Stomach and Intestines

Structure and Functions of Stomach and intestines

Structure of the Pharynx and Oesophagus

Pharynx: A tube through which a bolus passes when moved from the mouth to the oesophagus by the process of deglutition The pharynx is divided into three parts namely o Nasopharynx

• o Oropharynx
• o Laryngopharynx
• The Oropharynx is important for digestive system

The wall of the pharynx consists of three layers These are the Mucosa, Middle layer (Fibrous tissue) and Outer layer consisting of Smooth muscles involved in swallowing When food enters the pharynx swallowing is no longer under voluntary control

Stomach and Intestines

The oesophagus is a narrow muscular tube about 25 centimetres long diameter of 2cm, which starts at pharynx at the back of the mouth It passes through the thoracic diaphragm, and ends at the cardiac orifice of the stomach It lies in the median plane in the thorax in front of the vertebral column behind the trachea and the heart.

The oesophagus has 4 constrictions where adjacent structure produce impression

• 1st at the beginning approximately 15cm from the incisor teeth and caused by the cricopharyngeus muscles – upper oesophageal sphincter
• 2nd where it crossed by the arch of aorta 22.5cm from the incisor teeth
• 3rd where it crossed by the left main bronchus 27.5cm from the incisor
• 4th Where it passes through the diaphragm – lower oesophageal sphincter

At the top of the oesophagus, is a flap of tissue called the epiglottis that closes during swallowing to prevent food from entering the trachea (windpipe)

It has four layers; from outside:

o The adventitia

o Muscular layer; the muscles are striated (voluntary) in the upper third and mixed (striated & smooth) in the middle and smooth (involuntary) in the lower third There are two layers:

• The inner is circular
• The outer is longitudinal
• o Sub-mucosa layer

o Mucosa layer which is made up of stratified squamous epithelium Arterial blood supply of the oesophagus is through oesophageal arteries, branches of thoracic aorta, inferior phrenic arteries and the left gastric branch of the celiac artery Venous drainage is through the azygos and hemiazygos veins

Structure and Functions of the Stomach

What is a stomach?

The stomach: The most dilated part of the gastrointestinal tract

Has a J-like shape

Positioned between the abdominal oesophagus and the small intestine, the stomach is in the epigastric, umbilical and left hypochondrium regions of the abdomen Its shape and size varies from person to person; even within the same individual its size and shape change from time to time depending on its food contents and the moisture of the body and in adult capacity range from 1.0 to 1.5 litre

Structure of the Stomach

The stomach is divided for descriptive purposes into four regions by arbitrary lines drawn on its external surface which are o The outlet of the stomach (pyloric orifice) is marked on the surface of the organ by the pyloric constriction and surrounded by a thickened ring of gastric circular muscle (the pyloric sphincter) The pyloric orifice is just to the right of midline in a plane that passes through the lower border of vertebra LI (the trans pyloric plane) Other features of the stomach include:

o The greater curvature, which is a point of attachment for the gastro splenic ligament and the greater omentum o The lesser curvature, which is a point of attachment for the lesser omentum o The cardial notch, which is the superior angle created when the oesophagus enters the stomach o The angular incisure, which is a bend on the lesser curvature

The stomach is lined with simple columnar epithelium

The epithelium forms numerous tube like gastric pits, which are the opening of the gastric glands The epithelial cells of the stomach are of five types The first type is surface mucous cells which produce mucus, is on the surface and lines the gastric pits The remaining four cell types are in the gastric glands They are mucous neck cells which produce mucus; parietal (oxyntic) cells which secrete hydrochloric acid and intrinsic factor needed for vitamin B12 absorption; Chief (zymogenic) cells which produce pepsinogen and endocrine cells, which produce regulatory hormones

STOMACH

Blood supply of the stomach; a rich arterial supply, arise from the celiac trunk and its branches.

Most of the blood is supplied by anastomoses formed along the lesser curvature by the right and left gastric arteries and, along the greater curvature, by the right and left gastro omental artery (gastroepiploic artery) The fundus and upper body of stomach receive blood from the short and posterior gastric arteries branches of the splenic artery Venous drainage is through gastric veins parallel the arteries in position and course

Functions of GIS

• Motility
• Digestion
• Absorpton
• Excretion

Immunological (body protection)

Functions of the Stomach

• Reservoir for food until it is partially digested and moved further along the GI tract
• Secrete gastric juice to aid in digestion of food
• Breaks food into small particles and mixes them with gastric juice
• Secrete intrinsic factor Vit.B12
• Carries limited absorption
• Produces gastrin hormoneHcl

Helps protect body from pathogenic bacteria swallowed with food

Structure and functions of the Small Intestines

• Where is small intestines found?

What are the functions of small intestines?

The small intestine extends from the pyloric sphincter to the ileocecal valve

• It is highly adapted for digestion and absorption

Its glands produce enzymes and mucus The walls of small intestine are composed of four layers of tissues and there are some modifications of the peritoneum and mucous membrane The surface area of the mucosa is greatly increased by permanent circular folds, villi and microvilli Villi are tiny finger like projections of the mucosa layer into intestinal rumen about 0.5 to 1mm long, the function of villi is for absorption It consists of three parts:

• The duodenum
• The jejunum

The ileum

Duodenum is the first section of the small intestine

It begins with the duodenal bulb and ends at the ligament of Treitz o The duodenum also regulates the rate of emptying of the stomach via hormonal pathways; Secretin (inhibit release of gastric juice, promotes osmoregulation in renal collecting tubule) and cholecystokinin-CCK (which are released from cells in the duodenal epithelium in response to acidic and fatty stimuli present there when the pylorus opens and releases gastric chyme into the duodenum for further digestion CCK  stimulates release of Pancreatic enzymes and bile o The duodenum is divided into four sections for the purposes of description o The first three sections curve in a "C" loop concavity in which the head of the pancreas lies o Only the first 2 cm of the superior part is mobile (not covered by peritoneum), the distal 3cm of the first part along with the rest of the duodenum is retroperitoneal (immovable)

