The glomerulus ultrafiltrates plasma to form urine. This involves three processes: (1) filtration, (2) reabsorption, and (3) secretion. Despite filtering a large amount of fluid, only a small percentage is excreted in urine. The renal tubules play a crucial role in determining the volume and composition of urine, allowing the kidneys to regulate the extracellular and intracellular fluid compartments. Transport proteins in the nephron mediate the reabsorption and secretion of solutes and water. Kidney diseases can arise from defects in these transport proteins. The chapter discusses various aspects of solute and water reabsorption, including NaCl and water transport, organic anions and cations, and the regulation of NaCl transport. The proximal tubule is responsible for reabsorbing a significant portion of water, sodium, chloride, potassium, and other solutes. It also reabsorbs glucose, amino acids, and bicarbonate. Na+,K+-ATPase is a key element in proximal tubule reabsorption.
he formation of urine involves three basic processes: (1) ultrafiltration of plasma by the glomerulus, (2) reabsorption of water and solutes from the ultra-filtrate, and (3) secretion of selected solutes into tubular fluid. Although an average of 115 to 180 L/day in women and 130 to 200 L/day in men of essentially protein-free fluid is filtered by the human glomeruli each day, less than 1% of the filtered water and sodium chloride (NaCl) and variable amounts of other solutes are typically excreted in urine (
Table 34.1 ). By the processes of reabsorption and secretion, the renal tubules determine the volume and composition of urine (
Table 34.2 ), which in turn allows the 1Normal glomerular filtration rate (GFR) averages 115–180 L/day in women and 130–200 L/day in men. Thus the volume of the ultrafiltrate represents a volume that is approximately 10 times that of the extracellular fluid volume (ECFV). For simplicity, we assume throughout the remainder of this section that GFR is 180 L/day.
kidneys to precisely control the volume, osmolality, composition, and pH of the extracellular and intracellular fluid compartments. Transport proteins in cell membranes of the nephron mediate reabsorption and secretion of solutes and water in the kidneys. Approximately 5% to 10% of all human genes code for transport proteins, and genetic and acquired defects in transport proteins are the cause of many kidney diseases (
Table 34.3 ). In addition, numerous transport proteins are important drug targets. This chapter discusses NaCl and water reabsorption, transport of organic anions and cations, the transport proteins involved in solute and water transport, and some of the factors and hormones that regulate NaCl transport. Details on acid-base transport and on K+ , Ca++ , and inorganic phosphate (Pi) transport and their regulation are provided in through 37.
Solute and Water Reabsorption Along the Nephron
The general principles of solute and water transport across epithelial cells were discussed in
Quantitatively, reabsorption of NaCl and water represent the major function of nephrons. Approximately 25,000 mEq/day of Na+ and 179 L/day of water are reabsorbed by the renal tubules (see
Table 34.1).
In addition, renal transport of many other important solutes is linked either directly or indirectly to reabsorption of Na+ . In the following sections, the NaCl and water transport processes of each nephron segment and their regulation by hormones and other factors are presented.
The proximal tubule reabsorbs approximately 67% of filtered water, Na+ , Cl− , K+ , and most other solutes. In addition the proximal tubule reabsorbs virtually all the glucose and amino acids filtered by the glomerulus, as well as most of the HCO3 − . The key element in proximal tubule reabsorption is Na+,K+-ATPase in the basolateral membrane. Reabsorption of every substance, including water, is linked in some manner to the operation of Na+,K+-ATPase.