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Last Word is Â鶹´«Ã½â€™s long-running series in which readers give scientific answers to each other’s questions, ranging from the minutiae of everyday life to absurd astronomical hypotheticals. To answer a question or ask a new one, email lastword@newscientist.com
Why is our heart not in the centre of our body?
Mike Follows
Sutton Coldfield, West Midlands, UK
The heart is well-placed in the upper torso to serve the major organs, which are arranged like satellites around it. It isn’t displaced as far to the left as some people might imagine. However, it is rotated and twisted in that direction, resulting in the right ventricle forming most of the frontal view. The apex of the heart extends as far as a vertical line dropped from the midpoint of the left collarbone. These asymmetries form incredibly early during embryonic development.
The heart begins as a straight, midline tube comprised of segments that will form the atria, ventricles and more. This tube quadruples in length in under 24 hours when the embryo is a few weeks old. Since both ends remain fixed – at the arterial and venous poles – the tube cannot grow in a straight line. Instead, it buckles into a helical s-shape. This deformation is guided by asymmetric signalling pathways that are more active on one side of the embryo than the other. This looping is essential for correct chamber alignment, and errors in the process can cause congenital heart issues.
The looping significantly enhances the pumping efficiency of the valveless embryonic heart. Physical models show that a looped tube generates higher pressures and flow rates than a straight one. In other words, the heart loops to pump more effectively – a trait observed throughout chordates, the group of organisms that includes birds, mammals, fish, amphibians and reptiles – and so probably arose early in vertebrate evolution.
This adaptation can be compared to packing a hose into a suitcase: looping it neatly allows it to fit and function efficiently. Similarly, the internal components of a car engine are arranged asymmetrically to maximise performance and space, even if the exterior appears symmetrical. Only after this basic, efficient pump is established do further evolutionary developments arise. While chambers and other features add complexity, the primary purpose of looping appears to be biomechanical, ensuring effective pumping and compact organisation from the earliest stages of development.
Chris Daniel
Glan Conwy, Conwy, UK
The heart’s position in the chest is high relative to the whole body, but quite central relative to the head and trunk, allowing efficient perfusion of the vital organs and extremities. The heart lies essentially in the middle of the thoracic cavity and is asymmetric in shape, being dominated by the powerful left ventricle that pumps blood around the whole body, which means that about two-thirds of the heart lies to the left of the body’s midline. The location of the heart in the body is primarily determined by its close relationship with the lungs, which it nestles between. This means that oxygenated blood from the lungs only has a short distance to travel to the left atrium of the heart, and from there into the left ventricle to be pumped around the body with each heartbeat. The heart, being a muscle, also has first call on the oxygen that it needs to function, which is supplied via its cardiac arteries. As with the lungs, it is protected by the rib cage from everyday external trauma.
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This adaptation is a bit like packing a hose into a suitcase: looping it neatly allows it to fit and function efficiently
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The pulse pressure – the difference between systolic and diastolic pressure – will be experienced in a similar way throughout the body. However, hydrostatic blood pressure varies, being proportional to the vertical distance from the heart, so the pressure at the ankles will typically be higher than it is at the head. For this reason, mechanisms are necessary to regulate blood flow both above and below the level of the heart. For example, pressure receptors, also known as baroreceptors, in the arteries close to the heart will detect falling blood pressure to the head, as can happen when you go from sitting to standing, and send signals to increase the rate or strength of heart contractions to restore the pressure. Veins in the legs have valves to ensure that blood continues to return to the heart and doesn’t excessively pool in the feet and ankles under gravity. Muscle contractions in the legs also gently squeeze the veins, supplementing the circulation powered by the heart.
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