6.2 Applications and skills

6.2.2 Circulation of the blood

The heart is the only organ driving the circulatory system. Let’s have a closer look.

Pressure changes during the cardiac cycle

  • One cardiac cycle is one complete heartbeat.
  • The heartbeat occurs in a continuous wave pattern originating at the sinoatrial node. See Page 6.1.2.
  • Although the process is continuous, it can be divided into two main stages as follows:
Stage Atrioventricular (mitral and tricuspid) valves Semilunar (aortic and pulmonic)valves Description of atria and ventricles Blood pressure (aorta)
Atrial systole/ventricular diastole Open Closed
  • Atria contracting
  • Ventricles relaxing
Lower (diastolic)
Atrial diastole/ventricular systole Closed Open
  • Atria relaxing
  • Ventricles contracting
Higher (systolic)

Figure 6.2.2a – A normal cardiac cycle takes about 0.8 secondsFigure 6.2.2a – A normal cardiac cycle takes about 0.8 seconds

Data analysis

Figure 6.6.2b shows pressure and volume changes in the ventricles, atria and aorta during the cardiac cycle.

Figure 6.2.2b – Pressure changes during the cardiac cycleFigure 6.2.2b – Pressure changes during the cardiac cycle

Answer the following questions:

  • How many complete cycles are shown?
  • Locate the point on the graph where ventricular blood volume is highest. Explain the relationship between blood volume and aortic pressure during the systole.
  • Which valves open and close at the points labelled 1, 2, 3 and 4? (Click on the image for answers.)
  • Explain why the pressure stays low in the atria compared with pressure in the ventricles.
  • Explain the small spike in atrial pressure at point a.

Heart disease is caused by occlusion in the coronary arteries

  • Blood flow through an artery can be blocked by hard fatty acid deposits, called atheroma, that develop in the arterial wall.
  • Atheroma develops when fatty acids, such as low-density lipids and cholesterol, accumulate in the artery, and are incorporated by phagocytosis, underneath the smooth muscle layer.
  • The disease resulting from vascular occlusion is called atherosclerosis. Some of the risk factors are advanced age, family history, smoking, obesity, diabetes or a high sugar diet.
  • Atherosclerosis causes the arteries to become less flexible. The heart has to pump harder to supply oxygen to the tissues beyond the occlusion. This means that most sufferers also have hypertension.
  • Like all tissues, there is a steady blood supply to the walls of the heart by way of coronary arteries.
  • When coronary arteries are partially blocked, the chest pain that results is called angina. Some forms of angina are more severe than others.
  • When a small part of the heart muscle isn’t able to get enough oxygen because of the occlusion, a myocardial infarction (heart attack) occurs.

Figure 6.2.2c - Occlusion in coronary arteriesFigure 6.2.2c – Occlusion in coronary arteries
When coronary arteries cannot supply blood to the heart, a heart attack occurs.

Nature of Science: William Harvey and circulation

William Harvey was an English physician who first demonstrated that blood circulation relies on the heart as a pump. His methods were diverse:

  • Using deductive reasoning, he refuted the long-standing belief that the liver produces new blood. He calculated the volume of blood that the liver would need to produce in order for that to be true, and found the value to be unrealistic.
  • Using empirical evidence, he demonstrated the unidirectional nature of blood flow.
  • Using theoretical models, he predicted the existence of capillaries that join arteries to veins.
  • Harvey’s ideas overturned long-held beliefs that were attributed to the Greek philosopher/physician known as Galen of Pergamon. You will find a quick comparison in the following table.
Galen Harvey
Blood is formed in the liver from ingested food, is pumped through the right side of the heart, and is consumed daily. Blood is pumped through two sides of the heart and recycled over and over again.
The left and right sides of the heart are separated by a porous septum. The left and right sides of the heart are completely separate.
Blood does not circulate in one direction, but rather moves by ‘ebb and flow’. Blood flow is unidirectional.
Air diffuses in and out of the blood at the ends of veins and arteries. Predicted the existence of capillaries (though he couldn’t see them – he had no microscopes).


Figure 6.2.2d – Harvey blood flowFigure 6.2.2d – Harvey blood flow
William Harvey used this simple experiment to show that backflow is prevented in the circulatory system.

Concept help

  • Normally, the cardiac cycle is described beginning with the filling of the atria (ventricular diastole). However, as a cycle, it has no beginning to speak of!
  • Blood pressure is a measure of the pressure exerted by the blood on the walls of arteries. For example, a blood pressure of 120/80 (normal blood pressure) means that during the ventricular systole, 120mmHg of pressure is put against the arterial wall. During the diastole, it is 80mmHg.
  • The heartbeat makes a characteristic ‘lub-dub’ sound. The first sound (lub) is made when the atrioventricular valves close. The second sound (dub) happens when the semi-lunar valves close. Locate ‘lub’ and ‘dub’ on the phonocardiogram line in Figure 6.2.2b.

Language help

  • Systole and diastole are synonymous with muscular contraction and relaxation, respectively.

Figure 6.2.2e - William Harvey (1578–1657)Figure 6.2.2e – William Harvey (1578–1657)
William Harvey published his theories in De Motu Cordis (‘On the Motion of the Heart and Blood’) in 1628.

Nature of Science

Theories are generally regarded as uncertain and likely to be refined. William Harvey overturned theories developed by the ancient Greek philosopher Galen on movement of blood in the body. 

Figure 6.2.2f - GalenFigure 6.2.2f – Galen (c. AD200)
Galen held ideas on blood and circulation that were part of a greater philosophy of medicine common until the Renaissance.

Course link

In Page 6.2.1 you learned how fatty acids are deposited into the bloodstream as chylomicrons. Chylomicrons eventually become differentiated into different types of fatty acid and this may lead to atherosclerosis.