The cardiovascular system has four main functions:
- deliver oxygen and nutrients to the body
- remove the waste products such as carbon dioxide and lactic acid
- protection against disease and infection
- maintain body temperature
Blood pressure
When the heart contracts it pushes blood into blood vessels which creates blood pressure.
A blood pressure reading consists of two values:
- systolic value – blood pressure while the heart is squeezing
- diastolic value – blood pressure while the heart is relaxing
The average blood pressure for an adult is 120/80 mmHg. The first number is the systolic value and the second number is the diastolic value.
Blood pressure is determined by Q (cardiac output) and the resistance to the blood flow (R). Resistance to blood flow is caused both by the diameter of the blood vessels and by the thickness of the blood. Furthermore, if a person has a condition called atherosclerosis (plaque in the arteries), their resistance to blood flow will increase and so will blood pressure. This can have serious health implications such as causing chronic high blood pressure, angina or even heart attack or stroke.
The heart’s performance as a pump
The heart’s function is to pump the blood and circulate it round the body. We assess the heart’s performance by measuring how much blood it pumps out each minute. This is called cardiac output. To calculate cardiac output, we also need to know about heart rate and stroke volume.
Heart rate
Heart rate (HR) is the number of times the heart beats (or the ventricles pump blood out) in one minute. The average resting HR is approximately 70 beats per minute (bpm).
Stroke volume
Stroke volume (SV) is the amount of blood pumped out of the ventricles each time they contract. The average resting SV is approximately 70 ml.
Cardiac values
Cardiac output (Q) is the amount of blood pumped from the heart every minute and can be calculated by multiplying heart rate (HR) by stroke volume (SV).
Key fact
Q = HR × SV
SV is 70 to 90 millilitres at rest. The fitter you are, the larger your stroke volume.
We can calculate a typical cardiac output at rest and during exercise:
Q at rest = SV × HR
Q at rest = 70 × 70
Q at rest = 4900 ml/min
Q at rest = 4.9 l/min
Q during exercise = SV × HR
Q during exercise = 120 × 180
Q during exercise = 21600 ml/min
Q during exercise = 21.6 l/min
