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MCAT Question A Day - 3/4/14 - Answer!

Water and small proteins leak out of capillaries at their arterial ends because hydrostatic pressure (exerted mainly by blood pressure pushing outward against the capillary walls) is greater than colloid osmotic pressure (a fluid-retaining force caused by large solutes in the blood). Most of the fluid returns at the venule end because blood pressure

A.  increases and large solutes decrease.
B.  decreases and large solutes decrease.
C.  increases and large solutes stay the same.
D.  decreases and large solutes stay the same.

The correct answer is (D). A basic knowledge about the major osmotic components of blood plasma is important, as is an understanding of the relationship between blood pressure and blood vessel distance from the heart. The major osmotic blood proteins (such as albumin) are too large to leave the capillaries. Thus, colloid osmotic pressure (COP) normally remains the same at both ends of the capillary. However, blood pressure decreases in vessels as distance from the heart increases, resulting in hydrostatic pressure being lower than COP at the venule end.

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