F.1 STRUCTURE AND FUNCTIONS OF THE UPPER DIGESTIVE TRACT

• When food is ingested, it is put into the mouth, which is the beginning of the digestive tract (AKA alimentary canal or gastrointestinal tract).

• Chewing, a form of physical digestion, increases the surface area of the material, allowing digestive enzymes to conduct chemical digestion more efficiently.

• Three pairs of salivary glands release saliva into the mouth through ducts.

  • Glands like these that have ducts are known as exocrine

  • Saliva is a mixture of water and the enzyme salivary amylase (AKA ptyalin).

  • The secretion of water is essential throughout most of the digestive tract because its properties of lubrication and solvation help transport food materials. It is also a reactant for, and is consumed by, the hydrolytic reactions for digestion.

  • The enzyme in saliva breaks bonds between sugar molecules of starch, beginning its chemical conversion to maltose.

• The tongue is a muscle that facilitates chewing by pushing food between the grinding surfaces of the teeth. It also rolls sufficiently chewed food into a “lump” called a bolus and moves it to the back of the mouth in preparation for swallowing.

• Swallowing occurs reflexively (no thought required) as a result of the contractions of two layers of smooth muscles that line the pharynx region at the back of the mouth.

  • These muscle layers, known as the muscularis, extend the length of the digestive tract.

  • Usually, the muscularis has two layers oriented in different directions, as it does here.

  • The inner layer of circular muscles encircles and constricts the tube as they contract.

  • The outer longitudinal muscles run lengthways, shortening and extending segments of the digestive tract as they alternately contract and relax.

• Coordinated action of these muscles is required to move food materials through the lumen of the digestive tract.

• The serosa is a tough outer layer of epithelial (skin) cells covering these muscles.

• Next to the muscularis on the inside is the submucosa, a layer comprising connective tissue equipped with nerves and blood vessels.

• The innermost layer is the mucosa, another skin layer. This layer interacts directly with the food material.

• Both the esophagus and trachea start at the base of the pharynx and transport their intended contents to their respective destinations. In order to prevent the food materials from going down the trachea during swallowing, a ventral flap of tissue called the epiglottis reflexively covers its opening and exposes it for breathing.

• Alternating contractions of the smooth muscles result in peristalsis, a squeezing action that moves food materials, bolus by bolus, past the epiglottis and down the esophagus.

• The circular muscle at the base of the esophagus is more developed, creating a constriction called the cardiac sphincter that must relax and open for the contents of the tube to enter the stomach.

• This muscular ring also helps prevent the contents of the stomach from moving in the reverse direction, back up into the esophagus.

GASTRIC PROCESSES

• Gastric refers to the stomach, a large J-shaped organ.

• The muscularis of the walls of the stomach has an additional layer of smooth muscles, called the oblique muscles, that contract transversely, at an angle to both other layers.

• Equipped in this manner, the stomach’s muscularis adds another dimension to peristaltic activity, through churning the food materials and mixing them with a new set of digestive juices.

• The innermost layers of the stomach, the submucosa and mucosa, are folded into ridges called rugae.

  • This feature provides the stomach with the flexibility to expand when it fills.

  • The arrival of food material in the stomach is detected by nerve endings called mechanoreceptors .

  • These stimulate the release of the hormone gastrin from endocrine cells known as G-cells.

  • In contrast to exocrine cells, endocrine cells and tissues do not have ducts, and they deliver their hormone products directly into the bloodstream.

  • Like all hormones, gastrin circulates the body in blood and affects a target organ or tissue, which in this case are other cells in the gastric glands of the stomach’s inner mucosal layer.

  • These exocrine glands are elongate pouches of specialized epithelial cells that release a fluid called gastric juice into the interior of the stomach.

• Gastric juice contains water as well as HCl and pepsinogen, the inactive precursor of the digestive enzyme pepsin (a protease). The HCl component of gastric juice creates an environment with a low pH of about 2.5.

  • First, this promotes the unfolding of the tertiary structure of pepsinogen, cleaving off some of its amino acids and allowing the remainder to reconfigure as the enzyme pepsin. It also denatures salivary amylase, ending its activity.

  • Finally, it kills any bacterial growth present in the food material.

• The food material, mixed in this way, has a new set of properties.

  • No longer called bolus, it is renamed acid chyme (meaning “runny food mass”).

• Pepsin functions optimally in this low pH environment where it hydrolyzes peptide bonds between some amino acids, resulting in polypeptides, tripeptides, dipeptides, and possibly even some individual amino acids, depending on the amino acid sequence of the protein.

• In addition to the digestive glands, the mucosal layer is equipped with mucous-secreting cells. Mucous is a watery yet viscous fluid that protects the stomach tissues from the potentially corrosive effects of the acid chyme.

• The stomach temporarily stores food materials.

  • In an average adult, it can expand and retain about one litre of acid chyme, working it repeatedly and allowing the action of pepsin to have its maximum effect.

• At the posterior end of the stomach is another sphincter, the pyloric sphincter . Similar to the cardiac sphincter, this muscular constriction controls the passage of acid chyme, allowing only a small amount at a time to pass into the duodenom (first part of the small intestine).