The mammalian digestive system consists of the alimentary canal ( complete digestive tract) and various accessory glands that secret digestive juices into the canal through the ducts.  The food is moved along the tract by the contraction of smooth muscles in the walls of the canal.  These rhythmic contraction waves are called peristalsis.  The regulation of  passage of material from one chamber to another within the canal is controlled by ring-like valves called sphincters.


The accessory glands of the mammalian digestive tract are three pairs of salivary glands, the pancreas, the liver, and its storage organ the gall bladder.


Now lets  follow a meal through the human digestive canal. 


The Oral CavityMouth


Physical and chemical digestion of food begin in the mouth.  During chewing the food is made easier to swallow and the food’s surface area is increased.   The presence of food in the mouth triggers a nervous reflect that causes the salivary glands to secret saliva into the mouth.  Often saliva is secreted due to a smell or sight.  In humans , almost a liter of saliva is secreted into the mouth daily.  That may sound like a lot, but for horses it is gallons!  Human saliva contains mucin which is a slippery glycol-protein which protects the mouth from abrasion and lubricated  the food for swallowing.  Saliva contains buffers that help prevent tooth decay by neutralizing acids in the mouth.  There are also antibacterial agents  in the saliva.   The digestion of carbohydrates begins in the mouth.  Saliva contains salivary amylase, a digestive enzyme that hydrolyses starch ( a glucose polymer from plants and glycogen  ( a glucose polymer from animals).  This enzyme breaks down the carbohydrates into smaller polysaccharides and the disaccharide maltose.  The tongue located in the oral cavity helps to manipulate the food during chewing and shapes the food into a ball called the bolus which it pushes to the back of the mouth and into the pharynx.


The Pharynx


Out throat is the pharynx which leads to both the esophagus and the windpipe ( trachea).

When a human swallows the to of the windpipe moves up so that its opening , the glottis, is blocked by a flap of cartilage called the epiglottis.  This helps to ensure that the bolus enters the esophagus.


The Esophagus


The esophagus channels food from the pharynx to the stomach.  The muscles in the walls move the food.  The first part of swallowing is a voluntary act but then the involuntary waves of contraction of the smooth muscles take over.  Salivary amylase continues to hydrolyze starch as the bolus passes through the esophagus.


The Stomach


The stomach is located on the left side of the abdomen , just below the diaphragm.  Since its walls are elastic and it has accordion-like folds, the stomach can hold up to 2 liters of food and water.  The walls of the stomach secret gastric juices, a digestive fluid that mixes with the food.  Since this fluid has a high percentage of HCl , its pH is about 2 which is acidic enough to dissolve iron nails.   The functions of this acid include : 1. disrupt the extra-cellular matrix that binds cells together 2. kill most bacteria in the food.

Also present in this gastric juice is pepsin, an enzyme that begins the hydrolysis of proteins by breaking peptide bonds.  Cells of the stomach wall are protected from pepsin by a coating of mucus.  The epithelial cells which generate this mucus are eroded by the acid  and therefore the stomach lining must be replaced by mitosis every three days.


Much of the time the stomach is closed off at both ends.  The opening from the esophagus to the stomach is called the cardiac orifice.  This opening will open when a bolus is ready to pass.  Occasionally there will be backflow of acid chime from the stomach  into the lower ed of the esophagus, causing heartburn.  If heartburn persists,

An ulcer could develop in the esophagus.  The opening from the stomach to the small intestine is the pyloric sphincter.  This opening regulates the passage of chime into the small intestine.   This happens a squirt at a time, taking about 2-6 hours to empty the stomach after a meal.


Small  Intestine


The small intestine is the major organ of digestion and absorption.   The small intestine is the longest section of the digestive tract at more than 6 meters in length.  It is referred to as the small intestine because its diameter is smaller than the large intestine.  Most digestion and absorption happens in this organ.   The pancreas, liver and gall bladder  participate in digestion.


The first part of the small intestine is called the duodenum ( about 25 cm in length). This is where the chime from the stomach mixes with digestive juices from the pancreas, liver gall bladder and gland cells from the intestinal wall.   The pancreas produces bicarbonate which helps to offset the acidity of the stomach.   The liver produces bile which is stored in the gall bladder.  Bile does not contain digestive enzymes.  It contains bile salts which act as detergents and aid in the digestion and absorption of fats.  Bile also carries wastes from the liver ( where old red blood cells are destroyed).


Carbohydrate digestion

Pancreatic amylases hydrolyze starch, glycogen, and smaller polysaccharides into disaccharides including  maltose.  The enzyme maltase completes the digestion of maltose by splitting it into two molecules of glucoseSucrase hydrolyzes sucrose ( table sugar).  Lactase digest lactose ( sugar found in milk).  As people get older, they have less lactase in their system.  These disaccharidases are in the membranes of the intestinal epithelial where the final monomers are absorbed by the blood.

Protein digestion

Enzymes in the duodenum break the polypeptide chains down into amino acids.  These enzymes are supplies by the pancreas.


Nucleic acid digestion

Enzymes called nucleases hydrolyze DNA and RNA in food into their nucleic acids. Other enzymes break the nucleotides down.


Fat digestion

Nearly all the fat in a meal reaches the small intestine completely undigested.  Fat molecules are insoluble in water. Bile salts coat the tiny fat droplets to keep them from coalescing in a process called emulsification.  Since the droplets re small, a large surface area is exposed to lipase which is an enzyme which hydrolyzes fat.   Most of this digestion happens in the duodenum.   The remaining regions of the small intestine, jejunum and ileum, function mainly in the absorption of nutrients and water.


Absorption of nutrients

Most of the absorption of nutrients takes place in the small intestine while there is some absorption in the stomach and small intestine.  The lining of the small intestine has a surface area about the size of a tennis court.  Large circular folds in the lining have villi and each of the cells of the villi have microvilli.  These villi absorb nutrients which are then transported across the capillary  membranes. 


Large Intestine


The large intestine or colon is connected to the small intestine at a T-shaped junction where a sphincter regulates the movement of materials.  One arm of the T structure is a sac called the cecum which has a fingerlike extension called the appendix.  The main part of the colon is an upside down U about 1.5 meters in length.  Connected to the cecum is the right or ascending colon which is connected to the transverse colon which is connected to the descending colon which is connected to the sigmoid colon which is connected to the rectum.

The main function of the colon is to reabsorb water although most re-absorption of water happens in the small intestine with the absorption of nutrients.  The small and large intestines absorb about 90% of the water that enters the digestive tract.  The wastes of the digestive tract, feces, become more solid as they move along the colon.

Many harmless bacteria live in the colon.  E. coli is in the colon.  Intestinal bacteria live on organic bacteria that would otherwise be included in feces. By-products of colon bacteria metabolism include gases (like methane and hydrogen sulfide) and some vitamins.    Bacteria in the colon generate Vitamin K which is used in blood clotting.  The terminal portion of  the colon is called the rectum which is where feces are stored until they can be eliminated.  Between the rectum and the anus are two sphincters, one involuntary and one voluntary.  Once or more a day, strong contractions of the colon create an urge to deficate.