Ans. is 'a' i.e., Enddiastolic volume Factors affecting cardiac outputo Cardiac output is the product of stroke volume and heart rate. Hence any factor which affects either the stroke volume or the heart rate or both affects the cardiac output.A) Factors affecting stroke volume# Stroke volume, which is the amount of blood pumped by the heart during one stroke, depends mainly on three factors : -1) Preload (Degree of ventricular fdling 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-Star ling law or Starling rs taw of the heart. 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.| Stroke volume| Stroke volumeFactors increasing end-diastolic ventricular muscle fiber length (Factors increasing venous return or preload)#Increased total blood volume#Increased venous tone#Increased pumping action of skeletal muscle#Increased negative intrathoracic pressure (e.g. inspiration)o Lying down from sitting or standing position# Sympathatic discharge causing decrease in venous capacitance by decreasing venous complianceFactors decreasing end-diastolic ventricular muscle fiber length (Factors decreasing venous return or preload)o Decreased total blood volumeo Decreased venous toneo Decreased pumping action of skeletal muscleso Less negative or positive intrathoracic pressure (expiration)o Sitting or standing (venous pooling of blood)Contractility of ventricle : - If the contractile strength of ventricle increases, stroke volume increases. Contractility is increased by increase in sympathetic discharge or circulating catecholamines (epinephrine), for example during exercise.After load (resistance offered to ventricular pumping action): - Left ventricle has to pump out blood against aortic resistance. Increased aortic resistance (e.g., high BP) tends to decrease stroke volume. Decreased peripheral resistance increases cardiac output e.g., in exercise, AVfistula or shunt, severe anemia (due to vasodialtion by anemic hypoxia), thyrotoxicosis (due to vasodilation caused by in- creased O2 consumption), and wet beri-beri.B) Factors affecting heart rate# Heart rate is predominantly under neurohumoral influence. Sympathetic stimulation increases heart rate and therefore cardiac output. Thus, sympathetic stimulation increases cardiac output massively by : (i) Increasing stroke volume by increasing contractility, and (ii) Increasing heart rate; but without increasing the end-diastolic volume.