Omentum

The jejunum is the middle section of the small intestine

o In adult humans, the small intestine is usually between 5.5-6m long, 2.5m of which is the jejunum o The inner surface of the jejunum, its mucous membrane, is covered in projections called villi, which increase the surface area of tissue available to absorb nutrients from the gut content The epithelial cells which line these villi possess even larger numbers of microvilli o The villi in the jejunum are much longer than in the duodenum or ileum o The jejunum contains very few Brunner's glands (found in the duodenum-secreting mucus) or Peyer's patches (found in the ileum) o It has many large circular folds in its submucosa called plicae circulares which increase the surface area for nutrient absorption

The ileum is the final section of the small intestine

It is separated from the cecum by the ileocecal valve o The function of the ileum is mainly to absorb vitamin B12 and bile salts and whatever products of digestion which were not absorbed by the jejunum o The wall itself is made up of folds, each of which has many tiny finger-like projections known as villi on its surface o In turn, the epithelial cells which line these villi possess even larger numbers of microvilli.

o Therefore the ileum has an extremely large surface area both for the adsorption (attachment) of enzyme molecules and for the absorption of products of digestion o There is no line of demarcation between the jejunum and the ileum

There are, however, subtle differences between the two

• The ileum has more fat inside the mesentery than the jejunum

The ileum is a paler colour, and tends to be of a smaller caliber as well While the length of the intestinal tract contains lymphoid tissue, only the ileum has abundant Peyer's patches, un-encapsulated lymphoid nodules that contain large amounts of lymphocytes and other cells of the immune system

Structure and Functions of the Large Intestine

• The large intestine extends from the ileocecal valve to the anus
• Its subdivisions include the cecum, colon, rectum, and anal canal

The mucosa contains numerous goblet cells and the muscularis consists of taeniae coli The large intestine is the second to last part of the digestive system, the final stage of the alimentary canal is the anus It starts in the right iliac region of the pelvis, just at or below the right waist, where it is joined to the bottom end of the small intestine From here it continues up the abdomen, then across the width of the abdominal cavity, and then it turns down, continuing to its endpoint at the anus Its length is about 1.5 metres long, which is about one-fifth of the whole length of the intestinal canal

Colon

The following are the parts of the large intestine:

The cecum or caecum (from the Latin caecus meaning blind), is a pouch, connecting the ileum with the ascending colon of the large intestine It is separated from the ileum by the ileocecal valve is considered to be the beginning of the large intestine It is also separated from the colon by the cecocolic junction The ascending colon is smaller in caliber than the cecum, with which it is contiguous It passes upward, from its commencement at the cecum, opposite the colic valve, to the under surface of the right lobe of the liver, on the right of the gall-bladder, here it bends abruptly forward and to the left, forming the right colic flexure (hepatic) Hepatic (or the right colic) flexure is the sharp bend between the ascending and the transverse colon The right colic flexure is adjacent to the liver, and is therefore also known as the hepatic flexure The left colic flexure is also known as the splenic flexure (as it is close to the spleen)

Stomach and Intestines

The transverse colon the longest and most movable part of the colon, passes with a downward convexity from the right hypochondrium region across the abdomen, opposite the confines of the epigastric and umbilical zones, into the left hypochondrium region, where it curves sharply on itself beneath the lower end of the spleen, forming the splenic or left colic flexure Toward its splenic end there is often an abrupt U-shaped curve which may descend lower than the main curve The descending colon passes downward through the left hypochondrium and lumbar regions, along the lateral border of the left kidney At the lower end of the kidney it turns medial-ward toward the lateral border of the psoas muscle, and then descends, in the angle between psoas and quadratus lumborum, to the crest of the ilium, where it ends in the sigmoid colon It is smaller in caliber and more deeply placed than the ascending colon The sigmoid colon (pelvic colon; sigmoid flexure) 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, but on account of its freedom of movement it is liable to be displaced into the abdominal cavity.

Stomach and Intestines

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 (The name sigmoid means S-shaped.) 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 o The rectum (from the Latin rectum intestinum, meaning straight intestine) is the final straight portion of the large intestine and terminating in the anus Its length 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 The rectum intestinum acts as a temporary storage site for faeces 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 faeces results

The rectum shortens as material is forced into the anal canal and peristaltic waves propel the faeces out of the rectum The internal and external sphincters allows the faeces to be passed by muscles pulling the anus up over the exiting faeces

The anus is an opening at the opposite end of the digestive tract from the mouth

o Its function is to control the expulsion of faeces, unwanted semi-solid matter produced during digestion o Flow of substance through the anus is typically controlled by the anal sphincter muscle The peritoneum covers the ascending and descending colon on the anterior surface and sides, and therefore they are described as retroperitoneal The transverse colon and sigmoid colon are intraperitoneal

Key points

The stomach is the most dilated part of the gastrointestinal tract and has a J-like shape.

• The small intestine extends from the pyloric sphincter to the ileocecal valve.
• Small intestine consists of three parts i.e the duodenum, the jejunum and the ileum.

Subdivisions of large bowel include the cecum, colon, rectum, and anal canal.

evaluation

• What are the parts of the stomach?
• What are the parts of the small intestine?
• What are the functions of the stomach?

What are the functions of pharynx and oesophagus