The Heart — An Elegant Pump

The following sections are included:

• Dedication
• Preface
• About the Author
• Acknowledgements
• Contents

The following sections are included:

• The Origin of Heart
• Heart Metaphors
• Heart in European Languages
• Heart in Non-European Languages
• The Uses of Heart in Ancient Languages
• The Heart in Chinese
• The Heart in Chinese Medicine
• The Heart in Chinese Culture
• Heart Metaphors Expressing Emotion
• Mesopotamia
• The Heart in Mesopotamian Medicine
• Heart in Mesopotamian Culture
• Ancient Egypt
• The Heart in Egyptian Medicine
• Heart in Egyptian Culture
• Heart Metaphors in Ancient Egypt
• The Concept of Heart in 21st Century Culture
• Heart in Religion
• The Physical and Metaphorical Heart: Why We Need Both
• Conclusion
• References
• Appendix I
• Appendix II
• Appendix III
• Appendix IV
• Appendix V
• Appendix VI

The heart is the first organ to develop during human embryogenesis and it occurs at a very early stage. At around 18 days following fertilisation of the ovum, cells which will grow to form the heart have appeared and by 21–22 days a primitive heart has begun to pump. In another sense, the heart could be said to originate millions of years ago during the evolution of our ancestors. It has long been observed that during the development of an embryo, known as ontogeny, it has similarities with intermediate forms of animals at various times during the evolution of our ancestors, known as phylogeny. For example, during its life in the womb, a human embryo adopts transient features reminiscent of gills and a tail. This led to the idea that embryogenesis is a speeded-up version of evolution and during the 19th century this theory was widely held and known by the phrase “ontogeny recapitulates phylogeny”, coined by Ernst Haeckel. However, it became clear during the early 20th century that evolution did not progress in a linear manner but involved many branching divisions and so no single path could reflect embryogenesis. Nevertheless, the biology of modern humans incorporates the evolutionary innovations that occurred in ancient creatures millions of years ago and which we have inherited through our genes. Our chemistry and physiology are encoded in the DNA received from our parents as they and their parents did in turn over countless generations. Elements of the DNA which control human development including our hearts are also found in other animals, even in the simplest and oldest life forms such as bacteria. In that sense, our hearts can also be said to originate in ancient times during the evolution of life. Both origins, that of our own lives in the womb and our evolutionary inheritance, are encoded in the DNA which we received at our conception.

As conscious beings, our thoughts are mostly concerned with the external world and our interactions with it. We observe the movement of other people, animals and objects to plan our safe interactions with them. In health, we are only occasionally aware of movements within our bodies, and yet internally we are constantly on the move. Blood flows continuously through our arteries and veins, food passes through our intestines and air flows through our lungs. At a microscopic level, particles and molecules are constantly moving to fulfil functions such as conveying signals between different parts of the cell or for structural maintenance. Most of this is beyond the reach of our consciousness. Were it otherwise, we would be overwhelmed by the number and complexity of the processes involved, which far exceed the capacity of our conscious minds to regulate. Nevertheless, we are aware of certain movements within us from time to time. Exertion and excitement can make us aware of the heart beating and respiration. Occasionally we become aware of intestinal movements or the “kicking” of a growing foetus. And although we have little or no conscious control over them, they are important elements of our experience. As discussed in Chapter 1, we frequently use a shared understanding of how the heart responds to emotion to convey how we or others feel, and the first “kick” of a growing foetus is a moment to remember. This chapter is concerned with how these movements are powered, and, although it is focused on the heart, the basic motor involved is very similar in all of them…

Efficient pumping by the heart depends on coordinating its contraction so that its 2–3 billion myocytes and the molecular motors in each myocyte work together. This implies that there has to be a communication system to synchronise their behaviour. Chapter 3 showed that modulation of the intracellular concentration of Ca²⁺ acts as a signal to orchestrate contraction of all the sarcomere filaments within each myocyte. However, there must also be a mechanism to ensure that contraction of each myocyte is correctly coordinated to achieve efficient ejection of blood. This is achieved by a remarkable communication system which directs and controls the contraction and relaxation of every myocyte with each heartbeat.

