The Cardiac Cycle

Learning Objectives  To identify & describe the main parts of the cardiac cycle  To describe how the cardiac cycle is controlled  To describe the measurements associated with the cardiac cycle

The Cardiac Cycle

 The heart beats over 1 billion times in the average lifetime. However, in each heartbeat there are distinct phases where the heart is contracting and relaxing  When the heart contracts it is in Systole, when it is relaxing it is in Diastole  The whole cycle takes approximately 0.8 seconds to complete  Of those 0.8 seconds, the myocardium is in Systole for approximately 0.3 seconds leaving it in Diastole for 0.5 seconds. Therefore it is only actually working for less than 50% of the time

 The cycle consists of the following:-  Both atria relax and fill with blood from the veins (Vena Cava and the Pulmonary vein).  Atria contract and force the Atrio- ventricular valves open. Blood then enters the ventricles which then fill.  The blood entering the ventricles makes the AV valves close due to pressure to prevent blood flowing back into the atria.

 The ventricles walls contract (Systole) and this increases the pressure inside the ventricles.  When the pressure in the ventricles is higher than the pressure in the arteries it forces the semilunar valves open and blood enters the arteries.  Once blood has passed into the arteries the semilunar valves close to prevent backflow into the ventricles.

Cardiac Cycle Control

 The Cardiac cycle is controlled autonomously i.e. it is not under our direct control  There are bundles of fibres within the heart that act as a pacemaker. This is the Sinoatrial node (SAN) found in the right atrium

 Electrical impulses start at the SAN then spread over the atria causing them to contract.  It is blocked from passing to the ventricles by a layer of connective tissue.  Once blood has been pumped from the atria in to the ventricles, the Atrio-ventricular node (AVN) is stimulated by the original impulse.  This then causes stimulation of the Bundles of His which branches out into the Purkinje fibres.  These fibres carry the impulse upwards through the ventricular muscle causing it to contract from the apex upwards and ensures that the ventricles are completely emptied.

Measurements of the Cardiac Cycle

 There are various measuremenst that can be obtained from the cardiac cycle which give an indication of how healthy and fit the heart is.  These measurements include pulse rate, blood pressure, stroke volume, cardiac output and ECG.

Pulse rate  Pulse rate is the number of times your heart contracts per minute.  It is measured by counting the number of pulses felt at either the radial artery or the carotid artery in a set period of time.  It is normalised to beats per minute (BPM).  A fit, healthy person at rest should have a heart rate of between 60 and 100 BPM, the average being around 70.  An olympic athlete may have a resting heart rate of around BPM. Why?

 Pulse rate can be used as an indicator of someones fitness.  The faster it returns to normal, the fitter you are. Try the task to see how fit you are!!

Blood pressure

 Blood pressure is a measure of the pressure of the blood within your arteries when the heart is contracting and at rest.  A piece of equipment called a Sphygmomanometer. This can be manual or automatic.  It measures the diastolic pressure and the systolic pressure of your blood.

 Values for a fit healthy person should be in the region of systolic and below 80 diastolic.  Values outside these ranges indicate either hypertension (high blood pressure) if they are above or hypotension if they are much lower.  Both these conditions indicate potential problems with your circulatory system  Measure your blood pressure with the automated Sphygmomanometer

Stroke volume and Cardiac Output

 Stroke volume is the amount of blood pumped by the heart in each heart beat  In a fit healthy person this is normally around 70ml  In one minute you can calculate the cardiac output by multiplying the stroke volume by the heart rate  So CO = SV x HBR  Therefore as heart rate increases, CO increases to meet the extra demands for glucose and Oxygen in the cells

 An increase in the heart rate is caused by the Sympathetic nervous system in response to increase demands by the body  A decrease in the heart rate is caused by the parasympathetic nervous system in response to reduced demands by the body  In both cases, stroke volume and heart rate are affected

ECG

 A recording of the electrical impulses within the heart is possible  This is called an Electrocardiogram (ECG)  A normal ECG shows the normal function of the heart

 Abnormal ECG’s can indicate problems with the heart