It blood towards the heart whereas the

 

It is
evident that the heart is one of the most important organs in human physiology as
without it, there would be no way of transporting blood around the human body
and therefore the exchange of materials, required for survival, would not be
possible. The human heart is, essentially, a biological pump made up of muscle
which contracts in order to move oxygenated blood (blood saturated with oxygen)
and deoxygenated blood around the body. The importance of the hearts structure
in relation to its function can be broken up into smaller topics: structure,
function, how they both relate to each other, conditions and diseases, and
consequences of these diseases.

Structure

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To
further understand the structure of the heart, the surrounding blood vessels
attached to the heart must also be mentioned: aorta, pulmonary artery,
pulmonary vein, and the vena cava (Biology, no date). The veins, pulmonary
vein and vena cava, carry deoxygenated blood towards the heart whereas the
arteries, pulmonary artery and aorta, carry oxygenated blood away from the
heart; we shall learn more about this later(Bass and Kang, no date). The heart consists of
four chambers: the right and left atria and ventricles – blood passes into the
heart through the atria as they, essentially, gather blood whereas the
ventricles pump blood to various different parts of the body (Healthline Medical Team, 2015). The heart also
consists of four valves: the atrioventricular valves (located between the atria
and ventricles) and the semi-lunar valves (located in the aorta and pulmonary
artery) – 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 the
heart contain papillary muscle which join to the atrioventricular valves via
chordae tendineae (tendons mainly composed of collagen) which ensure that the
bicuspid and tricuspid valves do not prolapse (turn inside out) (Weinhaus and Roberts, 2005)

The
Epicardium (outer layer), myocardium (layer containing the cardiac muscle), and
endocardium (inner layer) make up the three layers of the heart wall (Bailey, 2017)(Taylor, no date).

The
right 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 which
would otherwise cause issues – more on this later (American College of Cardiology, 2008).

 Function

Essentially,
the function of the heart is to pump blood around the body in order for the
exchange of materials (such as oxygen and glucose) to occur (Lewis, 2016). In order to further
understand the function of the heart, the pathway of the blood must also be
understood. A double circulatory system is present in the human body meaning
that blood enters the heart twice in order to provide oxygenated blood to the
rest of the body – the process is as follows: blood present in the right atrium
is pumped into the right ventricle through the tricuspid valve. The blood then
flows from the right ventricle into the pulmonary artery through the semilunar
valve where the blood becomes saturated with oxygen in the lungs. The blood
returns to the heart into the left atrium through the pulmonary vein where it
is pumped into the left ventricle through the bicuspid valve – finally, the
blood is pumped from the left ventricle into the aorta (where the pressure is
lower) through the semilunar valve. The tricuspid and bicuspid valves prevent
blood flowing backwards into the atria during ventricular systole (contraction)
– semilunar valves have the same function as they, too, stop the back-flow of
blood however they do so from the arteries (pulmonary artery and aorta) into
the ventricles (Mr Pollock, 2014) (Pearson, no date).

It is
important to note that blood flows from an area of high pressure to an area of
low pressure. When the heart relaxes (cardiac diastole), blood enters the atria
at low pressure from the vena cava and pulmonary vein causing a pressure
increase within the atria. This, in turn, causes blood to flow from the atria
into the ventricles (through the atrioventricular valves) as the pressure is
lower in the ventricles. During atrial systole, blood is forced into the
ventricles from the atria causing a slight pressure increase in the ventricles
which causes the atrioventricular valves to close therefore preventing the
backwards flow of blood into the atria. Moreover, the ventricles then contract
(ventricular systole) which causes the pressure to increase inside the
ventricles and the blood to be pumped through the semilunar valves into the
pulmonary artery and the aorta where the pressure is lower (Mr Pollock, 2014) (Pearson, no date).

How
structure and function relate

The
structure of the heart relates to its function in many different ways for
example, the left ventricular wall is thicker because it contains more cardiac
muscle – this is because the left ventricle pumps oxygenated blood all the way
around the body (the systemic circuit) as opposed to the right ventricle which
only pumps blood to the lungs (pulmonary circuit) therefore, people who place
their body under physiological stress (for example through exercising and
conditioning)  can often have a thicker
left ventricular wall (Lee et al.,
2013) (Taylor, no date). Heart valves, for
example the aortic valve (one of the semilunar valves), must be able to endure a
lot of pressure as blood is continuously being pumped through them at high
pressure – one of the major tissues which a heart valve consists of is collagen
which provides the valves’ mechanical strength. Furthermore, the ventricularis
layer of the aortic valve is mostly comprised of collagen and elastin fibres.
These elastin fibres allow the valve to return to its arrangement after the
first part of cardiac diastole. During cardiac diastole, the cusps of the aortic
valve undergoes a stretch – it is during this phase in which the elastin fibres
are responsible for the work-load of the hydrostatic pressure. However, when
the cusps are almost closed, collagen bears the majority of the work-load. The
properties of both, collagen and elastin fibres in the aortic valve, ensure
that blood only flows in one direction whilst being pumped from the left
ventricle into the aorta (Balguid et al.,
2007).

Conditions

As
previously mentioned the septum divides the atria and the ventricles from the
right and left side, however in some instances after birth an atrial septal
defect (ASD) can occur (colloquially known as having a ‘hole in the heart’)
this that blood from both the pulmonary and systemic circulations are able to
mix (oxygenated and deoxygenated blood can mix) increasing the risk of
pulmonary 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, whereby
the both the septum primum (extends from the atrium’s anterior roof) and the
endocardial cushion (central heart tissues) are not fully fused together (Adler, 2017) (Belmont, 2015). This demonstrates the
importance of the septum’s function and how its correct structure allows
efficient functioning of the cardiac cycle.

We
previously touched on the left ventricular wall being thicker in a human heart
however, cardiomyocytes (cardiac muscle cells) can become hypertrophied in the
sense that the size of the cardiomyocytes become larger (Frey et al.,
2004). The
formation of the sarcomeres also become more sensitive – sarcomeres are muscle
fibre units which consist of thick and thin protein filaments; the thick
filaments contain mostly myosin protein whereas the thin filaments contain
mostly actin protein (Smith and Plowman, 2007). When muscle
contracts, the sliding filament theory proposes that that the thick filaments
slide past the thin filaments (Goodman, S. R., Zimmer, 2008). The increase in
cardiac muscle thickness happens through the addition of sarcomere units
(sarcomerogenesis) – this ensures that the muscle wall of the left ventricle is
even thicker therefore, the volume inside the chamber has decreased and less
blood can now be pumped around the body meaning that the heart now has to pump
more blood around the body in order to meet the oxygen and nutrient
requirements (Mayo Clinic Staff, 2017) (Wilson et al.,
2014). It
is also important to note that the cardiac muscle may also lose elasticity
which can potentially result in a stroke – this emphasises the importance of
the structure of the cardiac muscle and how a change in its thickness can alter
the functioning of the cardiac cycle (Mayo Clinic Staff, 2015).

It is interesting, then,
to conclude that structure is very important in relation to the heart’s function
– some of the structures could be argued to be of more importance for instance:
the cardiac muscle wall, valves, and septum; any defects in these structures usually
lead to a condition which, if left untreated, could prove to be lethal.  

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