Alimentary SystemCourse Title: Basic Anatomy Course No. PHR 111 Course Teacher: Shahana Sharmin (SS)

Oral Cavity

The cavity of the mouth is placed at the commencement of the digestive tube. it is a nearly oval-shaped cavity which consists of two parts:

The vestibule (an outer, smaller portion), and The mouth cavity proper (an inner, larger part).

Oral Cavity

The Vestibule :

It is a slit-like space, bounded externally by the lips and cheeks; internally by the gums and teeth. It communicates with the surface of the body by the rima or orifice of the mouth.

Function :It receives the secretion from the parotid salivary glands. Communicates, when the jaws are closed, with the mouth cavity proper by an aperture behind the wisdom teeth, and by narrow clefts between opposing teeth.

Oral Cavity

The Mouth Cavity Proper :

It is bounded laterally and in front by the alveolar arches with their contained teeth; behind. It is roofed in by the hard and soft palates, & floor is formed by the tongue, the remainder by the reflection of the mucous membrane.Function : It receives the secretion from the submaxillary and sublingual salivary glands.

Pharynx

The human pharynx is the part of the throat situated immediately posterior to (behind) the mouth and nasal cavity, and cranial, or superior, to the esophagus, larynx, and trachea. The pharynx is part of the digestive system and also the respiratory system; it is also important in vocalization. The human pharynx is conventionally divided into three sections: Nasopharynx (epipharynx), Oropharynx (mesopharynx), and Laryngopharynx (hypopharynx).

Pharynx

Nasopharynx :

The nasopharynx or epipharynx is the most important part in digestive and the respiratory system. The nasopharynx is the upper most portion of the pharynx. It extends from the base of the skull to the upper surface of the soft palate. The pharyngeal tonsils, more commonly referred to as the adenoids, are lymphoid tissue structures located in the posterior wall of the nasopharynx.

Pharynx

Oropharynx : The oropharynx or mesopharynx lies behind the oral cavity. The anterior wall consists of the base of the tongue and the epiglottic vallecula; the lateral wall is made up of the tonsil, tonsillar fossa, and tonsillar pillars. The superior wall consists of the inferior surface of the soft palate and the uvula. Because both food and air pass through the pharynx, a flap of connective tissue called the epiglottis closes over the glottis when food is swallowed to prevent aspiration.

Pharynx

Laryngopharynx : The laryngopharynx or hypopharynx is the caudal part of the pharynx. It is the part of the throat that connects to the esophagus. It lies inferior to the epiglottis and extends to the location where this common pathway diverges into the respiratory and digestive pathways. The laryngopharynx serves as a passageway for food and air and is lined with a stratified squamous epithelium.

Oesophagus

The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach. In humans the esophagus is continuous with the laryngeal part of the pharynx. It is usually about 2530 cm long and connects the mouth to the stomach. It is divided into cervical, thoracic and abdominal parts.

Oesophagus

Oesophagus

The layers of the esophagus are as follows: Mucosa Submucosa Muscularis externa Adventitia

Oesophagus

Normally, the esophagus has three anatomic constrictions at the following levels At the pharynx (14-16 cm from the incisor teeth). At aortic arch and the left bronchus (25-27 cm from the incisor teeth). At diaphragm (36-38 cm from the incisor teeth).

Stomach

Properties : The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract. It is involved in the second phase of digestion, following mastication (chewing). The stomach is located between the oesophagus and the small intestine. It secretes protein-digesting enzymes and strong acids to aid in food digestion.

Stomach

Anatomy :

The stomach lies between the oesophagus and the duodenum It is on the left upper part of the abdominal cavity. The top of the stomach lies against the diaphragm. Lying behind the stomach is the pancreas. Two sphincters, keep the contents of the stomach contained. The esophageal sphincter (found in the cardiac region) The Pyloric sphincter (dividing the stomach from the small intestine). In adult humans, the stomach has a relaxed, near empty volume of about 45 ml, but can hold as much as 2-3 litres. The stomach of a newborn human baby will only be able to retain about 30ml.

