During the learning of capillary fluid traffic, students are often puzzled about the negative values found in their textbooks for interstitial pressures in subcutaneous tissue and lungs. Positive pressures are reported for the brain, kidneys, and liver and particularly in various tumors. Although the reported values differ, pressures probably range from 8 mmHg in the lungs to 3 or 2 mmHg in subcutaneous tissue to 0, 1, and 2 mmHg in the liver and kidneys or 6 mmHg in the brain. Interstitial pressure can reach very high positive values in tumors (from 20 to 40 mmHg). Reported findings in freshly burned tissue describe even more negative interstitial pressures that can reach 20 to 30 mmHg. During seminars on capillary dynamics, teachers are often asked to clarify the whole concept in a brief explanation. One of the possible interpretations, in a form of a manual for students, is presented here.