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Week 13: Digestive System, image, image, image, image, image, image, image…
Week 13: Digestive System
Mouth
The mouth is where mechanical digestion takes place - teeth chew food to break it into smaller pieces and increase the surface area to volume ratio.
Amylase enzymes in saliva start digesting starch into maltose.
The food is softened into a bolus (semi-solid lump) by the tongue and lubricated by saliva so it can be swallowed easily.
Liver
The liver has several important functions. The blood vessels from the alimentary canal pass through the liver by means of the hepatic portal vein. The liver then adjusts the composition of blood before it is released for general circulation.
Functions of the liver:
1. Regulation of blood glucose
- The liver removes any excess glucose from the blood and stores it as glycogen. When the level of glucose in the blood falls in between meals, the liver converts some of the stored glycogen into glucose and releases it into the bloodstream. The two hormones that regulate blood glucose levels are insulin and glucagon.
2. Production of bile
- The cells in the liver produce bile continuously. It is stored in the gall bladder before being discharged into the small intestine.
3. Deamination
- Excess amino acids are converted into glycogen, the amino part is removed and it is converted into urea, which is later excreted by the kidney.
4. Storage of iron
- Millions of red blood cells are broken down everyday and the iron from their haemoglobin is stored in the liver.
5. Detoxification
- Poisonous compounds produced in the large intestine by the action of bacteria on amino acids enter the blood. The liver converts them into harmless substances and are later excreted in the urine.
Stomach
The stomach has strong, muscular walls and the muscles contract and relax to churn the food and mix it with the enzymes and mucus. The mixture is called chyme.
Protease enzymes start to chemically digest protein.
Hydrochloric acid is present to kill any bacteria present in the food and provide the optimum pH for protease enzymes to work.
The stomach can store food for a long time. After a few hours, the sphincter at the bottom of the stomach opens and lets the chyme into the duodenum.
Intestines
Small intestine
The first section of the small intestine is the duodenum, where food coming out of the stomach finishes being digested by enzymes produced in the small intestine and enzymes secreted from the pancreas.
The pH of the small intestine is slightly alkaline, around pH 8 or 9.
The third section of the small intestine is the ileum, where absorption of remaining digested food molecules takes place.
The ileum is long and lined with villi to increase the surface area and allow for faster absorption to take place.
The second section of the small intestine is the jejunum and it's function is to absorb sugars, amino acids and fatty acids.
Large intestine
Water is absorbed from the remaining material in the colon to produce faeces.
Faeces is stored in the rectum and removed through the anus.
Oesophagus
The oesophagus is the part of the alimentary canal along which food travels from the mouth to the stomach.
A piece of cartilage, called the epiglottis, covers the entrance to the trachea to stop food from going down into the lungs.
Smooth muscles contract behind the bolus to prevent it from being squeezed back into the mouth. Rhythmic, unidirectional waves of contractions work to rapidly force the bolus into the stomach.
Pancreas
The pancreas is a cream-coloured gland, lying just underneath the stomach. A tube called the pancreatic duct leads from the pancreas into the duodenum.
Pancreatic juice, which is a fluid made by the pancreas, flows along this tube. This fluid contains many enzymes, including amylase, protease and lipase.
These enzymes do not work well in acid environments, but the chyme from the stomach contains hydrochloric acid. Pancreatic juice contains sodium hydrogencarbonate which partially neutralizes the acid.
Gall bladder
The gall bladder is a small, hollow organ that lies beneath the liver.
Bile, a yellowish green alkaline, watery liquid that helps to neutralize the acidic mixture from the stomach, is made in the liver and stored in the gall bladder.
Bile flows to the duodenum along the bile duct.
Salivary glands
Salivary glands are a group of organs present in the mouth that secretes saliva.
Saliva has many benefits for the oral cavity and health in general. Saliva is composed of a variety of electrolytes, including sodium, potassium, calcium, magnesium, bicarbonate, and phosphates.
Saliva is mostly composed of water, so it moistens the mouth and helps compact food into softened particles to form the bolus.
Saliva contains the enzyme amylase which begins the breakdown of starch into maltose, a dissacharide.
