digestive system physiology

Digestive system:

The digestive system provides the body with nutrients and water as
well as various vitamins and minerals. it is consists of the alimentary canal
(= gastro- intestinal tract or GlT) and the glands that secrete digestive
juices in it (= salivary glands, li ver and pancreas). The GIT extends from
the mouth to the anus, and it includes from above downwards : the buccal
cavity, pharynx. esophagus, stomach, small intestine and large intestine (=
colon) which ends by the rectum and anal canal (figure I). The small
intestine consists of 3 parts, which include from above downwards : the
duodenum, jejunum and ileum.
The mouth (oral cavity)
The mouth or oral cavity is where the process of digestion begins. The oral cavity consists of several different structures, the lips and cheeks are muscular and connective tissue structures.
Tongue
The tongue is a muscular organ in the mouth. The tongue covered with tiny bumps called papillae, which give the tongue its rough texture. Thousands of taste buds cover the surfaces of the papillae.
Salivary glands (parotid, sublingual and submandibular).
Saliva is produced in and secreted from salivary glands. Salivary glands secrete a fluid that contains water, electrolytes, mucus ,enzymes and large quantities of bicarbonate ion secreted. Bicarbonate is important because it provides a critical buffer that neutralizes the massive quantities of acid produced in the stomach.
Oesophagus
The oesophagus extends from the laryngopharynx to the stomach. It is a thick walled structure and measures about 25 cm in length and lies in the thoracic cavity, posterior to the trachea. The mucosa of the oesophagus secrets thick mucous; this aids the passage of the food. The upper oesophageal sphincter regulates the movement of substances into the oesophagus and the lower oesophageal sphincter (also known as the cardiac sphincter) regulates the movement of substances from the oesophagus to the stomach.
Stomach
The stomach is a muscular organ located on the left side of the upper abdomen. As food reaches the end of the oesophagus, it enters the stomach through a muscular valve called the lower oesophageal sphincter. The vagus nerve innervates the stomach with parasympathetic fibres that stimulate gastric motility and the secretion of gastric juice. Sympathetic nerve reduces gastric activity, the stomach secretes acid and enzymes that digest food. The pyloric sphincter is a muscular valve that opens to allow food to pass from the stomach to the duodenum.
The lower gastrointestinal tract:
Small intestine
The small intestine is the part of the gastrointestinal tract following the stomach, and is where much of the digestion and absorption of food takes place. The entire small intestine measures about 6.5 metres. The small intestine consists of three sections.
Duodenum: It is approximately 25 cm long. The duodenum is largely responsible for the breakdown of food in the small intestine, using enzymes. The duodenum also regulates the rate of emptying of the stomach.


Jejunum: The section of the small intestine is beyond the duodenum. The inner surface of the jejunum covered in projections called villi, which increase the surface area to absorb nutrients from the gut contents.

