Gastrointestinal System

Movement: Mixes and passes nutrients through the system while eliminating waste.
Secretion: Involves the release of enzymes, hormones, and other substances necessary for digestion into the digestive tract.
Digestion: Chemical breakdown of nutrients into smaller units that can enter the internal environment of the body.
Absorption: Nutrients pass through plasma membranes into the blood or lymph after being digested.

Digestion begins in the mouth as food is chewed and mixed with saliva.
Only humans and other mammals chew their food.
Salivary glands are stimulated to secrete saliva, which contains enzymes to initiate the breakdown of starch.
Once swallowed, food moves from the mouth down the pharynx and into the esophagus towards the stomach.

The stomach is a flexible, muscular sac with three main functions:
- Mixing and storing food
- Dissolving and degrading food via secretions
- Controlling the passage of food into the small intestine
Protein digestion begins here; stomach acidity aids in breaking down food, making nutrients available for absorption.
Peristalsis: Smooth muscle contractions that move food by contracting and relaxing, facilitating movement into the small intestine.

The liver is the largest solid organ and gland in the body, weighing about three pounds in adults; located below the diaphragm on the right side of the abdomen.
Comprised of four lobes: right, left, quadrate, and caudate.
Secured to the diaphragm and abdominal walls by five ligaments:
- Falciform
- Coronary
- Right triangular
- Left triangular
- Round
The liver processes blood received from the intestines via the hepatic portal vein and is supplied with oxygen via the hepatic artery.
Blood leaves the liver through hepatic veins.
Functional units of the liver are called lobules which consist of layers of liver cells. Blood enters these lobules through branches of the portal vein and hepatic artery, and flows through sinusoids.

Production of Bile: Essential for digestion and absorption of fats.
Production of Blood Plasma Proteins.
Production of Cholesterol and certain proteins necessary for fat transport.
Storage of Glucose: Excess glucose is stored as glycogen and can be converted back to glucose as needed.
Regulation of Amino Acids.
Processing of Hemoglobin: Involved in storing iron.
Conversion of Ammonia to Urea: Ammonia, toxic to the body, is converted to a waste product (urea) excreted in urine.
Purification of the Blood: Clears out drugs and toxins.
Regulation of Blood Clotting.
Controlling Infections: Boosts immune factors and eliminates bacteria.

The pancreas is 6 to 10 inches long, located behind the stomach in the back of the abdomen, and is tapered in shape:
- The broader right side is called the head (near the duodenum), while the narrower left side is the tail (ending near the spleen).
- The body of the pancreas lies between the head and the tail.
Comprised of exocrine and endocrine tissues:
- Exocrine Tissue: Secretes digestive enzymes through a series of ducts forming the main pancreatic duct.
- Endocrine Tissue: Secretes hormones, such as insulin, into the bloodstream.
Blood supply comes from the splenic artery, gastroduodenal artery, and superior mesenteric artery.

The pancreas aids digestion by:
- Secreting enzymes to the small intestine that help break down fats and proteins.
- Producing inactive precursors of enzymes called zymogens, which are converted into active enzymes (e.g., pancreatic lipase, amylase) in the gut.
- Secreting sodium bicarbonate to neutralize stomach acid reaching the small intestine.
The exocrine functions are regulated by hormones released by the stomach and small intestine when food is present.

The majority of nutrient absorption occurs in the small intestine.
Enzymes from the pancreas, liver, and stomach contribute to digestion here.
Bile, a secretion of the liver, is crucial for fat breakdown and is stored in the gall bladder.
By the time food reaches the lining of the small intestine, it has transformed into small molecules.
The lining features villi, enhancing the surface area for interaction with chyme (semi-liquid mass of partially digested food).
Microvilli: Epithelial cells on the villi further maximize the absorptive capacity of the small intestine.

Also known as the colon, the large intestine concentrates, mixes, and stores waste material, measuring a little over a meter in length.
Structure: The colon ascends on the right side of the abdominal cavity, transversely cuts to the left side and descends to attach to the rectum (waste disposal tube).
Waste expulsion is triggered when the rectal wall distends, sending a nervous signal to the body.
A muscle sphincter at the anus facilitates the expulsion of waste.
The movement speed of waste through the colon is influenced by dietary bulk, particularly fiber; inadequate bulk may delay waste travel, potentially leading to health issues.