Describe the major anatomical differences in the circulatory system of a mammal and a fish. In a fish, blood leaving the heart first passes through a capillary bed in the gills, then passes through a second 'systemic' capillary bed before returning to the heart. During each beat your heart contracts for about 0.4 sec, then rests for about 0.6 sec as its chambers are refilling with blood. But the blood in your arteries travels towards the capillaries and veins throughout this whole time. Elastic recoil of the arteries, whose walls were expanded by the high blood pressure produced by heart contraction.
Water flows between blood and interstitial fluid across capillary walls.
Name two differences in these liquids that control the water flow between them. The pressure of blood in the circulatory system is greater than the pressure of interstitial fluid. The flow of water across the capillary wall varies along the capillary's length. Net flow of water is out of the capillary at the arterial end, and into the capillary at the venous end. If it were the same or lower, there would not be a net flow of O 2 into the blood.