The final segment of the nephron is the collecting duct. The collecting duct is where water levels are adjusted to meet the osmoregulatory needs of the organism and therefore maintain homeostasis. The concentration gradient from cortex to medulla established by the loop of Henle allows the collecting ducts to retain or lose water as needed depending on the water levels of the organism.

The dilute filtrate enters the collecting ducts, passing from the cortex to the medulla. These ducts drain into the renal pelvis at the base of the kidney. In response to the concentration gradient from the cortex to the medulla, water moves out of the collecting ducts into the interstitial fluid by osmosis as long as the collecting ducts are permeable to water.

The permeability of the collecting ducts to water can be adjusted to determine how dilute or concentrated the urine is. The water permeability of the collecting ducts is controlled by the peptide hormone antidiuretic hormone (ADH), or vasopressin. ADH is secreted by the posterior pituitary gland (Chapter 38) in response to signals from the hypothalamus, where receptors monitor the concentration of solutes in the blood. When a person is dehydrated, solute concentration is high and ADH is secreted. In the presence of ADH, the collecting duct walls become permeable to water (Fig. 41.21a) as aquaporins insert into cell membranes. Water in the collecting duct diffuses into the interstitial fluid through the aquaporins and the urine becomes more concentrated. When a person is dehydrated, the urine leaving the collecting ducts can become as concentrated as the interstitial fluid at the base of the kidney.

When a person is well hydrated, no ADH is released. In the absence of ADH, the collecting ducts are impermeable to water, preventing the water from being reabsorbed (Fig. 41.21b). The result is dilute urine. Some diuretic substances (“diuretic” is from a Greek word meaning “to flow through”), such as alcohol, promote the dilution of urine by interfering with ADH production and therefore making the collecting ducts impermeable to water. When dilute urine is produced, there is net water loss. Consequently, diuretic drinks often cause dehydration.

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The regulation of water and solute reabsorption by the kidney is remarkably precise. The kidneys of a human filter 180 L of blood each day (125 mL/min). The total blood volume of a human is about 5 L (of which 3 L is plasma), which means that the blood is filtered about 60 times per day. Urine production in humans averages 0.5 L per day, so under normal conditions, the kidneys reabsorb 99.9% of the water entering the renal tubules. In addition, the kidneys reabsorb all the glucose and amino acids that are filtered out of the blood, together with 99.9% of the electrolytes in the blood.

The exceptional ability of the kidneys to filter and reabsorb electrolytes, solutes, and water also explains why kidney damage can have such serious consequences. People with kidney disease must have their blood filtered by dialysis treatment as frequently as three times per week, each treatment lasting several hours. Otherwise, toxic wastes build up to dangerous levels in the body. Kidney disease remains a substantial health problem in the world today, costing more than $34 billion in the United States alone.