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The function of the circulation is to service the needs of the tissues--to transport nutrients to the tissues, to transport waste products away, to conduct hormones from one part of the body to another, and, in general, to maintain an appropriate environment in all the tissue fluids for optimal survival and function of the cells. It is a complex process to understand how the heart and circulation are controlled to provide the necessary cardiac output and arterial pressure.
Cardiac output is the quantity of blood pumped into the aorta each minute by the heart. This is also the quantity of blood that flows through the circulatory system and is responsible for transporting substances to and from the tissues. Cardiac output is an essential part of the circulatory function. When blood flows through a tissue, it immediately returns by way of the veins to the heart. The heart responds to this increased inflow of blood by pumping almost all of it immediately back into the arteries from where it came. In this sense, the heart acts as an automaton, responding to the demands of the tissues.
The Frank-Starling mechanism of the heart states that the heart does not have the only role because there are various factors of the peripheral circulation affecting the flow of blood into the heart from the veins called (venous return) that are the primary controllers.
The main reason peripheral factors are usually more important in controlling cardiac output is that the heart has a built-in mechanism that normally allows it to pump automatically whatever amount of blood flows into the right atrium from the veins. This mechanism is called the Frank-Starling law of the heart.
It is illustrated here to display the heart activity. When the total peripheral resistance increases, the cardiac output falls; conversely, when the total peripheral resistance decreases, the cardiac output increases. One can easily understand this by this analogy: if there is a huge resistance in a tunnel, then less water will be able to go through. If there is a small resistance in a tunnel, then more water will be able to go through. The same principle applies to the heart activity and its organ controllers.
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