DIGESTIVE SYSTEM : PHARYNX, ESOPHAGUS AND STOMACH-ENGLISH
Nursing Notesclick here to subscribe My Student Support System
PHARYNX, ESOPHAGUS
AND STOMACH-
• The
digestive system is also known as the gastrointestinal system or the alimentary
canal or GIT. This vast system is
approximately 10 m long. It travels the length of the body from the mouth
through the thoracic, abdominal and pelvic cavities, where it ends at the anus.
We will cover it in multiple lectures. Today we are going to discuss the
anatomy and physiology of pharynx, esophagus and stomach.
PHARYNX-
• The
pharynx consists of three parts: the oropharynx, the nasopharynx and
the laryngopharynx. The nasopharynx is considered a structure of the
respiratory system. The oropharynx and the laryngopharynx are passages for both
food and respiratory gases . The wall of pharynx is made up of adventitia,
muscles, submucosa and mucosa. The mucosa varies slightly in the different
regions.
DIGESTIVE FUNCTIONS OF PHRARYNX
• During
the pharyngeal phase of swallowing reflex action is initiated in response to
the sensation of the food bolus in the oropharynx. This reflex is coordinated
by the swallowing centre in the medulla oblongata, and the motor response is
contraction of the muscles of the pharynx. The soft palate elevates, closing
off the nasopharynx and preventing the food bolus from using this route. The
larynx moves up and moves forward, allowing the epiglottis to cover the
entrance to the larynx so the food bolus cannot move into the respiratory
passages.
ESOPHAGUS-
• The esophagus (food
pipe) is a fibromuscular tube forming
part of the gastrointestinal tract, which passes food and liquid aided by
peristaltic contractions from the pharynx to the stomach.
• The esophagus generally
starts around the level of the sixth cervical vertebra (C6) and travels behind
the trachea and the heart, passes through the diaphragm and enters in the stomach at the level of the eleventh thoracic
vertebra (T11). The overall length of the esophagus is about
25 cm
• The
superficial layer of the esophagus is known as the adventitia. The
adventitia attaches the esophagus to surrounding structures. Esophagus has two
sphincters—the upper esophageal sphincter (UES) which consists of skeletal muscle, and the lower
esophageal sphincter (LES), which consists of smooth muscle.
FUNCTIONS OF ESOPHAGUS
• The
esophagus secretes mucus and transports food into the stomach. It does not
produce digestive enzymes, and it does not take part in absorption. Food is
transported by peristalsis movement of esophagus.
STOMACH-
• The
stomach is a J-shaped dilated portion of the alimentary tract situated in the
epigastric, umbilical and left hypochondriac regions of the abdominal cavity.
It lies between the esophagus superiorly and the duodenum of the small
intestine inferiorly.
• The
entrance to the stomach from the oesophagus is through the lower esophageal sphincter or cardiac
sphincter. This leads to a small area within the stomach called the cardiac
region or cardia. The fundus is the dome‐shaped region in the
superior part of the stomach. The body region occupies the space between
the lesser and greater curvature of the stomach, and the pyloric region narrows
into the pyloric canal.
• The
pyloric sphincter controls the exit of chyme from the stomach
into the small intestine. Chyme is the name given to the food bolus as it
leaves the stomach.
WALL OF THE STOMACH
• The
stomach has the same four layers of tissue as the digestive tract, but with
some differences. The muscularis contains three layers of smooth muscle instead
of two. It has longitudinal, circular and oblique muscle fibres. The extra
muscle layer facilitates the churning, mixing and mechanical digestion of food
that occurs within the stomach, as well as supporting the onward journey of the
food by peristalsis.
• The
mucosa within the stomach is also different from the rest of the digestive
tract. When the stomach is empty, the mucosal epithelia falls into long folds
known as rugae. The rugae fill out when the stomach is full.
Numerous gastric glands are situated below the surface in the mucous
membrane and open on to it .They consist of specialised cells that secrete gastric
juice into the stomach.
• Stomach
is like a balloon it’s minimum volume is as little as 50ml. Normal volume after
eating full meal it has capacity of 1500-2000ml. Maximum capacity is 4000 ml.
About 2 litres of gastric juice are secreted daily by specialised secretory
glands in the mucosa It consists of mucus secreted by mucous neck cells in the
glands and surface mucous cells on the stomach surface inactive enzyme
precursors: pepsinogens secreted by chief cells in the glands.
FUNCTIONS OF GASTRIC JUICE-
• Water
further liquefies the food swallowed.
• Hydrochloric
acid: – acidifies the food and stops the action of salivary amylase, –
kills ingested microbes, – provides the acid environment needed for effective
digestion by pepsins.
• Pepsinogens
are activated to pepsins by hydrochloric acid and by pepsins already
present in the stomach. These enzymes begin the digestion of proteins, breaking
them into smaller molecules.
• Intrinsic
factor (a protein) is necessary for the absorption of vitamin B12 from the
ileum.
• Mucus
prevents mechanical injury to the stomach wall by acting as a barrier
between the stomach wall and the corrosive gastric juice. Hydrochloric acid is present in potentially
damaging concentrations and pepsins would digest the gastric tissues.
•
PHYSIOLOGY OF GASTRIC JUCE SECRETION-
• There
is always a small quantity of gastric juice present in the stomach, even when
it contains no food. This is known as fasting juice. Secretion reaches its
maximum level about 1 hour after a meal then declines to the fasting level
after about 4 hours. There are three
phases of secretion of gastric juice- cephalic phase, gastric phase and
intestinal phase.
• 1.
The cephalic phase: The sight, taste or smell of food stimulates the
secretion of gastric juice.
• 2.
The gastric phase: When food enters the stomach, the hormone gastrin is
secreted into the bloodstream, and this stimulates the secretion of gastric
juice. The secretion of hydrochloric acid reduces the pH of the stomach
contents, and when the pH drops below 2 the secretion of gastrin is inhibited.
• 3.
The intestinal phase: As the acidic contents of the stomach enter the
duodenum of the small intestine the hormones secretin and cholecystokinin (CKK)
are secreted. These hormones also act to reduce the secretion of gastric juice
and gastric motility. The rate of gastric emptying depends on the size and
content of the meal. A large meal takes longer than a small meal. Liquids
quickly pass through the stomach. (Normally in 4 hours)
FUNCTIONS OF STOMACH-
• Functions
of stomach include:
• Temporary
storage allowing time for the digestive enzymes, pepsins, to act on the food.
• Chemical
digestion – pepsins convert proteins to polypeptides
• Mechanical
breakdown – the three smooth muscle layers enable the stomach to act as a
churn, gastric juice is added and the contents are liquefied to chyme.
• Absorption - limited absorption of water,
alcohol and some lipid-soluble drugs. preparation of iron for absorption
further along the tract by the acid environment of the stomach.
• Non-specific
defence – non specific defence is
provided by hydrochloric acid in gastric juice by killing microbes.
• Secretion
of the hormone gastrin and enzyme
pepsinogen
• Production
and secretion of intrinsic factor needed for absorption of vitamin B12 in the
terminal ileum.
• Regulation
of the passage of gastric contents into the duodenum. When the chyme is
sufficiently acidified and liquefied, the pylorus forces small jets of gastric
contents through the pyloric sphincter into the duodenum.
No comments:
Post a Comment