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The Heart

The function of the heart is to circulate blood around the body. The heart comprises four chambers:

  • Right Atrium
  • Left Atrium
  • Right Ventricle
  • Left Ventricle

Functionally the heart comprises two pumps:

  • The right Atrium receives blood from the body (deoxygenated blood), and the right ventricle pumps it into the lungs for aeration (removing carbon dioxide and adding oxygen). .
  • The left atrium receives the oxygenated blood from the lungs, and the left ventricle pumps it around the body.

Blood Pressure

Blood pressure represents the force (pressure) exerted by blood against the arterial walls during a cardiac cycle (heartbeat) which consists of heart muscle contraction (systole) and heart muscle relaxation (diastole).

The higher the two pressure measurements, the more systolic blood pressure occurs as the heart muscles contract pumping blood into the aorta. Heart muscles then relax, allowing the heart to refill with blood, and the lowest pressure reached represents diastolic blood pressure.

Average systolic blood pressure in adults varies between 110 and 140 mm Hg, and diastolic pressure ranges between 60 and 90 mm Hg.

Blood Pressure Classification

The following table is the NICE[4] classification of blood pressure levels. (Note: "mm Hg" means millimetres of mercury)

Systolic (mm Hg) Diastolic (mm Hg) Classification
<130 <85 Normal
130-139 85-89 High Normal
140-159 90-99 Hypertension (stage 1)
160-179 100-109 Moderate Hypertension (stage 2)
>180 >110 Severe Hypertension (stage 3)

Resting Heart Rate

The average person's resting heart rate is between 70 and 90 beats per minute (bpm). The term tachycardia is applied to a rapid heart rate (over 100 bpm), and the term bradycardia indicates a slow heart rate (less than 50 bpm). Endurance athletes may have a resting heart rate of less than 50 bpm due to having an enlarged heart due to their training regime.

Cardiac Output

This is the amount of blood pumped from your heart and is calculated by multiplying heart rate by stroke volume (the amount of blood ejected by the heart in each beat). An endurance athlete will have a lower resting heart rate and a larger stroke volume than a non-athlete. The cardiac output for an endurance athlete is approx. thirty-five litres, while that for the non-athlete is 22 litres.

Starling's Law of the Heart

Starling's law of the heart states that the heart's stroke volume increases in response to an increase in the volume of blood filling the heart. A paper by Solaro (2007)[3] examines the mechanism of Starling's Law.

Blood Pressure daily variations

Millar-Craig et al. (1978)[1] found that blood pressure was lowest at 3-am and began to rise again during the early hours of the morning before waking.

Classification of the Hypertensive Disorders of Pregnancy

Higgins et al. (2001)[2] define the classifications as:

  • Chronic hypertension - hypertension (>140 mm Hg systolic or >90 mm Hg diastolic) that is present and observable before pregnancy or that is diagnosed before the 20th week of gestation
  • Pre-eclampsia-eclampsia - usually occurs after 20 weeks of gestation
  • Pre-eclampsia superimposed upon chronic hypertension - with hypertension and no proteinuria early in pregnancy >20 weeks, new-onset proteinuria (urinary excretion of 0.3 g Protein or higher in a 24-hour specimen)
  • Gestational hypertension - blood pressure elevation detected for the first time after mid-pregnancy without proteinuria.

How the Heart Functions

The "SA Node" (see diagram) is the heart's pacemaker which sends out an electrical signal causing the Atrium to contract and pump blood into the ventricle.

The electrical impulse is conducted to the ventricle via the "AV Node" causing the muscle to contract and pump blood out.

Oxygen-enriched blood from the lungs enters the left atrium, and the left ventricle pumps it out to the body.

Blood from the body, which contains carbon dioxide (CO2), enters the right atrium and the right ventricle and then pumps the blood out to the lungs to have the CO2 removed and replenished with oxygen.

How the Heart Functions

Normal Heart ECG Trace

The diagram opposite reflects a simplified normal heart electrocardiogram (ECG).

The "P Wave" represents the electrical activity of the SA Node and atrium.

The "QRS Interval" represents the electrical activity of the ventricle.

The "T Wave" represents the ventricle relaxing, ready for the next electrical impulse - known as repolarisation.

"Q-T Interval" is the measurement of repolarisation.

Simplified Normal ECG Trace
Normal Heart ECG Trace


  1. MILLAR-CRAIG, M. W. and BISHOP, C. N. and RAFTERY, E. B. (1978) Circadian Variation of Blood-Pressure. The Lancet, 311 (8068), p. 795-797
  2. HIGGINS, J. R. and SWIET, M. (2001) Blood-pressure measurement and classification in pregnancy. The Lancet, 357 (9250), p. 131-135
  3. SOLARO, R. J. (2007) Mechanisms of the Frank-Starling Law of the Heart: The Beat Goes On, Biophys J., 93 (12), p. 4095-4096
  4. HYSLOP, J. et al. (2011) Hypertension - Clinical management of primary hypertension in adults. NICE clinical guideline 127 - Hypertension p. 10

Page Reference

If you quote information from this page in your work, then the reference for this page is:

  • MACKENZIE, B. (2001) Blood Pressure [WWW] Available from: [Accessed