The to volume ratio, therefore only require a single

The heart has structures and mechanisms, which ensure it is
designed for acting as a pump to allow blood to circulate around the body via
the circulatory system. The heart needs to be able to pump oxygenated blood around
the whole body to ensure all cells have sufficient oxygen for respiration. This
is fundamental for organisms, to produce substances, such as ATP, for survival.
In this essay, we will discuss the different structures and mechanisms of the
heart, and how they are adapted to ensure sufficient blood is pumped around the
body. We will also explore what would happen if any of these structures or
mechanisms malfunctioned.

The heart and the circulatory system varies between each
organism, this is usually based on the organisms’ surface area to volume ratio.
For example, fish have a large surface area to volume ratio, therefore only
require a single circulatory system. Whereas elephants have a small surface
area to volume ratio requiring a double circulatory system. Humans specifically
have a double circulatory system, a separated systemic and pulmonary circuit. These
two separate systems allow for a major advantage ‘it allows pressure to be
different in both circuits.’ (Moyes and Schulte, 2008, p. 362). The lungs contain thin
capillaries to allow for efficient gas exchange, if blood entered these thin capillaries
at high pressure they would rupture, therefore low pressure if required for the
pulmonary circuit. However, a higher pressure if required for the systemic circuit
as when blood exits the aorta it needs to be able to pump oxygenated blood all
around the body. These two systems are vital to ensure the different conditions
needed in each part of the body are met.

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The heart is made of four chambers, the most vital being
the left ventricle as this is where blood is at its highest pressure, as it
needs to be able to reach all body extremities. The ventricular muscle is
usually made of two different myocardia. ‘The outer layer of compact myocardium
which is tightly packed cells arranged in a regular pattern, and an inner layer
of spongy myocardium, a meshwork of loosely connected cells’ (Moyes and Schulte, 2008, p. 369). In humans there is
usually more compact myocardium, perhaps due to the high pressure at which blood
is flowing. The left ventricle is specifically thicker to withstand the pressure,
however if the pressure increases too high (hypertension) this could lead to
serious health complications and a possible permanent damage to the heart.

Not only, are the heart chambers adapted to withstand different
pressures. The blood vessels are also. The arteries have a larger diameter,
with thick walls and a small lumen. The arteries have thicker walls ‘the closer to the heart
they are as they have to withstand high pressure, this means they have more elastic
fibres’ (‘Structure
and Function of Blood Vessels | Anatomy and Physiology II’, no date). These arteries are usually known as ‘muscular
arteries, due to the thicker tunica media’ (Moyes and Schulte, 2008, p. 359). The main purpose of the arteries is to
withstand high pressure in which blood is flowing. However, the veins have a
completely different structure as blood is flowing at a much lower pressure. They
have a larger lumen and not as thick walls, compared to arteries. They also
contain valves which help prevent back flow of blood, some veins are carrying
blood which work against gravity so therefore valves are required to ensure
blood flows in the correct direction. The last major blood vessel is the capillary
which is adapted to be efficient in the exchange of substances via diffusion.
The capillary only consists of an endothelium wall which makes it ideal for the
exchange of substances. There are different types of capillaries ‘Continuous capillaries,
found in skin and muscle. Fenestrated capillaries which are found in areas of
the body where exchange of substances occur and Sinusoidal capillaries, which
are found in specialised organs such as bone marrow and the liver’ (Moyes and Schulte, 2008, p. 359). The most commonly spoken about being
the fenestrated capillaries. Both the heart and blood vessels have various
adaptations to withstand variations in pressure. This is beneficial to ensure
the circulatory system works efficiently in providing all body cells with sufficient
oxygen.

Valves are not only present within veins but within the heart.
They also prevent the backflow of blood but also control the direction of flow
by opening and closing in a rhythm. This is determined by the cardiac cycle. When
the heart is in ventricular diastole ‘the atria and the ventricles relax and
the atrioventricular valves open’ (Bailey, no date). This allows the
ventricles to fill with blood. Next, atrial systole where the atria contract
and force any blood remaining in the atria into the ventricles. A contraction
in the ventricles ‘pushes AV valves closed and increases pressure inside the
ventricle’ (Moyes and Schulte, 2008, p. 374). The increase in
pressure within the ventricles causes the semi-lunar valves to open and blood
is pushed out of the ventricles and out of the heart via the aorta and the
pulmonary artery. This is known as ventricular systole. Then the cycle returns
to ventricular diastole where the ventricles relax and ‘pressure in the arteries
exceeds ventricular pressure, closing the semi-lunar valves’ (Moyes and Schulte, 2008, p. 374). The cycle then repeats.
By the description above, it is evident that valves are crucial in ensuring
blood flows in the correct direction. It could cause some serious damage if
blood flowed in the wrong direction.

The way the heart chambers contract are controlled by
electrical impulses, which are ultimately controlled by the brain (medulla oblongata).
The heart has its own pacemaker called a sinoatrial node (SAN). The SAN sends an
electrical impulse across the atria causing atrial systole. There is a short
delay before this signal is noticed by the atrioventricular node (AVN), this is
to allow blood to completely exit the atria and fill the ventricles. After this
short delay the AVN produces a second electrical impulse ‘which passes the
signal down to the apex of the heart. This is passed through cardiac muscle
fibres called the Bundle of His. From the apex, the electrical activity is spread
throughout the ventricles along Purkinje fibres’ (Heart,
no date). This
causes the heart to contract from the apex up, causing blood to be pushed out
of the ventricles, also known as ventricular systole. The medulla oblongata controls
how often an electrical impulse is produced by the SAN. For example, if there
is increased carbon dioxide within the blood, causing blood to become acidic, the
SAN produces more electrical impulses per minute to ensure the excess carbon
dioxide is exhaled. This is evident during exercise when heart rate increases. This
is known as a sympathetic nervous system response.

The heart is surrounded by the pericardium which is a fibrous
covering, the outer layer usually made of connective tissue, which surrounds
the heart, it also ‘contains fluid with lubricates the heart in the pericardial
space to prevent friction’ whilst the heart beats. (Know
the Structures and Functions about Your Heart, 2015). It is important there
is limited friction within the heart as ‘the rubbing together of inflamed
membranes of the pericardium, may be a symptom of pericarditis or myocardial infarction’. (pericardial
friction rub, no date). Furthermore, the pericardium isn’t only to prevent
friction within the heart but in the ‘maintenance
of cardiac position and protection against ventricular dilatation’ (Dubiel, 1991). Cardiac positioning refers to the heart remaining in the left-hand
side of the chest. Protection against ventricular dilation is key, as if the ventricular
walls become too thin, it will result in the ‘heart’s inability to supply the body with enough blood which can
also contribute to irregular heartbeats (arrhythmias), blood clots or sudden
death’ (Dilated
cardiomyopathy – Symptoms and causes – Mayo Clinic, no date).  

To conclude, in this essay we have focused specifically on
the human or mammalian heart. Exploring its structures, how the circulatory system
functions and how they are adapted to be more efficient in performing their required
function. Secondary, we have discussed two main mechanisms, the cardiac cycle
and the electrical activity within the heart. Overall, it is evident that all
the structures and adaptations the heart has are vital in ensuring the
performance of a healthy heart. Any deviation from the what is considered
normal may result in a serious condition. The heart is one of the most vital
organs, for any organism, but one which has many intricate structures which can
malfunction.