The cardiac cycle is the sequence of alternating relaxation and contraction of the atria and ventricles
to allow oxygen and nutrient rich blood to pump throughout the body. It begins with a spontaneous
action potential, generated in the Sino-atrial node. The SA node is known as a pacemaker, they
cardiac muscles in it are autorhythmic, which means they produce the action potentials but don’t
contract themselves, they make up 1% of cardiac muscles. The rest of the cardiac muscles are
branched and connected via discs with gap junctions and allow muscle action potentials to spread.
The first stage, known as atrial diastole in the cardiac cycle happens very slightly before the signal
from the SA node reaches the atria. During the early part of the first stage, blood enters the right
atrium through the inferior and superior vena cava and enters the left atrium by the pulmonary
veins. When the heart begins to beat faster, this stage gets shorter.
The next stage is known as atrial systole, it is the second event. Blood is moved from the highpressure atria to the low-pressure ventricle. The signal that was fired from the Sino-atrial node, now
initiates an action potential that travels throughout the atrial muscles, it causes the muscles to
contract, and forces the blood from the atria into the ventricles. The signal generated from the SA
node, also hits the atrio-ventricular node, which passes the signals down the Purkinje fibres to the
apex of the heart. This signal eventually spreads throughout the ventricles through bundles, and
smaller fibres and then spreads among contractile cells.
The blood is now in the ventricles, and the third stage known as ventricular diastole, begins. During
the early stages of ventricular diastole, the semilunar and atrioventricular valves are closed which
allows the pressure to fall. After the ventricular pressure becomes lower than that of the atrial
pressure, the atrioventricular valves open, which results in the ventricles quickly filling up with
blood. This is where the ventricles mostly get their blood.
The cardiac cycle now enters its final stage, ventricular systole. The ventricles start to contract.
During this early part, the pressure of the ventricles is higher than that of their corresponding
atrium, which makes the atrioventricular valves close. There’s no volume change however, and this
is known as isovolumetric contraction. Eventually, the pressure starts to build up and as a result of
this, the semilunar valves open and blood is released from the ventricles through the valves and into
the body.
This process repeats indefinitely until death.
The systolic aortic pressure, occurs when the left ventricle of the heart pumps or ejects blood into
the aorta, following this the aortic pressure rises. The systolic aortic pressure is the maximum aortic
pressure that ensues after the pumping of blood from the ventricle to the heart.
The diastolic aortic pressure, occurs when the left ventricle is beginning to relax, following the
systolic aortic pressure. The blood that is now pumped out of the left ventricle, allows the ventricle
to refill, and causes the pressure in the aorta to fall. The diastolic aortic pressure is the lowest
pressure in the aorta, which occurs right before the systolic aortic pressure.
These two pressures are akin to breathing, for example, if one inhales as much air as possible
(diastolic pressure), and then releases it with good pressure (systolic pressure).
A heart attack occurs when arteries begin to narrow, known as vasoconstriction, which decreases
blood flow to the heart. The heart then suffers from a lack of blood flow, and blood pressure will
begin to rise.
Nitro-glycerine is a rapid form of treatment designed to prevent heart attacks and treat them. Nitroglycerine is a chemical compound that has been used in medicine for several years. It is used
because it is an extremely potent nitrate vasodilator. A vasodilator is a type of medication that
opens or dilates blood vessels, allowing for better blood flow. When the body takes in nitroglycerine, it is quickly broken down into nitric oxide via biochemical reactions in the cell. Nitric oxide
is already found and produced by nearly every cell in the human body, it is also used for other
medication such as Viagra. Due to the large rapid increase of nitric oxide, it relaxes the inner muscles
of the blood vessels, causing them to widen and expand. This allows for an increase in oxygen and
nutrient rich blood flow to the heart and decreases blood pressure, thereby preventing a heart
attack.