The cardiac cycle

Describing the sequence of events in one heart beat

Learning Intentions

Describe the cardiac cycle with reference to the action of the valves in the heart

Introduction

It is important that the chambers of the heart contract in a coordinated fashion. The sequence of events involved in one heartbeat is called the cardiac cycle.

Definitions     

Systole = period of ventricular contraction.

Diastole = period of ventricular relaxation.

NOTE:  Normally diastole is longer than systole.

Cardiac cycle

General Principles.     

Contraction of the myocardium generates pressure changes which result in the orderly movement of blood.

Blood flows from an area of high pressure to an area of low pressure, unless flow is blocked by a valve.

Events on the right and left sides of the heart are the same, but pressures are lower on the right.

Atrial systole

The heart is full of blood and the ventricles are relaxed

Both the atria contract and blood passes down to the ventricles

The atrio-ventricular valves open due to blood pressure

70% of the blood flows passively down to the ventricles so the atria do not have to contract a great amount.

Ventricular systole

The atria relax.The ventricle walls

contract, forcing the blood out

The pressure of the blood forces the atrio-ventricular valves to shut (producing the heart sound ‘lub’)

Ventricular systole

The pressure of blood opens the semi-lunar valves.

Blood passes into the aorta and pulmonary arteries.

Diastole

The ventricles relaxPressure in the ventricles

falls below that in the arteries

Blood under high pressure in the arteries causes the semi lunar valves to shut. This produces the second heart sound, ‘dub’.

During diastole, all the muscle in the heart relaxes.

Blood from the vena cava and pulmonary veins enter the atria.

The whole cycle starts again.

Pressure changes during one heart beat

A Atrioventricular (bicuspid / mitral) valve(s) closes (“snaps shut”– makes 1st louder heart sound “LUB”

B Semilunar valve(s) (aortic valve) opens

C Semilunar valve(s) closes – makes second softer heart sound “DUB”- shut due to blood accumulating in their pockets

D Atriioventricular (bicuspid) valve(s) opens

“LUB”

“DUB”

Atrial Systole

Ventricular Systole

Diastole

Cardiac cycle= 0.8 sec

60/0.8 bpm= 72 bpm

Cardiac cycle

Match the letter on the graph to the following events

semilunar (aortic) valve starting to openatrio-ventricular (bicuspid) valve about to opensemilunar (aortic) valve about to closeatrio-ventricular (bicuspid) valve about to closeleft ventricle starting to contractboth left atrium and left ventricle relaxingminimum blood volume in left ventricle

16

14

12

10

8

6

4

2

0

–20 0.80

tim e / s

b loodpressure/ kPa

key

aortale ft ventric lele ft a trium

A

B

C

D

E

F

G

H

Length of 1 cardiac cycle ~ 0.8 secNo of beats per minute = 1 x 60/0.8 sec = 75 beats/min