Our last car was 17 years old when we replaced it. I was fond of its until then steady reliability. Its commodious boot had served well as family haulage space, and it was comfortable on long drives. By then I had come to the view that cars could be boring but should be reliable. I had once owned a sports car but then discovered that riding a bicycle provided more excitement, and the only occasion when I was ever stopped by a police patrol car was while riding a bike. After 17 years, the radiator hose joint snapped, signalled by a large eruption of steam. A temporary fix involving shoe laces allowed removal of the stricken vehicle to a nearby garage and although replacing the radiator proved straightforward, the episode undermined our confidence in its long-term durability. It might have gone on for many more miles, but when would another part fail? I felt disloyal to let it go after such long service and although the car salesman seemed to genuinely admire its old-fashioned durability, that was not reflected in the part exchange offer on its replacement…

The heart like any other tissue in the body is subject to disease and injury but its role in maintaining constant perfusion of other organs means that serious malfunction of the heart can rapidly put the entire body in jeopardy. However, in many instances, it is also capable of recovering, either partially or fully. To discuss this, I will use two examples. The first, “broken heart syndrome”, is an unusual illness in which the pump function of the heart is greatly diminished following an episode of severe emotional or physical stress, but nearly always recovers fully. The second, myocardial infarction (heart attack), which is caused by a blocked coronary artery, results in permanent damage to part of the heart. In most of these cases, it is capable of healing sufficiently well to sustain a normal life.

The ultimate purpose of the heart is to assist the transport of essential materials to and from specific locations around the body. Oxygen is delivered from the lungs to all tissues and carbon dioxide, the product of its metabolism, is taken back to the lungs to be exhaled. Nourishment from food is carried from the gut to all tissues and waste products are transported to the kidneys for excretion. Hormones, signalling molecules produced by endocrine glands to regulate the function of many organs, are circulated in the bloodstream to reach their target cells. Blood is the medium in which all of these are transported. Blood itself is far more than a passive transport medium, but rather a liquid organ with many unique functions, including acting as a surveillance system and first line of defence to maintain the integrity of blood vessels. All this transport takes place within a system of arteries and veins, referred to as the circulation, which carry blood from the heart to all regions of the body and return it for recirculation. Far from being a simple set of passive tubes, the circulatory system is highly evolved with distinctive features that are exquisitely suited to the pulsatile nature of the heart and on which the heart depends for its normal function.

The following sections are included:

• Introduction
• Intelligent Tubes in the Circulation
• Myogenic Tone
• Smooth Muscle
• Contractile Function
• Length Adaptability
• Smooth Muscle Structure
• A Dynamic Cytoskeleton
• Smooth Muscle Contraction
• Signalling Pathways Regulating Contraction
• Multiple Interacting Controls
• Mechano-sensing in Blood Vessels
• Evolution of Muscle
• Smooth Muscle Cells are Flexible in Structure and Function
• Actin Binding Proteins
• Tropomyosin: A Master Actin Filament Regulator
• Smooth Muscle Length Adaptability
• Conclusions
• References

The following sections are included:

• Introduction
• Composition of Blood
• Transport Medium
• Oxygen Consumption
• Oxidation of the Earth’s Atmosphere
• Photosynthesis
• Complex Organisms Needed More Efficient Oxygen Delivery
• Evolution of Oxygen Use
• Oxygen Transport
• Haemocyanins
• Haemerythrins
• Haemoglobin
• Haemoglobin and Oxygen Transport
• In Vivo Modulation of Haemoglobin Oxygen Affinity
• Evolution of Haemoglobin and Erythrocytes: Matching the Heart and Circulation
• Why Are Capillaries and Erythrocytes of Similar Size?
• Co-evolution of Erythrocytes and Haemoglobin
• Biconcave Disc Erythrocytes
• Erythrocyte Size, Metabolic Activity and Genome Size
• Diminution of Capillaries and Erythrocytes in Mammals and Bird
• Conclusions
• References

The following sections are included:

• Introduction
• Diets Then and Now
• Physical Activity Then and Now
• Tobacco and Alcohol Then and Now
• Cardiovascular Disease Then and Now
• Diabetes Then and Now
• Obesity Then and Now
• Blood Pressure Then and Now
• Impact of Lifestyle Changes on Longevity
• Learning from the Past
• What and How We Eat
• Dietary Modification to Reduce Cardiovascular Disease
• Individual and Population Dietary Challenges
• Physical Activity and Cardiovascular Health
• Physical Activity Guidelines
• Any Is Good, the More the Better
• Physical Activity for Life
• Tobacco and the Heart
• The Health Burden of Tobacco
• Recollections of an Ex-smoker
• Working Towards a Tobacco-free World
• Alcohol and the Heart
• Can We Drink Safely?
• Summary and Conclusions
• References

The following section is included:

• Index