structure and function of digestive organs; Mouth, oesophagus, stomach, intestines, rectum, anus-5

Regulation of Gastric Secretions

Even when there is no food in the stomach, gastric juice is still generated, though at varying rates. Histamine, gastrin, and parasympathetic neurons are a few of the factors that regulate it. Specialized cells in the stomach glands secrete the hormone somatostatin, which reduces acid output. Acetylcholine (ACh), which is released by parasympathetic impulses (vagus nerves), increases the secretion of copious gastric juice that is rich in hydrochloric acid and pepsinogen while suppressing the release of somatostatin. Additionally, they emit gastrin, a peptide hormone that stimulates the secretory function of the stomach glands. They encourage histamine release, which stimulates more gastric secretion.

Phases of Gastric Secretion

Digestive Activities in the Stomach

• Proteins are broken down into peptides by the digestive enzyme pepsinogen, which transforms into pepsin. Triglycerides are divided into fatty acids by gumastic lipase.
• Microbes in food are killed by HCL, which parietal cells release.
• Its ability to absorb vitamin B12, which is essential for producing red blood cells, is an intrinsic component that aids. The hormone gastrin encourages the chief and parietal cells to secrete more acid and promotes stomach motility.
• The peristaltic motion of the stomach mixes gastric juice with the food. Chime is also forced past the pyloric sphincter.
• The pyloric sphincter controls how easily chyme travels from the stomach to the duodenum.
• Some water, salts, and some medications are absorbed by the stomach wall. Both the stomach and the small intestine absorb alcohol.

Absorption in Stomach

Proteins start to be broken down by gastric enzymes, but the stomach wall is not designed to absorb digestive substances. It only absorbs some salts, lipid-soluble medications, and some water. Both the stomach and the small intestine absorb alcohol.

Small Intestine

The digestive tract's small intestine, which runs from the stomach's pylorus to the ileo-ceacal valve, is the longest section. The small intestine is so named because, despite being longer than the big intestine, it has a narrower average width of roughly 2.5 cm (1 inch). The small intestine is located in the abdominal cavity and is encircled by the large intestine when relaxed to its maximum length, which is approximately 5–6 m (16–20 feet). There are three sections of the small intestine:

• Duodenum:The shortest and first area, the duodenum, extends from the jejunum to the pyloric sphincter of the stomach. It is 25 cm (10 inches). Brunner's glands are located in the wall of the duodenum and release an alkaline mucus to shield the small intestine from the stomach's highly acidic chyme. Enzymes that break down proteins and carbohydrates are also secreted by the cells of the small intestine. Additionally, a tiny gap in the duodenum allows the digestive juices of the liver and pancreas to enter the small intestine (major duodenal papilla). The duodenum has four sections:
  • The first part, also known as the superior part, starts just to the right of the body of vertebra L1 and runs from the pyloric opening of the stomach to the neck of the gallbladder. It is 5 cm long.
  • The second segment, also known as the descending segment, runs from the gallbladder's neck to the lower border of vertebra L3, just to the right of midline. It is 7 to 10 cm long. The major duodenal papilla, which serves as the bile and pancreatic ducts' common entrance, and the minor duodenal papilla, which serves as the accessory pancreatic duct's entrance, are both located in this region of the duodenum.
  • It crosses the vertebral column, the aorta, and the third part, or horizontal part. It is 6–8 cm long.
  • The fourth part, also known as the ascending part, begins at the duodenojejunal flexure and travels upward on or to the left of the aorta until it reaches roughly the upper border of vertebra L2. It is 5 cm long.
• Jejunum: The Jejunum, which means "empty," is the following two fifths of the small intestine and extends to the ileum. It is roughly 1 m (3 ft) length. It is the intestine's middle section. In comparison to the ileum, the jejunum has a larger diameter and a wall that is thicker, more vascular, and more active.
• Ileum: The small intestine's longest and last section. It connects to the large intestine at the ileocecal sphincter and is about 2 meters (6 feet) long. It has more bacterial life and lymphatic nodules (Peyer's patches).

