Beachey: Respiratory Care Anatomy and Physiology, 2nd Edition

Answers to Concept Questions

Chapter 22: Filtration, Urine Formation, and Fluid Regulation

22-1	Osmotic pressure in efferent arteriole blood is much higher than in afferent arteriole blood because the blood loses much water in the glomerular filtration process. Thus efferent blood is highly concentrated and has great reabsorption capacity as it passes through the peritubular capillaries.

22-2	Untreated diabetes leads to very high blood-glucose levels, greatly increasing the amount of glucose that crosses the glomerular capillaries and enters the filtrate. The high level of glucose in the filtrate exceeds the tubules’ reabsorption capacity (the tubular transport maximum or Tm is exceeded). The extra unabsorbed glucose passes into the urine and draws much water with it because of its high osmotic pressure (i.e., filtrate glucose acts as an osmotic diuretic, causing excessive urine output and ultimately dehydration).

22-3	Renin brings about the production of angiotensin I, which in the presence of angiotensin-converting enzyme (ACE) is converted to the potent vasoconstrictor, angiotensin II. ACE-inhibitor drugs interfere with angiotensin II production, thus preventing the development of vasoconstriction and high blood pressure.

22-4	Efferent arteriole constriction increases glomerular filtration pressure, causing more water to be filtered out of the blood. This increases the osmotic pressure of the blood entering the efferent arteriole and ultimately the peritubular capillaries. This blood’s high osmotic pressure enhances its reabsorption capacity; thus more water is returned to the general circulation, restoring blood volume and pressure.

22-5	As it passes down the descending limb of the loop of Henle, filtrate fluid loses a great deal of water because of the tubule’s permeability and the surrounding interstitial fluid’s high osmotic pressure. Thus at the tip of the loop of Henle, the filtrate is quite reduced in volume and highly concentrated.

22-6	Decreased atrial filling pressure subjects the atrium to less stretch, decreasing production of atrial natriuretic hormone (ANH). Because it promotes sodium loss in the urine, ANH increases urine output via osmosis (i.e., water follows sodium). Decreased ANH thus leads to fluid retention and reduced urine output. This is one mechanism whereby mechanical ventilation may decrease urine output.

22-7	Without antidiuretic hormone (ADH) the collecting duct pores close, rendering the duct impermeable to water. The filtrate volume inside the duct thus remains relatively high and dilute. Hence the lack of ADH in diabetes insipidus causes the individual to lose abnormally high volumes of dilute urine, leading to dehydration.