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.

The nephron is composed of multiple sections, which are used to control filtrate solute concentration and form urine. The proximal convoluted tubule does not alter solute concentration, and the distal convoluted tubule decreases solute concentration in the filtrate. It is the loop of Henle that increases the solute concentration of the filtrate.

Filtration occurs in the renal corpuscle, which is composed of the glomerulus and Bowman's capsule. The glomerulus filters the blood, resulting in the filtrate, which will then go through the tubules. The proximal and distal convoluted tubules are primarily involved in solute reabsorption, and the loop of Henle helps concentrate the urine.

Water, small molecules, and ions are forced through the glomerular capillaries into Bowman’s capsule in the process of filtration, largely due to the high blood pressure in the capillaries of the glomerulus. Nitrogenous compounds, namely urea, are filtered into Bowman's capsule for excretion. Glucose is also filtered into the capsule, but is reabsorbed in the proximal convoluted tubule; the presence of glucose in the final filtrate or urine can be indicative of diabetes or other maladies.

The correct answer is that two answer options are correct: small ions, glucose, amino acids, water, and nitrogenous compounds are all found in the initial filtrate.

Large molecules and cells are never filtered through the podocytes (cells lining the capsule), so we never expect to find blood cells or large blood proteins like albumin in the filtrate.

The nephrons are the basic filtration units of the kidneys. In the cortex (plural "cortices") or outer portion of the kidney, blood enters a nephron consisting of a glomerulus (mass of capillaries), renal tubules and loop of Henle. Urine is formed by filtration in the glomerulus and reabsorption and secretion in the tubules. The inner medulla of the kidney contains the collecting ducts, which merge together to form papillary ducts that enter into a calyx (plural "calyces") or funnel-shaped structure. The hilum is the part where the renal artery enters and renal vein and ureter leave the kidney.

The correct answer is antidiuretic hormone (ADH), also known as vasopressin. ADH controls the permeability of the collecting duct of the nephron to water. If ADH levels are high in the blood, more water will be reabsorbed into the bloodstream, concentrating the urine. Alternatively, if ADH levels are low, less water is reabsorbed into the bloodstream, and the urine is dilute. 

The large intestine is responsible for further absorption of water and any remaining nutrients that the small intestine did not absorb. Also, the large intestine is responsible for the formation of fecal matter.