Cardiac output is the amount of blood pumped out of one side of the heart in a minute. The average for a normal adult is approximately 5 liters per minute. Cardiac output is determined by a combination of the amount of blood expelled with each beat, the number of beats per minute, the amount of blood returned to the heart and the resistance to blood flow through the vessels. Cardiac output normally increases in response to increased demand for oxygen by muscles during exercise, in reaction to danger, in response to certain classes of drugs and in some altered health conditions.
Response to Exercise
During exercise, an increase in heart rate is usually experienced, suggesting that as the cause of increased cardiac output and driving more blood to the muscles. Research published in the April 2012 issue of "The Journal of Physiology" suggests a more complicated process, however. Increases in heart rate alone did not increase blood flow in the tested volunteers and, in fact, decreased the amount of blood ejected with each beat. When muscles perform work, their demand for more oxygen causes their blood vessels to enlarge, allowing more blood to flow to them and back to the heart. The heart also responds by increasing the strength of its contractions and the amount of blood ejected with each beat, thereby increasing cardiac output.
The hypothalamus is a part of the brain called "primitive," because it is present and functions similarly in animals that do not have the "higher" brain structures that enable abstract thought, language and other functions seen in humans. Within the hypothalamus, some cells respond to threats, fear and anxiety by releasing chemicals that stimulate other organs to release adrenaline, steroids and other compounds that prepare the body to deal with perceived danger. This is the "fight or flight" response, part of which includes an increase in cardiac output to provide oxygen and nutrients necessary as fuel for sudden activity.
Drugs that Counteract Overactive Nervous System Responses
When low blood pressure is sensed by receptors in the blood vessels, the nervous system releases hormones -- principally adrenaline and related compounds -- that increase the force of the heart's contractions and tighten the blood vessels, producing an increase in cardiac output and blood pressure. In short bursts, these reactions are positive and helpful. In people with heart failure, however, the continuous production of these hormones begins to stiffen the heart and blood vessel muscles and can decrease cardiac output. As reviewed in the September 2013 issue of "Frontiers in Physiology," drugs such as carvedilol (Coreg) can block these negative hormonal effects and help support cardiac output. Other drugs can directly increase cardiac output, such as dobutamine used in critical care units to counteract shock and other causes of low output.
Conditions That Increase Cardiac Output
Cirrhosis of the liver causes blood vessels to open abnormally wide, decreasing resistance to blood flow and thus increasing cardiac output. Increased cardiac output is also an early response to severe infection, anemia, thiamine deficiency (Beriberi) and certain other disorders, caused primarily by similar changes in blood flow. In pregnancy, the mother's heart increases its output to help supply the fetus and placenta with adequate oxygen and nutrients. An excess of thyroid hormone acts very much like epinephrine and increases cardiac output as well.