Human Digestive System

Skatole or 3-methylindole is a mildly toxic white crystalline organic compound belonging to the indole family. It occurs naturally in faeces (it is produced from tryptophan in the mammalian digestive tract), and coal tar, and has a strong faecal odour.

It helps body get rid of undigested food.

Tobacco naturally contains nicotine, which is a highly addictive stimulant compound. Nicotine is the primary psychoactive ingredient in tobacco products that contributes to addiction.

Digestion begins in the mouth through both mechanical breakdown by teeth and chemical action of salivary amylase enzyme on starches. This is the first stage of the digestive process before food moves to the stomach and intestines.

The small intestine is the primary site of food digestion and nutrient absorption. While digestion begins in the mouth with salivary amylase, most enzymatic breakdown and absorption (90%) of nutrients occurs in the small intestine with help from pancreatic enzymes and bile.

The duodenum is the first and shortest section of the small intestine, approximately 25-30 cm long, connecting directly to the stomach at the pyloric sphincter. It receives partially digested food (chyme) from the stomach, along with bile from the liver and digestive enzymes from the pancreas. The caecum marks the beginning of the large intestine, while the ileum and jejunum are the middle and final sections of the small intestine, respectively. The duodenum's primary function is neutralizing stomach acid and continuing chemical digestion.

Plastic materials, if taken in by cows choke their wind pipe and make it difficult for them to breathe. Since it cannot be digested, then it cannot affect their digestive or excretory system in the way it affects the respiratory system. 

Enterogastrone is a hormone secreted by the intestinal mucosa when acidic chyme enters the small intestine. It inhibits gastric acid secretion and stomach motility, protecting the duodenum and slowing gastric emptying. Option C correctly describes this inhibitory function.

Secretin is the primary hormone that stimulates the pancreas to secrete pancreatic juice (rich in bicarbonate to neutralize acidic chyme). It is released from S cells in the duodenum when acidic chyme enters the small intestine. Trypsinogen is a pancreatic enzyme precursor, enterokinase activates trypsinogen, and cholecystokinin stimulates enzyme secretion but fluid/bicarbonate secretion is mainly secretin-driven.

Different phases of digestion take place including the cephalic phase, gastric phase and intestinal phase. The cephalic phase occurs at the sight, thought and smell of food, which stimulate the cerebral cortex. Taste and smell stimuli are sent to the hypothalamus and medulla oblongata.

Water and lipids are absorbed by passive diffusion throughout the small intestine.

Pancreatic amylase digests all the remaining starch in the duodenum. Amylase digests starch molecules from the ends of the chains in two-glucose units, forming the disaccharide maltose. Glycogen is also digested here.

The muscularis externa consists of an inner circular layer and a longitudinal outer muscular layer. The circular muscle layer prevents food from traveling backward and the longitudinal layer shortens the tract.

Pepsin (in gastric juice) digests proteins to peptides, 6-12 amino acids long. Pepsin is an endopeptidase, which means it hydrolyses peptide bonds in the middle of a polypeptide chain. It is unusual in that it has an optimum pH of about 2 and stops working at neutral pH.

After passing through the small intestine, food passes into the large intestine. In the large intestine, some of the water and electrolytes (chemicals like sodium) are removed from the food. Many microbes (bacteria like Bacteroides, Lactobacillus acidophilus, Escherichia coli, and Klebsiella) in the large intestine help in the digestion process.