Digestive System Notes


the digestive tract:-begins with the oral cavity and includes the pharynx, esophagus stomach, small intestine, large intestine and anus. Be able to label the parts in the diagram below


Oral cavity- teeth tear and grind food and the tongue mixes food with saliva. Functions of saliva


i. It contains an enzyme (salivary amylase) that breaks down starch to maltose


ii. It moistens the food and forms a food ball


iii. It lubricates the esophagus so swallowing in made easier


Pharynx and esophagus – swallowing


The top of the esophafus is under voluntary control. Eg. Swallowing a pill is a conscious effort


-The rest of the esophagus is under involuntary control, - swallowing is a reflex action along the entire digestive tract. Swallowing is such an involuntary muscle called the cardiac sphincter. This prevents the contents of the stomach from entering the esophagus (reflux)


Stomach – holding and churning


-Gastric glands are found on the surface of the stomach, they produce gastric juice


-Gastric juice contains two substances, HCl (hydrochloric acid) and pepsinogen


Function of HCl in the stomach


-kills bacteria and other living cells in food


-causes pepsinogen to be converted its active form pepsin


-denatures proteins which make it easier for enzymes to act on them


causes the stomach to have a low pH which is optimum pH for pepsin to function


Function of pepsinogen (inactive pepsin plus HCl yields pepsin)


-pepsin causes proteins to be broken down into peptides


-after eating a meal high in protein, the hormone gastrin is produced by the stomach


The secretion of gastric causes increased secretion of pepsinogen


The churning action of the stomach, the action of HCl, mucus, and pepsin eventually converts the food into a semi-liquid mass called acid chime. The acid chime gradually empties into the small intestine through a valve called the pyloric sphincter. Recognize the diagram of the stomach below.


Small intestines – complete digestion and absorption of food


-upper part (duodenum) is most active in digestion and the lower parts are concerned with absorption


Intestinal juice (produced by digestive glands in the small intestine) contains:


-peptidases, which digest peptides into amino acids (finish of carbohydrate digestion)


-maltase, which digest maltose into glucose (finish of carbohydrate digestion)


Absorption


The features of the small intestine that increase the surface area are:


a. a long convoluted organ


b. the lining of th the intestine is thrown into numerous ridges and folds


c. small fringelike projection called villi cover the inner surface of the intestine


The features of the small intestine that facilitate absorption of nutrients are the villi are richly supplied with blood capillaries and lacteals. The capillaries absorb the monosaccharide, amino acids, and peptides into the blood system. The lacteals absorb fats


To facilitate the absorption of nutrients the fold. The large, hollow organs of the digestive system contain muscle that enables their walls to move. The movement of organ walls can propel food and liquid and also can mix the contents within each organ. Typical movement of the esophagus, stomach, and intestine is called peristalsis. The action of peristalsis looks like an ocean wave moving through the muscle. The muscle of the organ produces a narrowing and then propels the narrowed portion slowly down the length of the organ. These waves of narrowing push the food and fluid in front of them through each hollow organ.


The esophagus is the organ into which the swallowed food is pushed. It connects the throat above with the stomach below. At the junction of the esophagus and stomach, there is a ringlike valve closing the passage between the two organs. However, as the food approaches the closed ring, the surrounding muscles relax and allow the food to pass.


The food then enters the stomach, which has three mechanical tasks to do. First, the stomach must store the swallowed food and liquid. This requires the muscle of the upper part of the stomach to relax and accept large volumes of swallowed material. The second job is to mix up the food, liquid, and digestive juice produced by the stomach. The lower part of the stomach mixes these materials by its muscle action. The third task of the stomach is to empty its contents slowly into the small intestine.


Several factors affect emptying of the stomach, including the nature of the food (mainly its fat and protein content) and the degree of muscle action of the