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). 

Kidneys contain small structures called nephrons that function to create urine. Urine is created by filtering the incoming blood. The blood enters the nephron via the glomerulus. It enters the Bowman’s space and then is transported through a series of nephron segments. Each segment has varying permeabilities for ions and water. The nephron segment that is only permeable to water is the descending loop of Henle. As urine travels down this segment, water gets reabsorbed back into the blood whereas the solutes remain in the urine (because the descending loop of Henle is impermeable to solutes). This makes the urine very concentrated.

The ascending loop of Henle has opposite permeabilities. Solutes, such as ions, are permeable and can cross the wall and be reabsorbed into the blood; however, water is impermeable. The proximal convoluted tubule is the site of most of the reabsorption and secretion of molecules; therefore, both water and solutes are permeable here. The collecting duct is not part of the nephron. Multiple nephrons drain into a collecting duct. Collecting ducts are sites of further reabsorption and secretion; therefore, both water and ions are permeable.

Note that reabsorption is the process of moving substances (solutes or water) from urine to the blood whereas secretion is the process of moving substances from the blood to urine.

Blood from afferent arterioles enter the Bowman’s space via glomerulus. The filtered blood in the Bowman’s space traverses through different nephron segments before being excreted as urine. The glomerulus and Bowman’s space together are called the renal corpuscle; therefore, blood is first filtered here.

The urine gets processed (reabsorption or secretion) in different nephron segments before being ultimately excreted. During this process, the urine (also called tubular fluid) is transported via tubes called renal tubules; therefore, this is the site of reabsorption and secretion.

A nephron begins at the renal corpuscle and ends at the distal convoluted tubule. The renal corpuscle consists of the glomerulus and the Bowman’s capsule; therefore, the glomerulus is a part of the nephron. The collecting duct, on the other hand, is not part of a nephron. Multiple nephrons drain their tubular fluid into a single collecting duct. The tubular fluid is further filtered in the collecting duct and is ultimately excreted as urine. The distal convoluted tubule (last part of a nephron) is connected to the collecting duct.

Most of the reabsorption and secretion of ions and water happens in the proximal tubule, which is the nephron segment located right next to the Bowman’s space. This means that most of the fluid that enters the Bowman’s space is immediately processed in the proximal tubule. The latter third of a nephron also processes the fluid; however, it is negligible when compared to the proximal tubule.

Most of the urine is created in the nephron. The tubular fluid traverses each nephron segment, where it is processed. The final fluid that enters the collecting duct is processed further, but most of the urine that is excreted comes from the nephron.

Kidneys can be divided into two parts: renal cortex and renal medulla. The cortex is the outer part of the kidneys that contains the renal corpuscles (glomerulus and Bowman’s space) and most of the renal tubules (except the loop of Henle). The medulla is the inner part; it contains the ascending and descending limbs of the loop of Henle, and the portion of renal tubule that descends into the medulla. The proximal convoluted tubule and the distal convoluted tubule are not part of the loop of Henle and, therefore, are found in the renal cortex.

Note that the nephron begins at the renal corpuscle in the cortex, becomes the proximal tubule (in the cortex), descends into the medulla as loop of Henle, and rises back into the cortex as the distal tubules. The tubular fluid in distal tubules drains into the cortical collecting duct (the region of the collecting duct found in the cortex), which eventually enters the medullary collecting duct and leaves the kidney as urine.