Digestion

Digestion is the process by which the food we consume is broken down into its smallest constituent componenents. This allows it to be absorbed by our gastrointestinal tract and used to provide nourishment; for growth; for cell maintenance; to provide energy.
Digestion occurs in the gastrointestinal (GI) tract. This is a series of organs which form a convoluted tube that stretches from our mouth to our anus. The process of digestion involves three main tasks: breaking the food down into small pieces both mechanically and chemically; mixing the food with digestive chemicals; moving the food along the tract.
Food is transported through the gut by successive wave-like muscular contractions in a process called peristalsis. This occurs due to the presence of two different layers of muscle that are present throughout the gut; an inner circular layer and an outer longitudinal layer. These are antagonistic (when one contracts, the other relaxes). When the circular layer of muscle contracts, the lumen (inner cavity or space) becomes narrower and longer. This pushes the food forward. When the longitudinal layer contracts, the lumen becomes wider and shorter to allow food to enter. This is what allows us to eat while upside down.

The digestive process

Mouth
Digestion begins in the mouth where our teeth break the food down into smaller pieces and mix it with saliva. Saliva in our mouth contains an enzyme called amylase which catalyses a chemical reaction in which starch is broken down. Saliva also functions to moisten the food which lubricates it and facilitates mixing.
When we swallow, the food is propelled through the pharynx (throat) into the oesophagus in a process controlled by our (involuntary) autonomic nervous system. Food is prevented from entering the airway by the epiglottis, a piece of tissue which blocks the trachea.

Oesophagus
The oesophagus is the tubular organ that carries food to the stomach by peristaltic contractions. The peristaltic wave opens the sphincter at the gastric-oesophageal junction, allowing passage of food into the stomach.

Stomach
The muscle in the upper part of the stomach can relax to accept large amounts of food. Digestive glands in the stomach mucosa secrete gastric acid (which kills bacteria in food) and pepsinogen (an enzyme which, when activated by acid, helps to digest protein into its constituent amino acids). A layer of mucous lines the stomach to protect it from the acid. A substance called intrinsic factor is also secreted, which binds to vitamin B12 enabling its absorbtion by the intestine. Alcohol is absorbed in the stomach.
The muscle in the lower part contracts to mix the chyme (partially digested food) and move it into the duodenum (the first part of the small intestine) via the pyloric sphincter. Gastric (stomach) emptying is inhibited by a hormone called cholecystokinin which is secreted in response to high levels of protein and fat in the stomach.

Small intestine
The majority of digestion takes place in the intestine. In order to maximise the surface available for absorption of nutrients, the mucosa is arranged into small finger like projections called villi.
The pancreas and liver are two organs which are not part of the gastrointestinal tract, but secrete chemicals which are essential to the digestive process - the liver and the pancreas. The pancreas secretes several enzymes such as amylase, lipase and proteases which are activated in the intestine. These break the different nutrients down enabling their absorption through the intestinal mucosa. The liver synthesises bile, which emulsifies (breaks down) fat in the small intestine to enable as much of it as possible to bind to lipase. Between meals, bile is stored in an organ called the gallbladder.
The duodenum (first and shortest part of the small intestine) is responsible for the absorption of vitamins A and D, glucose, iron, calcium and fatty acids.
The jejenum is the second part of the intestine, and absorbs glucose, galactose, monosaccharides, disaccharides, vitamins A, B (most), C and D, protein, amino acids and fatty acids.
The ileum is the longest and last part of the small intestine. Bile salts, protein, amino acids and vitamin B12 are absorbed here. This part of the small intestine also acts as a reserve, as it can absorb additional nutrients if required.
Blood carriies the absorbed nutrients to the liver via the portal vein where they are filtered and processed.
Parts of the food which are not absorbed, such as fibre, pass into the colon (large intestine).

Colon
The colon is about one and a half metres long and functions mainly to store faecal matter and transport it to the rectum where it is expelled as faeces. However the colon absorbs water, short chain fatty acids and electrolytes such as sodium, potassium and chloride.

Regulation

Digestion is regulated by a complex array of interconnecting systems which respond to different stimuli such as the types of food we have eaten, or the situations we find ourselves in. There are two main types of regulation, hormonal and neural.

Hormonal Regulation
Gastrin increases gastric acid production, stimulates growth of GI tract mucosa and increases motility of the stomach
CCK (cholecystokinin) increases secretion of digestive chemicals by the pancreas, increases contraction (emptying) of the gall bladder, stimulates growth of pancreas and gallbladder and reduces emptying of the stomach.
Secretin stimulates secretion of bicarbonate by the pancreas, which neutralises gastric acid in the intestine; it reduces gastric acid secretion and stimulates pepsin secretion; it also stimulates the liver to produce bile.
Somatostatin reduces secretions by the stomach and pancreas; it inhibits gallbladder contraction.
Ghrelin stimulates appetite. It is produced in the stomach and upper intestine in the absence of food.
Peptide YY is produced in response to food intake and inhibits appetite.

Neural Regulation
There are two nervous systems that regulate the digestive process; the autonomic (extrinsic) nervous system and the enteric (intrinsic) nervous system.
The autonomic system is subdivided into the parasympathetic and the sympathetic nervous systems. The parasympathetic nervous system secretes a chemical called acetylcholine which increses the contraction of the muscle of the gut, resulting in more rapid transit of the food through the tract. It also increases secretion of chemicals. The sympathetic nervous system produces a substance called adrenaline which relaxes the muscle (reducing transit time) and decreases the flow of blood to the GI tract.
The enteric (intrinsic) nervous system is located within the wall of the GI tract. It is organised into plexi (networks); Meissner's (submucosal) plexi regulate secretions, absorption and blood flow; Auerbach's (myenteric) plexi coordinate motility of the GI tract and are located between layers of muscle.
These plexi are connected to the autonomic (parasympathetic) nervous system.