Cardiac Cycle

Cardiac cycle

How the blood moves through the heart

From the specification

• 3.3.4.1 Mass Transport in Animals

• Students should understand:

1. Pressure and volume changes during the cardiac cycle

2. Valve movements during the cardiac cycle

• How these maintain a unidirectional flow of blood.

• Students should be able to:

• analyse and interpret data relating to pressure and volume changes during the cardiac cycle

The cardiac cycle

• A sequence of events that repeats about 70 times a minute in humans

• There are two phases:

❥ Diastole (relaxation)

❥ Systole (contraction or SQUEEZE!)

Remember the basic principles:

Blood flows from higher to lower pressure (down a pressure gradient)

Contraction of heart chambers increases the pressure

Valves open/close according to pressure gradients

Take your pulse! What is your heart rate?

The cardiac cycle

Diastole

• Relaxation of the muscle of the heart

• Blood returns to atria from vena cava/pulmonary vein

• Atria fill (as are at lower pressure than veins) and pressure in

atria rises

• Atrial pressure becomes higher than ventricular pressure

• AV valves open

• Blood flows into ventricles (gravity)

• PASSIVE FILLING

• As ventricles are relaxed, SL valves are closed

Atrial Systole

• Contraction of both atria, decreasing volume of

chamber and increasing pressure inside

chamber

• Finishes filling of ventricles

• blood flows → ventricles

• No backflow into vena cava or pulmonary vein

as veins have valves

• Ventricles still relaxed

Ventricular systole

• Atria relax

• Ventricles contract (0.1s after atria)

• Delay allows ventricular filling to finish

• Contraction causes decrease in volume = Increase in pressure in

ventricle

– Forces AV valves to close to prevent backflow (lub)

– Semi-lunar valves open

– blood flows from ventricles → arteries

• Lasts ~0.3 s

Ventricular Diastole

• High pressure in pulmonary artery & aorta closes SL valves to prevent

backflow into ventricles. (dup)

• Whole process restarts

• Blood returns to heart & atria fills again due to high pressure in vena

cava and pulmonary vein.

• Increase in pressure of atria, as ventricles continue to relax, their

pressure falls below that of the atria and so AV valves open

• Blood flows passively into ventricles from atria.

• Atria contract

Atrial Systole

The walls of the atria

contract. This reduces the

volume of the atria,

increasing the pressure.

More blood is forced through

the atrio-ventricular valves

into the ventricles.

Ventricular Systole

The walls of the ventricles
now contract, reducing the
volume in the ventricles. The
pressure increases and blood

is forced into the arteries.

Ventricular Diastole

The ventricle walls relax and
the pressure in the ventricles

falls. Blood starts to flow

from the atria into the

ventricles again.

Pressure changes

Video review

• https://www.youtube.com/watch?v=SwxA3L55yqs

• https://www.getbodysmart.com/circulatory-system/cardiac-cycle

The cardiac cycle

Cardiac Cycle: Summary

A

C

B

Questions

• 1. When does the blood start flowing in the aorta?

• 2. Are the semi-lunar valves open or closed at point B?

• 3.Why is ventricular volume decreasing at point C?

• 4. Calculate the heart rate

•Describe the cardiac cycle with reference
to the action of the valves in the heart and
pressure changes

ANSWERS

• 1. Point A - ventricles are contracting (AV valves are shut) forcing blood

into the aorta

• 2. Closed, ventricles are relaxed and refilling, so pressure is higher in the

pulmonary artery and aorta forcing SL valves to close

• 3. The ventricles are contracting, reducing the volume of the chamber

• 4. 60/0.8 = 75 beats per minute