Digestive System

The digestive system is a collection of organs that can be divided into two parts:

Pharynx

Rectum

LargeIntestine

Anus

SmallIntestine

Stomach

Esophagus

Mouth

1. The alimentary or gastrointestinal tract.

2. The accessory organs

The first group of organs constitute the alimentary or gastrointestinal tract. The organs of the

gastrointestinal tract are:

1. oral cavity (mouth) Pharynx

Rectum

LargeIntestine

Anus

SmallIntestine

Stomach

Esophagus

Mouth

2. pharynx

3. esophagus

4. stomach5. small intestine6. large intestine

7. rectum

8. anus

The alimentary tract is a continuous, hollow tube about 30 ft. long that runs from the oral cavity to the anus. Food taken into the tube remains within it and is broken down (digested) into usable, simpler components. These substances are absorbed into the body (outside the tract) while the unusable part of the food passes on and out of the tube.

• Take in food (eat and drink).

• Break down the food (digest) into simpler, usable components.

• Absorb the usable components (nutrients) into the body.

• Discard the unusable components (waste) from the body.

Therefore the functions of the alimentary tract, overall, would be to:

The second group of organs are the accessory organs. These contribute to the break down of food into usable components (digestion) but are not part of

the alimentary tract. The accessory organs are:

1. Tongue 2. Teeth

3. Salivary glands 4. Liver

5. Gall bladder 6. Pancreas

The oral cavity (mouth) –

This is the first area of the alimentary canal. It is also where both the mechanical and chemical digestive processes begin. The oral cavity extends from the lips to the oropharynx.

The oral cavity is equipped with three different accessory organs. These are the tongue, teeth, and salivary glands and these each help the digestive processes begin. These organs cut, grind, lubricate, taste, manipulate, and move food, and have secretions that chemically break down food.

1. Mechanical – Action of the digestive organs serve to move food materials along, moisten and liquefy, and to pulverize them to increase the surface area of the food. This makes things more susceptible to the actions of chemicals.

The two ways food is “digested” and to be further explored later:

2. Chemical – These actions convert the large, complicated food molecules into smaller, simpler units (like glucose, amino acids, lipids) that can pass through cell membranes and be used by cells.

Pharynx Common tube (to the respiratory and digestive systems) 5 inches long which functions as an

important junction point for several passages in the body.

Pharynx was covered in the respiratory system notes. A summary is as follows:

The functions of the pharynx, with respect to the digestive system, are to:

1.receive partially digested food from the oral cavity2.pass the partially digested food on to the esophagus.

The pharynx also contains the tonsils: (3 types)

1.          Pharyngeal tonsils 2.         Palatine tonsils3.          Lingual tonsils

#1

#2

#3

• Oropharynx• Laryngopharynx

• NasopharynxN

O

L

The pharynx is dividedinto three parts:

Esophagus

A straight, collapsible muscular tube which lies behind (posterior to) the trachea and is

about 1 inch in diameter and 10 inches

long.

The functions of the esophagus are to:

1.receive partially digested food from the pharynx2.pass the partially digested food on to the stomach.

FromPharynx

FromPharynx

FromPharynxThis is done by rhythmic

contractions of the smooth muscle layers called peristalsis. Peristaltic waves are the primary way in which all the digestive tract organs move their contents along.

The esophagus is composed mostly of smooth muscle arranged in two layers, one circular and one longitudinal, lined internally with a layer of mucus secreting cells.

lumen

LongitudinalSmooth muscle

Circular Smooth muscle

Mucous membrane

In fact, the entire alimentary tract is composed this way and secretions range from mucus to acid or digestive enzymes.

At the gastroesophageal junction (“bottom” of the esophagus where it connects to the stomach), the circular layer of smooth muscle fibers contract and relax creating a valve like action that is somewhat like that of a sphincter muscle, but is not a true sphincter valve. This portion of the circular muscle is called the cardiac sphincter or gastroesophageal sphincter because the esophagus joins the stomach in the cardiac region. This valve is controlled by pressure fluctuations in the esophagus and stomach.

Heartburn occurs when the cardiac sphincter relaxes and some of the digestive enzymes and stomach acid flow out into the esophagus. The enzymes can cause lesions in the esophageal lining and the acid irritates the esophagus. If the acid gets into any lesions this will irritate them further. The “burning” sensation felt around the area is perceived to be near the heart so the sensation is called “heartburn”. If chronic or severe, this is termed gastroesophageal reflux disease (acid reflux).

