CARDIAC OUTPUT

The cardiac output is the amount of blood ejected from each ventricle every minute. The amount expelled by each contraction of each ventricle is the stroke volume. 

The heart rate is the number of contraction recorded in one minutes. Factors affecting heart rate

• Gender

• Autonomic activity

• Age

• Circulating hormones

• Activity and exercise

• Temperature

• The baroreceptor reflex

• Emotional states

Cardiac output is expressed in litres per minute (L/min) and is calculated by multiplying the stroke volume by the heart rate (measured in beats per minute):

Cardiac output (CO) = Stroke volume (SV) × Heart rate (HR).

In a healthy adult at rest, the stroke volume is approximately 70 mL and if the heart rate is 75 per minute, the cardiac output is 5 L/minute.

CO = SV × HR

CO = 70 × 75

= 5250 ML/min

CO = 5.25L/min

This can be greatly increased to meet the demands of exercise to around 25 L/minute, and in athletes up to 35 L/minute. This increase during exercise is called the cardiac reserve.

CONDUCTING SYSTEM OF THE HEART

The heart possesses the property of autorhythmicity, which means it generates its own electrical impulses and beats independently of nervous or hormonal control, i.e. it is not reliant on external mechanisms to initiate each heartbeat. However, it is supplied with both sympathetic and parasympathetic nerve fibers, which increase and decrease respectively the intrinsic heart rate. In addition, the heart responds to a number of circulating hormones, including adrenaline (epinephrine) and thyroxine.

 

DIAGRAMATIC ILLUSTRATION OF THE CONDUCTING SYSTEM OF THE HEART

Small groups of specialized neuromuscular cells in the myocardium initiate and conduct impulses, causing coordinated and synchronized contraction of the heart muscle. These cells are:

i. Sinoatrial node (SA node)

ii. Atrioventricular node (AV node)

iii. Atrioventricular bundle (AV bundle or bundle of His)

iv. Purkinje fibres.

i. SINOATRIAL NODE (SA NODE)

These cells are located in the right atrium just at the entrance of the superior vena cava. It is known as the pace maker cell of the heart because they generate electrical impulse known as pace maker potential. This impulse generated is passed to the Atrioventricular node.

ii. ATRIOVENTRICULAR NODE (AV NODE)

AV node receives the impulse and then transfers it to the atrioventricular bundle. There is a delay in the transmission of the impulse because of the fibro skeleton separating the atria and the ventricles. This is why the atria and the ventricles contracts in different time.

iii. ATRIOVENTRICULAR BUNDLE (AV BUNDLE OR BUNDLE OF HIS)

AV bundle is present in the interventricular septum. It receives impulse from the AV node and transfers it to the purkinje fibers.

iv. PURKINJE FIBRES.

This fiber receives the impulse and transmits it to the apex of the myocardium causing ventricular contraction, pumping blood into the pulmonary artery and the aorta.

CARDIAC CYCLE

The term cardiac cycle refers to the events of one complete heartbeat, during which both atria and ventricles contract and then relax. The average heart beats approximately 75 times per minute, so the length of the cardiac cycle is normally about 0.8 second. This is gotten by dividing 60 by 75.

STAGES OF THE CARDIAC CYCLE

• Atrial systole: This is the time when the atria contract. It takes about 0.1seconds to complete this phase.

• Ventricular systole: This is the period of ventricular contraction pumping blood into the right and left ventricles. It takes about 0.3 seconds

• Complete cardiac diastole: This is the period of the relaxation of the atria and ventricles. It takes about 0.4 seconds.

 

THE CARDIAC CYCLE