This is a fundamental question that checks for clear understanding of the purpose of the digestive system. The digestive system includes the mouth, esophagus, stomach, small intestine, and large intestine. The primary functions of these structures are to break down and absorb nutrients from ingested food.

The respiratory system is responsible for providing a site for gas exchange (namely the alveoli). The circulatory system delivers oxygen to tissues, while the excretory system removes nitrogenous wastes and maintains water balance via kidney function.

It is important to know sites of production and action of the digestive enzymes. In the stomach, the very low pH environment facilitates gastric enzyme function, so we can already assume that an acidic compound is involved in the stomach.

HCl, or hydrochloric acid, is the acidic compound released by parietal cells and functions to kill microorganisms, digest acid labile substances, and activate pepsinogen to create pepsin.

The second main digestive compound of the stomach is pepsin, a protease, which digests amino acids into smaller peptides. It is released as a zymogen, or inactive form, by chief cells before it is activated to pepsin. The inactive form of pepsin is called pepsinogen.

Bile is a substance produced by the liver, stored by the gall bladder, and secreted into the duodenum of the small intestine for emulsification of fat globules. HCO₃^-, or bicarbonate ion, is a basic substance released by the pancreas into the duodenum to neutralize the entering chyme, reducing acidity to allow for optimal enzyme activity in the duodenum. Neither bile, nor bicarbonate are active in the stomach.

The stomach contains the enzyme pepsin, which helps sever the peptide bonds between amino acids and breaks protein molecules into smaller fragments. As the contents of the stomach enters the small intestine, more digestive enzymes are added to further digest the proteins; however, the first step occurs in the stomach.

It is important to note that the mouth is the first location at which carbohydrates are digested, due to the protein amylase found in saliva.

The presence of food in the stomach stimulates the release of gastrin, which stimulates the secretion of gastric acid (). The acid in the stomach aids in digestion, and also functions as a nonspecific line of defense from infection, destroying most pathogens. Cholecystokinin is a hormone produced by the small intestine, and is responsible for stimulating release of digestive enzymes by other gastrointestinal organs. Amylase is an enzyme that breaks down starch. Motilin is a hormone that aids in digestion via pathways other than stimulating production of gastric acid in the stomach. 

Amylase is the enzyme secreted in salvia and into the small intestine by the pancreas that breaks down startch. Insulin is a hormone released from the pancreas that decreases blood glucose levels. Lysozyme is an enzyme present in saliva, tears, and sweat, and functions as a natural antibacterial. Pepsin is an enzyme responsible for the breakdown of proteins. Gastrin is a hormone released by the cells of the gastrointestinal organs which stimulates secretion of  from the stomach, ultimately aiding in digestion. 

When food is swallowed and reaches the stomach, the distension of the stomach is the number one stimulator for acid production. Smelling food and chewing food does stimulate acid production, however, the rise in acid produced is not as significant as produced by distention of the stomach. 

Many desert animals have adapted physiologically with certain mechanisms to retain more water and survive the in a dry, desert environment. Such desert animals have long loops of Henle, allowing greater opportunity to reabsorb water in the medulla in the descending loop of Henle.

The correct answer is the glomerulus, a convoluted capillary bed that is directly adjacent to the nephron. Solutes from the blood are filtered in the glomerulus and enter the nephron, specifically, Bowman's capsule. Both blood pressure and oncotic force (pressure in the glomerulus due to proteins) affect filtration rate. Together, the glomerulus and Bowman's capsule are known as the renal corpuscle.

The glomerular basement membrane—which sits between capillary endothelial cells and the selectively permeable podocytes that line Bowman's capsule—is comprised of negatively charged proteins that repel other negatively charged proteins. This helps prevent these compounds from entering the nephron with the filtrate.

Sodium is coupled with glucose via a cotransporter, allowing it to enter the proximal tubule cells from the tubule lumen. Once in the proximal tubule cells, glucose then passes through a Glut-2 transmembrane carrier protein, but only on the basolateral membrane of the proximal tubule cells (from proximal tubule cells to the interstitual fluid).