Exercise-Induced Redistribution of Cardiac Output

Why Exercise Changes Both Cardiac Output and Its Distribution

During exercise, working skeletal muscle needs more oxygen and substrates and produces more metabolic waste and heat. The heart itself also works harder and needs greater oxygen delivery. Meeting those demands requires more blood to be pumped per minute and a different allocation of systemic blood flow. Blood flow to the kidneys and gastrointestinal organs commonly receives a smaller share, particularly as exercise becomes strenuous.

These are related but distinct changes. Cardiac output is the total volume of blood pumped by a ventricle per unit time. An organ’s blood-flow fraction is the share of systemic cardiac output reaching that organ; its absolute flow is the volume reaching it per unit time. A smaller fraction does not, by itself, prove that an organ’s absolute flow has fallen: the total cardiac output may have risen enough to offset the reduced share.

Definition: Let \(CO\) denote systemic cardiac output, \(Q_i\) the blood flow to organ \(i\), and \(f_i\) that organ’s fraction of systemic cardiac output. For a consistent accounting of systemic organ beds, \(Q_i=f_iCO\), and the organ fractions sum to 1. Redistribution means that these fractions change; absolute organ flows depend on both the fractions and total cardiac output.

The accounting assumes that flows and fractions refer to the same physiological state and measurement basis, and that the listed organ beds account for the systemic output without omitted pathways or double counting. For steady or time-averaged circulation, systemic output equals pulmonary output. Exercise does not create an additional supply of blood flow separate from cardiac output: as some organ flows rise, the total and the relative allocation must remain consistent.

How Cardiac Output Rises

Cardiac output is the product of heart rate and stroke volume, where stroke volume is the volume ejected by a ventricle per beat. Exercise increases sympathetic drive and reduces parasympathetic influence on the heart. Heart rate rises, and stroke volume usually rises as well, especially from rest to moderate exercise. Greater venous return and stronger cardiac contraction contribute to the increase in stroke volume. At high exercise intensities, stroke volume may change less, so further increases in output rely more heavily on heart rate.

CO = HR \times SV

Here \(HR\) is measured in beats per minute and \(SV\) in volume per beat, so their product has units of volume per minute. The increase is not identical in every person or at every workload: fitness, posture, temperature, hydration, and cardiovascular health all matter. The equation describes the accounting relationship; it does not say that heart rate and stroke volume respond independently or predict their values under a particular exercise condition.

Worked Example: Separating the Heart-Rate and Stroke-Volume Contributions

Suppose a hypothetical adult has a resting heart rate of 70 beats/min and stroke volume of 70 mL/beat. During a specified exercise stage, suppose these values are 150 beats/min and 100 mL/beat. Estimate cardiac output in each state and compare them.

At rest, \(CO=HR\times SV\), so \(70\ \text{beats/min}\times70\ \text{mL/beat}=4{,}900\ \text{mL/min}=4.90\ \text{L/min}\). During exercise, \(150\ \text{beats/min}\times100\ \text{mL/beat}=15{,}000\ \text{mL/min}=15.0\ \text{L/min}\). The ratio is \(15.0/4.90=3.06\), so the hypothetical exercise output is about 3.06 times the resting output. Both heart rate and stroke volume contributed; neither alone accounts for the full change. These values illustrate the relationship and are not reference values for every person.

Why Systemic Flow Is Redistributed

Two broad control processes help explain the pattern. First, local chemical signals in active skeletal muscle promote vasodilation. Increased metabolic activity changes the local environment, and the resulting reduction in resistance supports a substantial increase in muscle perfusion. The heart also receives increased flow as its oxygen demand rises; coronary flow is regulated in relation to cardiac work and local metabolic need.

Second, sympathetic activation during exercise causes vasoconstriction in several vascular beds, including renal and gastrointestinal beds. This helps limit their shares of systemic output while the cardiovascular system supports the working muscles and heart. The response is not an on-off switch: its magnitude varies with exercise intensity and duration, and local control, circulating signals, and the need to maintain arterial pressure interact. In active muscle, strong local metabolic vasodilation can counteract sympathetic vasoconstrictor influence.

