The Circulatory System

The circulatory system is composed of the heart, the blood vessels and the blood. The heart is a pump that creates the pressure that drives the flow of blood throughout the system. The blood vessels serve as passageways that distribute the blood to all parts of the body. The blood is a transport medium for the movement of nutrients into and wastes out of tissues.

Blood flows through 2 distinct circuits; the pulmonary circuit and the systemic circuit. In the Pulmonary Circuit, blood that is high in carbon dioxide and low in oxygen flows from the right heart to the lungs. In the capillaries of the lungs, blood takes on oxygen and offloads carbon dioxide. Oxygenated blood then flows from the lungs to the left heart.

In the Systemic Circuit, oxygenated blood flows from the left heart to the systemic tissues (meaning all cells of the body). Systemic capillaries are the site of exchange of nutrients and wastes. The blood offloads oxygen to the tissues and picks up carbon dioxide wastes. Deoxygenated blood then flows from the systemic tissues to the right heart, completing the circuit.

The Heart as a Pump

The heart is a hollow, muscular organ that is divided into four chambers. The right and left atria (atrium is the singular form) receive venous blood from the veins, while the right and left ventricles pump blood into the arteries. Although the heart is a single organ, it functions as 2 separate pumps. The right half pumps blood into the pulmonary circuit, while the left half pumps blood into the systemic circuit. A muscular wall called the septum separates the two halves of the heart. The atria are separated from the ventricles by a sheet of tough connective tissue called the fibrous skeleton of the heart. Imbedded within this sheet of tissue are the heart valves.

Heart Valves

Heart valves function to ensure a one-way flow of blood through the heart. The valves are not made of muscle, but rather are composed of sheets of tough connective tissue (leaflets) that act like flaps. The heart valves open and close passively because of pressure differences on either side of the valve. When pressure is greater behind the valve, the leaflets are blown open and the blood flows through the valve. However, when pressure is greater in front of the valve, the leaflets snap shut and blood flow is stopped. The motion of a heart valve is analogous to the motion of the front door of your house. The door, which only opens in one direction, opens and closes due to pressure on the door.

Atrioventricular (AV) and Semilunar Valves

The atrioventricular valves (AV valves), which separate the atria from the ventricles, allow blood to flow from the atria to the ventricles, but prevent flow in the opposite direction. The right AV valves is called the tricuspid valve. The left AV valve is called the mitral valve. The opening and closing of the AV valves is dependent on pressure differences between the atria and ventricles. When the ventricles relax, atrial pressure exceeds ventricular pressure, the AV valves are pushed open and blood flows into the ventricles. However, when the ventricles contract, ventricular pressure exceeds atrial pressure causing the AV valves to snap shut.

To ensure that the AV valves do not evert (turn inside-out), they are attached to small papillary muscles by tough tendons called the cordae tendineae. Papillary muscle contract in synchrony with the ventricles, thus maintaining constant tension on the valve leaflets.

The semilunar valves (pulmonary valve and aortic valve) are one-way valves that separate the ventricles from major arteries. The aortic valve separates the left ventricle from the aorta, while the pulmonary valve separates the right ventricle from the pulmonary artery. As the ventricles contract, ventricular pressure exceeds arterial pressure, the semilunar valves open and blood is pumped into the major arteries. However, when the ventricles relax, arterial pressure exceeds ventricular pressure and the semilunar valves snap shut.

Heart Sounds are associated with Valve Closure

Normal heart sounds are caused by the closing of heart valves. As valves snap shut, the walls of the chambers and major arteries vibrate. We hear these vibrations as two distinct sounds; lub-dup. The first sound, "lub", is associated with the closing of the AV valves. The second sound, "dup", is associated with the closing of the semilunar valves.

Heart Murmurs Abnormal heart sounds, or murmurs, are usually (but not always) associated with heart disease. Blood flow through the valves should occur in a smooth fashion (laminar blood flow). However, if blood flow through a valve is turbulent, then an abnormal sound will be produced. Turbulent blood flow will occur if the the valve fails to fully open (stenotic valve) or if the valve fails to fully close (insufficient valve). A Stenotic Valve has an abnormally narrow opening that impedes the smooth, laminar flow of blood. An Insufficient Valve (incompetent valve) is a leaky valve that allows blood to flow backwards (regurgitate) into the adjacent chamber. The severity of heart murmurs varies across a wide range. Mild murmurs are of no consequence, but severe murmurs can significantly effect heart function.

Rheumatic fever
One of the most common causes of heart murmurs is rheumatic fever. Rheumatic fever is thought to be a autoimmune disorder initiated by a streptococcal bacterial infection. In some individuals, the antibodies directed against the steptococcus bacterium cross-react with heart valve tissue. This causes inflammation and subsequent scarring of the valves, leading to valve stenosis or insufficiency. Prior to the discovery of penicillin, rheumatic fever-induced heart murmurs were relatively common. The prevalent use of antibiotics nowadays has lowered to risk of rheumatic fever and therefore lowered the incidence of heart murmurs.

