"Medullary collecting ducts" are divided into outer and inner segments, the latter reaching more deeply into the medulla.

They receive renal filtrate (precursor to urine) from several medullary collecting ducts and empty into a minor calyx.

Papillary ducts continue the work of water reabsorption and electrolyte balance initiated in the collecting tubules.

The duct is lined by a layer of simple columnar epithelium resting on a thin basement membrane.

[6] These cells work in tandem to reabsorb water, sodium, and urea and secrete acid and potassium.

These processes are mediated by hormones (aldosterone, vasopressin) and the osmolarity (concentration of electrically charged chemicals) of the surrounding medulla.

Osmolarity influences the strength of the force that pulls (reabsorbs) water from the papillary duct into the medullary interstitium.

Thus the force driving the reabsorption of water from the collecting system is the greatest in the papillary duct.

[10]: 336  Vasopressin determines the expression of aquaporin channels that provide a physical pathway for water to pass through the principal cells.

[12][13] For their contribution to acid–base homeostasis, the intercalated cells play important roles in the kidney's response to acidosis and alkalosis.

[17][18] The collecting duct system is the final component of the kidney to influence the body's electrolyte and fluid balance.

The wide variation in water reabsorption levels for the collecting duct system reflects its dependence on hormonal activation.

The collecting duct system participates in the regulation of other electrolytes, including chloride, potassium, hydrogen ions, and bicarbonate.