The brain’s share of output may decline as total output rises, even when cerebral flow is relatively well maintained over ordinary exercise conditions. Skin flow is also context dependent: it can increase to help dissipate heat, so its allocation is influenced by temperature and exercise duration as well as workload. Consequently, an exercise state should not be represented by a single universal set of fractions.

Worked Example: A Smaller Fraction Can Accompany Lower Absolute Flow

Consider hypothetical resting output of 5.0 L/min, with 20% going to the kidneys. At a strenuous exercise stage, suppose output is 15.0 L/min and the renal fraction is 5%. Determine the corresponding renal flows and explain what changed.

At rest, renal flow is \(0.20\times5.0\ \text{L/min}=1.00\ \text{L/min}\). In the exercise scenario, it is \(0.05\times15.0\ \text{L/min}=0.750\ \text{L/min}\). Thus, the renal fraction falls from 20% to 5%, and the absolute renal flow also falls, from 1.00 to 0.750 L/min, a decrease of \(0.250\ \text{L/min}\). This conclusion follows from both the stated fraction and the stated total output; it could not be inferred from the fraction change alone. All figures are hypothetical.

Fraction Changes and Absolute-Flow Changes

The distinction can be expressed directly from \(Q_i=f_iCO\). Between two states, the absolute flow changes according to both the change in total output and the change in the organ fraction. Dividing the exercise-state flow by the resting-state flow gives a useful comparison when the resting values are positive.

\frac{Q_{i,\mathrm{exercise}}}{Q_{i,\mathrm{rest}}} = \frac{f_{i,\mathrm{exercise}}}{f_{i,\mathrm{rest}}} \times \frac{CO_{\mathrm{exercise}}}{CO_{\mathrm{rest}}}

This identity gives a simple test of interpretation. An organ’s absolute flow rises if the increase in total output more than offsets the reduction in its fraction. It falls if its fraction decreases proportionally more than total output increases. It stays the same if the two changes exactly offset. The equation is an accounting result, not a physiological law predicting the fractions.

Justification: In each state, \(Q_i=f_iCO\). Therefore \(Q_{i,\mathrm{exercise}}/Q_{i,\mathrm{rest}}=(f_{i,\mathrm{exercise}}CO_{\mathrm{exercise}})/(f_{i,\mathrm{rest}}CO_{\mathrm{rest}})\). Rearranging the factors yields the displayed identity. It applies when the quantities are defined on a consistent basis and the denominators are nonzero.

Worked Example: A Lower Fraction but Higher Muscle Flow

Suppose, hypothetically, that a muscle group receives 20% of a resting \(CO\) of 5.0 L/min and 40% of an exercise \(CO\) of 15.0 L/min. Compare its absolute flow across the two conditions.

At rest, the flow is \(0.20\times5.0=1.00\ \text{L/min}\). During exercise, it is \(0.40\times15.0=6.00\ \text{L/min}\). The flow ratio is \(6.00/1.00=6.00\). The fraction doubled, while total output tripled, giving a sixfold increase in this illustrative muscle flow. The example shows how a rise in the fraction and a rise in total output combine; the assumed fractions are not proposed as universal physiological values.

Representing Exercise in a PBPK Model

For a PBPK model, exercise is a change in physiological conditions, not simply a label attached to the same reference-adult flow table. The modeler must specify a scenario—such as exercise intensity and whether the target is a steady or time-averaged state—and choose a coherent cardiac output and set of regional flow fractions for that scenario. Earlier material on cardiac output, organ blood-flow fractions, flow conservation, and resting systemic distribution provides the accounting basis for this representation.

A model should distinguish a change in \(CO\) from a change in each \(f_i\). If only total output is increased while every fraction is held at its resting value, every organ flow increases in the same proportion. That may be a useful simplified scenario, but it does not represent the characteristic redistribution toward active muscle and the heart with reduced renal and gastrointestinal shares. Conversely, changing selected fractions without checking the whole set can violate flow conservation. The systemic fractions must still sum to 1 under the stated accounting.

Physiological organ flow is not the only determinant of drug disposition. A flow change can alter delivery to a tissue and, for some drugs and organs, influence uptake or elimination, but concentration changes also depend on properties such as tissue partitioning, binding, and metabolism. A flow-allocation scenario alone does not determine the direction or magnitude of a concentration change.

