The Urinary System

Max’s Maximum: A Case Study on the Urinary System”

1. What does the color of Max’s urine tell Tracey about how concentrated or dilute it is? How does Max’s urine color/concentration compare to the urine specific gravity at the same time?

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Max’s pale urine tells Tracey that his urine is diluted. The yellow urine is more concentrated, and the dark yellow urine is super concentrated. Specific gravity is the ratio of the density of a substance compared to the density of an equal volume of distilled water. Urine, with its various solutes, has a greater specific gravity than water which is (1.001-1.035), and depending on the concentration of the solute it can vary. The more concentrated the urine, the darker the color will be and specific gravity will be higher.

2. Based on the urine color and specific gravity, what might Tracey conclude about the hydration status of Max’s body at the three different times?

When Max’s urine is pale with a low specific gravity, his body is likely to be well hydrated; the kidneys are reabsorbing less water, allowing it to be eliminated in urine. When his urine is dark yellow with a higher specific gravity, his kidneys are concentrating urine and reabsorbing more water in an attempt to maintain body fluid homeostasis (defend plasma osmolality). Dehydration will trigger this response. The yellow urine signals euhydration; the corresponding high-normal specific gravity is likely due to the small amount of glucose in the urine.

3. Antidiuretic hormone (ADH) regulates the formation of concentrated or dilute urine. In which time period is Max’s body secreting its highest amount of ADH? Explain your answer.

ADH is going to be highest when the body is most dehydrated. In Max’s case, this is right after his 2-hour run. During his run, Max loses water via sweat and exhalation, and doesn’t effectively replace it. The osmolality of his blood increases, signaling the posterior pituitary to release ADH. ADH targets the principle cells in the collecting ducts of the kidney tubules, causing them to increase reabsorption of water from the filtrate. This negative feedback mechanism attempts to conserve water in order to decrease blood osmolality and restore body fluid homeostasis.

4. Tracey knows that proteinuria (protein in the urine) after intense exercise is physiological (normal). However, protein is typically not present in urine. Why is that?

The glomerulus is a passive filter that acts as a barrier to larger molecules, such as most plasma proteins. Also, because most proteins carry a net negative charge, they are repelled by the membrane, thereby hindering their passage. However, some smaller proteins can and do pass through the filtration membrane. Most of them are reabsorbed in the proximal convoluted tubule by endocytosis and then broken down into amino acids to be returned to the blood in the peritubular capillaries. The mechanism of exercise-induced proteinuria is unclear, but it is typically a transient occurrence. Any kidney disease can render the glomerulus dysfunctional, leading to a more sustained proteinuria. In fact, Max’s hypertension, if left unchecked, could damage the filtration membrane of the glomerulus. The proteinuria will increase the specific gravity of his urine (along with his relative dehydration).

5. Tracey had been slightly concerned about the trace glucose that was found in Max’s urine six hours after his exercise until she discovered that he had eaten an entire large pizza an hour before the urinalysis. Explain why glucose might show up in Max’s urine after a particularly heavy meal.

Glucose passes easily through the glomerular filtration membrane, and normally 100% of it is reabsorbed in the proximal convoluted tubule (PCT). The reabsorption of glucose is accomplished by secondary active transport and requires a transport protein in the membrane to facilitate the movement of glucose across the membrane. Typically, there are more than enough of these transport proteins, but they do have an upper limit beyond which no more glucose can be transported. This is called the transport maximum (Tm) for glucose. Generally, the Tm for glucose is not reached until blood levels of glucose exceed 180 mg/dL, which is known as the renal threshold. The most common cause of hyperglycemia (high blood glucose) is diabetes mellitus. So, if the renal threshold is exceeded for glucose in the PCT, it will “spill” into the filtrate and be eliminated in urine. Without further testing, it’s unknown whether Max has a lower-than-normal renal threshold for glucose, or has exceeded the normal renal threshold of 180 mg/dL with his heavy meal.

6. Lactic acid accumulation can be a consequence of intense exercise. Tracey notes that Max’s kidneys are working to defend his body against acidosis. How can she tell? Describe this mechanism.

Max’s pH is more acidic right after his run. This is evidence of a higher hydrogen concentration in urine. When pH drops, the tubule cells secrete hydrogen into the filtrate, allowing it to be lost in urine. They also reabsorb bicarbonate (a base) to help buffer the body fluids. The result is a restoration of normal pH.

7. Based on Max’s urinalysis data, should he drink more water prior to exercise to ensure that he doesn’t dehydrate during intense activity? Explain your answer.

Max’s urine is already extremely dilute before exercise, indicating he is probably well hydrated. If he were to drink more, then he would simply end up with a full bladder during his run (the kidneys would eliminate the excess water). Better advice would be to drink more strategically during his run. For instance, when he is doing his training runs, Max could take a drink every 15 minutes (more if he is sweating heavily) and practice different drinking regimens when he trains until he finds one that keeps him hydrated but does not require him to stop to urinate during his run.

8. Max’s regular exercise regimen has reduced his high blood pressure, allowing him to achieve normal blood pressure on a single antihypertensive medication. The medication he takes is called an angiotensin converting enzyme inhibitor, or ACE inhibitor, which blocks the activation of angiotensin II. Describe at least two mechanisms by which angiotensin II targets the kidneys to increase extracellular fluid volume and, therefore, increase blood pressure.

Angiotensin II directly signals the renal tubules to reabsorb sodium. Where sodium goes, water follows by osmosis, causing blood volume and pressure to increase.

Angiotensin II signals the release of ADH from the posterior pituitary. ADH acts on the principle cells of the collecting ducts to cause reabsorption of water from the filtrate into the blood. Blood volume and pressure are increased.

Angiotensin II is a potent vasoconstrictor, which leads to a decline in peritubular capillary hydrostatic pressure and more fluid reabsorption. Blood volume and pressure are increased.

Angiotensin II prompts the release of aldosterone from the adrenal medulla. Aldosterone stimulates renal tubules to reabsorb sodium (with water following) and this leads to an increase in blood volume and pressure.

Angiotensin II stimulates the glomerular mesangial cells to contract and reduce glomerular filtration rates. Less filtrate is produced, less fluid is lost in urine, and blood volume and pressure are increased.

Angiotensin II triggers thirst by acting at the hypothalamus. This increases blood volume and blood pressure.To continue with the answer check on

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