The Heart As a Pump, Heart Sounds and Cardiac Output

Hello :) In this post we'll be discussing how the heart acts as a pump, how the sounds of the heart are produced as well as how cardiac output is regulated.

Diagram of the Heart
source:http://commons.wikimedia.org/wiki/File%3ADiagram_of_the_human_heart_(cropped).svg
please see this website if you'd like to use this diagram.

The heart is really two pumps which work in series and the heart pumps in pulmonary and systemic circulations. Pulmonary circulation is the part of the circulatory system which transports deoxygenated blood from the right side of the heart to the lungs and returns oxygenated blood to the left side of the heart. The systemic circuit is the part of the circulatory system which pumps oxygenated blood from the left ventricle to to the tissues in the body and returns deoxygenated blood to the right atrium of the heart. When we talk about ventricular contraction, it is termed systole. While ventricular relaxation is referred to as diastole.  

Pressure in the heart chambers vary between different stages of the cardiac cycle. Blood flow is driven by the changes in pressure. The normal direction of blood flow is from the atria to the ventricles, then from the ventricles to the arteries. Valves in the heart ensure that the blood only flows in the correct direction, they open and close in response to pressure gradients.

The Cardiac Cycle and Pressure Changes

Ventricular Diastole

When the ventricles relax (diastole) no ventricular filling will occur until the pressure inside the ventricles is less than the atrial pressures and the AV (atrioventricular) valves open. When the pressure inside the ventricles is less than the pressure inside the atria, the ventricles fill with blood. This period of rapid ventricular filling is followed by a phase of reduced ventricular filling (known as diastisis). Diastisis persists until atrial contraction occurs. 


Ventricular Systole:

Firstly, isovolumetric ventricular contraction occurs. This is when the ventricular pressure increases but the volume remains unchanged. This increase in the ventricular pressure closes the AV valves. However, the semilunar (aortic and pulmonary) valves remain closed because the ventricular pressures are less than the aortic or pulmonary artery pressure. The ventricles eject blood when the ventricular pressure exceeds the aortic or pulmonary pressure pressure causing the semilunar valves to open. This leads to the rapid ejection of blood into the aorta or pulmonary artery. This is followed a reduction in the amount of blood ejected as the ventricular pressures begin to decline. Ventricular pressures continue to decrease which closes the semilunar valves and causes the end of systole.


Heart Sounds

There are four sounds that may be heard when the heart beats:

1. A low frequency "lub" (S1). This sound is associated with turbulence as the AV valves close. 
2. A high frequency "dup" (S2). This is associated with turbulence as the semilunar valves close. 
3.  A short, low frequency sound. This is not heard in most normal animals and is associated with turbulence as the ventricles fill.
4. A sound similar to S1. This is associated with turbulence as the atria contract.

Regulation of Cardiac Output

 There are some important terms that you'll need to become familiar with. End diastolic volume (EDV) is the volume of blood in each ventricle at the end of diastole. End systolic volume (ESV) i the volume of blood in each ventricle at the end of systole. Stroke volume is the EDV minus the ESV. For a large dog this is usually around 30 mL. Ejection fraction is the fraction of blood ejected during systole. It is calculated by dividing the strove volume (SV) by the EDV (SV/EDV). For a resting dog this is typically 50%.


Cardiac output (CO) is the total amount of blood pumped by each ventricle (L/min). 

CO = SV x HR 

Where HR is heart rate. Cardiac output can be affected by many factors, this can be summarised in the flow chart below:

Cardiac output increases only if HR increases, SV increases or both increase. Stroke Volume will increase with an increase in EDV (which can occur through increased ventricular filling) or with a decrease in ESV (which can occur through more complete ventricular emptying).  

End Diastolic Volume (EDV) is determined by:

• ventricular preload: this is the end diastolic ventricular pressure which equals the atrial pressure and the vena caval pressure.
• ventricular compliance: this is the ease with which ventricular walls stretch to accommodate diastolic filling. A non compliant ventricle increases the amount of ventricular preload required to increase the EDV. 
• diastolic filling time: this is the length of time available for ventricular filling during diastole. This is determined mainly by heart rate. As the HR increases, the ventricles have a shorter time in which to relax and fill with blood, reducing the stroke volume.

End Systolic Volume (ESV) is determined by:

• ventricular contractility: contractility refers to the ventricle's pumping ability. An increased contractility decreases the ESV because the ventricles empty more completely. A decrease in cardiac contractility is the hallmark of cardiac failure. The sympathetic nervous system can increase the heart rate and contractility. The parasympathetic nervous system can decrease the heart rate and contractility.
• systolic duration: an increase in the activity of the sympathetic NS (such as during exercise) decreases the duration of ventricular systole. This helps to preserve the diastolic filling time which will help to preserve the cardiac output
• ventricular after load: this is the pressure against which the ventricle must pump to eject blood, this is equal to the pressure in the arteries. A substantial increase on the arterial blood pressure impairs the ejection of blood by the ventricles.  

That's it for this post, see you next time :)