Question

Joe is studying for ANP. He is relaxed and resting. He just ate a big meal, so his digestive organs are very active.

1. Explain why the precapillary sphincters feeding his resting muscles are closed tight, but the precapillary sphincters feeding the smooth muscle of his digestive organs are relaxed/open. How (what stimulus) causes those sphincters to relax?
2. Explain how/why plasma is leaving his capillaries.
   1. Is this happening at the arterial end or the venous end?
   2. Use the terms capillary hydrostatic pressure, blood colloid osmotic pressure, and net filtration pressure. Indicate whether CHP is greater than or less than BCOP, and whether NFP is positive or negative.
3. Explain how/why some of the fluid is returning to his capillaries.
   1. Is this happening at the arterial end or the venous end?
   2. Use the terms capillary hydrostatic pressure, blood colloid osmotic pressure, and net filtration pressure. Indicate whether CHP is greater than or less than BCOP, and whether NFP is positive or negative.
4. Explain how/why this phenomenon is making lymph.

Joe begins to exercise.

5. What changes will his heart AND vasculature make to ensure that his active muscles get the extra O2 and glucose they need?
   1. Heart:
   2. Vasculature:
      1. Most blood vessels
      2. Arterioles and precapillary sphincters serving his digestive organs
      3. Arterioles and precapillary sphincters serving his active muscles
6. This time, his muscles are active enough that he needs to increase his total body blood pressure. What stimulus causes the heart and vascular changes seen above?
7. Using the terms “change in pressure” and “flow,” and comparing pressure in the arteries with that in the capillaries, and between capillaries perfusing the digestive organs vs those perfusing skeletal muscle, explain how/why these changes work to keep his muscles adequately fed.

When Joe is preparing to shower after his workout, he suddenly remembers that he has a big assignment in his microbiology class that night, and he hasn’t studied! He has an extreme burst of sympathetic activation. Almost immediately, his brain realizes this isn’t an emergency worthy of extreme stimulation, and has an overcompensation reaction: his sympathetic tone goes WAY down and his parasympathetic tone goes WAY up. Joe faints. Joe faints because his brain isn’t getting enough O2. (this is a real reason for some fainting spells; it's called vasovagal syncope)

8. Why isn’t his brain getting enough O2? Use the terms “change in pressure” and “flow,” and compare pressure in the arteries with that in the capillaries.

Answer 1

#. Precapillary sphincter

surrounds root of each true capillary at the metarteriole and acts as a valve to regulate blood flow into the capillary

a.Precapillary sphincters are relaxed= blood flows through the true capillaries (resting)

b.Precapillary sphincters are contracted= blood flows through vascular shunt (exercise)

#. A local drop in pO2 is the most important factor causing relaxation of the precapillary sphincters.

#. The blood contains plasma proteins giving the blood a relatively high solute potential (and therefore a low water potential), tending to draw water into the blood. Since the hydrostatic pressure has a greater effect than the solute potential at the arteriole end, the net effect is that fluid leaves the capillary

#.