Human Digestive System

All the constituents of food are digested but roughage or fibres are not digested, but they play a very important role in the formation and smooth elimination of bowels.

• Glucose is the major source of energy in the body. excess glucose is stored in the form of glycogen in muscles and liver.
  
• Most of the glycogen is stored in liver i.e 10 % of the mass of liver and the muscles contain a low amount of glycogen i.e 1% of the muscle mass.
  
• When the glucose is completely utilized the animal feels exhausted, to overcome this the glycogen is degraded to lactic acid by enzymes in muscles and liver. 
• So, the correct answer is 'Exhausted'.

Enterogastrone is a hormone secreted by the duodenal mucosa that inhibits gastric secretion and motility when fat is present in the duodenum.

Pepsin enzymes are the enzymes that help in degradation of the proteins into various components like peptones. These enzymes are mostly found in stomach which has acidic pH.

Excretion is process by which nitrogenous waste is thrown out of body. It is done to avoid toxicity in body by the waste, maintain pH and water level. It's done by various organs like kidney in human, nephridia in worms, etc.

The epiglottis is a flap-like structure made of elastic cartilage tissue covered with a mucous membrane, attached to the entrance of the larynx. The main function of the epiglottis is to close off the windpipe during eating which prevents the entry of the food into the respiratory tract so that food is not accidentally inhaled. Hence option D is correct.

Rennin (chymosin) is an enzyme that helps in the digestion of milk proteins. It is found in the gastric juice of young mammals (like calves) and human infants to digest milk, but its production decreases significantly in human adults.

Rennin is an enzyme primarily found in the gastric juice of infant mammals to coagulate milk. In adult humans, rennin is typically absent or present in negligible amounts.

Rennin is the specific gastric enzyme in infant mammals that converts soluble caseinogen into insoluble casein, which is then digested by pepsin.

Zymogen cells, also known as chief cells or peptic cells, are located in the gastric glands and are responsible for secreting pepsinogen, the inactive precursor to pepsin.

Pepsin is a protease that requires a low pH environment, provided by hydrochloric acid in the stomach, to become active and function optimally.

Gastric acid secretion is stimulated by multiple pathways: gastrin (hormonal), histamine (paracrine), and vagal activation (neural). Therefore, all these factors contribute to the process.

Chief cells (or zymogen cells) are the primary source of pepsinogen secretion in the gastric glands of the stomach.

Liquids, especially those like beer or water, pass through the stomach much faster than solid foods like proteins, carbohydrates, or fats, which require significant mechanical and chemical breakdown.