The volume of blood pumped by the heart over a given interval of time is its cardiac output (CO). This quantity is a key measure of heart function. Cardiac output is determined by calculating the product of heart rate (HR) and the volume of blood pumped during each beat, the stroke volume (SV): CO = HR × SV. Animals can therefore increase their cardiac output by increasing heart rate or stroke volume or both. Whereas humans and other mammals rely most heavily on increases in heart rate to increase cardiac output, fish rely more on increases in stroke volume. Cardiac output rises or falls in response to the metabolic demand for O₂, which in turn depends on an animal’s state of activity. When an animal sleeps, cardiac output is minimal, but when it runs, cardiac output increases to meet the demand for O₂ of its active muscles.

The nervous system controls the heart rate. Nerve signals influence the rate at which the cardiac pacemaker cells depolarize in between action potentials. Stimulation by sympathetic nerves causes the pacemaker cells of the sinoatrial node to depolarize more rapidly and the heart rate to speed up. Stimulation by parasympathetic nerves causes these cells to depolarize more slowly and the heart rate to slow down. Adrenaline released by the adrenal gland into the circulation also increases the heart rate for more prolonged periods of time. This hormone is a key component of the fight-or-flight response of many vertebrate animals.

Stroke volume is adjusted directly in response to changes in blood flow triggered by a change in activity. During exercise, when an animal’s limb muscles regularly contract, the return of blood to the heart is increased. The influx of additional blood stretches the walls of the atria and ventricles to a greater extent, causing them to contract more forcefully. As a result, more blood is ejected during each heartbeat. The adjustment of stroke volume to changes in activity ensures that heart emptying is matched to heart filling (that is, to venous return). The relationship between the volume of blood filling the heart and stroke volume is described by Starling’s Law: A greater volume of blood returned to the heart increases stretching of the muscle, which leads to more forceful ejection of blood and a greater stroke volume.

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Increased sympathetic stimulation to the heart and circulating adrenaline also increase the contractile strength of the heart muscle, further increasing the heart’s stroke volume. These changes in heart rate and stroke volume underlie the increase in cardiac output that occurs when an animal changes from resting to activity (see Fig. 39.18).

As we have noted, circulation is linked to several other functions in addition to the transport of respiratory gases, nutrients, and wastes. The distribution of blood flow, controlled by changes in arterial resistance through vasodilation or vasoconstriction, is an important factor controlling heat loss or gain between an animal’s body and its environment. Thus, the circulatory system plays a central role in the temperature regulation of many animals (Chapters 35 and 40). The circulatory system also provides the route by which hormones (Chapter 38) and immune cells (Chapter 43) are distributed throughout the body, enabling the endocrine system to maintain homeostatic control of diverse bodily functions and the immune system to defend the body against pathogens. In doing so, the circulatory system serves as a general pathway by which hormonal and immune communication occurs to regulate, integrate, and protect the functional state of the whole organism.