Overview of Regional Function

We conclude this chapter with a broad overview of the tasks performed by the various nephron segments. Later, we examine renal function substance by substance and see how tasks performed in the various regions combine to produce an overall result that is useful for the body.
The glomerulus is the site of filtration—about 180 L/day of volume and proportional amounts of solutes that are freely filtered, which is the case for most solutes (large plasma proteins are an exception). The glomerulus is where the greatest mass of excreted substances enter the nephron. The proximal tubule (convoluted and straight portions) reabsorbs about two thirds of the filtered water, sodium, and chloride. The proximal convoluted tubule reabsorbs all of the useful organic molecules that the body wishes to conserve (eg, glucose, amino acids). It reabsorbs significant fractions, but by no means all, of many important ions, such as potassium, phosphate, calcium, and bicarbonate. It is the site of secretion of a number of organic substances that are either metabolic waste products (eg, urate, creatinine) or drugs (eg, penicillin) that physicians must replace to make up for renal excretion.
The loop of Henle contains different segments that perform different functions, but the key functions occur in the thick ascending limb (a region that begins in the outer medulla for all nephrons and continues outward into the renal cortex until it reaches the renal corpuscle from which the tubule arose (which can, depending on the nephron, be near the corticomedullary border or close to the cortical surface)). As a whole, the loop of Henle reabsorbs about 20% of the filtered sodium and chloride and 10% of the filtered water. A crucial consequence of these different proportions is that, by reabsorbing relatively more salt than water, the luminal fluid becomes diluted relative to normal plasma and the surrounding interstitium. During periods when the kidneys excrete dilute final urine, the role of the loop of Henle in diluting the luminal fluid is crucial.
The end of the loop of Henle contains cells of the macula densa, which senses or assays the sodium and chloride content of the lumen and generates signals that influence other aspects of renal function, specifically the renin-angiotensin system.
The distal tubule and connecting tubule together reabsorb some additional salt and water, perhaps 5% of each.
The cortical collecting tubule is where several (6–10) connecting tubules join to form 1 tubule. Cells of the cortical collecting tubule are strongly responsive to and are regulated by the hormones aldosterone and ADH. Aldosterone enhances sodium reabsorption and potassium secretion by this segment, and ADH enhances water reabsorption. The degree to which these processes are stimulated or not stimulated plays a major role in regulating the amount of solutes and water present in the final urine. With large amounts of ADH present, most of the water remaining in the lumen is reabsorbed, leading to concentrated, low-volume urine. With little ADH present, most of the water passes on to the final urine, producing dilute, high-volume urine.
The medullary collecting tubule continues the functions of the cortical collecting tubule in salt and water reabsorption. In addition, it plays a major role in regulating urea reabsorption and in acid-base balance (secretion of protons or bicarbonate).