Over the next 24 hours, Carlos' condition worsened. His blood pressure remained elevated, and his kidneys seemed to stop working-since his admission, he had only produced 50 ml of urine. Carlos was receiving intravenous fluids in an attempt to reverse the dehydrated state with which he presented, and he now needed a urinary catheter, a tube that drains urine from the bladder, to closely compare urine output with fluid intake.

Although the results of Carlos' stool culture would not be available until the following day, the results of two more of his tests, the urinalysis and a second lab test, were returned. The second lab test allowed Dr. Williams to estimate Carlos' glomerular filtration rate. The findings are summarized here:

Test

Carlos'Value

Normal Value

Interpretation

Glomerular filtration rate (GFR)

25 ml/min

125ml/min

Low

Creatinine

5.2 mg/dl

0.3-0.7 mg/dl

Critically high

Blood urea

nitrogen (BUN)

110 mg/dl

7-20 mg/dl

Critically high

Urinalysis

Protein, erythrocytes, and leukocytes; dark, cloudy

No proteins, erythrocytes, and leukocytes

Abnormal

This latest round of tests add another problem to our list-acute renal failure (ARF), also called acute kidney injury. ARF results when the GFR is too low for the kidneys to function adequately. As we will see, impairment of glomerular filtration will also impair the ability of the kidneys to carry out tubular reabsorption and tubular secretion. Additionally, the lab results for creatinine and BUN are both very elevated, which also indicates renal failure. Gastroenteritis can lead to dehydration, but usually not enough to cause such a serious condition.

Though these new findings give us more information about Carlos' condition, the question remained: Why were Carlos' kidneys failing? To develop a treatment plan for him, we still need more information.

Carlos' kidneys are only capable to producing small amounts of very concentrated urine. What happens in the nephron loop and countercurrent multiplier to produce the concentrated urine?

a. Sodium ions pumped into the interstitial fluid from the thick ascending limb form medullary osmotic gradient that pulls water from the interstitial fluid into the thin descending limb.

b. Filtrate flows in the same direction in the descending and ascending loops to create the countercurrent multiplier.

c. Water is drawn out of the thin descending limb toward the sodium and chloride ions in the interstitial fluid.

d. Sodium and chloride ions are actively transported from the interstitial fluid into the filtrate in the thick ascending limb of the nephron loop.