What is the Difference Between Cardiac Output and Stroke Volume

The main difference between cardiac output and stroke volume is that cardiac output  (Q) is the volume of blood pumped per minute out of the left ventricle of the heart to the body. But, stroke volume (SV) is the volume of blood pumped out of the left ventricle of the heart in each heartbeat. Furthermore, cardiac output is 3 to 5 liters per minute while stroke volume is 60 to 70 milliliters per heartbeat. Also, cardiac output is determined by multiplying stroke volume by heart rate, while stroke volume is determined by subtracting the end-systolic volume from the end-diastolic volume. 

In brief, cardiac output and stroke volume are two terms used in cardiac physiology to describe the volume of blood pumped by the heart per unit time. Generally, these values change constantly in order to meet the needs of your body. 

Key Areas Covered 

1. What is Cardiac Output
     – Definition, Calculation, Importance
2. What is Stroke Volume
     – Definition, Calculation, Importance
3. What are the Similarities Between Cardiac Output and Stroke Volume
     – Outline of Common Features
4. What is the Difference Between Cardiac Output and Stroke Volume
     – Comparison of Key Differences

Key Terms 

Cardiac Output (Q), EDV, ESV, Heart Rate, Stroke Volume (SV)

What is Cardiac Output 

Cardiac output (Q) is the volume of blood that the heart pumps out from its left ventricle to the body per minute. Also, it is calculated by multiplying the stroke volume and heart rate.  

Cardiac output (Q) = stroke volume (SV) x heart rate (HR) 

Importantly, the heart is a muscular organ, which produces the pumping pressure on the blood in order to transport it throughout the body through systemic circulation and to the lungs through the pulmonary circulation. Besides, the blood that enters the systemic circulation from the left ventricle is both oxygen- and nutrient-rich. Ultimately, it reaches the metabolizing tissues of the body. Furthermore, cardiac output is 4.7 liters of blood per minute in a healthy adult.

Figure 1: Relationship Between Cardiac Output and Stroke Volume

Moreover, in a healthy person, cardiac output constantly depends on the oxygen demand of the metabolizing tissue and organs in the body. For example, during a low-exercise aerobic load (not maximal load), cardiac output increases due to a linearly increasing heart rate and non-linearly increasing stroke volume response. However, a mismatch in the oxygen delivery to a particular organ may cause hemodynamic instability, leading to eventual organ dysfunction.

Figure 2: Factors Affecting Cardiac Output

Pathologically, during acute myocardial infarction, cardiac output may decrease. Thus, in order to maintain a mean arterial pressure, the heart rate has to be increased. 

What is Stroke Volume 

Basically, stroke volume (SV) is the volume of blood that the left ventricle of the heart pumps out of the left ventricle of the heart per heartbeat. Basically, a heartbeat is a round of systolic cardiac contraction. Also, the stroke volume can be calculated by subtracting the end-systolic volume from the end-diastolic volume. Of these, the end-systolic volume is the volume of the blood in the ventricle at the end of a beat while the end-diastolic volume is the volume of blood just prior to the beat. Additionally, stroke volume can be applied to the left ventricle as well as the right ventricle in a separate manner. Besides, the cardiac volume is also applicable to both left and right ventricles. 

Stroke volume (SV) = end-diastolic volume (EDV) – end-systolic volume (ESV)

Figure 3: Factors Affecting Stroke Volume

Furthermore, multiple factors can affect stroke volume. Among them, the three primary factors regulating the stroke volume include preload, contractility, and afterload. Of these, preload is the passive ventricular wall stress at the end of diastole and is synonymous with the end-diastolic volume. Also, contractility is the force of myocyte contraction, referred to as the heart’s inotropy. Meanwhile, afterload is the pressure the heart must work against to eject blood during systole (ventricular contraction), and it is proportional to the average arterial pressure. 

Similarities Between Cardiac Output and Stroke Volume  

• Cardiac output and stroke volume are two interrelated measurements of cardiac function by means of the volume of blood pumped out of the heart per unit time.  
• Here,  the blood volume is the volume of blood pumped out by the left ventricle.  
• As they constantly change depending on the needs of the body, it is important to measure these values in cardiac physiology to understand different conditions of the heart.  
• They are measured by noninvasive methods. 

Difference Between Cardiac Output and Stroke Volume 

Definition 

Cardiac output refers to the volume of blood the heart pumps out per minute, but stroke volume refers to the volume of blood pumped out of the left ventricle of the heart during each systolic cardiac contraction. 

Method of Calculation 

Cardiac output is determined by multiplying stroke volume by heart rate, while stroke volume is determined by subtracting the end-systolic volume from the end-diastolic volume. 

Normal Values 

Cardiac output is 3 to 5 liters per minute in a healthy adult, but stroke volume is 60 to 70 milliliters per heartbeat. 

Importance 

The cardiac output varies depending on the oxygen requirements of the metabolizing tissue, while the three variables affecting the stroke volume include contractility, preload, and afterload.  

Conclusion 

In brief, cardiac output is the volume of blood that the left ventricle of the heart pumps out per minute. Generally, it varies depending on the oxygen demand of the body. And, the normal value of the cardiac output is 3-5 L per minute. Meantime, stroke volume is the volume of blood that the left ventricle of the heart pumps out of the left ventricle per heartbeat. However, it depends on contractility, preload, and afterload. Also, the normal value of stroke volume is 60-70 mL per heartbeat. Importantly, the product of the stroke volume and heart rate gives the cardiac output. Therefore, the main difference between cardiac output and stroke rate is the method of calculation and importance. 

References:

1. Bruss ZS, Raja A. Physiology, Stroke Volume. [Updated 2019 Sep 19]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available Here. 
2. Shiel, William C. “Definition of Cardiac Output.” MedicineNet, MedicineNet, 27 Dec. 2018, Available Here.

Image Courtesy:

1. “2036 Summary of Factors in Cardiac Output” By OpenStax College – Anatomy & Physiology, Connexions Web site. (CC BY 3.0) via Commons Wikimedia   
2. “2031 Factors in Cardiac Output” By OpenStax College – Anatomy & Physiology, Connexions Web site. (CC BY 3.0) via Commons Wikimedia   
3. “2035 Factors in Stroke Volume” By OpenStax College – Anatomy & Physiology, Connexions Web site. (CC BY 3.0) via Commons Wikimedia