Structure of the Small Intestine

There are four layers of tissues forming the walls of the small intestine.

• The sclerosing layer: The mesentery is the peritoneum's protective covering.
• The layer of muscle Under the serous membrane, there are two layers of smooth muscle fibers.
  • Longitudinal fibers in the outermost layer
  • Circular fibers in a deeper layer.
• The Sub-mucous layer: Consists of the autonomic nervous system's nerves, lymphatic vessels, and areolar tissue.
• The mucus membrane: Numerous peculiarities in the arrangement of the mucous membrane greatly increase the surface area of the small intestine. These structural characteristics make digestion and absorption easier.
• The circular folds: The circular folds are not smoothed out when the small intestine is swollen, unlike the rugae of the stomach. By increasing the small intestine's surface area, the circular folds aid in absorption.
• The villi: These are minuscule, finger-like protrusions into the organ's lumen. Their wall is made up of columnar epithelial cells that surround a web of lymphatic and blood capillaries. The lacteals are the name for the lymph capillaries. Through the villus's wall, nutritional substances are absorbed into the blood and lymph capillaries.
• Micro villi: They are incredibly tiny projections that reach into the small intestine's lumen. They increase the plasma membrane's surface area to allow the absorptive cells to absorb a lot of nutrients.
• Intestinal glands: These straightforward tubular glands are situated in between the villi. They release digestive juice that contains the enzymes needed to finish the chemical digestion of proteins, carbs, and lipids.
• Absorptive cells: These small intestinal mucosa cells use the chyme in the small intestine to digest and absorb nutrients.
• Lymph nodes: The mucous membrane of the small intestine contains numerous lymph nodes spaced erratically along its length. Solitary lymphatic nodules are the smaller ones, while aggregated lymphatic nodules (Payer's patches), which are roughly 20 or 30 bigger nodules located near the distal end of the ileum, are the larger ones. They capture and eliminate antigen and stop the growth of harmful microorganisms in the intestines.

Blood Supply of the Small Intestine

The superior mesenteric artery, a branch of the abdominal aorta, supplies the arterial blood. The superior mesenteric vein drains its impure blood through venous drainage.

Nerve Supply of the Small Intestine

The sympathetic and parasympathetic nerves provide the small intestine with its nerve supply.

Function of the Small Intestine

• By using peristaltic, which is facilitated by parasympathetic stimulation, it aids in the forward flow of its contents.
• It releases the digestive fluids.
• It completes the breakdown of proteins, carbs, and lipids in the villi's enterocytes.
• It guards against microbial infections that have evaded the HCL's antimicrobial effects.
• Cholecystokinin (CCK) and secretin are hormones that are secreted by it.
• It absorbs about 99% of the nutrients' final form.

Intestinal Juice

Intestinal juice is released in amounts of 1-2 liters. It is a 99% water, 0.5–1% solid, and mucus-containing transparent yellow fluid. Peptides, enterokinase, sucrose, maltase, lactase, dextrose, lipase, mucus, and intrinsic factors are among the components of the organic solid. Among the inorganic solid components are phosphate, potassium, calcium, sodium, and bicarbonate. The small intestine's ability to absorb substances is aided by intestinal and pancreatic juices.

Enzymes of the Digestion

The digestive system's salivary glands, lingual glands, stomach, pancreas, liver, and intestinal mucosa all manufacture and secrete digestive enzymes. In the villi of enterocytes, the latter stages of digestion frequently occur. Nearly all of these enzymes are hydrolases. The process of splitting an organic molecule by adding water is known as hydrolysis. Different enzymes hydrolyze the proteins, lipids, and carbohydrates to be absorbed. The primary enzymes include lipases from the tongue, stomach, and pancreas as well as pepsin, trypsin, chymotrypsin, and carboxypolipeptidase.