The heart is a four chambered pump which is responsible for moving blood through the vascular system. In addition, it is an endocrine organ secreting atrial natriuretic hormone (ANH).

STRUCTURE

The four chambers of the heart are the two superior atria and the two inferior ventricles. The walls of the heart are made up of three layers: 1) the Epicardium; 2) the Myocardium; and 3) the Endocardium. The epicardium is the outer layer of the heart and contains fat and blood vessels. The outermost layer of the epicardium is an epithelial layer termed the visceral pericardium. If you were to hold a heart in your hand, the tissue in contact with your fingers would be the visceral pericardium. At the base of the heart, near the major blood vessels, the visceral pericardium is reflexed and continues out to form the parietal pericardium, a thick, tough sac which encloses the heart. The space between the visceral pericardium and the parietal pericardium constitutes the pericardial cavity of the heart. The pericardial cavity contains a small amount of pericardial fluid which friction between the membranes as the heart moves.

The middle layer of the heart is the myocardium, which contains the cardiac muscle tissue responsible for the contraction of the heart. Cardiac muscle is striated and undergoes spontaneous contraction. The cardiac muscle cells are branched and the junctions between cardiac muscle cells are termed Intercalated Discs. Intercalated discs contain intercellular junctions which fuse the cells together and they also contain gap junctions which readily pass muscle impulses from cell to cell. Cardiac muscle tissue can have its rate of intrinsic contraction modified by the action of the autonomic nervous system (ANS). The parasympathetic division of the ANS slows down the heart rate and the sympathetic division speeds up the heart rate and also increases the strength of the muscular contraction. In addition to cardiac muscle, there are two types of modified cardiac muscle tissue, nodal tissue and conducting tissue, which have lost their contractile ability but carry out specialized functions. The Sinoatrial (SA) node and the Atrioventricular ( AV) node are the two forms of nodal tissue. The SA node is located on the surface of the right atrium at the junction of the superior vena cava. The SA node is the pacemaker of the heart and establishes the intrinsic rate of heart contraction. The atrio-ventricular node is located on the floor of the right atrium adjacent to the interventricular septum. The primary role of the AV node is conducting impulses from the atria to the ventricles. The two major conducting tissues are the AV bundle (Bundle of His) and the Purkinje fibers. These tissues function to transmit the muscle impulses from the AV node to the ventricular muscle.

The innermost layer of the heart wall is the endocardium, an epithelial layer which covers the chamber walls of the atria and ventricles. The endocardial epithelium also covers the heart valves and is continuous with the endothelium lining the blood vessels.

The four chambers of the heart are the two superior Atria (sing. Atrium) and the two inferior Ventricles. The atria are the receptive chambers of the heart receiving the blood returning to the heart from the tissues. The walls of the atria are relatively thin to prevent resistance to the returning blood. Blood is pumped out of the heart by the thick walled, muscular ventricles. Blood returning to the heart from the body and the Coronary circulation would enter the right atrium. Blood returning to the heart from the lungs (pulmonary) enters the left atrium. The right ventricle pumps blood to the lungs and the left ventricle pumps blood to the body (systemic system). 

A circulatory system is a system in which the blood is pumped from the heart to the tissue and then returns to the heart. Arteries are blood vessels which carry the blood away from the heart and Veins are vessels which carry the blood to the heart. There are two circulatory systems in humans:

A) The Pulmonary circulatory system B) The Systemic circulatory system

The Pulmonary circulatory system would be as follows:

The Systemic circulatory system would be as follows:

The heart valves are located between the atrial and ventricular chambers ( Atrioventricular valves) and between the ventricular chambers and the major arteries (Semilunar valves). The heart valves do not contain muscle but are comprised of connective tissue sheets ( Cusps of the AV valves) or connective tissue bags ( semilunar valves). Opening and closing of the valves is dependent on differential pressure on the opposing sides of the valves. The AV valves are comprised of sheets of connective tissue termed cups. The right AV valve has three cusps and is termed the Tricuspid valve. On the left side of the heart the AV valve has two cusps and is termed the Bicuspid valve. The bicuspid valve may also be called the Mitral valve. Heart strings ( chordae tendineae) run from the AV valves to specialized muscles (Papillary muscles) on the wall of the ventricular chambers. When the ventricles contract and generate high pressures, the heart strings prevent the valves from being blown into the atrial chambers. The semilunar valve on the right side of the heart between the right ventricle and the Pulmonary artery is termed the Pulmonary valve and the semilunar valve on the left side of the heart between the left ventricle and the Aorta is termed the Aortic valve. Three bag-like sacs make up the semilunar valves. When the ventricles contract and generate pressure, the bags are pushed against the wall emptying them of the blood and allowing the blood to pass from the ventricles into the arteries. When the ventricles relax the pressure drops and the blood in the arteries tries to run back into the ventricles. The blood moving toward the ventricles from the arteries fills the bags and closes the valves.