Ileum: The ileum is the final and longest segment of the small intestine. It is specifically responsible for the absorption of vitamin B12 and the reabsorption of conjugated bile salts. The ileum extends from the jejunum to the ileocecal valve, which empties into the colon.
Large intestine:
The large intestine consists of four regions: cecum, colon, rectum and anus. The large intestine is wider and shorter than the small intestine (approximately 1.5 metres in length) and has a smooth inner wall. In the upper half of the large intestine bacteria produce B vitamins (B12, thiamin and riboflavin). The primary function of the large intestine is to absorb water and turn the food residue into semi solid faeces. The large intestine also absorbs some vitamins, minerals, electrolytes and drugs.
Functions of the GIT
(1) Digestion of food : Digestion is the process of breakdown of proteins,
fats and carbohydrates into simple particles that can easily be absorbed.
(2) Secretion of digestive juices :These contain the various enzymes that
are required for the process of food digestion.
(3) Absorption of the products of digestion (as well as various vitamins,
minerals and fluids) into the blood and lymph vessels.
(4) Excretion of the undigested and nonabsorbed substances in feces.
(5) Motility : This is essential for the processes of digestion, absorption
and excretion through mixing and propulsion of the GIT contents.
Regulation of gastrointestinal functions:
The functions of the G IT are regulated by 2 main mechanisms :
(A) Nervous regulation
This occurs through the following reflexes
(1) Local axon (or local enteric) reflexes : These are short reflexes
that lead to peristaltic movement and secretion (~/many gastrointestinal hormones.
(2) Ganglionic reflexes : These reflexes start from receptors in the
G IT to the prevertehral sympathetic ganglia then back to the G IT (i .e.
these ganglia act as nerve centres in this case) e.g. the enterogastric.
?,ltSirocolic and colonoileal reflexes.
(3) Central ner vous reflexes : These arc long reflexes that start from
rlceprors in the G IT to the spinal cord or brain stem then back to the G IT
e.g. the defecation reflex and the vagovagal reflexes that lead to secretion
of many digestive juices.
(B) Hormonal regulation
A group or polypeptide hormones arc secreted by certain cells in
the mucosa of the G IT (= enteroendocrinc cells) and via the portal vein
they reach the liver from which they arc delivered to the systemic
circulation.
Depending on structural similarity, the gastrointestinal divided into 2 main families :
(1) Gastrin family : This includes gastrin and cholecystokinin (CCK).
(2) Secretin f~l mi l y : This includes secretin, gastric inhibitory peptide(=
GIP) There arc other hormonally-active polypeptides that arc secreted


Digestion
It is the mechanical and chemical breaking down of food into smaller components, to a form that can be absorbed into a blood stream.
A: Mechanical
Mechanical digestion is simply the aspects of digestion achieved through a mechanism or movement. There are two basic types of mechanical digestion.
Mastication: The first step when it comes to digestion actually begins as soon as food enters the mouth. Mastication (chewing) begins the process of breaking down food into nutrients. Chewing our food is an important part of the digestive process because smaller pieces are more readily digested through chemical digestion.
Swallowing: Peristalsis is simply the involuntary contractions responsible for the movement of food through the oesophagus and intestinal tracts.

B: Chemical

Chemical digestion is achieved with the addition of chemicals to the food. Digestive enzymes and water are responsible for the breakdown of complex molecules such as fats, proteins and carbohydrates into smaller molecules. These smaller molecules can then be absorbed by cells. The presence of these digestive enzymes accelerates the digestion process, where absence of these enzymes slows overall reaction speed. Digestive enzymes mainly responsible for chemical digestion include:
Protease.
Collagenase.
Lipase.
Amylase.
Trypsin.
Chemotrypsin

FUNCTIONS OF SALIVA:

( I)- Enzyme p1yalin starts digestion of starch (specially cooked starch). It splits starch into smaller molecules i.e.
(2) It keeps the oral cavity moist, which helps articulation (i.e.
mding speech) by facilitating the movements of the lips and tongue.
(3) Its mucin content lubricates food, thus facilitating the process or
swallowing f= deglutition).
(4) It serves as a solvent for the molecules that stimulate the taste
receptors, thus facilitating the taste sensation.
(5) It keeps the mouth and teeth clean, and it contains nn antibacterial
emJmle (= lysozyme) as well as lgA (= immunoglolmli11 A).
Therefore, deficient sa li vation (= xerostomia) predisposes to buccal
in llammation (= stomatitis) and dental caries, and causes dysphagia (=
difficult swallowing) and dysphasia (= difficult speech).
(6) The salivary buffers (bicarbonates and mucin) keep the oral pH at
about 7 and antagoni : e its change. This is impo11ant since acidity increases
calcium solubi lity leading to its loss from the enamel of the teeth.
(7) The saliva acts as a dilution medium for initant substances.
(8) It helps heat loss (through evaporation), specially in panting
animals e.g. clogs (refer to body temperature regulation in metabolism).
(9) Decreased salivation in cases of dehydration leads to the sensation
of thirst, which drives the patient to drink. This helps to maintain a normal
water balance in the body.
110) It is an excretory channel for certain organic substances (e.g. urea)
as well as some inorganic sails (e.g. salts or mercury, lead, and iodine