Facultative Reabsorption and Its Features
The process of reabsorbing substances in the distal nephron segments is called facultative reabsorption. Its primary function is to flexibly adjust the qualitative and quantitative characteristics of urine to suit the body's needs. On average, about 10% of sodium ions ($Na^+$) and water are returned to the bloodstream at this stage.
A critical feature of the distal convoluted tubules is the high energy demand of transport. Unlike the proximal segments, $Na^+$ reabsorption here is significantly hindered. This is because ion transport occurs against a substantial electrostatic gradient of 50 mV. This gradient is formed by the potential difference across the tubular epithelial cell membranes: it is 70 mV at the basolateral membrane and 20 mV at the apical membrane.
The transport of other ions is fundamentally similar to the mechanisms operating in the proximal tubules. However, the behavior of potassium ($K^+$) is highly variable: if an excess of potassium occurs in the blood, transport reverses, and passive secretion of potassium from the bloodstream into the forming urine begins. In addition to potassium, the epithelium actively secretes hydrogen ions ($H^+$), ammonia, and p-aminohippuric acid.
Hormonal Regulation of Ion and Water Transport
The function of the distal tubules and collecting ducts is under strict hormonal control, which determines the final urine volume:
- Aldosterone. An adrenal cortex hormone that is a potent stimulator of $Na^+,K^+$-ATPase. Its action leads to a sharp increase in sodium ion reabsorption. As osmotic pressure changes, water passively follows sodium into the bloodstream.
- Natriuretic peptide. Acts as an antagonist to aldosterone. It inhibits $Na^+,K^+$-ATPase activity, which naturally leads to a decrease in the reabsorption of sodium and, consequently, water.
- Antidiuretic hormone (ADH, or vasopressin). Produced by the posterior pituitary gland, ADH is the main regulator of urine concentration in the collecting ducts. Its mechanism of action involves the activation of specific proteins called aquaporins. This significantly increases the water permeability of the tubular walls, enhancing water reabsorption and increasing the concentration of solutes in the urine (salts, urea, ammonia).
Mechanism of Final Urine Concentration
Upon entering the collecting ducts, urine undergoes its final concentration stage before being excreted into the renal pelvis. Maximal concentration occurs precisely at the exit into the pelvis.
The key factor in this process is the concentration gradient. The activity of the preceding loops of Henle creates a zone of very high salt and $Na^+$ concentration surrounding the collecting ducts. Because of this osmotic gradient, water can passively leave the lumen of the ducts through specialized pores into the intercellular space, returning to the body.
An additional role is played by the urea recycling loop. A portion of urea and other nitrogenous compounds diffuses from the terminal urine into the renal interstitium (intercellular fluid). A unique cycle arises: urea passively enters the loops of Henle from the interstitium, travels with the fluid flow to the distal convoluted tubules, and re-enters the collecting ducts, maintaining the high osmolarity of the renal medulla.