Contraction of the heart and initiation of the cardiac cycle begins in the pacemaker of the heart - the sinoatrial (SA) node. The muscle impulse generated in the SA node is transmitted to the atrial muscle through the gap junctions. Although the muscle impulse passes as a wave from the SA node to the atrioventricular border, there are also internodal conducting pathways which rapidly transmit muscle impulses. The anterior, medial and posterior internodal pathways conduct impulses from the SA node to the AV node. A conduction pathway ( BachmanÕs Bundle) runs from the right atrium to the left atrium which results in the left atrium contracting is synchrony with the right ventricle which is triggered by the SA node. The muscle impulse generated by the SA node travels over the membranes of the atrial muscle cells resulting in contraction (systole) of the atrial muscles. However, the atrial muscle and the ventricular muscle are separated by a connective tissue sheet which can not conduct impulses resulting in the atrial muscle impulse stopping at the atrioventricular border. If connective tissue blocks the impulse being transmitted from the atria to the ventricles, how is the ventricular muscle stimulated? Ventricular muscle stimulation is a function of the AV node and the conducting system. Recall that the internodal pathways carry the impulse from the SA node to the AV node. There are several characteristics of the AV node which are important:

1) Impulse conduction can only be transmitted toward the ventricle - transmission is unidirectional. 2) There is a maximum rate at which impulses can be conducted through the AV node. When this rate is exceeded, impulse transmission is blocked. 3) Impulse transmission through the AV node is slow. This slowing of impulse transmission by the AV node allows time for the atria to relax before the ventricles contract.

The muscle impulse moves from the AV node into the AV bundle which then passes into the interventricular septum. In the interventricular septum the AV bundle divides into a right and left branch. Impulse conduction in the AV bundle is very fast. Approximately half way down the interventricular septum the AV branches become continuous with the Purkinje fibers. The Purkinje fibers conduct the impulse to the ventricular muscle. Purkinje fibers first contact ventricular muscle at the apex of the heart and then progressively up the wall of the heart. Thus, contraction of the ventricular muscle begins at the apex of the heart and forces the blood toward the arteries.

The organization of the conducting system would be as follows:

The sequential filling and emptying of the heart chambers is referred to as a cardiac cycle. Systole is the condition when the cardiac muscle is contracted (emptying) and diastole is the condition when the cardiac muscle is relaxed ( filling). During a cardiac cycle the atria and ventricles go through separate cycles of systole and diastole. Events that occur during the cardiac cycle include electrical phenomenon ( the electrocardiogram, ECG), blood volume changes, pressure changes, valve activity and heart sounds.

A cardiac cycle would be as follows:

1) The Heart At Rest

a) The atria and ventricles are in diastole and the cardiac muscle is maintaining a resting potential.

b) Blood is flowing into the right and left atria from the vena cavae and pulmonary veins respectively.

c) The AV valves are open due to atrial pressure exceeding ventricular pressure

d) The majority of blood (approximately 70%) entering the atria flows into ventricle due to gravity . Recall that the atria are superior to the ventricles.

e) The semilunar valves are closed due to the ventricles being in diastole resulting in the arterial pressure being greater than the ventricular pressure.

2) Atrial Systole

a) Depolarization occurs preceding atrial systole. Depolarization of atria results in the P wave of the ECG.

b) Contraction of the atria forces the remaining 30% of blood into ventricles.

c) The AV valves remain open as atrial pressure exceeds ventricular pressure

d) The Ventricles are in diastole e) The semilunar valves remain closed

3) Atrial diastole - Ventricular systole

a) The ventricles depolarize preceding systole of ventricular muscle. The atria repolarize but atrial repolarization is not seen on the ECG because it is masked by ventricular depolarization. Ventricle depolarization results in the QRS wave of ECG.

b) The ventricles enter sytole

c) The pressure in ventricles exceeds the pressure in the atria closing the AV valves. Closing of the valves, with reverberation of the blood, generates first heart sound of heartbeat - lub.

d) When the pressure in ventricle exceeds the pressure in the arteries, the semilunar valves open and blood is ejected into the arteries. Pressure in ventricle drops as blood passes from the ventricles into arteries.