Stomach

Sections : The stomach is divided into 4 sections, each of which has different cells and functions. The sections are: Cardia : Where the contents of the oesophagus empty into the stomach. Fundus : Formed by the upper curvature of the organ. Body or Corpus : The main, central region. Pylorus : The lower section of the organ that facilitates emptying the contents into the small intestine.

Stomach

Layers of Stomach

Stomach

Layers of stomach : The stomach walls are made of the following layers, from inside to outside: Mucosa : The first main layer. This consists of an epithelium, the lamina propria composed of loose connective tissue which has gastric glands and a thin layer of smooth muscle called the muscularis mucosae. Submucosa : This layer lies over the mucosa and consists of fibrous connective tissue, separating the mucosa from the next layer. The Meissner's plexus is in this layer. Muscularis externa : Over the submucosa, the muscularis externa in the stomach differs from that of other GI organs in that it has three layers of smooth muscle instead of two.

Serosa : This layer is over the muscularis externa, consisting of layers of connective tissue.

inner oblique layer middle circular layer outer longitudinal layer

Small intestine

The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine It is the part where the vast majority of digestion and absorption of food takes place. The primary function of the small intestine is the absorption of nutrients found in food.

Small intestine

Small intestine

Size and divisions : The small intestine in an adult human measures on average about 5 meters (16 feet), with a normal range of 3 7 meters. The surface of the small intestine is increased by its special structure, and it is about 200-250 square meters. The small intestine is divided into three structural parts:

Duodenum 26 cm in length Jejunum 2.5 m Ileum 3.5 m

Small intestine

Function :Digestion1. Most of the chemical digestion takes place in the small intestine (duodenum). The pancreas secretes digestive enzymes, which enter the small intestine via the pancreatic duct. Moreover, the pancreas also releases the hormone secretin (bicarbonate) to be released into the small intestine to neutralize potentially deleterious acids coming from the stomach. The major classes of nutrients undergoing digestion in the small intestine are carbohydrates, proteins and lipids. 2. In the small intestine, carbohydrates are broken down into simpler sugars (monosaccharides - glucose). For example, carbohydrates are degraded into oligosaccharides by pancreatic amylase, after which two other enzymes: dextrinase and glucoamylase further break them down. Proteins and peptides, on the other hand, are broken down into amino acids. 3. The degradation of protein commences in the stomach and continues to take place in the small intestine. Proteolytic enzymes secreted by the pancreas break down peptides into smaller peptides. 4. Moreover, the pancreatic brush border enzyme called carboxypeptidase split one amino acid at a time. Pancreatic lipase degrades triglycerides into monoglycerides and free fatty acids. 4. The salts secreted by the liver and the gall bladder work along with pancreatic lipase to digest the lipids. Read more on benefits of digestive enzymes.

Small intestine

Function (cont..) :Absorption1. Once the food has been digested, it is ready to pass into the blood vessels situated in the intestinal walls, by a process known as diffusion. Both active, passive and facilitated diffusion of nutrients takes place in the small intestine. 2. Moreover, the mucosal lining in the intestinal walls featuring plicae circulares and rugae absorb maximum nutrients as possible from the food that is passed through the small intestine. 3. The absorbed nutrients are then transported to different organs of the body, via blood vessels, wherein, they are used to build proteins and other substances required by the body. This process is known as assimilation. 4. Most of the nutrients are absorbed by the jejunum of the small intestine and nutrients not absorbed by the jejunum are absorbed by the ilium. 5.The remaining undigested food is passed to the next part of the digestive system i.e. to the large intestine.

Ceacum

Cecum is a part of the large intestine and is situated between ileum and the ascending colon. The cecum is a pouch-like structure that is connected to the small intestine It also has the appendix attached to it. The opening of small intestine into the cecum is regulated by the ileocecal valve, that regulates the entry of food and stops any digested material from re-entering the small intestine. Its size is variously estimated as on an average may be to be 6.25 cm. in length and 7.5 in breadth.

