# 7.13 Calculation of Cardiac Output

## Calculation of Cardiac Output

• Cardiac output (CO) is the term used to describe the volume of blood that is pumped by the heart (the left and right ventricle) per unit of time
• An average adult has a cardiac output of roughly 4.7 litres of blood per minute when at rest
• Individuals who are fitter often have higher cardiac outputs due to having thicker and stronger ventricular muscles in their hearts
• Cardiac output increases when an individual is exercising
• This is so that the blood supply can match the increased metabolic demands of the cells
• The CO of an individual can be calculated using their heart rate and stroke volume
• Heart rate is the number of times a heart beats per minute
• This can also be described as the number of cardiac cycles per minute
• Stroke volume is the volume of blood pumped out of the left ventricle during one cardiac cycle

Graph showing the changes in the volume of the left ventricle within one cardiac cycle; this is the stroke volume

#### Calculating cardiac output

• Cardiac output is found by multiplying the heart rate by the stroke volume:

Cardiac output = heart rate x stroke volume

• The equation can be rearranged to find the heart rate and stroke volume if required:
• Heart rate = cardiac output ÷ stroke volume
• Stroke volume = cardiac output ÷ heart rate
• Cardiac output is measured in cm3 min-1
• Heart rate is measured in beats per min (bpm)
• Stroke volume is measured in cm3

#### Worked example

A woman took 0.833 seconds to complete a single cardiac cycle. The stroke volume of her heart was measured at 75 cm3. Calculate the cardiac output. Give your answer in dm3.

Step 1: Find the heart rate

One cardiac cycle (atrial systole, ventricular systole and diastole) takes 0.833 seconds

To find the number of cardiac cycles completed in a minute, divide 60 (seconds) by the time taken for one cycle, 0.833 seconds

Heart rate = 60 ÷ 0.833

= 72 bpm

Step 2: Insert relevant figures into the equation

Cardiac output = heart rate x stroke volume

Cardiac output = 72 x 75 = 5 400 cm3

Step 3: Convert to dm3

1 000 cm3 = 1 dm3

Cardiac output = 5 400 ÷ 1 000 =

5.40 dm3

#### Worked example

An athlete runs a 10 km race, after which his heart rate was measured at 110 bpm and his cardiac output was determined to be 9,800 cm3.

Calculate the stroke volume after the race.

Stroke volume = cardiac output ÷ heart rate

Stroke volume = 9,800 ÷ 110

Stroke volume = 89.1 cm3

#### Exam Tip

1 dm3 is equal to 1000 cm3. It can be useful to convert all the figures found in the first question into the same units before starting your working out, that way you are less likely to make any mistakes!

## Effects of Variability of Cardiac Output

• During exercise, muscle contraction occurs more frequently, requiring more energy
• The rate of aerobic respiration increases to meet the increase in energy demand
• This means that cells require more oxygen to be delivered to them, while producing more carbon dioxide as a waste product of respiration
• The body will accommodate this by making the following changes:
• Increase the rate and depth of breathing which will increase the amount of oxygen entering the lungs and bloodstream, while getting rid of more carbon dioxide
• Increase the heart rate which will transport the oxygen (and glucose) to the muscles much faster, while removing the additional carbon dioxide produced due to the increased rate of respiration

#### Control of the breathing rate

• Breathing rate is controlled by the ventilation centres (also called respiratory centres) in the medulla oblongata
• This is one of the three regions that make up the brainstem, it transfers nerve messages from the brain to the spinal cord
• The inspiratory centre controls the movement of air into the lungs (inhalation)
• The expiratory centre controls the movement of air out of the lungs (exhalation)
• The inspiratory centre in the medulla oblongata has the following effect on breathing:
• It sends nerve impulses along motor neurons to the intercostal muscles of the ribs and diaphragm muscles
• These muscles will contract and cause the volume of the chest to increase
• This lowers the air pressure in the lungs to slightly below atmospheric pressure
• An impulse is also sent to the expiratory centre to inhibit its action
• Due to the difference in pressure between the lungs and outside air, air will flow into the lungs
• Stretch receptors in the lungs are stimulated as they inflate with air
• Nerve impulses are sent back to the medulla oblongata which will inhibit the inspiratory centre
• The expiratory centre is no longer inhibited and will bring about the following changes:
• It sends nerve impulses to the intercostal and diaphragm muscles
• These muscles will relax and cause the volume of the chest to decrease
• This increases the air pressure in the lungs to slightly above atmospheric pressure
• Due to the higher pressure in the lungs, air will flow out of the lungs
• As the lungs deflate, the stretch receptors become inactive which means that the inspiratory centre is no longer inhibited and the next breathing cycle can begin

