When the heart rest?

Emobileclinic Trending Topic : The heart never rest 

Emobileclinic Specialist 

The human body needs blood to function well and this is strictly dependent on the Cardiac Output (CO) which is the amount of blood the heart pumps through the circulatory system in a minute. The amount of blood put out by the left ventricle of the heart in one contraction is known as the stroke volume. The stroke volume and the heart rate determine the cardiac output. A normal adult has a cardiac output of 4.7 litres (5 quarts) of blood per minute. 

Since the right and left heart are in series it is necessary that in the normal heart the output of the right and left ventricles are the same in steady state. The cardiac output is determined by the stroke volume (SV) and the heart rate (HR).  Stroke volume is the volume of blood ejected by the heart in a single heart beat.  

CO = HR x SV

For a resting individual HR is typically about 70 beats per minute and SV is about 70-75 ml.   Thus the cardiac output of a resting individual is typically about 5 L/min.  The cardiac output of a normal healthy individual can increase to as much as about 20 L/min during intense exercise. Clearly, either changes in heart rate or stroke volume can change cardiac output.  In a typical individual, heart rate can vary from roughly 60 to 180 beats per minute. 

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Stroke volume varies over a smaller range, typically from about 70 to 120 ml. For a normal resting individual the cardiac output (CO, the amount of blood pumped by the heart per minute) is about 5 L/min.  

A moderately active individual will pump about 10,000 L of blood each day, or roughly 4million litres of blood per year,   that is a lot of blood to be pumped by an organ about the size of your fist.  The heart never rests.

Stroke volume is determined by three variables, these are:

1. The inotropic state (or contractility) of the heart. 

2. The filling of the heart (preload). 

3. The pressure against which the heart must work (afterload, for the left ventricle this is the aortic pressure).

Increased inotropic state and increased filling of the heart (preload) both increase the force with which the heart contracts and therefore increase stroke volume.  Increased afterload will decrease stroke volume.   At the cellular level inotropic state (or contractility) is determined primarily by the amount of Ca2+ that enters the myoplasm during each heartbeat (this will be described in greater detail later).  At the level of the whole heart, other factors may also effect contractility as it is broadly defined, these include the number of functional myocytes in the heart (which can be decreased by a myocardial infarct) and the coronary blood supply (inadequate coronary blood flow can reduce contractility).  The filling of the heart is primarily determined by the central venous pressure.

CO = SV x HR

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HINTS ON CARDIAC OUTPUT

Heart rate and contractility are intrinsic factors, characteristics of cardiac tissues, influenced by neural and humoral mechanisms. Preload and afterload depend on the characteristics of both the heart and the vascular system.

Four factors control CO; heart rate, myocardial contractility, preload, and afterload. 

Preload and CO relationships can be described in two curves: cardiac function curve and vascular function curve.

The cardiac function curve is a characteristic of the heart and is an expression of the Frank-Starling relationship. 

The vascular function curve defines the dependence of the central venous pressure on the CO. 

Cardiac Output (CO) is the volume pumped by the left ventricle each minute.



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