 Hello friends, let us discuss the factors which affect the cardiac output. So let's start with this equation and see what are the factors affecting the cardiac output. So anything which increases heart rate and increases stroke volume will increase the cardiac output. Now this stroke volume is in turn dependent on two things. One is shortening of the muscle fibre. More the shortening of the muscle fibre that is more it is contracting it will cause increase in the stroke volume. And the second aspect is the size of the ventricle. More the size of the ventricle that means there is more muscle mass. So obviously its contraction will cause increase in the stroke volume. So now let us see what things will affect these aspects. So first thing we will see is two limbs of the autonomic nervous system. So first is sympathetic activity. Sympathetic activity increase in sympathetic activity leads to both increase in heart rate as well as causes increase in the stroke volume by causing increase in the shortening of the muscle fibre. On the other hand there is parasympathetic activity. Parasympathetic activity decreases the heart rate but has minimal effect on the stroke volume. This we should be very clear that parasympathetic activity basically supplies the SA node and it affects the heart rate and not stroke volume because there is minimal parasympathetic innovation on the muscle as such. Then next is the effect of the preload as we have told that there are mainly four factors which are important heart rate, stroke volume, preload and afterload. So preload also affects both heart rate as well as the stroke volume. So preload increases heart rate. This occurs by a phenomenon known as bain bridge reflex. This is basically that whenever there is increase in the preload certain nerve fibres in atrium are stimulated and via reflex mechanisms they increase the heart rate. Plus there is also some direct stretching of SA node which leads to increase in heart rate. Then this preload also increases the stroke volume by increasing the shortening of the muscle fibre. This happens because of Frank Starling law. Now Frank Starling law says that force of contraction is directly proportional to the initial length of the muscle fibre. So whenever preload increases what happens that it stretches the muscle fibre that is increases the initial length of the muscle fibre causing increase in the shortening of the muscle fibre. So see this preload is already incorporated into this equation. Now when we think about preload we should be very clear that there are multiple factors which affect preload itself. So preload is first of all affected by blood volume. So increase in the blood volume will increase the preload. Then there is a effect of gravity that means suppose we change our position from lying to standing then in that case the preload decreases. Then there is contraction of our skeletal muscles and abdominal muscles that also affect the preload. Then there is ventricular compliance that means how stiff or how compliant the ventricle is that means if it is very stiff so it will not relax and if it doesn't relax it will affect the feeling of the heart so that ventricular compliance is another factor. Then there is atrial contraction because atrial contraction causes the last push. It is responsible for approximately 10% of feeling of the ventricles. So atrial contraction will also affect the preload. Then another very important factor is venous compliance that means how much blood can veins accommodate. Now this depends on the diameter of the veins. So if veins have lesser diameter then that means they will push the blood towards the heart so that is venous compliance. Then push from the heart itself that is contraction of the heart that means we can say the cardiac output that also affects the preload. If heart is not contracting obviously the blood will not move. So this phenomena is known as vis atergo push from behind is vis atergo. So if there is a push from behind there is also a pull from front that is known as vis affronte. So there is another factor vis affronte which affects the preload. This is basically whenever there is increase in the size of the thoracic cavity as during inspiration. So it creates a negative pressure in the thoracic cavity and this creates a pull force for the blood. So this also affects the preload. Okay there is quite a overlap of the arrows but don't worry we will just in the end we will make a clearer flowchart for you to take it. Okay now what about afterload? Well afterload actually decreases stroke volume. So it actually decreases the shortening of the muscle fiber. So it is at this level that afterload acts and decreases the shortening of the muscle fiber. So see this is also incorporated in the equation itself because afterload is affecting the stroke volume. Now right now we have drawn just one on one arrows which are affecting these four factors preload, afterload, heart rate and stroke volume. But there is lot of crisscross also because the sympathetic activity affects the venous compliance as well. When there is increase in the sympathetic activity it causes venoconstriction and venoconstriction will in turn lead to increase in the preload. So sympathetic activity is affecting heart rate, stroke volume as well as the preload. Also it increases the afterload because sympathetic activity causes constriction of the arterioles which are responsible for peripheral resistance. So this sympathetic activity increases the afterload as well. Then there is something known as force frequency relationship that means increase in heart rate causes increase in the stroke volume. This is known as force frequency relationship or also known as Bowditch phenomena. So this happens because when heart rate is more it causes with each cardiac impulse there is increase in calcium inside the cardiac muscle and as you might be aware that calcium is important for contraction. So this increase in calcium leads to increase in the stroke volume. So guys these are the factors which affect the cardiac output. Let me just show you a summary of all these factors in a clearer project. So this is a clearer summary. If you like the video do press the like button and share the video and don't forget to subscribe to the channel Physiology Open. Thank you.