Page 23, CV Review, Essentials, Develop. Physiol., Dr. D. Penney

Misconception 24. Blood flow falls to zero only when perfusion pressure reaches zero (i.e. there is a linear relationship).

Perfusion pressure/flow diagrams are easily constructed, as with the rabbit ear preparation. With zero stimulation of the efferent nerves, the pressure/flow relationship approximates a straight line. At 40 mmHg of pressure the flow rate is 3 ml/min, while at a pressure of 20 mmHg the flow rate is about 0.8 ml/min. At a pressure of 15 mmHg the flow rate is zero. Thus, the flow line does not go through zero, instead flow reaches zero at a perfusion pressure well above zero. That pressure is known as the critical closure pressure, for a given vascular bed under particular conditions.

Below the critical closure pressure, complete vessel closure occurs at any given state of smooth muscle contraction, and flow becomes zero. If the rate of sympathetic stimulation to the rabbit ear vascular bed is increased, the flow line moves downward and to the right. For example, at the rate of 1 stimulus per second, flow is decreased at any perfusion pressure. Further increasing the rate of stimulation further increases the critical closing pressure.

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