Human Digestive System

Mastication or chewing is the process by which food is crushed and ground by teeth. It is the first step of digestion and it increases the surface area of foods to allow more efficient break down by enzymes.

Mass movement of feces into the rectum stimulates a defecation reflex that opens the internal anal sphincter. Unless the external and sphincter is voluntarily closed, feces are evacuated through the anus. Defaecation is the main function of large intestine.

Parietal (oxyntic) cells are scattered along the neck and lower walls of the ducts. They secrete hydrochloric acid (HCl) and intrinsic factor. Intrinsic factor is necessary for the absorption of Vitamin B12 in the small intestine.

The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. It is not an accessoary part of digestive system.

The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, such as bones cellulose or lignin.It is not a part of small intestine.

An ulcer is fundamentally a break or sore in the epithelial lining. This can occur on the skin (like bed sores or diabetic foot ulcers) or in the mucous lining of internal organs (like peptic ulcers in the stomach/duodenum). Both manifestations involve tissue breakdown and impaired healing, whether external (skin) or internal (organ lining).

Gastric specifically refers to the stomach. Therefore, a gastric ulcer is a sore that develops in the stomach lining. Peptic ulcers is the broader category that includes both gastric ulcers (stomach) and duodenal ulcers (upper part of small intestine). The term helps distinguish the location of the ulcerative damage.

Bile is a digestive fluid produced by the liver and stored in the gallbladder. It helps break down fats in the small intestine during digestion. The liver continuously produces bile, which is then concentrated and stored in the gallbladder.

Saliva contains the enzyme salivary amylase (also called ptyalin), which begins the breakdown of starch into simpler sugars like maltose. This is the first step of carbohydrate digestion. Saliva does not contain enzymes that digest proteins (requires pepsin/trypsin) or fats (requires lipase). Fiber is indigestible by human enzymes.

Cysteine strengthens the protective lining of the stomach and intestines, which may help prevent damage caused by aspirin and similar drugs.

Digestive enzymes (like amylase, protease, lipase) break down food molecules into smaller absorbable units during digestion. They catalyze specific reactions - carbohydrates to sugars, proteins to amino acids, fats to fatty acids. While enzymes play roles in many body processes, the digestive system is their primary domain.

Ptyalin (also known as salivary amylase) is an enzyme present in saliva, which is produced in the mouth by salivary glands. It begins the digestion of starch into simpler sugars like maltose. The enzyme works optimally in the slightly acidic to neutral pH of the mouth (pH 6.7-7.0). No ptyalin is present in the stomach, pancreas, or liver.

Nutrition is the biological process of obtaining food and utilizing it for energy, growth, and repair. It encompasses ingestion, digestion, absorption, and metabolic utilization of nutrients.

The liver produces bile but doesn't secrete any digestive enzymes. Bile emulsifies fats (breaks them into smaller droplets) but doesn't chemically digest them. Salivary glands secrete amylase, small intestine glands secrete various enzymes, and the pancreas secretes multiple enzymes like amylase, lipase, and proteases.

Peristalsis is the wave-like muscular contractions of the digestive tract (especially intestines) that propel food contents forward. This rhythmic contraction and relaxation of smooth muscle is essential for digestion and absorption. Metabolism is all chemical reactions, osmosis is water movement, and catabolism is breakdown of complex molecules.