Is the Heart Really a Pump? New Perspectives on Cardiovascular Physiology

A metabolic view of blood flow suggests that heart may play a secondary role in the circulation.

Cardiovascular physiology has long held that the heart is a mechanical pump and that the heart’s propulsive power is the main driver of blood flow throughout the body. However, the cardiocentric view of circulation fails to explain blood flow regulation during exercise and other unique scenarios. Thus, a new model is proposed in which metabolic demands at the tissue level are the primary driver of circulation — the heart plays a permissive role.

One of Leonardo da Vinci's most famous sketches is his detailed drawing of the human heart. Created circa 1500, this anatomical illustration is a remarkable representation of the heart's interior and exterior.

Da Vinci’s studies led him to recognize the four chambers of the heart and the complex system of valves and vessels within it. He recognized that the heart has four chambers: two atria (upper chambers) and two ventricles (lower chambers) — a significant departure from the prevailing belief of his time that the heart had only three chambers.

Da Vinci studied the valves within the heart and the large vessels connected to it. He understood that these valves played a crucial role in controlling the flow of blood. He also recognized that blood from the right ventricle of the heart is pumped to the lungs and then returns to the left atrium. While he didn't fully grasp the complete circulatory system, his insights into the pulmonary circulation were notable for the time.

Da Vinci also conceptualized the heart as a muscular pump, a notion that laid the groundwork for later understandings of the heart's function. His anatomical studies suggested that the contraction and relaxation of the heart muscles played a role in pumping blood through the circulatory system.

https://www.openculture.com/2020/09/leonardo-da-vincis-elegant-studies-of-the-human-heart.html 

The heart: a brief history

Throughout history, others — including physiologists and philosophers — have held unique theories to explain blood circulation.

William Harvey — who “discovered” circulation — maintained that blood moved due to the heart’s pressure-generating function and that the blood had an “innate heat” that was responsible for its movement. He proposed that the blood “gives heat to the heart, as it does to all the other parts of the body” and that this innate heat was “the common instrument of every function, the prime cause of the pulse among the rest.”

Innate properties of the blood were also discussed by Rene Descartes, who believed that the blood was a mixture of materials and food particles that fueled the fire that was maintained by the heart. Put another way, the combustion of these materials and “vital heat” from the blood fueled the mechanical pump of the heart — the blood was merely a passive fluid.

These initial observations led to two views at the time:

One of these views suggested that blood’s movement was controlled by a “force from behind” — so-called vis a fronte. 

The opposing view held that forces acting at the periphery or so-called “capillary power” moved the blood — or vis a tergo.

These can be thought of as the cardiocentric view of circulation (vis a fronte) and the venous return model (vis a tergo).

https://natureofscienceib.wordpress.com/2015/11/18/6-2-circulation-galen-and-harvey/ 

Cardiovascular physiology basics

In the cardiocentric view of circulation, the heart’s mechanical beating generates a pressure gradient  — with the highest pressure at the aorta (where blood leaves the heart), higher pressure throughout the arterial system, and a low-pressure venous system (through which blood returns to the heart). 

Pressure is lowest at the right ventricle (where blood returns from the body). This high-to-low pressure gradient is the primary driver of blood flow throughout the body.

The venous return model is characterized by the fact that blood flow from the heart is the side effect of more blood returning to the heart from the rest of the body. Indeed, increased venous return means that more blood fills the heart — this greater filling is met with a greater stretch of the left ventricle, a stronger force of contraction, and a larger amount of blood ejected during the heart’s contraction (known as stroke volume). 

This fundamental concept in cardiovascular physiology is known as the Frank-Starling mechanism.

Despite their differences, both views hold that the heart provides the propulsive force for blood flow. In other words, without the mechanical force of the heart, blood flow would stagnate.

Is there an alternative model that better explains how the body coordinates blood flow?

The heart as a siphon

Given the limitations of the previous views on the heart’s role in the circulation of blood, some propose that the heart — rather than play a leading role as a propulsive force for blood flow — may play a mediating role between circulation in the lungs (pulmonary circulation) and the periphery (the systemic circulation). 

In other words, the heart integrates information on oxygen supply in the lungs, oxygen demand in the body, and regulates blood flow accordingly.

A few interesting proposals for how the heart moves blood emerge.¹ 

In one such explanation, the heart functions like a hydraulic ram, whereby blood flows from the atrium (the initial reservoir) into the ventricle during diastole and during systole, a build-up of ventricular pressure causes the closure of heart valves and the ejection of blood through its “delivery pipe” (the aorta or pulmonary artery).

In addition to the ram-like function, blood flow is assumed to occur due to a closed-loop siphon effect in the circulation. A balance of forces in the arteries and veins eliminates the need for energy from the heart to overcome gravity (to help blood return to the heart) — this is accomplished through the siphoning of blood from the lower extremities.

In short, some modern (though still not universally accepted) views of the heart suggest that rather than the primary source of energy that delivers blood to the rest of the body, the heart plays a “permissive” role in blood flow.

Other forces seem to better explain how the body regulates blood flow in unique scenarios such as exercise and suggest that a newer explanation of circulation may be necessary.