This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Circulation, Subject name: Physiology
When capillary and interstitial hydrostatic pressures are 30 mm Hg and 10 mm Hg, respectively, and colloid oncotic pressure in capillaries and in interstitium are 22 mm Hg and 5 mm Hg, respectively, how much is the net filtration pressure? 
Select and explain. 
A. + 3 mm Hg, B. - 3 mm Hg, C. + 13 mm Hg, D. - 13 mm Hg
A: + 3 mm Hg
Answer: A: + 3 mm Hg 
Explanation: Forces oring filtration out of the capillaries ~ (1) Capillary hydrostatic pressure (30 mm Hg) (2) Interstitial fluid colloid oncotic pressure (5 mm Hg). Thus, total force that ors filtration = 30 + 5 = 35 mm Hg. Forces that pull the fluid back into the capillaries ~ (1) Plasma colloid oncotic pressure (22 mm Hg) (2) Interstitial fluid hydrostatic pressure (10 mm Hg). Thus, total force that opposes filtration = 22 + 10 = 32 mm Hg. Hence, net filtration pressure = 35 - 33 = + 3 mm Hg.