The heart is a tough, hollow muscle about the size of a fist that pumps blood. The heart is enclosed in a tough sack of tissue called the pericardium. The average human heart beats about 70 times a minute, and with each beat it pumps about 5 ounces of blood, or about 4,000 gallons of blood each day. The heart rests in between beats, so it works less than half the time and rests more than half the time. In a 70 year lifetime, the average human heart works for about 30 years and rests for about 40 years.
The heart is not shaped like a valentine heart. Instead, it is pear-shaped. The heart lies in the center of the chest. The apex (bottom) of the heart lies to the left of the center of the chest. Since the top of the heart is anchored by large blood vessels and the apex is not, we feel a heart beat in the apex. This is why the heart appears to be a left-sided organ.
There are three kinds of muscle: striated muscle which controls voluntary actions. Smooth muscle which controls involuntary actions. Cardiac muscle which is found only in the heart. Cardiac muscle is like a combination of smooth and striated muscle.
The heart has four chambers. Two atria or auricles, whose primary function is to receive blood, and two ventricles whose primary function is to pump blood. The right half of the heart receives oxygen depleted blood from the veins of the body. The function of the right half of the heart is to pump blood to the lungs where it picks up a new supply of oxygen. The left half of the heart receives oxygenated blood from the lungs and pumps it into the body through the arteries.
The right atria or auricle receives blood from two large veins. The superior vena cava brings oxygen poor blood from the upper part of the body and the inferior vena cava brings oxygen poor blood from the lower part of the body. The right ventricle pumps this oxygen poor blood to the right and left pulmonary arteries which take it to the lungs. After blood is oxygenated in the lungs it flows back to the left side of the heart by way of the pulmonary veins. This oxygenated blood is received by the left atria and the left ventricle pumps this oxygenated blood out of the heart into the artery called the aorta.
The heart has many valves. The valves between the atria and ventricles are called the atrioventricular valves. The valves which lead out of the ventricles are named for the blood vessels that they guard. Where the right ventricle meets the pulmonary vein is the pulmonary valve. Where the left ventricle meets the aorta is the aortic valve.
The heart functions in cycles. These cycles are:
1. The right atria is filled with blood from the vena cavas.
2. This blood is passed from the right atria to the right ventricle.
3. At the same time, oxygenated blood from the lungs comes from the pulmonary veins to the left side of the heart, filling the left atria.
4. This blood is passed from the left atria to the left ventricle.
5. To pass blood from atria to ventricles, the atria contract to force the blood to pass from atria to ventricles. This atrial contraction is called the atrial systole.
6. The ventricles contract. This is called the ventricular systole.
Valves of the heart will open in only one direction so they keep the blood flowing in the proper direction. They also give us blood pressure. When the auricles fill with blood, the atrial systole forces open the atrioventricular valves so that blood enters the ventricles. When the ventricles contract (ventricular systole), this forces the atrioventricular valves to close and forces open the pulmonary and aortic valves. The closing of these valves makes a noise.
The sound of a heart beating is often described as "lub dub."
The lub is the closing of the atrioventricular valves. The dub is the closing of the pulmonary and aortic valves.
When the heart is contracting it is called systole. When the heart is relaxing it is called diastole. Therefore, you have two kinds of blood pressure. The pressure which exists in the arteries when the heart is contracting is called systolic blood pressure. The pressure which exists in the arteries when the heart is relaxing is called diastolic blood pressure. A measure of blood pressure will always have these two numbers. The systolic number is always higher than the diastolic number. The blood pressure cuff which is used to get a blood pressure reading is called a sphygmomanometer.
On the back side of the heart above the right atria is located a bundle of nerve cells called the sinoatrial node (SA node). The SA node controls the pace or rate which the heart beats. For this reason the SA node is also known as the pacemaker of the heart. It is the SA node which cause the atria to contract.
The wall which separates the right and left halves of the heart is called the septum. Near the septum is another bundle of nerve cells called the atrioventricular node (AV node). The AV node has nerve cells which spread down the septum between the ventricles and curves around the apex (bottom) of the heart. This bundle is connected to fibers called the Purkinje fibers which connect to walls of the ventricles.
Electricity from these nerve cells allow the Purkinje fibers to make the ventricles contract. So although it is the SA node which causes the atria to contract, it is the AV node which causes the ventricles to contract.
HEART DISEASE
Even though the heart functions to pump blood, it has its own arteries, veins and capillaries which supply its cells with blood. The arteries which supply the heart with blood look like a crown upon the heart. Because of this, they are referred to as being the coronary arteries.
If a coronary artery gets clogged, the heart cells which formerly depended upon that artery for blood get less blood. This may cause the heart to hurt. Since the brain has never received a pain message from the heart before the brain has trouble telling exactly where the pain is coming from. Because of this, the brain often thinks that the pain is in the general area of the chest, back, or in the arm pit. This pain is called angina pectoris.
If a coronary artery becomes completely blocked, the heart tissue which is blocked from receiving blood dies. If a large area is affected, this can cause a major heart attack and death. Heart disease which occurs because of clogged or blocked coronary arteries is called coronary heart disease.
To correct the clog or blockage, a vein is taken from the upper leg and is used to by-pass the clogged or blocked artery. This surgical procedure is called a coronary by-pass.
Various abnormalities of heart beat are called arrhythmia. Arrhythmia can be cause by many things.
The sounds of a heart can be heard through a devise called a stethoscope. A normal human heart makes the "lub dub" sound. If a soft swishing or hissing sound is heard, this may indicate a heart "murmur." A heart murmur often indicates that blood is leaking back through an imperfectly closed valve.
If there is a hole in the septum (the wall which separates the right from left heart) old blood in the right heart can mix with oxygenated blood in the left heart. The most common septal defect is called patent ductus arteriosus. The lungs of the unborn are filled with amniotic fluid. Because of this, there is an artery that runs through the septum so that blood can by-passes the lungs. This artery, called the ductus arteriosus, must close shortly after birth. If it does not, surgery may be required to close it so that old and oxygenated blood will not mix.
An instrument called an electrocardiograph can detect and record the electrical
activity of the heart. The record of the electrical activity of the heart is
called an electrocardiogram or EKG. The EKG is often used to help diagnose heart
disease.