McKenna's Pharmacology for Nursing, 2e

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P A R T 9  Drugs acting on the renal system

In contrast, the ascending loop of Henle is imper- meable to water, and so water that remains in the tubule is trapped there. Chloride is actively transported out of the tubule using energy in a process that is referred to as the chloride pump; sodium leaves with the chloride to maintain electrical neutrality. As a result, the fluid in the ascending loop of Henle becomes hypotonic in compari- son to the hypertonic situation in the peritubular tissue. Antidiuretic hormone (ADH) , which is produced by the hypothalamus and stored in the posterior pitu- itary gland, is important in maintaining fluid balance. ADH is released in response to falling blood volume, sympathetic stimulation, or rising sodium levels (a con- centration that is sensed by the osmotic cells of the hypothalamus). If ADH is present at the distal convoluted tubule and the collecting duct, the permeability of the membrane to water is increased. Consequently, the water remaining in the tubule rapidly flows into the hypertonic tissue sur- rounding the loop of Henle, where it either is absorbed by the peritubular capillaries or re-enters the descending loop of Henle in a countercurrent style. The resulting urine is hypertonic and of small volume. If ADH is not present, the tubule remains impermeable to water. The water that has been trapped in the ascending loop of Henle passes into the collection duct, resulting in

hypotonic urine of greater volume. This countercur- rent mechanism allows the body to finely regulate fluid volume by regulating the control of sodium and water (Figure 50.3). Chloride regulation Chloride is an important negatively charged ion that helps to maintain electrical neutrality with the movement of cations across the cell membrane. Chloride is prim­ arily reabsorbed in the loop of Henle, where it promotes the movement of sodium out of the cell. Potassium regulation Potassium is another cation that is vital to proper function- ing of the nervous system, muscles and cell membranes. About 65% of the potassium that is filtered at the glomer- ulus is reabsorbed at Bowman’s capsule and the proximal convoluted tubule. Another 25% to 30% is reabsorbed in the ascending loop of Henle. The fine-tuning of potas- sium levels occurs in the distal convoluted tubule, where aldosterone activates the sodium–potassium exchange, leading to a loss of potassium. If potassium levels are very high, the retention of sodium in exchange for potas- sium also leads to a retention of water and a dilution of blood volume, which further decreases the potassium concentration (see Figure 50.3).

Proximal convoluted tubule

Bowman’s capsule

Distal convoluted tubule

65%

H 2 O , Na + , Cl - Ca ++ , K +

ADH

Collecting duct

Efferent arteriole

PTH

Ca ++ , H 2 O

Impermeable to Na + /Cl -

Na +

Glomerulus

Impermeable to H 2 O Aldosterone K +

ADH

H 2 O

H 2 O

Afferent arteriole

H 2 O , Na + , Cl - Ca ++ , K +

Loop of Henle

FIGURE 50.3  Nephron and points of regulation of sodium, chloride, potassium, calcium and water. ADH, antidiuretic hormone; PTH, parathyroid hormone.