As we saw earlier in this activity, the mammal heart is a four-chambered structure. Blood flows from the right ventricle to the lungs through the pulmonary artery. In the lungs, it picks up oxygen and then flows back to the left atrium through the pulmonary vein. The oxygenated blood is pumped to the left ventricle. From there it is pumped out to the organs of the body via the aorta.
After reaching the capillaries, this blood is collected in veins and then flows back into the right atrium through a vein called the anterior vena cava. And the process begins once again.
The sequence of events from entry of the blood into the atria to the pumping of the blood out to the body is called the heart cycle.
A system of valves inside the heart prevents the blood from moving in the reverse direction. These valves are flaps of connective tissue. There are valves between the atria and the ventricles, and between the ventricles and the major arteries connected to the heart.
The blood flow through the lungs is referred to as pulmonary circulation. The blood flow through the body’s organs is called systemic circulation.
The noises that your physician hears with a stethoscope are in fact the sounds of the heart valves closing.
The first sound is made by the blood reversing its flow and hitting the closed valves between the atria and the ventricles.
The second sound is caused by blood reversing its flow and hitting the valves leading from the ventricles to the major arteries.
You may think there is a complex series of nerve impulses from the brain to control the heart beat, but this is not so. If all nerves to the heart are severed, the heart will still beat, although the nervous system can speed up or slow down the rate at which the heart beats.
Recall that heart muscle, or cardiac muscle, consists of a network of interconnected fibers. Part of the right atrium—the pacemaker or SA node — generates electrical impulses that travel across the heart and set the rate at which the cardiac muscle contracts.
These impulses don’t pass across to the ventricles in a wave. Instead they move to a second node called the AV node at the junction of the atria and the ventricles. Here, the impulses are delayed for about a tenth of a second, enabling the ventricles to fill with blood.
The impulses then shoot down to the apex of the heart via a series of specialized muscle fibers that form a conduction pathway, and the impulses then spread over the ventricles from the apex back toward the ventricles.
Let’s watch this sequence of contractions again, this time watching an electrocardiograph trace of the impulses. An electrocardiograph is a machine that records electrical currents generated by cardiac muscle impulses.
First the SA node generates the wave of impulses.
Next the atria contract.
After this the impulses are delayed for a short time.
Now they move to the heart apex.
And finally they initiate contraction of the ventricles.
Now that we’ve looked at the electrocardiograph trace of the heart beat, see if you can match up the different phases of the electrocardiograph with the correct part of the heart cycle.
Click Submit to see if you’re correct.
Correct: That’s right!
All others: Sorry, that’s not quite right. Think about what the spikes on the electrocardiograph represent, and try again.
All: A shows the relaxed heart as blood flows into the atria and ventricles.
In B, blood is pumped from the atria into the ventricles.
C shows impulses traveling to the apex of the heart.
In D, blood is pumped from the ventricles to the major arteries.
The circulatory system is of vital importance in the well-being of the body. More than one half of all deaths in the United States are caused by failure of the heart or blood vessels.
Most circulatory disease occurs because arteries become clogged by the build up of plaque, causing a thickening of the artery wall made up of fatty deposits. This process is called athero-sclerosis. In some cases, the thickening becomes hardened by the formation of calcium deposits, a process called arterio-sclerosis.
Various factors add to the risk of plaque formation, including high blood pressure, cholesterol level, activity, smoking, and being overweight.
Smoking is a major cause of cardiovascular disease. If you are under 50, smokers are five times as likely as non smokers to die of coronary heart disease. If you stop smoking, you’ll also avoid lung diseases like cancer and emphysema.
If the clogged circulation affects the coronary arteries, which provide oxygen to the heart muscle, a heart attack may result. If the clogged arteries affect the blood supply to the brain, death of nervous tissue may occur, causing a stroke.
Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education