4) Ventricular diastole

a) The ventricular muscle repolarizes prior to diastole generating the T wave of the ECG.

b) The ventricles enter diastole

c) Pressure in the ventricles drop below atria pressure resulting in the AV valves opening.

d) When the ventricular pressure drops below the arterial pressure, the blood attempts to flow back into the ventricles. Blood from the arteries fills the semilunar sacs closing semilunar valves. Closing of semilunar valves and reverberation of the blood generates second heart sound of the heartbeat - dub.

As stated earlier, arteries are vessels that carry blood away from the heart and veins carry blood toward the heart. Factors such as oxygen level, carbon dioxide level, or nutrient levels do NOT play a role in defining arteries and veins. The criteria to define arteries and veins is the direction of blood flow with respect to the heart. Arteries and veins have similar structural components except that, in addition to having a thicker muscle layer, arteries possess elastic tissue. Thus, in comparison to a vein wall, an artery wall is thicker, elastic and stronger. These artery characteristics are essential in dealing with the high blood pressures found in arterial systems. As arteries pass further from the heart they become smaller and their walls become thinner. The name of the arteries change as they become smaller and this transition would be as follows:

Arteries do not connect directly to veins but empty into the capillaries which are found in the tissues between the cells. Capillary walls are only one cell thick and are comprised of endothelial cells. The capillaries are the primary vessels in the circulatory system where materials can be exchanged between the plasma inside the capillary and the interstitial fluid surrounding the cells. The ability of materials to pass from the capillaries to the interstitial fluid, the porosity of the capillary, varies greatly in different tissue. The capillaries are very porous in the kidneys with materials passing between the endothelial cells, while in the brain the endothelial cells are joined by tight junctions and material can only pass through the endothelial cells ( the blood-brain barrier). The capillaries empty into the venules of the venous system. As the veins pass toward the heart they become progressively larger, with the largest veins emptying into the heart. The vascular system would be outlined as follows:

The flow of blood through the arterial system can be expressed as follows:

Blood Flow = ( P1- P2 ) R

In order for the blood to flow through a vessel, the pressure at one end (P1) must be greater than the pressure at the other end (P2). P1 is considered to be the blood pressure generated at the heart (systemic arterial pressure) and P2 is considered to be the pressure at the end of the capillary ( 10- 16 mmHg). R represents the resistance to the flow of blood through the vessel. Resistance to flow can be expressed as:

R = Viscosity X Length r4 Consideration of the formula shows that there is a direct relationship between the resistance and the viscosity and length. Thus, an increase in viscosity or length will result in an increase in resistance. The viscosity (thickness) of the blood is directly related to the number of erythrocytes (red blood cells) present in the blood (the hematocrit). In the normal individual the number of red blood cells is kept constant, so unless there is some form of pathology this factor does not play a significant role in determining resistance. Once an individual reaches maturity, the length of the vascular system should be stable. Barring any pathologies, this factor does not play a role in resistance to blood flow. In contrast to viscosity and length, there is an inverse relationship between resistance and the radius of the vessel. As the radius of the vessel increases resistance will decrease and as a vessel becomes smaller the resistance will increase. Notice that the relationship between radius and resistance is not based on the simple radius but the radius to the fourth factor (r4). Thus, very small changes in the radius can have profound effects on vascular resistance. Doubling the radius would reduce the resistance sixteen fold.

In the arterial system it is necessary to have a high blood pressure to move the blood through the system. As the arteries move away from the heart the radius of the vessels get progressively smaller and thus the resistance to blood flow increases. This results in a decrease in blood pressure within the arteries. In contrast, in the venous system the radius of the vessels steadily increases as they move toward the heart resulting in little or no resistance to blood flow in the venous system. Lacking a pressure differential to generate blood flow, the venous system relies on the venous ÒpumpÓ to move blood through the system. The venous pump is composed of two major components: 1) one way valves and 2) skeletal muscle contractions. Throughout the venous system there are one way valves which only permit the movement of blood toward the heart. If blood attempts to flow away from the heart it shuts the valves.

The veins are located between skeletal muscle masses. When the muscles contract it causes an expansion of the bellies of the muscles which push against the veins causing an increase in pressure. The increased pressure causes the blood to move and the valves only allow the blood to move toward the heart.