What determines systemic blood flow in vertebrates?

Area of Science:

• Cardiovascular Physiology
• Comparative Physiology
• Vascular Biology

Background:

• Arthur C. Guyton challenged the traditional view of cardiac output regulation in the 1950s.
• Cardiac output (systemic blood flow, Q̇sys) is often assumed to be primarily controlled by heart rate.
• This review examines the concept that peripheral vasculature plays a key role in regulating cardiac output.

Purpose of the Study:

• To review recent and classic advances in comparative physiology regarding cardiac output regulation.
• To evaluate the role of heart rate versus vascular factors in controlling systemic blood flow.
• To highlight unique vertebrate adaptations in venous return regulation.

Main Methods:

• Review of experimental evidence from comparative physiology.
• Analysis of classic and recent research on cardiovascular regulation.
• Examination of vertebrate cardiovascular responses to altered metabolic states.

Main Results:

• Tachycardia (increased heart rate) is not always necessary or sufficient to alter cardiac output during increased oxygen demand.
• Systemic blood flow (Q̇sys) is primarily determined by vascular conductance and capacitance, influenced by the venous circulation.
• Heart function (myocardial inotropy) contributes variably to Q̇sys.
• Vertebrates exhibit unique adaptations for venous return, such as sphincters in diving mammals and atrial muscle in turtles.

Conclusions:

• Cardiac output regulation across vertebrates is predominantly influenced by vascular properties rather than solely by heart rate.
• Future research should equally consider venous return and cardiac filling factors alongside cardiac function and heart rate when studying metabolic rate effects.
• Understanding the interplay between vascular and cardiac factors is crucial for a comprehensive view of cardiovascular physiology.