The walls of the thick ascending limb of loop of Henle are impermeable to water; so in this section of the nephron, water is not reabsorbed along with sodium.

Vasopressin causes an acute increase of sodium absorption across the ascending loop of Henle. This adds to the countercurrent multiplication which aids in proper water reabsorption later in the distal tubule and collecting duct.

The excretory organ of Balanoglossus is a glomerulus or proboscis gland lying in front of the central sinus and projecting into the proboscis coelom.

Constriction of efferent arterioles as blood exits the glomerulus provides resistance to blood flow, preventing a pressure drop. Efferent arterioles are smaller in diameter than afferent arterioles. As a result, pressurised blood enters the glomerulus through a relatively wide tube, but is forced to exit through a narrower tube. Together, these unique features plus the fact that heart is supplying kidneys with over a litre of blood per minute maintain a high glomerular capillary pressure.

Epithelium layer consists of specialised cells called podocytes. These cells are attached to the basement membrane by foot processes (pedicels). They wrap around the capillaries, but leave slits between them, known as filtration slits.

Peritubular capillaries are tiny blood vessels that run parallel to and surround the proximal and distal tubules of the nephron, as well as the loop of Henle, where they are known as the vasa recta. The peritubular capillaries, i.e. vasa recta are important for countercurrent exchange, the process that concentrates urine.

Atrial natriuretic peptide is a hormone that can increase the glomerular filtration rate. This hormone is produced in heart and is secreted when plasma volume increases, which increases urine production.

Most of the reabsorption (65%) occurs in the proximal tubule. In the latter part, it is favoured by an electrochemical driving force. Sodium passes along an electrochemical gradient (passive transport) from the lumen into the tubular cell, together with water and chloride which also diffuse passively. Water is reabsorbed to the same degree, resulting in the concentration in the end of the proximal tubule being the same as in the beginning.

The release of antidiuretic hormone by the pituitary gland is controlled by sensors in heart and blood vessels that detect drop in blood pressure, or increased concentrations of salt in bloodstream that may occur when dehydration occurs.

Aldosterone acts on the distal convoluted tubule. Aldosterone increases sodium reabsorption in DCT.