Cardiac output increases by ?

Ans. is 'c' i.e., Increased cardiac contractility Cardiac output is the product of stroke volume and hea rate. Hence any factor which affects either the stroke volume or the hea rate or both affects the cardiac output. A) Factors affecting stroke volume Stroke volume, which is the amount of blood pumped by the hea during one stroke, depends mainly on three factors : ? Preload (Degree of ventricular filling during diastole) : - Cardiac preload is represented by volume of venous blood that distends the ventricle, i.e., venous return determines the preload. An increase in preload, i.e., increase in venous return results in a higher end-diastolic volume (Preload). This results in stretching of myocardial fiber and this increase in length of myofibril increases the strength of cardiac contraction in accordance with the Frank-Starling law or Starling's law of the hea. According to Starling's law, greater the initial length of muscle fiber, greater is the force of contraction. The initial length of muscle fiber (length of fiber at the initiation of contraction/systole) refers to length of the fiber at the end of the diastole, i.e., end-diastolic fiber length. Thus, the factors which improve venous return increase the cardiac output by increasing end-diastolic ventricular volume and length, i.e., preload. Opposite is true for factors which decrease venous return.