2021-10-11 09:29:48
Human Circulatory System
Heart- Four chambered, derived from mesoderm. It is present between the two lungs in the thoracic cavity and protected by the pericardium.
A bicuspid (mitral) valve is present between the left, atrium and ventricle.
A tricuspid valve is present between the right, atrium and ventricle.
The right ventricle opens into the pulmonary artery and the left ventricle opens into the aorta. Both the openings are guarded by semilunar valves.
The heart is made up of cardiac muscles. There are nodal tissues also present in the heart.
The sinoatrial node (SAN) is present on the upper right corner of the right atrium and the atrioventricular node (AVN) is present at the lower-left corner of the right atrium.
The AVN divides into right and left bundle at the interventricular septum and branches into minute fibres, that are present throughout the ventricle and are called Purkinje fibres.
The nodal musculature is auto-excitable, i.e. it can generate action potential without any external stimuli.
Bundle of His- Purkinje fibres along with the AV bundles form the bundle of His. It regulates the heartbeat and conducts the electrical impulses.
The SAN is called ‘pacemaker’, it initiates and maintains the heart’s rhythmic activity. It can generate 70-75 action potentials per minute.
Arteries and veins are together called blood vessels, which transport blood to various parts of the body and bring back to the heart
Arteries and veins are made up of three layers:
Inner tunica intima – squamous endothelium
Middle tunica media – elastic fibres and smooth muscles (thin in the veins)
Outer tunica externa – fibrous connective tissue having collagen fibres
Cardiac Cycle
At the start of the cycle, all the four chambers remain in a relaxed state known as joint diastole.
Joint diastole is followed by atrial systole, on the generation of action potential from SAN node
The action potential then gets transferred to AVN and then to the bundle of His leading to the contraction of the ventricle, i.e. ventricular systole and at the same time atrial diastole occurs.
Ventricular systole causes closure of tricuspid and bicuspid valves. The semilunar valves open resulting in the flow of blood to vessels in the circulatory pathways.
Ventricular diastole follows with the closure of semilunar valves.
Then bicuspid and tricuspid valves open and the state of joint diastole is achieved once again.
This completes the one full cycle of the cardiac cycle.
The SAN generates a new action potential and the full process repeats.
As the heart beats ~72 times per minute, so there are ~72 cardiac cycles in a minute.
Stroke Volume is the amount of blood each ventricle pumps in a cardiac cycle, i.e. 70 ml.
Cardiac Output is the total output of blood from each ventricle in a minute, i.e. stroke volume multiplied by the no. of heartbeats per minute, which is ~5 L in a healthy individual. The cardiac output varies from person to person. An athlete will have much more cardiac output compared to an ordinary person.
In each cardiac cycle, two distinct sounds are produced, ie. ‘lub’ and ‘dub’, which can be heard using a stethoscope.
The first sound ‘lub’ is produced when bicuspid and tricuspid valves close at the time of ventricular systole.
The second sound ‘dub’ is produced when semilunar valves close at the time of ventricular diastole.
@Science_Wallah
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