Worked Example: Checking Whether an Interpretation Is Supported

A hypothetical report states that during exercise an organ’s share of systemic output falls from 12% to 6%, while cardiac output rises from 5.0 to 15.0 L/min. Does this information imply that the organ’s absolute flow falls?

The resting flow is \(0.12\times5.0=0.600\ \text{L/min}\). The exercise flow is \(0.06\times15.0=0.900\ \text{L/min}\). Despite the halved fraction, absolute flow rises by \(0.300\ \text{L/min}\), or a factor of \(0.900/0.600=1.50\). The data therefore contradict the claim that the organ’s absolute flow falls in this example. The result does not establish that such a pattern applies to every organ or exercise intensity; it follows only from the hypothetical values provided.

Assumptions, Diagnostics, and Limitations

A simple exercise flow table usually treats the selected condition as steady or time averaged. Actual exercise is dynamic: heart rate and regional perfusion adjust during onset, sustained work, and recovery. A steady-state table cannot describe these time courses unless the model adds time-varying inputs. It also compresses heterogeneous flow within an organ into one average, which may miss regional differences that matter for drug delivery.

Before using an exercise allocation, check that the total output and organ fractions refer to the same condition, that their measurement bases are compatible, and that the listed fractions conserve systemic flow. Examine whether the qualitative directions are plausible for the stated scenario: active muscle and coronary perfusion generally increase, while renal and gastrointestinal fractions commonly decrease. A reduced fraction should not be reported as reduced absolute flow without evaluating the product \(f_iCO\). Large differences among sources may reflect different exercise intensities, populations, measurement methods, or timing rather than a simple arithmetic error.

The main alternative to an exercise-specific flow allocation is to retain resting flows or scale all organ flows in proportion to \(CO\). Retaining resting flows ignores the increased output; scaling all flows equally represents increased output but no redistribution. Either can be appropriate as a deliberate simplification or comparator, but neither captures the characteristic regional pattern. An exercise-specific allocation is more physiologically informative, while requiring scenario-specific input data and explicit assumptions.

Common Mistakes

Equating a lower fraction with a lower flow. This ignores the change in total output. Compare \(f_iCO\) in both states before drawing a conclusion.
Increasing every organ flow by the same percentage. This assumes unchanged fractions and therefore omits redistribution.
Changing fractions without checking their sum. If the fractions do not conserve systemic output on the stated basis, the allocation is internally inconsistent or incomplete.
Treating one exercise allocation as universal. Intensity, duration, heat, hydration, and individual physiology can alter the response.
Inferring drug concentrations from flow alone. Flow is one physiological input; it is not a complete prediction of tissue concentrations or systemic exposure.

Key takeaway: Exercise commonly raises cardiac output and shifts the systemic flow distribution toward active skeletal muscle and the heart, while reducing the fractions directed to renal and gastrointestinal beds. To determine an organ’s absolute-flow change, consider both its flow fraction and total cardiac output.

Check Your Understanding

Answer each question using the distinction between total output, flow fraction, and absolute organ flow.

If cardiac output doubles while an organ’s fraction remains unchanged, what happens to its absolute flow?
Why can a kidney receive a larger absolute flow during one exercise scenario even if its fraction of output falls?
In a simplified model, why is increasing cardiac output while retaining all resting fractions different from exercise-induced redistribution?
A hypothetical organ receives 10% of 6.0 L/min at rest and 4% of 12.0 L/min during exercise. Calculate its flow in both states and state whether it rises or falls.

Solutions. (1) Its absolute flow doubles because \(Q_i=f_iCO\) and \(f_i\) is unchanged. (2) The total output may rise enough to outweigh the lower fraction, so the product \(f_iCO\) can increase. (3) Retaining resting fractions makes all organ flows scale in the same proportion as output; redistribution changes regional shares unequally. (4) Resting flow is \(0.10\times6.0=0.600\ \text{L/min}\). Exercise flow is \(0.04\times12.0=0.480\ \text{L/min}\). It falls by \(0.120\ \text{L/min}\), despite the doubled total output, because the fraction falls sufficiently.