Cardiac Muscle

The walls of the heart are primarily composed of cardiac muscle. Cardiac muscle cells are called myocardial cells. Like skeletal muscle, myocardial cells are striated in appearance. Unlike skeletal muscle, myocardial cells are branched and connect with other myocardial cells. The site where one myocardial cell joins with another is called an intercalated disc.

Myocardial cells are electrically excitable cells, capable of initiating and propagating action potentials. The propagation of an action potential from one myocardial cell to another occurs by the movement of sodium ions through the gap junction. The gap junction is, in essense, an electrical synapse. The branching architecture of myocardial cells coupled with the presence of gap junctions at intercalated discs creates a high degree of interconnection between myocardial cells. All of the myocardial cells connected through gap junctions are collectively called a myocardium. A myocardium functions as a single unit. When stimulated, the entire myocardium contracts simultaneously. The two ventricles form a myocardium. The two atria, which are separated from the ventricles by the non-conducting fibrous skeleton, form a second myocardium. The synchronous contraction of myocardial cells that make up the myocardium is required to produce the force necessary to pump blood. When myocardial cells contract asynchronously, as occurs in ventricular fibrillation (during a heart attack), then little no blood is pumped.

Cardiac Cycle

The heart undergoes a constant cycle of contractions and relaxations. The period of ventricular contraction is called systole. The period of ventricular relaxation is called diastole.

Events in Diastole

Diastole begins as the ventricles start to relax. Soon the pressures within the aorta and pumonary artery exceed ventricular pressures, causing the semilunar valves to close. As the ventricular pressure falls below the atrial pressure the AV valves open and the ventricles fill with blood. The ventricles fill to about 80% of capacity prior to contraction of the atria, the last event in diastole. Atrial contraction forces the final 20% of the end-diastolic volume (the volume of blood that exists in the ventricles at the end of diastole) into the ventricles.

Summary of Diastole

1. Ventricles relax
2. pulmonary and aortic valves close
3. AV valves open
4. ventricles fill (about 80% of capacity)
5. atria contract (ventricles fill another 20%)

Events in Systole

As the ventricles start to contract, the ventricular pressure soon exceeds the atrial pressure, causing the AV valves to close. As the ventricles continue to contract, the ventricular pressure exceeds the arterial pressures causing the semilunar valves open. Blood is forcefully ejected out of the ventricles and into the aorta and pulmonary artery.

Summary of Systole

1. ventricles contract
2. AV valves close
3. aortic and pulmonary valves open
4. blood is ejected

Electrical Activity of the Heart

Although the heart is enervated by the autonomic nervous system, the heart does not require the nervous system to function. If all the nerves going into the heart are cut, the heart will continue to beat. This is because the heart is autorhythmic, meaning it generates its own rhythmic action potentials independent of the nervous system.

The rhythmic beating of the heart is controlled by a small group of cells in the wall of the right atrium, collectively called the sinoatrial node (typically referred to as the SA node). Because the SA node controls heart rate, it is called the pacemaker of the heart. The autorhythmic cells of the SA node intiate action potentials at a constant rate because they do not have a resting membrane potential. Rather their membrane potential is always drifting towards threshold. This slow drift toward threshold is called the pacemaker potential. When threshold is reached, an action potential is fired. Again the membrane potential drifts toward threshold and another action potential is fired. The cycle of drift and fire repeats itself over and over again in a rhythmic manner.

Conduction System

Action potentials that originate in the SA node spread to the myocardial cells of the atria through gap junctions between cells. Depolarization of the atria stimulates contraction of the atrial myocardium. Action potentials cannot directly spread from the atrial myocardium to the ventricular myocardium due to the presence of the non-conducting fibrous skeleton that separates them. Rather, the impulse travels to the ventricles through a system of specialized cells called the conduction system. The conduction system is composed of the atrioventricular node (AV node), Bundle of His, bundle branches and Purkinje fibers.

Defects of the Conduction System
All of the cells of the conduction system are autorythmic, but differ in the rate at which they discharge action potentials. The SA node has the fastest rate of discharge, therefore it sets the pace of the heart. Sometimes a region of the heart other than the SA node develops a even faster rate of discharge and thus takes over the role of pacemaker. A pacemaker other than the SA node is called an ectopic pacemaker, or more commonly an ectopic foci. Ectopic foci are abnormally excitable regions of the heart and occur most commonly in damaged regions of the conduction system. Ectopic foci are caused by heart disease, especially by ischemia (oxygen deprivation) resulting from poor coronary blood flow. Ectopic foci cause an abnormal sequence of contraction of different regions of the heart and greatly diminish the effective pumping of the heart.

Another common electrical defect of the heart is heart block. In Heart Block, the impulse traveling from the SA node to the AV node is either impeded or, in the most severe case (complete heart block), entirely blocked. In this case, the AV node becomes the pacemaker for the ventricles (remember that the AV node cells are also autorhythmic), but at a much slower rate than the SA node. In complete heart block, the atria continue to beat at their normal rate, while the ventricles beat at a slower rate. The asynchrony of atrial and ventricular contractions coupled with the slow ventricular beat leads to poor pumping efficiency. Individuals that suffer from heart block are candidates for an artificial pacemaker, an electrical device that sets the heart rate automatically.