It isevident that the heart is one of the most important organs in human physiology aswithout it, there would be no way of transporting blood around the human bodyand therefore the exchange of materials, required for survival, would not bepossible.
The human heart is, essentially, a biological pump made up of musclewhich contracts in order to move oxygenated blood (blood saturated with oxygen)and deoxygenated blood around the body. The importance of the hearts structurein relation to its function can be broken up into smaller topics: structure,function, how they both relate to each other, conditions and diseases, andconsequences of these diseases. StructureTofurther understand the structure of the heart, the surrounding blood vesselsattached to the heart must also be mentioned: aorta, pulmonary artery,pulmonary vein, and the vena cava (Biology, no date). The veins, pulmonaryvein and vena cava, carry deoxygenated blood towards the heart whereas thearteries, pulmonary artery and aorta, carry oxygenated blood away from theheart; we shall learn more about this later(Bass and Kang, no date). The heart consists offour chambers: the right and left atria and ventricles – blood passes into theheart through the atria as they, essentially, gather blood whereas theventricles pump blood to various different parts of the body (Healthline Medical Team, 2015). The heart alsoconsists of four valves: the atrioventricular valves (located between the atriaand ventricles) and the semi-lunar valves (located in the aorta and pulmonaryartery) – these valves prevent blood flowing in the incorrect direction(backwards) therefore, the blood only flows in one direction (Bass and Kang, no date).
The ventricles of theheart contain papillary muscle which join to the atrioventricular valves viachordae tendineae (tendons mainly composed of collagen) which ensure that thebicuspid and tricuspid valves do not prolapse (turn inside out) (Weinhaus and Roberts, 2005) TheEpicardium (outer layer), myocardium (layer containing the cardiac muscle), andendocardium (inner layer) make up the three layers of the heart wall (Bailey, 2017)(Taylor, no date). Theright and left chambers of the heart are separated by a wall called a septum,this ensures that deoxygenated blood and oxygenated blood do not mix whichwould otherwise cause issues – more on this later (American College of Cardiology, 2008). FunctionEssentially,the function of the heart is to pump blood around the body in order for theexchange of materials (such as oxygen and glucose) to occur (Lewis, 2016).
In order to furtherunderstand the function of the heart, the pathway of the blood must also beunderstood. A double circulatory system is present in the human body meaningthat blood enters the heart twice in order to provide oxygenated blood to therest of the body – the process is as follows: blood present in the right atriumis pumped into the right ventricle through the tricuspid valve. The blood thenflows from the right ventricle into the pulmonary artery through the semilunarvalve where the blood becomes saturated with oxygen in the lungs. The bloodreturns to the heart into the left atrium through the pulmonary vein where itis pumped into the left ventricle through the bicuspid valve – finally, theblood is pumped from the left ventricle into the aorta (where the pressure islower) through the semilunar valve. The tricuspid and bicuspid valves preventblood flowing backwards into the atria during ventricular systole (contraction)– semilunar valves have the same function as they, too, stop the back-flow ofblood however they do so from the arteries (pulmonary artery and aorta) intothe ventricles (Mr Pollock, 2014) (Pearson, no date).It isimportant to note that blood flows from an area of high pressure to an area oflow pressure.
When the heart relaxes (cardiac diastole), blood enters the atriaat low pressure from the vena cava and pulmonary vein causing a pressureincrease within the atria. This, in turn, causes blood to flow from the atriainto the ventricles (through the atrioventricular valves) as the pressure islower in the ventricles. During atrial systole, blood is forced into theventricles from the atria causing a slight pressure increase in the ventricleswhich causes the atrioventricular valves to close therefore preventing thebackwards flow of blood into the atria. Moreover, the ventricles then contract(ventricular systole) which causes the pressure to increase inside theventricles and the blood to be pumped through the semilunar valves into thepulmonary artery and the aorta where the pressure is lower (Mr Pollock, 2014) (Pearson, no date).RelationThestructure of the heart relates to its function in many different ways forexample, the left ventricular wall is thicker because it contains more cardiacmuscle – this is because the left ventricle pumps oxygenated blood all the wayaround the body (the systemic circuit) as opposed to the right ventricle whichonly pumps blood to the lungs (pulmonary circuit) therefore, people who placetheir body under physiological stress (for example through exercising andconditioning) can often have a thickerleft ventricular wall (Lee et al.
,2013) (Taylor, no date). Heart valves, forexample the aortic valve (one of the semilunar valves), must be able to endure alot of pressure as blood is continuously being pumped through them at highpressure – one of the major tissues which a heart valve consists of is collagenwhich provides the valves’ mechanical strength. Furthermore, the ventricularislayer of the aortic valve is mostly comprised of collagen and elastin fibres.These elastin fibres allow the valve to return to its arrangement after thefirst part of cardiac diastole. During cardiac diastole, the cusps of the aorticvalve undergoes a stretch – it is during this phase in which the elastin fibresare responsible for the work-load of the hydrostatic pressure. However, whenthe cusps are almost closed, collagen bears the majority of the work-load.
Theproperties of both, collagen and elastin fibres in the aortic valve, ensurethat blood only flows in one direction whilst being pumped from the leftventricle into the aorta (Balguid et al.,2007). Conditionsand diseases/consequences Aspreviously mentioned the septum divides the atria and the ventricles from theright and left side, however in some instances after birth an atrial septaldefect (ASD) can occur (colloquially known as having a ‘hole in the heart’)this that blood from both the pulmonary and systemic circulations are able tomix (oxygenated and deoxygenated blood can mix) increasing the risk ofpulmonary hypertension (high blood pressure in the arteries of the lungs)causing angina and fatigue (Geva, Martins and Wald, 2014) (NHS, 2017). An example of an ASD is Ostium primum, wherebythe both the septum primum (extends from the atrium’s anterior roof) and theendocardial cushion (central heart tissues) are not fully fused together (Adler, 2017) (Belmont, 2015).
This demonstrates theimportance of the septum’s function and how its correct structure allowsefficient functioning of the cardiac cycle. Wepreviously touched on the left ventricular wall being thicker in a human hearthowever, cardiomyocytes (cardiac muscle cells) can become hypertrophied wherebythe size of the cardiomyocytes become larger (Frey et al.,2004). Theformation of the sarcomeres also become more sensitive – sarcomeres are musclefibre units which consist of thick and thin protein filaments; the thickfilaments contain mostly myosin protein whereas the thin filaments containmostly actin protein (Smith and Plowman, 2007).
When musclecontracts, the sliding filament theory proposes that that the thick filamentsslide past the thin filaments (Goodman, S. R., Zimmer, 2008).
The increase incardiac muscle thickness happens through the addition of sarcomere units(sarcomerogenesis) – this ensures that the muscle wall of the left ventricle iseven thicker therefore, the volume inside the chamber has decreased and lessblood can now be pumped around the body meaning that the heart now has to pumpmore blood around the body in order to meet the oxygen and nutrientrequirements (Mayo Clinic Staff, 2017) (Wilson et al.,2014). Itis also important to note that the cardiac muscle may also lose elasticitywhich can potentially result in a stroke – this emphasises the importance ofthe structure of the cardiac muscle and how a change in its thickness can alterthe functioning of the cardiac cycle (Mayo Clinic Staff, 2015). It isinteresting, then, to conclude that structure is very important in relation tothe heart’s function – some of the structures could be argued to be of moreimportance for instance: the cardiac muscle wall, valves, and septum; anydefects in these structures usually lead to a condition which, if leftuntreated, could prove to be lethal.