Structure and function of the heart

Describe the structure and function of the heart and explain how it responds to exercise or disease.  How could you measure this clinically or in the laboratory?

The heart is anchored and protected by the pericardial sac.  The pericardium has two layers; the fibrous pericardium and the serous pericardium.  These two layers work in conjunction with each other to prevent the heart from over-stretching, thus, preventing over filling of the heart but also allows for rapid and vigorous contraction where required.

Between the two pericardial layers is the pericardial cavity which is filled with serous fluid.  This fluid contributes to the protection of the heart as it provides lubrication allowing the heart to contract free from friction.

The heart is made up of three layers; the epicardium, myocardium and the endocardium. 

The heart acts as a dual pump.  The left side of the heart pumps blood to and from the lungs, this is the pulmonary circulation and the blood being distributed by the right side of the heart is pumped around the rest of the body via the systemic circulation.

Each “pump” consists of atria and ventricles.  The atria are located superior to the ventricles and are made up of thin, smooth walls covered with endothelium.

The ventricles have thicker muscles than the atria; this allows the ventricles to provide enough force for the blood to be pumped out of the heart to tissues and organs.  The walls of the left ventricle are thicker than those of the other chambers; this is due to the greater stress that is being applied.

Between the four chambers are thin partitions held in place by connective tissue, these prevent blood from one chamber being contaminated with the blood from another.  The interatrial septum separates the atria and the interventricular septum separates the ventricles.

Valves located in the heart play a vital role in preventing back flow of blood.  The atrioventricular valve (tricuspid valve) is found between the right atrium and the right ventricle (Fig. 3); this stops the blood being pumped back in to the atrium as the ventricle contracts.

Between the left atrium and the left ventricle, the mitral valve (bicuspid valve) is found; the role of this particular valve is similar to that of the atrioventricular valve.  This valve has two lobes; this is due to the high pressure and force of contraction from the left ventricle.

When the blood leaves the left ventricle, it enters the aorta.  The aortic valve acts as a barrier and does not allow the blood to flow back into the heart as the ventricle relaxes.

It is the Sino-atrial Node (SV Node), the pace maker, that intrinsically regulates the heart rate.  The electric pulse then passes across the atria, causing them to contract, to the Atrio-ventricular Node (AV Node).  A delay of 0.1 seconds allows the atria to fully contract and force the blood into the ventricles.  The AV Bundle (Bundle of His), then, speeds the impulse rapidly through the ventricles over the Purkinje Fibres (fibres penetrating the left and right ventricles), causing contraction of the ventricles.

Stimulation of the sympathetic cardio-accelerator nerves releases neural hormones, such as epinephrine and norepinephrine.  These hormones increase myocardial contractibility by accelerating depolarization of the SA Node, this increases the heart rate.  The parasympathetic nervous system releases hormones, such as acetylcholine.  These hormones have opposite effect on the heart’s activity.  This change in neural hormone release occurs when there is an increase or decrease in physical activity.

Cardiac output components in endurance-trained and untrained subjects during rest and maximal exercise

Cardiac output (ml)

Heart rate (b. Min-1)

Stroke Volume (ml. B-1)

Untrained

Rest

Maximal Exercise

5000ml

22,000ml

70 b. Min-1

195 b. Min-1

71 ml. B-1

113 ml. B-1

Endurance Trained

Rest

Maximal Exercise

5000ml

35,000ml

50 b. Min-1

195 b. Min-1

100 ml. B-1

179 ml. B-1

Table taken from: McArdle, W. D.et al.  Essentials of Exercise Physiology. 2nd Edition. 2000

These changes are achieved as hypertrophy (increase of muscle fibre) occurs.  Echocardiography involves the use of sound waves to produce an image showing the structure of the heart, including its four chambers and the myocardium structure.  It has been used to determine what training types can inflict myocardium enlargement and whether there are consequences to health from hypertrophy due to physical activity. 

During exercise blood pressure and heart rate increase.  This is due to the demand from working muscles to remove waste (I. e. CO2) in order to keep perform effectively.