Blood entering the heart, venous return, plays an important role in regulating the volume that will be ejected with each beat--stroke volume. The volume ejected each minute, cardiac output, is also dependent on heart rate.

Cardiac Output and Venous Return

The box represents the heart. The block arrow leaving the top of the box represents cardiac output (CO), the volume of blood that is ejected from the heart each minute. The block arrow entering the bottom of the heart represents venous return (VR), the volume of blood that is returned to the heart each minute.

Neural Mechanisms

Stretch receptors (SR) in the heart wall are stimulated by venous return; the solid arrow between VR and SR represents this direct relationship. Sensory neurons send this information to the brain via the glossopharyngeal (GP) nerve, a stimulatory nerve represented by the solid line. The brain responds by sending stimulatory signals--solid line--back to the heart via the cardiac nerves (CN). This stimulatory nerve (solid line) innervates the pacemaker (region that controls heart rate (HR) and the ventricles which are responsible for contractility (C).

The pacemaker (HR) of the heart has 'dual innervation.' This means that it is innervated by both stimulatory and inhibitory nerves. The vagus nerve (VN) is an inhibitory nerve as indicated by the dashed arrow. It originates in the brain and its signals reduce the heart rate (HR).

Force of Contraction

Contractility (C) has to do with the force of contraction that is due to the availibility of calcium ions; more calcium ions cause a greater force of contraction. The neurotransmitter from the cardiac nerves causes more calcium to be available for cells in the ventricles.(See tutorial on 'Cardiac Contractile Cells'.)

The heart does not fully empty with each contraction but more forceful contractions do eject more blood (i.e., stroke volume, SV). Accordingly, less blood is left behind; this remaining volume is called the (end systolic volume, ESV). Systole means 'contraction.' The dashed arrow from C to ESV represents this inverse relationship--greater contractility results in reduced end systolic volume. Likewise, the dashed arrow from ESV to SV represents the inverse relationship between these two factors--greater end systolic volume means less was ejected--stroke volume.

Filling the Heart

End diastolic volume (EDV)--'diastole' means relaxation-- is the volume of blood in the heart the instant before contraction. If the heart rate is slow there is more time for filling and the EDV of each contraction will be greater. Also, if the venous return (VR) is increased there will be more blood entering the heart prior to each contraction. The direct relationship between VR and EDV is represented by the solid arrow--greater venous return (VR) will lead to greater end diastolic volume (EDV).

Stroke Volume

Starling's Law of the Heart states, "the more the heart muscle is stretched when it is filling (greater EDV), the more forcefully it will contract".--ejecting more blood (SV). In the model, the direct relationship between end diastolic volume (EDV) and stroke volume (SV) is shown by the solid arrow. Venous return (VR) and end diastolic volume (EDV) are also directly related as indicated by the solid arrow.

Cardiac Output

The two factors that control cardiac output (CO) are stroke volume (SV) and heart rate (HR). Both relationships are direct (solid arrows); an increase in either, or both, will increase cardiac output.

Two factors also control stroke volume; end diastolic volume (EDV) and end systolic volume (ESV). The first relationship is direct (solid arrow) and the second is inverse (dashed arrow.) End diastolic volume (EDV) increases when venous return (VR) increases--a direct relationship (solid arrow). However, end systolic volume (ESV) is inversely related (dashed arrow) to the availability of calcium ions--contractility (C). This will be discussed in the tutorial on 'Neural Controls.'

Last updated: 7/14/2005