Ceacum

Cecum function :Digestion

Cecum is a vital part of digestive system and it has numerous functions in the digestion. The functions are 1. The cecum houses a large number of bacteria that help in

digestion of plant materials, mostly cellulose, that remains undigested in the stomach and small intestine. 2. This is done by the process of fermentation that helps in breaking down the plant fibers. The nutrients from cellulose are later absorbed by the large intestine.

Absorption

1. The cecum helps to receive the undigested food as well as liquids

from the small intestine. As the small intestine does not absorb liquid, that becomes the large intestine function in digestion. 2. Cecum helps in absorption of salts and electrolyte, mostly sodium and potassium, back into the body. 3. The cecum is made of muscle tissues, that churn the food waste. This is done with the help of mucus membrane lining the cecum.

CeacumCecum Function (cont.) :Lubrication 1. The solid waste that is passed to the cecum from the smallintestine, is lubricated by the cecum. 2. The cecum is lined by a thick mucus membrane that produces mucus. This mucus is mixed with the solid waste to lubricate it. This is necessary because the liquids are almost totally absorbed in the large intestine and to pass the solid waste along the large intestine easily, it becomes extremely important to lubricate it.

Figure : Cecum

Appendix

In the lower right part of the belly, where the large intestine and the small intestine meet, is a four-inch, finger-like organ called the appendix. The appendix serves as a depot for good bacteria, helping digestive organs recover from diarrhea and similar condition. It can be surgically removed without impairing the bodys overall health.

Appendix

When inflamed or infected, the appendix is said to be suffering from appendicitis. An inflamed or infected appendix can readily burst, the effect of which is profound pain in the abdomen, accompanied by vomiting and nausea. The cause of appendicitis is largely unknown though.

Appendix

Sigmoid colon

The sigmoid colon (pelvic colon) is the part of the large intestine that is closest to the rectum and anus. It forms a loop that averages about 40 cm. in length, and normally lies within the pelvis. It begins at the superior aperture of the lesser pelvis, where it is continuous with the iliac colon, and passes transversely across the front of the sacrum to the right side of the pelvis. It then curves on itself and turns toward the left to reach the middle line at the level of the third piece of the sacrum, where it bends downward and ends in the rectum. It is completely surrounded by peritoneum, which forms a mesentery.

Sigmoid colon

Function :

The main purpose of the sigmoid colon and also the other parts of the colon, is to help in giving the finishing touches to the digestion process. This is the final stage where digestion takes place. The colon absorbs the water and minerals from the digested food and helps not only in the formation but also in the passing out off feces. Due to muscular contractions, the colon moves the digested products or the waste products to the anus. The sigmoid colon, in order to help in smooth passing of the stool and also in order to protect the digestive system from any kind of toxins, also produces a mucus. But sometimes sigmoid colon might produce excess mucus. It usually occurs if we take in unhealthy food and this means production of more mucus to protect the digestive system from the harmful toxins. As a result the person might suffer from constipation.

Large Intestine

Rectum

The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long. Its caliber is similar to that of the sigmoid colon at its commencement, but it is dilated near its termination, forming the rectal ampulla.

Rectum

Role in human defecation : The rectum intestinum acts as a temporary storage site for feces. As the rectal walls expand due to the materials filling it from within, stretch receptors from the nervous system located in the rectal walls stimulate the desire to defecate. If the urge is not acted upon, the material in the rectum is often returned to the colon where more water is absorbed. If defecation is delayed for a prolonged period, constipation and hardened feces results. When the rectum becomes full, the increase in intrarectal pressure forces the walls of the anal canal apart, allowing the fecal matter to enter the canal. The rectum shortens as material is forced into the anal canal and peristaltic waves propel the feces out of the rectum. The internal and external sphincter allow the feces to be passed by muscles pulling the anus up over the exiting feces.

Human digestive system

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