Porth's Essentials of Pathophysiology, 4e

605

Structure and Function of the Kidney

C h a p t e r 2 4

concentration that facilitates the downhill movement of sodium across the luminal membrane; it is this downhill diffusion of sodium to the interior of the cell that pro- vides the energy for the simultaneous uphill transport of glucose across the luminal membrane. A few substances, such as hydrogen (H + ), are secreted into the tubule. Proximal Tubule. Approximately 65% of all reabsorp- tive and secretory processes that occur in the tubular system take place in the proximal tubule. There is almost complete reabsorption of nutritionally impor- tant substances, such as glucose, amino acids, lactate, and water-soluble vitamins (Fig. 24-8). Electrolytes, such as Na + , K + , Cl – , and bicarbonate (HCO 3 – ), are 65% to 80% reabsorbed. As these solutes move into the tubular cells, their concentration in the tubular lumen decreases, providing a concentration gradient for the osmotic reabsorption of water and urea. The proximal tubule is highly permeable to water, and the osmotic movement of water occurs so rapidly that the concentration difference of solutes on either side of the membrane seldom is more than a few milliosmoles. Many substances, such as glucose, are freely filtered in the glomerulus and reabsorbed by energy-dependent cotransport carrier mechanisms. The maximum amount of substance that these transport systems can reabsorb per unit time is called the transport maximum. The trans- port maximum is related to the number of carrier proteins that are available for transport and usually is sufficient to ensure that all of a filtered substance such as glucose can be reabsorbed rather than being eliminated in the urine. The plasma level at which the substance appears in the urine is called the renal threshold . Under some circum- stances, the amount of substance filtered in the glomeru- lus exceeds the transport maximum. For example, when the blood glucose level is elevated in uncontrolled diabe- tes mellitus, the amount that is filtered in the glomerulus often exceeds the transport maximum (approximately 320 mg/minute), and glucose spills into the urine.

Peritubular capillary

Tubular lumen

Proximal tubular cell

Blood

Tubular fluid

Interstitial fluid

H 2

O

Na +

Na +

ATP

Glucose Amino acids

K +

K +

Na +

Glucose Amino acids

H +

Basolateral cell membrane

Luminal cell membrane

down its concentration gradient, the energy released is used to move another substance (for instance, glucose or an amino acid) against its concentration gradient. Thus, the secondary active transport of a substance such as glucose does not require energy directly from adenosine triphosphatase (ATPase), but depends on the energy-dependent Na + /K + -adenosine triphosphatase (ATPase) pump on the basolateral side of renal tubular cells. This pump maintains a low intracellular sodium FIGURE 24-7. Mechanism for secondary active transport or cotransport of glucose and amino acids in the proximal tubule. The energy-dependent sodium–potassium pump on the basal lateral surface of the cell maintains a low intracellular gradient that facilitates the downhill movement of sodium and glucose or amino acids (cotransport) from the tubular lumen into the tubular cell and then into the peritubular capillary.

Proximal tubule • Reabsorption: Na + , Cl – , HCO 3 – , K + , H 2 O, glucose, amino acids • Secretion: H + , organic acids and bases

Early distal tubule • Reabsorption: Na + , Cl – , Ca ++ , Mg ++

Late distal tubule and collecting duct • Principal cells Reabsorption: Na + Cl – Secretion: K + ; ADH-mediated H 2 • Intercalated cells Reabsorption: HCO 3

Thin descending loop of Henle • Reabsorption: H 2 O

O reabsorption

– , K +

Thick ascending loop of Henle • Reabsorption: Na + , Cl – , K + , Ca ++ , HCO 3 – , Mg ++ • Secretion: H +

Secretion: H +

FIGURE 24-8. Sites of tubular water (H 2

O), glucose, amino acids, Na + (sodium), Cl – (chloride),

HCO 3 – (bicarbonate), K + (potassium), Ca ++ (calcium), and Mg ++ (magnesium) reabsorption; and sites of organic acids and bases, H + (hydrogen), and K + secretion.