The Circulatory System

The heart and system of blood vessels are important in the normal healthy functioning of the human body.  The organs and tissues of the body need oxygen and nutrients to function properly.  Blood circulates oxygen from the lungs and nutrients from the intestines throughout the body and carries waste products away from the tissues so they can be filtered and eliminated.

In a simplified version the circulatory system consists of a pump, the heart, and a series of tubes, the vessels.  Blood vessels that carry blood away from the heart are called arteries.  Typically these arteries are filled with blood fully saturated with oxygen, call oxygen rich blood.  As blood leaves the heart it enters the main artery in the body called the aorta.  When the aorta loops and descends to the lower body it branches and these branches provide blood to the arms and head.  Each time arteries divide they gradually become smaller.  After a series of divisions the small arteries become arterioles.  Later theses arterioles branch into capillaries.  It is in these capillaries that oxygen, nutrients and wastes are exchanged between the blood and body tissues.  Blood leaving the capillaries is devoid of of oxygen and is called oxygen poor blood.  It then travels through a system venuoles to veins and back to the heart.  This describes the systemic circulation of blood.  There is also another loop the provides blood through the lungs to become oxygenated.  This is called the pulmonary circulation.

The heart is the vital organ of the circulatory system.  It is the main pump that forces blood through the system of arteries and veins.  The heart is composed of four chambers.  The two atria function to collect blood from the veins and pump it into the two ventricles.  The ventricles, in turn, serve to collect blood from the atria and pump the blood to the system of arteries.

Oxygen poor blood enters the heart from the systemic veins at the right atrium.  When the right atrium contracts it forces blood into the right ventricle.  The blood flows through a valve which prevents a back flow of blood back into the right atrium.  Contraction of the right ventricle causes blood to flow through another one way valve and to the lungs where it becomes saturated with oxygen.  As blood leaves the lungs it re-enters the heart at the left atrium.  Like the right side, the blood flow on the left travels from the left atrium through a valve into the left ventricle, through yet another valve and to the systemic circulation.

The heart is composed of muscle tissue that has the unique property of conducting electrical impulses.  These electrical stimuli are what causes the cardiac muscle to contract and form a beat.  The electrical signals begin a the top of the heart in the SA node.  The electrical wave pass through the atria and cause both right and left atria to contract simultaneously.  When the signal reaches the wall between the atria and ventricles it passes through at one point called the AV node.  This is much like running water though a funnel.  The AV node sends the electrical signal through the wall between both ventricles where it travels along electrical bundles.  As the electrical signal reaches the ventricular muscle fibers it causes a coordinated contraction of both right and left ventricles.

At times the normal electrical pattern of the heart malfunctions or the signal become blocked which causes the heart to beat irregularly.  These irregularities can often be detected and identified by reading the flow of electrical waves produced by the heart on a heart monitor.  Normal conduction consists of a P wave, a QRS complex and a T wave.  The P wave represents the impulse which cause the atria to contract.  Shortly after the P wave is the QRS complex which is the signal for the ventricles to contract.  As the heart prepares for the next contraction it experiences a repolarizing wave called the T wave.  Normally there is an equal spacing between QRS waves signaling a constant methodical series of contractions.

This page is meant for public education in basic medical safety issues.  The Cedarville Township Fire Department is not responsible for any further complications resulting from improper use of information contained on this page.

Cedarville Township Volunteer Fire Department
19 South St.
Cedarville, OH, 45314
Phone: 937-766-5851 - Email: