# What Is Hemodynamics?

Hemodynamics is the study of blood flow. It focuses on how the heart distributes or pumps blood throughout the body. The study of hemodynamics integrates a number of sciences, including biology, chemistry, and physics.

As the heart pumps blood through the blood vessels, it helps to supply oxygen to the organs and tissues of the body. This process is vitally important so that the body can maintain itself. Problems with the hemodynamic system can cause serious health issues, the most common of which is hypertension.

### Key Terms

• Hemodynamics: the study of blood flow
• Heart rate (or pulse): the number of times a heart beats in a minute
• Stroke volume: the volume of blood pumped by a ventricle each time it contracts
• Cardiac output: a measure of how efficiently the heart moves blood through the body
• Systemic vascular resistance: the resistance the heart must overcome to successfully pump blood through the body
• Blood pressure: the force exerted against blood vessel walls by the blood as it flows through them

## The Hemodynamic System

The hemodynamic system's key elements include heart rate, stroke volume, cardiac output, systemic vascular resistance, and blood pressure.

Heart rate, or pulse, is the number of times a heart beats in a minute. The stroke volume is the amount of blood pumped by a ventricle when it contracts. Based on pulse and stroke volume, we can calculate the cardiac output, which is a measure of how much blood the heart (specifically, the left or right ventricle) can pump per time unit. It is calculated using the following formula:

Cardiac Output = Heart Rate x Stroke Volume

The average stroke volume for humans is 75 ml per heartbeat. With that stroke volume, a heart beating 70 times per minute will have a cardiac output roughly equivalent to the total volume of blood in the body.

Cardiac output is thus a measure of how efficiently the heart can move blood throughout the body. In our normal daily activities, the output needs to be such that the body can distribute blood based upon the demands placed on it. Exercising is a common example of the need for increased cardiac output.

Cardiac output is related to Ohm's law. Ohm's law states that the current passing through some conductor is proportional to the voltage over the resistance. Similar to a circuit, the blood flow pathway through the body is related to resistance to the flow exerted by the blood vessels. The systemic vascular resistance is the resistance the heart must overcome to successfully pump blood through the body. Cardiac output multiplied by systemic vascular resistance is equal to blood pressure.

When cardiac output is impaired (e.g. due to heart failure), the body will have a difficult time managing its daily needs. A decrease in cardiac output results in a decrease in the oxygen available to the tissues and organs of the body.

## Hemodynamic Monitoring

The study of hemodynamics is vitally important since the body needs oxygen to function. In medicine, hemodynamic monitoring is used to assess this relationship between the cardiovascular system and the oxygen needs of the body's tissues. Such assessments are designed to allow medical professionals to make proper decisions for their patients.

Similarly, when these assessments indicate that a patient is having trouble meeting their own oxygen needs, they are classified as hemodynamically unstable. These patients are provided with mechanical or pharmacological support so that they can maintain the needed blood pressure and cardiac output.

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