions.

Duodenum

Structure: The duodenum is one of the three parts of the small intestine and is a C-shaped organ located at the start of the small intestine below the stomach where it curves around the pancreas. It is the shortest section of the small intestine, and is about 25-30cm long. The duodenum is divided into four sections: superior, descending, horizontal and ascending duodenum.

Function: Tubes from the pancreas secrete pancreatic juice and bile drained from the liver and gall bladder enter the duodenum. These secretions help in the digestion of food. The duodenums’ main function is to receive chyme which is a combination of partly digested food and stomach acids. The chyme from the stomach released into the duodenum continues digestion and is done with the help of digestive enzymes and intestine juices secreted by the chambers in the intestinal wall. The duodenum regulates the rate of gastric emptying, (stomach emptying), which is the process of food transferring from the stomach to the duodenum. The duodenum triggers hunger signals with the help of hormones that are produced and released by the duodenal epithelium. Duodenal epithelium includes cells that secrete two hormones known as secretin and cholecystokinin. When excess acid is in the small intestine or duodenum, hormone secretin is released. Secretin and cholecystokinin help the secretion of bile and pancreatic juice. The duodenum also absorbs the nutrients and it does it even more than the stomach.

Large Intestine

Structure: The large intestine is a 1.8m long muscular tube that connects the small intestine to the rectum. The large intestine is made up of the caecum, the ascending colon, the transverse colon, the descending colon and the sigmoid colon which is connected to the rectum.

Function: The large intestine is responsible for processing waste so that emptying the bowels is easy. Stool (faeces) that is left over from the digestive process, is passed through the ascending colon and transverse colon by using peristalsis and absorbing water through this process in a liquid state. The waste becomes firm when it reaches the sigmoid colon, it is stored there until it is emptied into the rectum once or twice a day. It usually takes about 36 hours for stool to get through the colon. The bacteria and food debris in the stool perform several functions: synthesising vitamins, processing waste products and food particles and protecting against harmful bacteria. When the descending colon becomes full of stool, it empties into the rectum.

Rectum

Structure: The rectum is about a 20cm compartment that connects the colon to the anus.

Function: The rectum holds stool from the colon and has nerve sensors that send messages to the brain, producing the sensation or urge to pass out the stool.

Anus

Structure: Being up to 5cm long, this is the last part of the digestive tract, and consists mainly of an internal muscle sphincter and an external muscle sphincter that is connected to the pelvic floor muscles.

Function: The anus controls the release of rectal contents. When you are asleep, the internal sphincter is tightly closed to hold in rectal contents. When the brain receives the urge to pass out the stool, the external sphincter contracts to help hold in the stool until you reach the toilet. The sphincters then relax to allow the stool to be passed out and the rectum is emptied.

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