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MCAT

Unit 2: Lesson 3

Foundation 3: Organ Systems

Congestive heart failure and diuretics

Problem

Congestive heart failure is caused by an inability of the heart to pump blood and maintain proper blood flow throughout the body. This inability can occur because at least one chamber of the heart does not pump at peak capacity. As a result of poor pumping action, blood can pool in various areas of the body. Since 55% of blood volume is plasma, and plasma contains approximately 92% water, excess extracellular fluid (ECF) accumulates in the areas where the blood pools. The accumulation of excess ECF can occur in various parts of the body, depending on which chambers of the heart are not pumping effectively.
Excess ECF causes many symptoms of chronic congestive heart failure. To relieve the symptoms, diuretics are commonly used to remove the excess ECF. Furosemide is a diuretic that works by blocking the Na-K-2Cl symporter in the loop of Henle. Once this ion pump is blocked, the volume of urine is increased and the amount of ECF is reduced.
To measure the effectiveness of how diuretics like furosemide are delivered to the body, physicians compared patients who had continuous-intravenous doses of furosemide to patients who had oral dosages of furosemide. For each patient, researchers measured the amount of Na+ in the urine per hour. Patients with continuous-intravenous dosages on average had 18.5% higher millimoles per hour of Na+ in their urine than those with oral dosages.
Figure 1: Oral dosages of furosemide behave as intermittent sources of furosemide while an intravenous injection of furosemide delivers a continuous delivery of furosemide. This intermittent behavior occurs because of the time delay between ingesting oral dosages of furosemide and their action in the kidney. The spikes of intermittent oral dosages caused 18.5% less millimoles of Na+ per hour compared to continuous delivery through an intravenous injection.
Sources: Figure 1 was adapted from Lahav M, Regev A, Ra’anani P, Theodor E. Intermittent administration of furosemide vs continuous infusion preceded by a loading dose for congestive heart failure. Chest 1992; 102: 725-31 and Ellison D. Diuretic Therapy and Resistance in Congestive Heart Failure. Cardiology; 2001; 96: 132-143.
Where in the loop of Henle does furosemide block the ion symporters?
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