Cardiac output (known as ‘Q’) is a measure of the amount of blood that is pumped out of the heart in one minute.

Stroke volume (SV) refers to the quantity of blood pumped out of the left ventricle with every heart beat.

The equation for cardiac output is:

HR x SV = Q.

What is diastolic blood pressure? It is the pressure that is exerted on the walls of the various arteries around the body in between heart beats when the heart is relaxed. It is the minimum pressure in the entire cardiac cycle. So it basically represents amount of blood in arterial system during diastole.

What happens when there is a decrease in heart rate? Because the heart rate is decreased, there is more time for the blood to run out of the arteries during diastole. Arterial volume, therefore, drops to a lower value than before and diastolic pressure decreases as a result.

What happens when there is a decrease in stroke volume? A decrease in stroke volume decreases the amount of blood in the arterial system, decreasing the diastolic blood pressure.

What happens in our body: When heart rate is decreased, stroke volume increases to maintain cardiac output. That's like two opposite things! So what happens to diastolic blood pressure?

Hi Terun Desai,

Increase or decrease in the stroke volume has some influence in the diastolic blood pressure (DBP). But It is directly proportional the total peripheral resistance (TPR) offered by blood vessels, whereas systolic blood pressure (SBP) is directly proportional to the stroke volume.

If there is increase in the peripheral resistance there will be increase in the DBP as seen in atherosclerosis in which condition there is decrease in the diameter of the blood vessel, will offer more resistance to blood flow. In order to pump adequate amount of blood to the tissues in this condition, there will be increase in the heart rate and stroke volume, therefore increase in the SBP. Hence there will be increase both SBP and DPB (Increase in BP).

Exercise causes increase in the SBP and a slight increase in the DBP. During exercise there will be increased production of metabolic waste product which will dilate the blood vessels. Therefore there will be decreased TPR, which will not allow to increase in the DPB much. At the same time, increase in the sympathetic during activity during exercise will increase the heart rate and stroke volume increases the SBP. Though sympathetic activity has vaso-constriction effect on blood vessels, where increase in TPR is expected, overwhelming activity by metabolic waste product eventually increase the diameter of the blood vessels causes decrease in DBP.

Therefore SBP is directly proportional to stroke volume and DBP is directly proportional to the TPR.

To answer the question asked by Sergey Kozhukhov in the last para, you need to understand the regulatory mechanism of BP. Whenever there is decrease in the heart rate (mostly happens if there is decrease in the sympathetic activity or increase in parasympathetic (vagal) activity), cause decrease in stroke volume (amount of blood pumped out per ventricle per beat(stroke) ~ 70 ml/beat), and cardiac output (amount of blood pumped out in one minute ~ 5L/min), therefore decreases in the SBP. Since there is no vaso-constrictor effect by sympathetic fibers, will cause dilatation/ relaxation of blood vessels. Hence decrease in the DBP. An overall decrease in the BP (SBP & DBP), cause a decrease in the activity of the baro-receptor (a pressure receptor that sense changes in the blood pressure). This will decrease inhibitory effect baro-receptors on the on the vasomotor center (which regulates sympathetic activity) in the brain. This will increase the sympathetic activity eventually will increase the SBP by increasing the hear rate & stroke volume and increase in the DBP by vaso-constrictor effect.

Hope you doubt is cleared. If not, mail me for more explanation

Best

Ramesh

Thanks a lot Ramesh for additional information.

It is clear helpful.

KR,

Sergey.

Stroke volume (SV) refers to the quantity of blood pumped out of the left ventricle with every heart beat. The exact volumes are not easily measured, so they are often estimated based on what we know about stroke volume and the factors that it affects such as blood pressure which we can measure.

The equation for cardiac output is:

HR x SV = Q. Therefore to calculate Q we must first establish HR and SV. An example at rest is shown below.

HR (70BPM) x SV (70ml) = 4900ml/min or 4.9 litres per minute

An increase in HR, SV or both will increase someones Q. SV on the whole does not fluctuate too much, with only relatively small increases with exercise. HR on the other hand increases quite dramatically and thus is the biggest influencer of increasing somesones Q.

Increases in Q with exercise are vital, as it is essentially your CV system trying to meet the demands of the body for the supply of oxygen rich blood and the removal of waste.

However it is highly unlikely that you will ever have to measure a client’s Q, but because Q affects blood pressure, which you will measure, its important that you know what HR & SV are and how they along with Q influence blood pressure.

Blood pressure (BP) is a measure of the force being exerted on the walls of arteries as blood is pumped out of the heart. BP measurements are usually taken on the upper arm with a ‘sphygmomanometer’ and a stethoscope as pictured on the adjacent diagram.

The sphygmomanometer consists of an inflatable cuff with a pressure gauge. When inflated the cuff blocks the flow of blood to the arm below the cuff. As the cuff is allowed to slowly deflate, the measurer listens through the stethoscope to sounds as the artery opens and allows blood flow to continue again.

The measurer is listening for two specific sounds as the blood flows through the artery, as shown on the below image.

The first sound heard as the artery opens enough for the first pumps of blood to come through is known as the ‘systolic’ pressure. This measures the force the heart has to pump against to get the blood to flow around the body.

The second sound recorded is known as the ‘diastolic’ pressure. This measure is recorded at the point where the measurer stops hearing the ‘pump’; essentially it measures the pressure in the arteries as the heart relaxes.

The systolic number is placed over the diastolic number and is always the higher of the two numbers. For example blood pressure of 110 (systolic) and 70 (diastolic) is read as 110/70mmHg. The measurement of blood pressure is expressed in millimetres of mercury (mmHg).

High blood pressure at rest is an indicator that the cardiovascular system is in a less than ideal state of health.

High blood pressure (known as ‘hypertension) occurs when systolic blood pressure readings consistently exceed 140mmHg and or diastolic readings exceed 90mmHg.

‘Normal’, healthy BP is regarded as 120/80mmHg or thereabouts.

BP is also used as a ‘risk factor’ for many diseases and illnesses, such as heart disease. Doctors and fitness professionals alike use blood pressure to screen for potential problems before making judgements as to what exercise a person can safely take part in.

A person’s BP is determined by the following three factors;

1. Cardiac output (as we have already discussed)

2. Blood viscosity (the thickness of the blood)

3. Total peripheral resistance ‘TPR’ (the resistance the blood encounters on its voyage within the blood vessels)

Cardiac output as you know is made up of heart rate and stroke volume. At rest these are relatively constant however with exercise the heart beats faster and more blood is pumped out with each beat. These factors both contribute to a rise in BP, as would any other factor that caused the heart to speed up.

Changes in the volume of blood within the cardiovascular system will also affect BP. If a person was severely dehydrated or lost a large quantity of blood through a wound there would be less blood for the heart to pump, thereby reducing cardiac output and BP.

If the volume of blood increased (waste products not being removed to the kidneys due to kidney failure for example) then there would be a greater quantity of blood within the system increasing the pressure within. Think about putting more air into an already inflated balloon and you’ll get the picture!