Cardiac Output (CO)

Cardiac Output is simply the amount of blood pumped by the heart per minute. It is the quantity of blood pumped into the aorta each minute by the heart. (Guyton 10th) or, the amount of blood is pumped out by each ventricle into the circulation at each minute. Necessarily, the cardiac output is the product of the heart rate, which is the number of beats per minute, and the stroke volume, which is amount pumped per beat. It is an important measure of cardiovascular function.

Cardiac Output (CO) is the product of stroke volume and heart rate.

CO = Stroke volume x heart rate.

= 70 x 72 = 5042 ml = 5.04 (L/minute)

[Here, Stroke volume = 70ml and Heart rate 72].

Normal value = about 5-6 liters. A normal adult has a CO of 4.7 liters (5 quarts) of blood per minute.

Factor regulating cardiac output is

CO depends upon the following 4 major factors-

• Venous return
• Force of heart contraction
• Frequency of heartbeat (Heart rate)
• Peripheral resistance.

Other minor factors are-

• Arterial pressure
• Ejection fraction

Description

(i) Venous return: ↑ Venous return →↑ EDV →↑ stroke volume →↑ CO.

It depends on –

• Means systemic filling pressure
• Right atrial pressure
• Respiration
• Skeletal muscle pump (Muscular exercise)
• Tissue vasomotor Tone

(ii) Force of contraction: ↑ Force of heart contraction →↑ CO.

It depends on-

• Within the physiological limit, greater the initial length, stronger will be the force of contraction.
• The duration of the diastolic phase.
• Nutrition and O2 supply of heart.
• Sympathetic stimulation.
• Circulating hormone level
• Ca2+ level of blood.

(iii) The frequency of heartbeat (Heart rate):

CO = stroke volume x Heart rate. So, CO is directly proportional to heart rate.

(iv) Peripheral resistance: ↑ TPR →↓ VR →↓ EDV →↓ SV →↓ CO.

(v) Arterial pressure: ↑ Arterial pr →↑ After load →↓ stroke volume →↓ CO.

• (vi) Ejection fraction: It also influences the amount of CO.