The process of breathing in (inhalation)

The process of breathing out (exhalation)

#### Effect of exercise

• The extra carbon dioxide that is produced due to the increase in the rate of respiration during exercise dissolves in the blood to form carbonic acid
• This quickly dissociates into hydrogen ions (H+) and hydrogencarbonate ions (HCO3-)
• The increase in the concentration of H+ ions will decrease the pH of the blood (it becomes more acidic)
• The decrease in pH is detected by receptors sensitive to changes in the chemical composition of blood
• These are called chemoreceptors and they are located in several places
• In the ventilation centre of the medulla oblongata
• They are also present as clusters of cells in the aorta (aortic bodies) and the carotid arteries (carotid bodies)
• Once they are stimulated a nerve impulse is sent to the medulla oblongata
• The medulla oblongata will then send more frequent nerve impulses to the intercostal and diaphragm muscles to increase the rate and strength of contractions
• This increases the breathing rate and depth
• This results in more oxygen entering the lungs (and bloodstream), while more carbon dioxide can be exhaled and thus be removed from the bloodstream
• The decrease in carbon dioxide levels will result in the blood pH returning back to normal, which leads to the breathing rate returning to normal
• The volume of air that moves in and out of the lungs during a set time period (e.g. a minute) is known as the ventilation rate
• The ventilation rate increases during exercise due to the increase in breathing rate and depth

#### Control of the heart rate

• The cardiovascular control centre in the medulla oblongata unconsciously controls the heart rate by controlling the rate at which the sinoatrial node (SAN) generates electrical impulses
• These electrical impulses cause the atria to contract and therefore determines the rhythm of a heartbeat
• Changes in the internal environment of the body (e.g. blood pressure, oxygen levels) can result in a change in the heart rate
• These changes act as stimuli which is detected by baroreceptors and chemoreceptors
• Baroreceptors are found in the aortic and carotid bodies and they are stimulated by high and low blood pressure
• Chemoreceptors are found in the medulla oblongata, as well as in the aortic and carotid bodies
• They are stimulated by changes in the levels of carbon dioxide and oxygen in the blood, as well as blood pH
• Once stimulated, these receptors will send electrical impulses to the medulla oblongata
• The cardiovascular control centre in the medulla oblongata will respond by sending impulses to the SAN along sympathetic or parasympathetic neurones
• Each of these neurones release different neurotransmitters which will affect the SAN in a different way
• Sympathetic neurones will increase the rate at which the SAN generates electrical impulses, thus speeding up the heart rate
• These neurones form part of the sympathetic nervous system which prepares the body for action ('fight or flight' response) and increases the heart rate during exercise
• Parasympathetic neurones will decrease the rate at which the SAN fires, thus slowing down the heart rate
• These neurones form part of the parasympathetic nervous system which calms the body down after action ('rest and digest' response) and decreases the heart rate after exercise

Nervous control of the heart rate by the cardioregulatory centre (also known as the cardiovascular control centre). Sympathetic neurones (indicated in red) will speed up the heart rate while parasympathetic neurones (indicated in blue) will slow the heart rate down

#### Changes in heart rate

• The heart will respond in different way depending on the stimulus that it receives
• High blood pressure
• Detected by baroreceptors which send impulses to cardiovascular control centre
• It sends impulses along parasympathetic neurones which secrete the neurotransmitter acetylcholine
• Acetylcholine binds to receptors on SAN causing it to fire less frequently
• Heart rate slows down and blood pressure decreases back to normal
• Low blood pressure
• Detected by baroreceptors which send impulses to cardiovascular control centre
• It sends impulses along sympathetic neurones which secrete the neurotransmitter noradrenaline
• Noradrenaline binds to receptors on SAN causing it to fire more frequently
• Heart rate speeds up and blood pressure increases back to normal
• High blood O2 / Low CO2 / high pH levels
• Detected by chemoreceptors which send impulses to cardiovascular control centre
• It sends impulses along parasympathetic neurones which secrete the neurotransmitter acetylcholine
• Acetylcholine binds to receptors on SAN causing it to fire less frequently
• Heart rate slows down and O2 / CO2 and pH levels return to normal
• Low blood O2 / High CO2 / low pH levels (during exercise)
• Detected by chemoreceptors which send impulses to cardiovascular control centre
• It sends impulses along sympathetic neurones which secrete the neurotransmitter noradrenaline
• Noradrenaline binds to receptors on SAN causing it to fire more frequently
• Heart rate speeds up and O2 / CO2 and pH levels return to normal

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