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Pulse rates and occlusion of the coronary arteries

Organisms (Form and function) · Transport · note 3 of 5

Spec B3.2.4, B3.2.6Download all Transport notes (PDF)Download all notes (PDF)7 pages

Pulse rates and occlusion of the coronary arteriesSpec B3.2.4, B3.2.6

In short

Pulse rate is measured by feeling the carotid or radial pulse with the fingertips and counting beats per minute. Occlusion of the coronary arteries is blockage by fatty plaque or a blood clot, which cuts the supply of oxygen to cardiac muscle and can cause a heart attack, where part of the heart muscle dies.

Measuring pulse rate

Each heartbeat sends a surge of blood along the arteries, felt as a pulse. The pulse rate equals the heart rate.

Practical skill:

Press two fingertips (not the thumb, which has its own pulse) on the radial artery on the inside of the wrist or the carotid artery in the neck. Count beats for 30 s and multiply by 2, repeat and find the mean. Compare the result with a digital heart rate monitor or pulse oximeter.

Pulse rate

A student counts 19 beats in 15 seconds. Calculate the pulse rate. (Practice data.)

  1. Beats per minute = 19 × (60 ÷ 15)
  2. = 19 × 4 = 76

Answer: 76 beats per minute

Occlusion of the coronary arteries

The coronary arteries branch from the aorta and supply cardiac muscle with oxygen and glucose.

  1. Fatty deposits (atheroma or plaque) build up in the wall of a coronary artery, under the endothelium: atherosclerosis.
  2. The lumen narrows and the wall hardens, so blood flow is reduced.
  3. If a plaque ruptures, a blood clot (thrombus) can form and block the artery completely: occlusion.
  4. Cardiac muscle beyond the blockage gets too little oxygen for aerobic respiration, causing chest pain (angina) or, if severe, a heart attack (myocardial infarction), when part of the heart muscle dies.

Risk factors include a diet high in saturated fat, high blood cholesterol, smoking, high blood pressure, obesity, lack of exercise, increasing age, male sex and family history.

Exam tip:

A correlation coefficient (r) from −1 to +1 measures how strongly two variables are correlated. Even a strong correlation, such as between saturated fat intake and coronary heart disease, does not prove that one causes the other; a low r or no correlation can be evidence against a hypothesis.

Written and checked against the IB Biology SL specification · Updated October 2026

Frequently asked questions

What is the difference between arteries and veins?

Arteries carry blood away from the heart at high pressure, so they have thick walls of smooth muscle and elastic fibres and a narrow lumen. Veins return blood to the heart at low pressure, so they have thin flexible walls, a wide lumen and valves that stop blood flowing backwards.

What causes a heart attack?

A heart attack is caused by occlusion of a coronary artery. Fatty plaque builds up in the artery wall and narrows the lumen, and if it ruptures a blood clot can block the artery. Cardiac muscle beyond the blockage is starved of oxygen and part of it dies.

How does water move up a plant?

Water moves up a plant by the cohesion–tension mechanism. Transpiration from leaf cell walls draws water out of the xylem by capillary action, creating tension that pulls the water column up from the roots. Cohesion between water molecules, from hydrogen bonding, keeps the column continuous.