Regulation of blood flow. DR. Shafali Singh. Learning objectives. Define autoregulation of blood flow. Describe how the theory of metabolic regulation of blood flow accounts for active hyperemia and reactive hyperemia.
DR. Shafali Singh
An increase in transmural pressure, caused either by an increase in venous pressure or by dilation of arterioles, results in contraction of the terminal arterioles. A decrease in transmural pressure causes precapillary vessel relaxation .
Active hyperemia : blood flow to an organ is proportional to its metabolic activity.
Reactive hyperemia : increase in blood flow in response to or reacting to a prior period of decreased blood flow (arterial occlusion).
Autonomic Nervous system
Two basic mechanisms are proposed to explain the phenomena of autoregulation and active and reactive hyperemia:
Autoregulation is the maintenance of a constant blood flow to an organ in the face of changing arterial pressure.
Several organs exhibit autoregulation of blood flow, including the kidneys, brain, heart, and skeletal muscle.
If BP fallscompensatory vasodilation of the coronary arterioles decreases the resistance of the coronary vasculature flow is maintained constant .
Skeletal muscle vasculature during exercise
The main mechanism controlling flow in resting skeletal muscle and all other major extrinsically regulated systemic circuits is tonic changes in sympathetic adrenergic activity, i.e., norepinephrine acting on α receptors, causing constriction.
Generally, the parasympathetic system does not affect arterioles, and thus it has little or no influence on TPR (exception: the penis).
Circulations with mainly extrinsic regulation (those most affected by nervous reflexes):
Resting skeletal muscle
ml/100 g Muscle/min
Increase in flow results from intra-muscular vasodilation caused by the direct effects of increased muscle metabolism.
Correct statements about the increase in pulmonary blood flow during vigorous exercise include which of the following?
a. The percentage of increase in flow is greater in the bases of the lungs than in the apices
b. The increase in flow is caused by a greater-than-fivefold increase in pulmonary arterial pressure
c. The increase in pulmonary blood flow is less than the increase in systemic blood flow
d. The increase in pulmonary blood flow is accommodated by dilation of pulmonary arterioles and capillaries
e. The increase in pulmonary blood flow is caused by sympathetic nerve stimulation of the pulmonary vasculature
Initial decrease, then an increase to dissipate heat
Increase due to increase in volume work of the heart
No significant change (arterial CO2 remains unchanged)
Any change would be a decrease. This is more likely in the splanchnic circuit.