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    80 Time of systemic circulation
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    80 Time of systemic circulation

    Systemic circulation time: mean transit time through the body, indicator dilution and causes of faster or slower circulation.

    The systemic circulation time is the time blood takes to travel from the left ventricle through the aorta, arteries, capillaries of the organs and veins back to the right atrium. Because the systemic circulation holds most of the blood volume (about 4.5 of 5 litres), the mean transit time through it at rest is about one minute; blood passing through short circuits (for example the coronary vessels) returns in seconds, while blood passing through slow vascular beds (skin, fat, resting muscle) takes longer. Circulation time depends on cardiac output, blood volume, vascular tone and blood viscosity, and therefore on the regulation of water and electrolyte balance.

    HaemodynamicsSystemic circulationTransit time
    Principletime = volume / flow
    Systemic blood volume≈ 4.5 L
    Mean transit at rest≈ 1 min
    Measured byindicator dilution
    Double circulation: the right ventricle pumps blood through the lungs (pulmonary circulation, a few seconds) and the left ventricle through the body (systemic circulation, about a minute on average) Two circulations in series Heart right · left Lungs ≈ 0.5 L of blood Body organs ≈ 4.5 L of blood pulmonary: ≈ 5–8 s systemic: ≈ 1 min (mean) mean transit time ≈ blood volume of the circuit / cardiac output
    Figure 1. Blood passes alternately through the pulmonary and the systemic circulation. Times are approximate averages for an adult at rest with a cardiac output of about 5 L/min; individual paths through short or long vascular beds are faster or slower.

    How it is measured

    Mean transit time = volume of the vascular bed / blood flow (cardiac output)

    According to indicator-dilution theory, when an indicator (a dye, cold saline, a radioactive tracer or contrast agent) is injected, its concentration curve downstream gives both the flow and the mean transit time through the vascular bed. Historically, "arm-to-tongue" tests (a bitter or sweet substance injected into an arm vein) were used at the bedside; today transit times are derived from indicator dilution, contrast imaging or calculated from cardiac output and blood volume.

    Clinical significance

    Shorter time

    High cardiac output: exercise, fever, hyperthyroidism, severe anaemia, pregnancy, arteriovenous fistulas, early sepsis.

    Longer time

    Low cardiac output: heart failure, cardiogenic shock, severe valve disease, bradycardia; high blood viscosity (polycythaemia); hypothyroidism; low blood volume with marked vasoconstriction.

    When to see a doctor

    Breathlessness, swelling of the legs, fatigue and cold extremities may indicate low cardiac output and need cardiological assessment.

    References

    1. Meier P, Zierler KL. On the theory of the indicator-dilution method for measurement of blood flow and volume. J Appl Physiol. 1954;6(12):731–744. doi:10.1152/jappl.1954.6.12.731
    2. Vincent JL. Understanding cardiac output. Crit Care. 2008;12(4):174. doi:10.1186/cc6975

    This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.

    Published on 1 May 2024