In gastric bypass surgery, rows of staples are used to divide the stomach into two parts. Food can enter only the smaller upper part, called a stomach pouch. The small intestine is cut, and the section of the small intestine below the cut is attached to the stomach pouch. Thus, the lower part of the stomach and the upper part of the small intestine are bypassed.

As a result, the amount of food that can be eaten at one time is drastically reduced and less food is absorbed. The section of the small intestine above the cut is then attached to the small intestine further down. Thus, digestive juices produced in the bypassed part of the stomach can reach the rest of the small intestine and can be mixed with food.

The stomach extends from the esophagus at the cardiac sphincter to the duodenum, the uppermost portion of the small intestine. The stomach is divided into four parts:

Stomach – The stomach lies at the upper part of the abdominal cavity just under the diaphragm (which separates the thoracic and abdominal cavities).

***Note – the stomach has three layers of smooth muscle.

1. Fundus or fundic region – This is the uppermost portion of the stomach. It actually extends above the level of the cardiac sphincter. It acts as a temporary storage area and can “balloon up” to accomplish this function. Normally it is filled with air.

esophagus

esophagus

2. Cardia or cardiac region – This is a small region very near the cardiac sphincter where the esophagus meets the stomach.

esophagus

3. Body – The middle portion of the stomach or main portion located between the fundic and pyloric region. This is the largest region.

esophagus

4. Pylorus or pyloric region – This is the terminal portion of the stomach. It constricts as it approaches the small intestine forming thepyloric canal. It ends at an area of thickened smooth muscle called the pyloric sphincter. This acts as a valve preventing back flow of material from the small intestine to the stomach.

Overall, the stomach, which has a structure like a collapsible pouch, has only a few basic functions. These functions are, as with the mouth, are related to the digestion or breakdown of food in either a mechanical or a chemical manner and are as follows:

1. Receives partially digested food from the esophagus.

2. Tosses and churns the partially digested food, mixes it with gastric juices, and mechanically breaks up the food (now called chyme) further.

3. Begins the chemical breakdown of proteins.

4. Begins the process of absorption. (Very little actual absorption takes place, however small amounts of water & glucose, fat soluble drugs, some salts and especially alcohol are absorbed.)

5. Passes the chyme on to the small intestine. ***NOTE – “chyme” is the term used to describe the partially digested food once it has been mixed with the digestive secretions of the stomach.

Small intestine

A collapsible, muscular tube that extends from the pyloric sphincter of the stomach to the cecum of the large intestine. The small intestine is the site where the majority of the chemical breakdown of food and absorption of nutrients takes place.

Duodenum

Ileum

Jejunum

Three parts of the small intestine:

Duodenum

Ileum

Jejunum

Duodenum

1. Duodenum – about 10 – 12 inches long. It connects to the stomach via the pyloric sphincter. This is the “fixed” portion of the small intestine.

2. Jejunum – The only distinction is that it is a bit larger than the ileum. It represents about 2/5 of the length of the small intestine .

Duodenum

Ileum

Jejunum

Jejunum

3. Ileum – This is the final portion of the small intestine and represents about 3/5 of its length.

Ileum

Duodenum

Ileum

Jejunum

The small intestine is covered on its inside wall with a “velvety” looking formation. These are called the intestinal villi.

The villi project into the lumen where they are bathed in the intestinal contents. They function to increase the surface area of the intestinal lining for absorption. Absorption takes place throughout the length of the small intestine.

Functions of the small intestine:

1. Receives nutrient rich chyme from the stomach.

2. Receives digestive secretions from the pancreas.

3. Receives digestive secretions and waste secretions from the liver and gall bladder.

4. Completes the chemical digestion of the chyme received from the stomach.

5. Absorbs nutrients from the fully digested chyme.

6. Transports unusable portion (nutrient depleted) of chyme to the large intestine.

1. Cecum – the entrance to the large intestine which also has the vermiform appendix (appendix) hanging off it.

Large Intestine – This is called “large” due to an increase in diameter compared to the “small” intestine. Its total length is about 5 feet. The large intestine is divided into two parts:

2.  the Colon – This is the major portion of the large intestine. It is divided into 4 parts:

a) ascendingb) transversec) descendingd) sigmoid

Functions of the large intestine:

1. Receives unusable portion of chyme from the small intestine.

2. Reabsorbs the rest of the water and electrolytes.

3. Forms and stores feces until defecation.

4. Passes feces on to the rectum.

Rectum – final storage of feces before defecation. When the rectum fills with feces it is stimulated to contract (peristalsis). This action is called the defecation reflex.

Rectum

of three sphincter muscles (one made of smooth muscle, the other two of skeletal muscle), and finally through the anal opening.

Anus – Feces expelled from the rectum passes through the anal canal, then through a series