Describe some characteristics of blood flow in capillaries (4).
1) Blood flow is intermittent
2) Vasomotion causes intermittency - metarterioles and precapillary sphincters contract/relax
3) Tissue oxygen concentration detrmines vasomotion; low O2 relaxes, high O2 contracts sphincters
4) There are billions of individual capillaries so despite intermittent opening and closing we generally consider an average rate of flow, an average capillary pressure, and average rate of transfer of substances