The digestive system is a continuous tube composed of 6 primary regions.
i. Mouth ii. Pharynx & Oesophagus iii. Stomach iv. Pancreatic & biliary Secretions
v. Small Intestines vi. Large Intestines
The digestive systems primary role is to transfer nutrients, water and electrolytes into the internal environment and to circulate it via the circulatory system.
The Digestive system is primarily composed of smooth muscle and therefore under involuntary control. Areas of control such as the sphincter are composed of skeletal muscle. With in each area of the tract there are specialised tissues such as the salivary glands in the mouth. The digestive system is aided by accessory structures that are not considered to be part of the tract but secrete in the tract to aid in digestion. (Salivary glands, pancreas, liver and gallbladder.)
Each section of the tract will play a role in the 4 process that define the role of this system.
i. Digestion – Breaking down and dissolving process ii. Secretion iii. Absorption iv. Motility
The digestive system is primarily controlled by a subdivision of the sympathetic nervous system known as the Enteric Nervous System (ENS). The ENS is also known as the little brain. To a lesser degree the CNS also controls digestion. This combination of Controllers results in the digestive system receiving both neural and hormonal input.
Chewing begins the chemical digestion of food, breaking it down into smaller portions (Mastication). When food is masticated and it forms a ball of the food mixed with saliva it is known as a bolus.
Salivary Secretion The body produced between 1-2L of saliva a day. Salivary secretion is under autonomic (neural) control. Both sympathetic and parasympathetic innervations results in secretion. Parasympathetic innervations leads to serous (watery, clear to pale yellow) saliva production. Sympathetic innervation leads to thick mucus like saliva production (less volume). Saliva is produced in 3 locations.
i. Sublingual Gland – Secretes mucous like saliva ii. Parotid Gland – Secretes serous saliva iii. Submandibular Gland – Secretes both thick and serous saliva.
Innervations can be triggered by many stimuli sources including smell, sight, taste and even thought. Saliva softens and lubricates the food. It also gives us the ability to taste the food. Salivary amylase begins the digestion of carbohydrates in the mouth to a small extent. Components within the saliva also play a protective role utilising their antibacterial properties. There is no absorption in the mouth.
Pharynx and Oesophagus
Swallowing is a reflex reaction. Swallowing pushes the bolus of food or liquid into the oesophagus. Both the pharynx and oesophagus are under the control of the central nervous system and the Autonomic system. While the idea of swallowing is simple the process is complex.
Pressure is created when the tongue pushes the bolus against the soft pallet and back of the mouth. The Epiglottis then folds down, sealing the larynx and trachea from the food/liquid. At the same time the soft pallet seals the nasal cavity. At this point respiration is inhibited and the upper oesophageal sphincter relaxes allowing passage of the bolus to the oesophagus. Waves of peristaltic contractions then push the bolus towards the stomach aided by gravity. The sphincter is composed of circular orientated muscle (smooth or skeletal). Contraction of the sphincter leads to reduced lumen size. The oesophagus is composed of both smooth and skeletal muscle, although the ratios are dependent on the region of the oesophagus. The First 1/3 is composed primarily of skeletal muscle, the middle 1/3 is a mixture of both muscle types and the final 1/3 is primarily smooth muscle.
Peristalsis is the movement of the tubular structures characterised by waves of alternate circular contraction and relaxation. This results in the contents being moved forward. This sort of movement…