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• The final processing of filtrate in the late distal convoluted tubule and collecting ducts comes under direct physiological control. • In this region, membrane permeabilities and cellular activities are altered inresponse to the body's need to retain or excrete specific substances.
Page 2. Goals
• To understand the role of the hormone aldosterone in the reabsorption of sodium and secretion o f potassium. • To examine the role of the antidiuretic hormone in the concentration of urine. • To understand the role of the medullary osmotic gradient in the concentration of urine.
Late FiltrateProcessing: Analogy
• The bulk of reabsorption occurs in the early tubular segments. In these regions the rates of both reabsorption and secretion are relatively constant, because the membrane permeabilities are relatively fixed. • In the later tubular segments you are about to tour, the membrane permeabilities change in response to changing physiological conditions and hormone levels. Thisvariability provides a mechanism for precisely regulating the final balance of fluid and solutes returned to the blood. • An analogy for this two-stage process would be to use a steady but unregulated flow to fill a container to almost the level needed--that’s early filtrate processing. Then use a precisely regulated flow o f water to top off to the exact level--that’s late filtrate processing. Bulkfilling is analogous to the reabsorption of water and solutes occurring in the early tubular segments. Fine-tuning is analogous to late filtrate processing.
Page 4. Filtrate Processing in the Late DCT and CCD: Hydrogen Ion Secretion
• The epithelium of the late distal convoluted tubule and the collecting ducts consists of two cell types. Each of these cells plays a different role in the finalprocessing of filtrate. 1. Intercalated cells The intercalated cells help to balance the blood pH by secreting hydrogen ions into the filtrate through ATPase pumps in the luminal membrane. 2. Principal cells The principal cells perform hormonally regulated water and sodium reabsorption and potassium secretion. • Label this diagram of the two cell types in the late distal convoluted tubule and thecollecting ducts:
Filtrate Processing in the Late
DCT and CCD: Role of Aldosterone
• The principal cells are permeable to sodium ions and water only in the presence of the hormones aldosterone from the adrenal gland and antidiuretic hormone, or ADH, from the posterior pituitary gland. • Let’s first look at the role of aldosterone, which preciselyregulates the final amount of sodium reabsorbed. When levels of sodium and potassium ions in the blood are balanced, aldosterone levels remain low. As a result, there are few sodium/potassium ATPase ion pumps in the basolateral membrane and few sodium and potassium channels in the luminal membrane. Therefore, sodium ion reabsorption and potassium ion secretion are both low. • Label this diagram to showwhat happens when levels of sodium and potassium ions in the blood are balanced and aldosterone levels remain low:
• However, a decrease in the level of sodium ions or an increase in potassium ions will trigger the release of aldosterone. • Label this diagram to show what happens when aldosterone levels are high:
• In response to increased aldosterone, bothsodium ion reabsorption and potassium ion secretion increase. • This occurs because the principal cells increase the number and activity of sodium/potassium pumps in the basolateral membrane. The number of sodium and potassium channels in the luminal membrane is also increased. • Notice the absence of potassium channels in the basolateral membrane. Potassium ions enter the cell through the...