Learning Objectives
By the end of this section, you will be able to:
- Define cardiac output and explain how heart rate and stroke volume effect it
- Describe the effect of exercise on cardiac output
- Identify cardiovascular centers and cardiac reflexes that regulate heart function
- Describe factors affecting heart rate and force of contraction
- Explain the connection between preload, contractility, afterload and stroke volume
- Distinguish between positive and negative inotropic factors
- Summarize factors affecting stroke volume, heart rate and cardiac output
The autorhythmicity inherent in cardiac cells keeps the heart beating at a regular pace; however, the heart is regulated by and responds to outside influences as well. Neural and endocrine controls are vital to the regulation of cardiac function. In addition, the heart is sensitive to several environmental factors, including electrolytes.
Resting Cardiac Output
Cardiac output (CO) is a measurement of the amount of blood pumped by each ventricle in one minute. To calculate this value, multiply stroke volume (SV), the amount of blood pumped by each ventricle, by heart rate (HR), in contractions per minute (or beats per minute, bpm). It can be represented mathematically by the following equation:
CO = HR × SV
SV is normally measured using an echocardiogram to record EDV and ESV, and calculating the difference: SV = EDV – ESV. SV can also be measured using a specialized catheter, but this is an invasive procedure and far more dangerous to the patient. A mean SV for a resting 70-kg (150-lb) individual would be approximately 70 mL. This is because typical EDV and ESV values are approximately 120 mL and 50 mL, respectively and 70 mL = 120 mL – 50 mL. Normal range for SV would be 55–100 mL. An average resting HR would be approximately 75 bpm but could range from 60–100 in some individuals. There are several important variables, including size of the heart, physical and mental condition (via hormones and the ANS) of the individual, gender, contractility, duration of contraction, preload or EDV, and afterload or resistance that can affect SV and HR.
Using these numbers, the mean resting CO is 5.25 L/min, with a range of 4.0–8.0 L/min. The CO of 5.25 L/min, was calculated using the following values.
CO L/min = 75 beats/min x 0.070 L/beat (where 0.070 L is equal to 70 mL).
Remember, however, that these numbers refer to CO from each ventricle separately, not the total for the heart. In a healthy heart the CO from each ventricle is the same. CO is influenced by HR and by SV. If SV decreases, CO can be maintained by increasing HR. Factors that influence HR are referred to as chronotropic factors. Chrono- refers to time. Positive chronotropic factors increase HR and negative chronotropic factors decrease HR. HR is influenced by the autonomic nervous system, chemicals, and other factors. The factors influencing CO are summarized in Figure 19.4.1.
SVs are also used to calculate ejection fraction, which i