Future Scientists

Revision note 2.7

The Heart and Circulatory System

The heart creates the pressure that moves blood through a double circulation, linking the lungs with every tissue in the body.

CoreAnatomyCalculation

What you need to know

  • Describe pulmonary and systemic circulation.
  • Trace blood through the chambers, valves and major vessels.
  • Relate wall thickness and valves to function.
  • Explain the roles of coronary arteries and pacemakers.
  • Interpret heart-rate data and calculate cardiac output.

A double circulatory system

Blood passes through the heart twice during one complete circuit.

Pulmonary circuit

The right side pumps deoxygenated blood to the lungs. Oxygenated blood returns to the left side.

Systemic circuit

The left side pumps oxygenated blood to the body. Deoxygenated blood returns to the right side.

Separating the circuits allows blood to reach the lungs at a lower pressure while the body receives blood at a higher pressure.

The route of blood

Vena cavaReturns deoxygenated blood from the body.
Right atriumReceives blood and passes it through an atrioventricular valve.
Right ventriclePumps blood through the pulmonary artery.
LungsBlood loses carbon dioxide and gains oxygen.
Pulmonary veinReturns oxygenated blood to the left atrium.
Left ventriclePumps blood through the aorta to the body.

Valves prevent backflow. They open and close because of pressure differences rather than actively contracting.

Misconception AlertArteries carry oxygenated blood and veins carry deoxygenated blood.Select to reveal the correctionSelect to hide the correction
Correct understanding

Arteries carry blood away from the heart and veins carry blood towards it. The pulmonary artery and pulmonary vein are important exceptions to the oxygenation pattern.

How heart structure supports function

  • Atria have relatively thin walls because they push blood only into nearby ventricles.
  • Ventricles have thicker muscular walls because they pump blood out of the heart.
  • The left ventricle is thickest because it generates pressure for the whole body.
  • The septum separates oxygenated from deoxygenated blood.
  • Cardiac muscle respires continuously and resists fatigue.
  • Coronary arteries supply heart muscle with oxygen and glucose.

A blockage in a coronary artery reduces oxygen supply to cardiac muscle and can cause a heart attack.

Heartbeat, pacemakers and ECGs

A group of specialised cells in the right atrium acts as the heart's natural pacemaker and produces electrical impulses. These coordinate contraction. If the natural rhythm is too slow or irregular, an artificial pacemaker can deliver electrical impulses.

An electrocardiogram, or ECG, records electrical activity at the body surface. Regular repeating waves suggest a regular rhythm; unusual rate or pattern may indicate a problem, but diagnosis requires expert interpretation.

Exercise and cardiac output

During exercise, active muscles need more oxygen and glucose and produce more carbon dioxide and heat. Heart rate rises, and stroke volume often rises, increasing blood flow.

cardiac output = heart rate × stroke volumeExample units: beats min−1 × cm3 beat−1 = cm3 min−1.

Heart rate is the number of beats per minute. Stroke volume is the volume pumped by one ventricle in each beat. Cardiac output is the volume pumped per minute.

Pulse-rate investigation

Measure resting pulse after a standard rest. Complete a fixed exercise, then measure pulse immediately and at regular intervals during recovery. Keep exercise intensity, duration, posture and measurement interval constant.

Repeat trials and calculate means. Faster recovery may indicate greater fitness, but age, temperature, hydration, stress, caffeine and measurement error are confounding factors. A small study cannot establish a general rule.

Safety

Choose an exercise appropriate to participants, provide space, stop if anyone feels unwell and do not exclude recovery time from the procedure.

Quick retrieval check

1. Which vessel carries blood from the right ventricle?
The pulmonary artery.
2. Why is the left ventricle wall thicker than the right?
It must generate higher pressure to pump blood around the whole body.
3. What is the function of a heart valve?
To prevent backflow and maintain one-way blood movement.
4. Calculate cardiac output at 75 beats min−1 and 70 cm3 beat−1.
75 × 70 = 5250 cm³ min−1, or 5.25 dm³ min−1.

Exam connection

Question

Explain why cardiac output rises during exercise.

Show the mark points
  • Muscle respiration increases.
  • Muscles require more oxygen and glucose and produce more carbon dioxide.
  • Heart rate increases and stroke volume may increase.
  • Because cardiac output = heart rate × stroke volume, more blood is delivered each minute.