A renal afferent arteriole is directed toward the glomerulus, while a renal efferent arteriole is directed away from the glomerlus. If the radius of the afferent arteriole is increased, there is more flow through it toward the glomerulus, and if there is a smaller radius in the efferent arteriole, there is a resultant back pressure in the glomerulus. This is can be imagined as trying to squeeze a high-pressure hose through a small pipe. This pressure increases the force within the glomerulus to increase filtration, and subsequently increase reabsorption.

Antidiuretic hormone (ADH) secretion has a neglible effect on the radius of renal arterioles. Blood flow to the kidney is increased when afferent arteriole radius is increased (this also increases the arteriole flow).

The adrenal gland is situated superior to the kidney, and is responsible for the production of several key hormones.

One such hormone is cortisol. Cortisol is often released when an individual is highly stressed over an extended period. It is a steroid hormone that functions to increase blood glucose levels by inducing gluconeogenesis, the process that converts glycogen stores in the liver to glucose. Cortisol is released from cells in the adrenal cortex. Recall that the adrenal cortex is the outer portion of the gland, whereas the adrenal medulla is the inner portion of the gland.

Aldosterone is another hormone that is released by cells in the adrenal cortex, and is essential for sodium reabsorption in the kidney. Epinephrine is released from the adrenal medulla in response to stimulation by the sympathetic nervous system.

Recall that the renal medulla is the inner portion of the kidney. A nephron spans both the renal cortex and the renal medulla. Structures such as the glomerulus (capillary bed), Bowman's capsule, and the proximal and distal convoluted tubules are found in the renal cortex, whereas the loop of Henle is found in the renal medulla. The collecting duct (the structure that transports urine to the renal pelvis) spans both the renal cortex and the renal medulla.

As mentioned above, distal tubules are found in the renal medulla and function to reabsorb sodium ions. Reabsorption of sodium ions inside the nephron (for example in the distal tubules) is facilitated by the hormone aldosterone. Aldosterone is a steroid hormone that is produced in glands inside the adrenal cortex (in the adrenal gland, rather than the kidney).

The loop of Henle descends into the medulla before ascending back into the cortex. The collecting duct, which ends the nephron, extends into the medulla.

The main function of the Loop of Henle is to establish a concentration gradient so that water can be reabsorbed from the collecting duct and avoid being lost as urine. Although the ascending limb does absorb water, this water would be lost as urine if it were not for the concentration gradient established in the medulla of the kidney. Neither sodium nor potassium is absorbed in the Loop of Henle.

Diabetes mellitus is the product of decreased insulin effectiveness in the body. As a result, blood glucose levels are extremely high. When filtrate enters the nephron through Bowman's capsule, glucose is generally transported as well. In a healthy individual, this glucose is rapidly removed from the filtrate in the proximal convoluted tubule. In a diabetes patient, however, the level of glucose in the filtrate can overwhelm the reabsorption of the tubule, resulting in glucose in the urine. This increases urine osmolarity, causing the filtrate to retain water. The result is an increase in urine volume, resulting in more frequent urination.

The kidney uses all three of the following processes: filtration, secretion and reabsorption. All three of these processes aid in allowing the body to filter waste products from the blood while retaining nutrients, salts, and water when needed.

Filtration occur when filtrate is separated from blood in the renal corpuscle. Reabsorption is the removal of ions from the filtrate to retain salts. Secretion is the input of salts to the filtrate to eliminate them. All of these processes occur in the nephrons.

The correct path of filtrate through a nephron starts in the renal corpuscle, which is comprised of the glomerulus and Bowman's capsule. Filtrate then passes through the proximal convoluted tubule, where the majority of reabsorption takes place. It then travels through the descending and ascending limbs of the Loop of Henle, creating the counter current multiplier gradient that will allow urine to be concentration in the collecting duct. From the Loop of Henle, filtrate enters the distal convoluted tubule for final reabsorption before entering the collecting duct and being trasported to the bladder.

The renal artery is used to carry blood into the kidneys. Filtrate originates from the renal artery, but it is not a part of the nephrons.

The Loop of Henle creates a countercurrent multiplier system. As the filtrate descends through the Loop of Henle, water leaves the filtrate and is reabsorbed, making the filtrate very concentrated. When the Loop of Henle ascends, salt ions leave the filtrate and are reabsorbed making the filtrate less concentrated. This creates a strong concentration of ions in the interstitial fluid toward the bottom of the loop, as compared to the concentration at the top. When filtrate flows down the collecting duct, this gradient helps concentrate the urine by removing water.

The ascending and descending limbs of the Loop of Henle are responsible for creating a countercurrent multiplier system, which concentrates urine and allows water and electrolytes to passively diffuse down their concentration gradients.

All the other options are part of the nephron, but are not responsible for the process of urine concentration. The glomerulus and Bowman's capsule are responsible for collecting and producing initial filtrate from the blood, and form the renal corpuscle. The proximal convoluted tuble is the initial site of reabsorption.