7.9 - Cardiac output and ventilation control

Blood flow per minute

The amount of blood moved depends on both how much leaves a ventricle in one beat and how often the ventricle beats.

Stroke volume is the volume pumped by one ventricle in one beat. Heart rate is the number of heartbeats per minute. Multiplying volume per beat by beats per minute gives volume per minute.

The right and left ventricles pump through the pulmonary and systemic circulations in series. Over time their outputs must match; adding their outputs would count the same circulating blood twice.

Worked calculation and complete dataset

These are constructed teaching values for one person, rather than measurements from a study.

For the resting row:

The conversion divides by 1000 because $1000\,\mathrm{mL}=1\,\mathrm{L}$. The exercise row gives $120\times150=18000\,\mathrm{mL}\,\mathrm{min}^{-1}=18.0\,\mathrm{L}\,\mathrm{min}^{-1}$. Both heart rate and stroke volume have increased, so cardiac output increases more than either factor alone. These values illustrate the relationship; they are not universal resting or exercise targets. A higher heart rate does not by itself guarantee a particular output if stroke volume changes too.

Finding an unknown

The same equation can determine stroke volume or heart rate, provided the volume and time units are compatible.

Start with $Q=SV\times HR$. Divide both sides by $HR$ to make stroke volume the subject, or by $SV$ to make heart rate the subject:

If cardiac output is $9.60\,\mathrm{L}\,\mathrm{min}^{-1}$ and heart rate is 120 beats per minute, convert the output to $9600\,\mathrm{mL}\,\mathrm{min}^{-1}$. Then solve:

As a check, $80.0\times120=9600$: multiplying the calculated volume per beat by the number of beats restores the supplied volume per minute. Alternatively, divide $9.60$ by 120 to get $0.0800\,\mathrm{L}$ per beat, and then convert to millilitres. Both routes give the same physical volume.

For an unknown heart rate, suppose $Q=14.4\,\mathrm{L}\,\mathrm{min}^{-1}$ and $SV=120\,\mathrm{mL}$ per beat:

Do not divide 14.4 by 120 while treating litres and millilitres as the same unit. That would make the numerical answer 1000 times too small.

Matching the time unit

An output given per second must also be converted if heart rate is given per minute. For $Q=0.120\,\mathrm{L}\,\mathrm{s}^{-1}$ and $HR=96\,\mathrm{beats}\,\mathrm{min}^{-1}$:

There are 60 seconds in one minute, so a volume delivered in each second is delivered 60 times in a minute. Another valid route converts 96 beats per minute to 1.60 beats per second, then divides $0.120\,\mathrm{L}\,\mathrm{s}^{-1}$ by this rate. Keep enough digits during working and round only the final answer to the precision requested.

Ventilation and blood transport

Moving air through the lungs and moving blood through tissues solve different stages of the same transport problem.

Ventilation is movement of air into and out of the lungs. Breathing rate is the number of breaths per minute; tidal volume is the volume of air moved in one normal breath. Minute ventilation is tidal volume multiplied by breathing rate. For example, 0.50 L per breath at 12 breaths per minute gives 6.0 L per minute; 2.0 L per breath at 30 breaths per minute gives 60 L per minute. These are air volumes, whereas cardiac output is a blood volume per unit time. Neither quantity is the volume of oxygen consumed. Some inhaled air remains in conducting airways and does not reach gas-exchanging alveoli.

At the lungs, ventilation replaces alveolar air: it replenishes oxygen and removes carbon dioxide. Oxygen diffuses from alveoli into blood and carbon dioxide diffuses from blood into alveoli, down their respective partial-pressure gradients. A partial pressure expresses the contribution of one gas to a gas mixture and helps describe the direction in which that gas diffuses. Blood flow carries oxygen away from the lung exchange surface and brings carbon dioxide to it, helping maintain these gradients.

At working muscle, faster aerobic respiration uses more oxygen and produces more carbon dioxide per unit time. Oxygen moves from blood into tissue; carbon dioxide moves from tissue into blood. Increased blood flow can bring more oxygenated blood to the tissue and carry carbon dioxide away more quickly. The blood then returns carbon dioxide to the lungs for exhalation. Increasing cardiac output supports this transport, although the flow reaching a particular tissue also depends on how circulation distributes blood.

The two responses must work together. Ventilating well cannot deliver oxygen to a distant muscle without blood transport. Increasing blood flow cannot indefinitely replace inadequate ventilation: the lungs must keep replenishing oxygen and removing carbon dioxide. For a given oxygen content of arterial blood, a larger blood flow delivers more oxygen per minute, but actual tissue oxygen uptake also depends on how much oxygen the tissue extracts.

Chemical control of breathing

The brainstem adjusts breathing when chemical signals indicate that carbon dioxide removal is not matching production.

When carbon dioxide accumulates, its reaction with water shifts the carbonic-acid/bicarbonate equilibrium towards more hydrogen ions. More hydrogen ions mean a lower pH. Carbon dioxide can pass from blood into the fluid around the brain; central chemoreceptors in the medullary region respond chiefly to the resulting change in the pH of that surrounding fluid. They do not simply measure a shortage of oxygen in muscle.

Peripheral chemoreceptors in the carotid bodies near the carotid arteries and in the aortic bodies supply information about arterial gases. Carotid bodies respond to increased carbon dioxide and decreased arterial pH, and are important sensors of markedly reduced arterial oxygen partial pressure. Low arterial oxygen is an additional stimulus, not the necessary explanation for every increase in breathing during ordinary exercise.